orion electronics ltd. gps technical reference guide

ORION ELECTRONICS LIMITEDglobal leader of gps tracking solutions

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T E C H N I C A L R E F E R E N C E G U I D E r e v 1 . 3For more information on any of the topics discussed in this manual, please refer to the URL reference index. 3

O R I O N E L E C T R O N I C S L I M I T E D

Special Thanks and Acknowledgements

This technical reference guide was a substantial undertaking and could not have been completed without

the support of many exceptional people. The information that is included in this handbook has been

derived from several sources, and it is only with the valuable input of these many specialized individuals,

that the information presented here is as valid and accurate as it resulted in being. We sincerely thank all

those who have provided content over the years, and to those who continue to provide new information

on emerging technologies.

Above all else, thank-you, the readers and loyal patrons of Orion Electronics for making us strive to

serve you bett er everyday. None of this would have been possible if not for your ongoing support of our

products and services. We always welcome your feedback and look forward to improving this manual to

serve you bett er in future years!

T E C H N I C A L R E F E R E N C E G U I D E r e v 1 . 3For more information on any of the topics discussed in this manual, please refer to the URL reference index.4


TABLE OF CONTENTSCHAPTER 1 INTRODUCTION TO GPS Chapter 1 - Introduction .......................................................................................................................................... 6 1.1 Global Positioning System (GPS) ............................................................................................................... 7 1.2 Law Enforcement Uses of GPS ................................................................................................................... 9 1.3 GPS - One More Tool in the Belt ..............................................................................................................10 1.4 Starting a New Case Using GPS ...............................................................................................................11 1.5 Assisted GPS (AGPS) ....................................................................................................................................13 1.6 Enhanced GPS ...............................................................................................................................................15 1.7 Differential GPS .............................................................................................................................................16 1.8 Errors in GPS ...................................................................................................................................................17 Improving GPS Accuracy ...........................................................................................................................18 1.9 GPS Has Its Day in Court ............................................................................................................................20

CHAPTER 2 GPS TRACKING FOR LAW ENFORCEMENT Chapter 2 - Introduction ........................................................................................................................................22 2.1 Data Logging / Memory Tracking ...........................................................................................................23 2.2 Near Real Time Tracking .............................................................................................................................24 2.3 Real Time / Live Tracking ............................................................................................................................25 2.4 Multi / Team Tracking ..................................................................................................................................26

CHAPTER 3 WIRELESS COMMUNICATIONS Chapter 3 - Introduction ........................................................................................................................................30 3.1 Wireless RF Spread Spectrum Unlicensed Radio ..................................................................................................31 Radio Frequency Direction Finding - (RF) Tracking ..........................................................................32 Long Range RF (Loran) ................................................................................................................35 Short Range RF (Zigbee & Bluetooth) ...................................................................................37 Paging Overview ..........................................................................................................................................40 Cellular Intercept Receiver Tracking ......................................................................................................42 3.2 Introduction to Digital Communication ..............................................................................................43 3.3 Wireless Data Networks .............................................................................................................................44 Circuit Switched Communication ..........................................................................................................45 Packet Switched Communication ..........................................................................................................46 Analog Cellular - AMPS...............................................................................................................................47 Digital Cellular CDMA ................................................................................................................................................48 GPRS ..................................................................................................................................................49 CDMA 2000 .....................................................................................................................................50 W-CDMA ..........................................................................................................................................51 GSM ............................................................................................................................................52 TDMA/ D-AMPS .............................................................................................................................53 Digital Data over AMPS CDPD .................................................................................................................................................54 Low Speed Packet Radio DataTAC ............................................................................................................................................55 Mobitex ............................................................................................................................................56 High Speed Packet Radio iDEN ...................................................................................................................................................58 Data Over Low-Orbitting Satellite ..........................................................................................................59 3.4 Wireless Communications Summary ....................................................................................................61 3.5 Coverage Maps Examples .........................................................................................................................62 3.6 International Standards and Roaming .................................................................................................63



CHAPTER 4 GPS ACCESSORIES Chapter 4 - Introduction ........................................................................................................................................66 4.1 Antenna Overview .......................................................................................................................................67 GPS Antennas ................................................................................................................................................69 4.2 Camera Surveillance Overview ...............................................................................................................70 4.3 Microphone Overview ................................................................................................................................71 4.4 Batteries ...........................................................................................................................................................72

CHAPTER 5 INSTALLATION TIPS & TOOLS Chapter 5 Introduction ...........................................................................................................................................78 5.1 Installation Preparation ..............................................................................................................................79 5.2 Covert GPS Tracking Installation .............................................................................................................80 5.3 GPS Installation Equipment Checklist ..................................................................................................81 5.4 Legal Aspects of Covert GPS Tracking ..................................................................................................83 5.5 GPS Covert Tracking Unit Installation Tips ..........................................................................................84 5.6 Security Systems ...........................................................................................................................................85 5.7 Example of GPS Digital Switching Setup .............................................................................................86 5.8 Homemade GPS Battery Calculator .......................................................................................................87 5.9 Modem Overview & AT Command Sets ...............................................................................................89

CHAPTER 6 INTRODUCTION TO ORION EQUIPMENT Chapter 6 Introduction ...........................................................................................................................................92 6.1 Product Requirements Flowchart ..........................................................................................................93 6.2 Orion Product Features Matrix ................................................................................................................94

CHAPTER 7 WARNINGS & SAFETY CONCERNS Chapter 7 Introduction ...........................................................................................................................................98 7.1 Battery Disposal ............................................................................................................................................99 7.2 Radio Frequency Communications and Your Health .......................................................................99 7.3 How to Solder Safely ................................................................................................................................ 100 7.4 Electrostatic Discharge (ESD) ................................................................................................................ 101

APPENDICES A Basic Electronics and Defi nitions ......................................................................................................... 104

B Basic Electronic Formulas ....................................................................................................................... 108 C Basic Battery Reference ........................................................................................................................... 109 D Basic Antenna Reference ........................................................................................................................ 110 E Metric Prefi xes ............................................................................................................................................. 111 F At Command Reference Chart .............................................................................................................. 112 G Installation Tool Sources ......................................................................................................................... 113 H Cellular Network Providers (US & CDN) ............................................................................................. 114

I Cellular Network System / Band Frequency Chart ......................................................................... 115

GLOSSARY Glossary of Acronyms and Terms ..................................................................................................................... 118

REFERENCE INFORMATION References and Useful Websites ....................................................................................................................... 134

ORION ELECTRONICS LTD CONTACT INFORMATION Address, Website, Contact Names .................................................................................................................... 142



C h a p te r 1

I n t ro d u c t i o n to G P S



CHAPTER 1 - INTRODUCTION

This chapter will discuss a wealth of information covering GPS technologies and how they can be applied to serve the Law Enforcement community. Each section will review the details of some of the more common forms of GPS, and how these particular systems works.



An Introduction to GPS Each satellite is equipped with an accurate clock, allowing it to broadcast signals coupled with its location and a precise time message. The signals from the satellites travel through the atmosphere at the speed of light, so the time that it takes for the message to reach the receiver can be used, together with the speed of light, to determine the distance that the satellite is from the receiver. To measure precise latitude, longitude, and altitude, the receiver measures the time it took for the signals from four separate satellites to get to it. For a GPS receiver to fi nd your location, it has to determine two things:

The location of at least three, preferably four, satellites above you. (With only 3 satellites, the receiver will be able to give you your location, but not your altitude.)

The distance between you and each of those satellites triangulation.

How Accurate Is It?Using a commercially available GPS receiver, the system can tell you your location anywhere on or above the Earth to within approximately 300 feet. Even greater accuracy, usually within less than three feet, can be obtained with corrections calculated by a GPS

1.1 GLOBAL POSITIONING SYSTEM (GPS)

What is GPS?

The Global Positioning System (GPS) is a system that is able to show you your exact position on the Earth at anytime, and in any weather. It is funded and controlled by the United States Department of Defense (DOD). GPS satellites, 24 in all, orbit at 11,000 nautical miles above the Earth. Five ground stations distributed worldwide, continuously monitor these satellites. The satellites transmit signals that can be detected by anyone with a GPS receiver. Using the receiver, you can determine your location with great precision (Figure 1).

Figure 13 GPS segments: the space segment, the user segment and the control segment

receiver at a known fi xed location. This is what is known as Differential GPS, and will be discussed in greater detail, later in this chapter.

Remember that all reports on accuracy depend on a variety of factors. If you are doing a covert installation, you are rarely going to get ideal reception of the GPS signals. This will decrease your accuracy. Be wary of people telling you that they get accuracy of 3 feet with their commercial grade receivers. Three hundred feet might sound like a lot but if you installed the system on a car, fi nding that car in a 300 foot circle should be pretty easy.

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The application of GPS generally falls into one of the following fi ve real-world categories:

Location - determining a basic position Navigation - getting from one location to another Tracking - monitoring the movement of objects and

people Mapping - creating maps of the world Timing - bringing precise timing to the world

The GPS system was designed by and is controlled by the United States Department of Defense and can be used by anyone, free of charge. Russia operates an independent system called GLONASS (global navigation system), although with only 12 active satellites as of 2004 the system is of limited usefulness. The GPS system is divided into three segments: space, control, and user.

SpaceThe space segment is comprised of the GPS satellite constellation.

ControlThe control segment is comprised of ground stations around the world that are responsible for monitoring the fl ight paths of the GPS satellites, synchronizing the satellites onboard atomic clocks, and uploading almanac data for transmission by the satellites.

UserThe user segment is comprised of GPS receivers used for both military and civilian applications. A GPS receiver (GPSR) decodes time signal transmissions from multiple satellites and calculates its position through trilateration.

Each satellite circles the Earth twice every day at an altitude of 20,200 kilometers (12,600 miles). The satellites carry atomic clocks and constantly broadcast the precise time according to their own clock, along with administrative information including the orbital elements of their own motion, as determined by a set of ground-based observatories.

1. The basis of GPS is triangulation from satellites. 2. To triangulate, a GPS receiver measures distance

using the travel time of radio signals. 3. To measure travel time, GPS needs very accurate

timing which it achieves with some tricks. 4. Along with distance, you need to know exactly

where the satellites are in space. High orbits and careful monitoring are the secret.

5. Finally you must correct for any delays the signal experiences as it travels through the atmosphere. By very, very accurately measuring our distance from three satellites we can triangulate our position anywhere on earth.

By ranging from three satellites we can narrow our position to just two points in space. To confi rm the true GPS location, we can make a fourth measurement. But usually one of the two points is a ridiculous answer (either too far from Earth or moving at an impossible velocity) and can be rejected without a measurement.

Getting Perfect Timing Accurate timing is the key to measuring distance to satellites. Satellites are accurate because they have atomic clocks on board. If measuring the travel time of a radio signal is the key to GPS, then our stop watches had better be darn good, because if their timing is off by just a thousandth of a second, at the speed of light, that translates into almost 200 miles of error! Receiver clocks dont have to be too accurate because an extra satellite range measurement can remove errors.

The Pseudo Random Code (PRC) is a fundamental part of GPS. Physically its just a very complicated digital code, or in other words, a complicated sequence of on and off pulses. The signal is so complicated that it almost looks like random electrical noise. Hence the name Pseudo-Random.

Since each satellite has its own unique Pseudo-Random Code, this complexity also guarantees that the receiver wont accidentally pick up another satellites signal. So all the satellites can use the same frequency without jamming each other. And it makes it more diffi cult for a hostile force to jam the system. In fact, the Pseudo Random Code gives the DoD a way to control access to the system.

Getting to Know GPS How GPS Works in 5 Logical Steps

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Now that we have seen how GPS technology works, and what methods we can use it in, lets examine a few of the GPS specifi c features that pertain to law enforcement offi cials, in particular.

Auto Call OutAlmost all real-time, as well as some near-real time units can perform these functions. Auto call outs are used to monitor a targets activity without 24 hour live monitoring. Most units will notify users of motion, boundary, input, as well as low battery voltage variations. Different manufacturers offer different call out options so check with them for specifi c details.

Motion This specifi cation notifi es the user as soon as the vehicle begins to move, or stops moving.

Boundary The unit can be programmed to page the user on entry or exit of a geographical area, as determined by the user. A notifi cation will be sent if the target should enter or exit this pre-determined boundary.

Input This setting is an internal switch that will notify the user of a sensor being tripped. For example, if a switch has been put in place to monitor the activity of a targets car trunk, a notifi cation will be sent each and every time that trunk is opened.

Low Battery This convenient feature allows the user to set their own battery threshold. Based on this pre-determined level of battery life, the device will page and notify the user once this level has been reached. This is a great tool to warn investigators in advance when the battery is getting low and will soon need replacing.

Offi ce TrackingThis tracking method permits observing real or near real time events from the safety of the offi ce via a modem and land line telephone. Because of the ability to download remotely in real or near real time tracking, offi ce tracking can be applied in either of these cases, thus freeing up the time of the offi cer.

Mobile TrackingTaking a laptop (with the appropriate installed software), a cellular phone (either analog or digital), and obtaining a wireless modem, the user can track in real time from their own vehicle at a safe distance. Self-tracking is also a common feature used in mobile tracking whereby the user can view the exact location of themselves, as well as the target, on a street level map. This allows the agent to determine their proximity to the target at all times, and allows them to follow without being noticed. This feature, only possible in real time units, is ideal for use with cases where the suspect is considered dangerous or may need to be intercepted and apprehended at any point. With mobile tracking, the user can pursue the target through dense traffi c, poor weather conditions, and unfamiliar surroundings, while still being able to visually observe the target once it reaches its destination.

Figure 2GPS satellite orbiting the earth

1.2 LAW ENFORCEMENT USES OF GPS



The goal of most any investigation is to try to get information about a crime that has happened, an ongoing crime, or a crime that is about to happen. Investigators use a variety of tools to get this information; informants, under-cover agents, electronic surveillance devices and plain old legwork. GPS technology is just one more tool in the arsenal of the modern day technical investigator.

The most important part of a GPS system is its ability to get precise positional information. Combine this with an ability to store this information in memory for later retrieval, or transmit it in real time, and you have a really useful tool. It can be used for deducing a persons whereabouts at a given time, or for tying someone to a location.

Figure 4Track across the country or even across the world!

Example: If you have a drug runners car tagged with a GPS system and you see them visit a certain house on a regular basis, you can start a visual surveillance to fi nd out whats in that house. Hopefully its that big cocaine importer youve been tasked with fi nding.

The two best things about a GPS tracking system are its scope and its precision. Look at the maps below. In Figure 21, you have a street level track through Kansas City - perfect for identifying where the person went and when. On Figure 22, you have a track stretching from Halifax, Nova Scotia, Canada to Baton Rouge, Louisiana, USA.

Figure 3Urban tracking on street level maps

1.3 GPS - ONE MORE TOOL IN THE BELT

Figure 5GPS Track Data with

Satellite Imagery



1. Make sure that the units memory has been properly downloaded and stored from any previous cases, so that it may be cleared in preparation for a new case.

2. Make sure that the unit has been supplied with a

fresh set of batteries, as even new batteries, kept in storage will deteriorate over time.

3. Power up the unit and allow it to acquire a current GPS almanac, before installing it for a new case. If the device has not been used for a period of time or the location of the device has changed greatly between the last time it was taking fi xes and now, then the device may need to re-acquire its GPS almanac. The defi nition of the GPS almanac is as follows:

Information transmitted by each satellite on the orbits and state (health) of every satellite in the

GPS constellation. Almanac data allows the GPS receiver to rapidly acquire satellites shortly after it is turned on.

Be advised that this process may take up to 20 minutes, making it vital that this step is performed before installing the unit on a target. Once a unit is installed, its gone and missing this step could easily make or break the case.

4. Perform a brief examination of the unit to ensure it is functioning properly. This should include checking the units settings using any applicable test boxes. This guarantees that the unit is able to accept incoming calls (where applicable), and is taking fi xes correctly.

1.4 STARTING A NEW CASE USING GPS

Acquisition / Almanac

Some very important steps should be taken into account before considering any stored GPS tracking device for a new case. Please be aware that before installing any GPS unit that has been stored for any number of weeks, users should:



Greenwich Mean Time (GMT)

Greenwich Mean Time (GMT) is the international standard time reference, based on the time of day in London, England. As you know, the time on the East Coast is different from that on the West, and as you cross from one Time Zone to the next you change your watch. GPS satellites report current time in Greenwich Mean Time (a.k.a, Universal Time or zulu time). Satellites and Tracking Systems use GMT, because it is the same all over the world.

You can input a local offset (or difference) from GMT so that the unit will display your local time. Your local

Figure 6Global Time Zone Map

time is behind or ahead of GMT depending on where you are in relation to Greenwich, England. Although all GPS data is collected and stored in GMT (or Greenwich Mean Time), data downloaded is often associated with a GMT offset so that it can be viewed in local time. Be sure to set a correct GMT offset, if required, to view your downloaded data in your local time. It will also be worthwhile to note the daylight savings time, where applicable, for the same reason.

A global Time Zone Map is provided below to assist you in determining the appropriate value for local offset in your area.



1.5 AGPS - ASSISTED GPS

Assisted Global Positioning System

Assisted GPS is becoming a commonplace term. Simply put, assisted GPS (AGPS) describes a system where outside sources, such as an assistance server and reference network, help a GPS receiver (as found in more and more of todays cellular phones) map out the position of the GPS receiver. Although GPS provides excellent position accuracy, position fi xes require lines of sight to the satellites. In urban areas, when the user is located in urban canyons, under heavy tree cover, or even indoors, Assisted GPS is a technology that can be used to triangulate the position. It is becoming more popular and is commonly associated with Location Based Services.

There are at least three components to assisted GPS. The fi rst is determining an approximate location of the receiver using some mechanism other than the GPS system. An example of this might be a database which uses the cell tower id to look up the geographical co-ordinates of the cell tower (assuming the GPS receiver is in this case part of a cell phone). The second is using that approximate location to determine where the receiver should be looking for GPS satellites. This requires a combination of observation and sophisticated calculations on the part of the network. The third component is using network resources to move the work of calculation from the GPS receiver, into the network, allowing the receiver to be simpler, cheaper, and less power-hungry.

An assistance server communicates with the GPS receiver via a wireless link. With assistance from the network, the receiver can operate more quickly and effi ciently than it would unassisted, because a set of tasks that it would normally handle is shared with the assistance server. The resulting AGPS system, consisting of the integrated GPS receiver and network components, boosts performance beyond that of the same receiver in a stand-alone mode.

Cellular telephones with embedded GPS engines will be a reality for many in the near future. The development of these phones is being fuelled, in part, by the U.S. Federal Communications Commissions E-911 mandate requiring the position of a cell phone to be available to emergency call dispatchers.

There are three basic types of data that the assistance server provides to the GPS receiver: precise GPS satellite orbit and clock information; initial position and time estimate; and for AGPS-only receivers, satellite selection, range, and range-rate information. The assistance server is also able to compute position solutions, leaving the GPS receiver with the sole job of collecting range measurements.

Figure 7Assisted GPS

Localization Based Systems

LBS are Localization Based Systems. One can fi nd several types: Cell ID The precision of this method is 200 meters

in urban areas, 2 km in suburban areas and 3-4 km in rural zones.

Enhanced Cell ID With this method one can get a precision similar to Cell ID, but for rural areas, with circular sectors of 550 meters.

TOA Time Of Arrival AOA Angle Of Arrival E-OTD This is similar to TOA, but the position is

estimated by the mobile phone, not by the base station. The precision of this method depends on the number of available LMUs in the networks, varying from 50 to 200m.

Assisted-GPS requires a worldwide tracking network for obtaining the navigation messages of all satellites and data processing hubs, along with a server which feeds data to a Serving Mobile Location Center (SMLC) or Mobile Position Center (MPC) operated by a network service provider. Data is sent to individual cell phones using Hypertext Transfer Protocol (HTTP) and the Short Messaging Service (SMS).

AGPS architectures increase the capability of a stand-alone receiver to conserve battery power, acquire and track more satellites, thereby improving observation geometry, and increase sensitivity over a conventional GPS architecture. These enhanced capabilities come from knowledge of the satellite position and velocity, the initial receiver position, and time supplied by the assistance server.



Assisted GPS & Time-to-First-Fix

Time-to-fi rst-fi x (TTFF) is reduced as a result of the fewer frequency bins which must be searched to acquire the signal. TTFF is further reduced because the receiver no longer has the task of decoding the navigation data bits, a task that takes tens of seconds. Instead, the assistance server provides the satellite orbit and clock parameter values to the receiver. Shorter TTFF results in reduced power consumption because the system does not have to wait for the GPS receiver to decode the navigation data for each visible satellite. See Figure 8 for a visual comparison between TTFF for assisted and unassisted GPS.

Increased receiver sensitivity is directly related to the TTFF and the number of frequency bins which must be searched to fi nd a satellite signal. Because the receiver has fewer frequency bins to search in an AGPS architecture, it can dwell in each bin for longer periods of time. This additional dwell time increases the sensitivity of the receiver, so that it can use signal strengths below the conventional thresholds to make range measurements. In addition, when the higher sensitivity is required, the navigation data bits would be diffi cult if not impossible to decode. Therefore, this technique allows the use of satellite data which would have otherwise been unavailable.

Although discussions of TTFF and navigation data bits are compelling to engineers, the real reason for implementing AGPS is customer satisfaction when using location or E-911 services. With AGPS, the position can be computed more quickly, on the order of a few seconds. If the position solution took minutes, as is common with warm starts in conventional GPS receivers, the consumer might become frustrated while waiting and wonder whether there was anything wrong with the phone. The typical cell phone consumer has grown accustomed to applications which work in a few seconds. Location services should behave the same way to gain customer acceptance beyond those already familiar with, and accustomed to, the performance of GPS receivers.

AGPS assistance data will be transmitted by cellular network operators using CDMA packet data (1X) or GPRS. Assistance is possible since the cellular operator can determine, roughly, where the mobile phone is by using techniques such as cell site identifi cation, or triangulation of cellular voice or data signals.

Figure 8Assisted GPS vs Unassisted GPS



The words Code-Phase and Carrier-Phase may sound like electronic mumbo-jumbo but, in fact, they just refer to the particular signal that we use for timing measurements. Using the GPS carrier frequency can signifi cantly improve the accuracy of GPS.

Remember that a GPS receiver determines the travel time of a signal from a satellite by comparing the pseudo random code its generating, with an identical code in the signal from the satellite. The receiver slides its code later and later in time until it syncs up with the satellites code. The amount it has to slide the code is equal to the signals travel time.

Thats the problem with code-phase GPS. Its comparing pseudo random codes that have a cycle width of almost a microsecond. And at the speed of light a microsecond is almost 300 meters of error!

So the trick with carrier-phase GPS is to use code-phase techniques to get close. Resolving this carrier phase ambiguity for just a few cycles is a much more tractable problem and as the computers inside the receivers get smarter and smarter its becoming possible to make this kind of measurement without all the ritual that surveyors go through.

Enhanced GPS (or Super Sensitive GPS) are terms used to describe a new generation of GPS receivers. These receivers are much more sensitive than older types, and are often able to determine their position in very tough areas. The E-GPS seeks to capture the best-quality signals from either satellites or cellular base stations, whichever has a preferred position fi x at that precise time. Enhanced GPS receivers dont need to use information from any external source, however, enhanced GPS receivers can be more reliable with the help of assistance data.

What does this mean? When considering a GPS system, the use of Assisted GPS or Enhanced GPS is a plus. Any tool that can allow you to get positional information in areas where traditional GPS cant perform is a good thing.

1.6 E-GPS ENHANCED GPS

The Evolution of GPS Enhanced GPS

Figure 9Diagram of DBS E-GPS, also known as DE-GPS

Using a conventional GPS receiver in dense urban areas, a large portion of the sky is frequently obstructed by buildings, making it impossible to see a suffi cient number of GPS satellites. The DE-GPS technique allows a GPS receiver to use the signal from a DBS satellite when it is in a bad spot. To do so, the DE-GPS receiver must obtain assistance information from a remote server. This assistance information enables the DE-GPS receiver to calculate the range information from the non-GPS satellites for the location fi x.

Sensitivity Comparison Chart for Various Enhanced GPS Receivers

u-Blox LEA-LA u-Blox TIM-LH Fastrax iTrax03

Tracking Sensitivity -149dBm -158dBm -152dBm

Acquisition -140dBm -142dBm



Differential GPS Systems

Differential GPS (DGPS) is the blanket term for a number of different systems that have evolved to overcome the inaccuracies caused by SA and the physics involved in sending radio signals through the earths atmosphere. Differential GPS, a way to correct the various inaccuracies in the GPS system thereby pushing its accuracy even farther. Differential GPS or DGPS can yield measurements good to a couple of meters in moving applications and even better in stationary situations.

Thats the idea behind Differential GPS. We have one receiver measure the timing errors and then provide correction information to the other receivers that are roving around. Instead of using timing signals to calculate its position, it uses its known position to calculate timing. It fi gures out what the travel time of the GPS signals should be, and compares it with what they actually are. The difference is an error correction factor.

These systems are separated into two classifi cations: Real-Time Differential and Post Processed Differential.

Real-Time Diff erential is correction where signals are received from the differential provider, and used by the receiver at the same time as signals from the GPS satellite are received to calculate a much more precise position instantaneously.

Post Processing is Differential correction that takes place after the fact: the GPS data collected in the fi eld is saved, and the Differential correction is added at a later date.

The roving receivers get the complete list of errors and apply the corrections for the particular satellites theyre using. Many new GPS receivers are being designed to accept corrections, and some are even equipped with built-in radio receivers.

Theres another permutation of DGPS, called Inverted DGPS that can save money in certain tracking applications. With an inverted DGPS system, vehicles would be equipped with standard GPS receivers and a transmitter and would transmit their standard GPS positions back to the tracking offi ce. Then at the tracking offi ce, the corrections would be applied to the received positions.

Wide Area Augmentation System, or WAAS is basically a continental DGPS system. The FAA set up a monitoring system with an incredibly fast response time. In fact, they fi gured they could park a geosynchronous satellite somewhere over the U.S. that would instantly alert aircraft when there was a problem. Then they reasoned that they could transmit this information right on a GPS channel so aircraft could receive it on their GPS receivers and would prevent the need for any additional radios. The ramifi cations of this go well beyond aviation, because the system guarantees that DGPS corrections will be raining out of the sky for everyone to use.

To complete the system, the FAA is planning on eventually establishing Local Area Augmentation Systems near runways. These would work like WAAS, but on a smaller scale.

1.7 DIFFERENTIAL GPS



1.8 ERRORS IN GPS

The US Air Force has injected each GPS satellite into a very precise orbit, according to the GPS master plan. The basic orbits are quite exact but just to make things perfect the GPS satellites are constantly monitored by the Department of Defense. They use very precise radar to check each satellites exact altitude, position and speed. The errors theyre checking for are called ephemeris errors because they affect the satellites orbit or ephemeris. These errors are caused by gravitational pulls from the moon and sun and by the pressure of solar radiation on the satellites. The errors are usually very slight but if you want great accuracy they must be taken into account.

There are a couple of ways to minimize this kind of error. For one thing we can predict what a typical delay might be on a typical day. This is called modeling and it helps but, of course, atmospheric conditions are rarely exactly typical. Another way to get a handle on these atmosphere-induced errors is to compare the relative speeds of two different signals. This dual frequency measurement is very sophisticated and is only possible with advanced receivers. Trouble for the GPS signal doesnt end when it gets down to the ground. The signal may bounce off various local obstructions before it gets to our receiver. This is called multipath error and is similar to the ghosting you might see on a TV. Good receivers use sophisticated signal rejection techniques to minimize this problem.

Sources of GPS Errors

Satellite clock drift (1.5 m) (1usec = 300m)

Orbit estimation errors (2.5 m)

Atmospheric and relativistic effects (5.5 m)

Receiver noise (0.3 m)

Multipath interference (0.6 m)

Intentional randomization to reduce civilian grade accuracy (30m)

Ephemeris data errors: 1 meter

Tropospheric delays: 1 meter.

Unmodeled ionosphere delays: 10 meters.

Blunders can result in errors of hundred of kilometers.

For standard GPS (without Differential correction), the expected accuracy of a handheld GPS receiver is +/-15 meters. Horizontal Dilution of Precision (HDOP) is caused by satellite geometry. If the GPS receiver is using satellites in the same area of the sky as opposed to being distributed across the horizon, the location of the receiver becomes increasingly uncertain. Good geometric position of the satellites is ideal.

GPS errors are a combination of noise, bias, blunders. Noise errors are the combined effect of Pseudo Random Number (PRN) code noise (around 1 meter) and noise within the receiver noise (around 1 meter). Bias errors result from Selective Availability and other factors. SA is the intentional degradation of the SPS signals by a time varying bias. For more information on Selective Availability, please refer to the next page.



Diff erential GPS: A way of collecting Global Positioning Systems data with increased accuracy. Using this technique, data from a receiver at a known location is used to correct the data from a receiver at an unknown location. Using a network of fi xed ground based reference stations. These stations broadcast the difference between the measured satellite pseudo ranges and actual (internally computed) pseudo ranges, and receiver stations may similarly correct their pseudorange estimates. Differential GPS can eliminate almost all error.

Clocks: Both GPS satellites and receivers are prone to timing errors. These errors can be calculated and eliminated once the receiver is tracking four satellites.

Ionosphere: The Ionosphere is one of the leading causes of GPS error. The speed of light varies due to atmospheric conditions. By comparing the phase difference between the L1 and L2 signals, the error caused by the ionosphere can be calculated and eliminated. Special dual frequency receivers are required to make use of both L1 and L2.

Wide-Area Augmentation System (WAAS): This uses a series of ground reference stations to calculate GPS correction messages, which are uploaded to a series of additional satellites in geosynchronous orbit for transmission to GPS receivers. The correction message includes information on ionospheric delays, individual satellite clock drift, and so on. The current WAAS system only works for North America, and due to the satellite location the system, is most suited for use in the eastern and western coastal regions. GPS receivers may also use WAAS satellites for navigation, similarly to the other GPS satellites.

Local-Area Augmentation System (LAAS): This is similar to WAAS, in that similar correction data are used. But in this case, the correction data are transmitted from a local source, typically at an airport or another location where accurate positioning is needed. These correction data are typically useful for only about a thirty to fi fty kilometer radius around the transmitter.

Wide Area GPS Enhancement (WAGE): is an attempt to improve GPS accuracy by providing more accurate satellite clock and ephemeris (orbital) data to specially-equipped receivers.

Relative Kinematic Positioning (RKP): is another approach for a precise GPS-based positioning sytem. In this approach, accurate determinination of range signal can be resolved to an accuracy of less than 10 centimeters. This is done by resolving the number of cycles in which the signal is transmitted and received by the receiver. This can be accomplished by using a combination of differential GPS (DGPS) correction data, transmitting GPS signal phase information and ambiguity resolution techniques via statistical tests - possibly with processing in real-time (real-time kinematic positioning, RTK).

Selective Availability: In the past, the civilian signal was degraded, and a more accurate Precise Positioning Service was available only to the United States military, its allies and other, mostly government users. On May 1, 2000, however, then US President Bill Clinton announced that this Selective Availability would be turned off, and so now all users enjoy nearly the same level of access, allowing a precision of position determination of less than 20 meters. For military purposes Selective Deniability may still be used to, in effect, render civilian GPS units useless in a particular geographic area, while still allowing military units to have full functionality.

Multipath: The antenna receives not only direct GPS signals, but also multipath signals: refl ections of the radio signals off the ground and/or surrounding structures (buildings, canyon walls, etc). Allow your GPS receiver to see the sky for optimal performance.

Improving GPS Accuracy

100 meters Accuracy of the original GPS system, which was subject to accuracy degradation under the government-imposed Selective Availability (SA) program.

15 meters Typical GPS position accuracy without SA.

3-5 meters Typical differential GPS (DGPS) position accuracy.

< 3 meters Typical WAAS position accuracy.

Table 1Comparison of GPS Correction Methods



What is WAAS?The Wide Area Augmentation System (WAAS) is a form of differential GPS (DGPS) giving enhanced position accuracy. Systems such as WAAS are known as satellite-based augmentation systems (SBAS). This system of satellites and ground stations provide GPS signal corrections, resulting in up to fi ve times better position accuracy. WAAS corrects for GPS signal errors caused by ionospheric disturbances, timing, and satellite orbit errors, and it provides vital integrity information regarding the health of each GPS satellite. A WAAS-capable receiver can give you a position accuracy of better than three meters, 95% of the time. Users do not have to purchase additional receiving equipment or pay service fees to utilize WAAS. As long as you own a WAAS-enabled GPS receiver, and use it within the reach of this system, you can reap the benefi ts of this system.

How it WorksWAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through two satellites with a fi xed position over the equator (in geostationary orbit). The goal of WAAS was to obtain at least a 7-meter horizontal and vertical accuracy, but has been confi rmed at 1 - 2 meters horizontal and 2 -3 meters vertical throughout the majority of the continental U.S. and portions of Alaska.

The Origins of WAASThe Federal Aviation Administration (FAA) and the Department of Transportation (DOT) developed the

WAAS program to be a GPS-based navigation and landing system that provides precision guidance to aircraft at thousands of airports and airstrips where there is currently no precision landing capability. Currently, GPS alone does not meet the FAAs navigation requirements for accuracy, integrity, and availability. WAAS was designed to improve the accuracy and ensure the integrity of information coming from GPS satellites.

Who benefi ts from WAAS?Anyone with a WAAS-Enabled GPS receiver may receive the enhanced positional information provided by WAAS. Currently, WAAS satellite coverage is only available in North America. In some instances however, WAAS signals may be obscurred even if GPS reception is possible. In that case, the GPS signals will not be corrected. WAAS signal reception is ideal for open land and marine applications. WAAS provides extended coverage both inland and offshore compared to the land-based DGPS (differential GPS) system.

Other governments are developing similar satellite-based differential systems. Japan developed the Multi-Functional Satellite Augmentation System (MSAS), while Europe has the Euro Geostationary Navigation Overlay Service (EGNOS). Eventually, GPS users around the world will have access to precise position data using these and other compatible systems.

A Closer Look at The Wide Area Augmentation System

Figure 10Diagram of Wide Area Augmentation System



The days when GPS technology was new and unheard of are long gone. Today, most car manufacturers offer a GPS navigation system in one or more models, and commercial GPS devices are readily available. This increase in public awareness has also resulted in GPS based technology being introduced into courts of law across North America. Introducing GPS technology can be a diffi cult process as defense lawyers raise questions about the legality of installation, credibility of information, and what exactly GPS data tells the courts. Described below are a few hints taken from Orions experience with testifying in various court cases on the use of GPS based surveillance systems.

What does it tell us? Although modern covert GPS systems are much more accurate than they used to be, there still can be some apparent errors such as spikes between fi xes or tracks that are not quite on the road, etc. The error inherent in GPS location systems, combined with the often less then ideal location of the GPS antennas may introduce small errors into the GPS record. If asked about the accuracy of the GPS records, emphasize the general accuracy, but state the possibility of error and discuss the fact that it is not one record in particular that is important, but rather the story that the GPS data tells, as a whole. Static during a TV movie doesnt change the plot. For example: repeated visits back to a murder site, or regular visits to a known drug suppliers house could be indications of wrongdoing, especially when combined with your other evidence.

If some of your GPS data is backed up by visual surveillance, use that to give credibility to the rest. If you have the ability to get satellite overview images of the area, use them in court. They are much more impressive to Jury members, who may not be as convinced by traditional maps.

Motion Sensor: Make sure that they know that the motion on the print out comes from a separate motion detector, and therefore it might say motion when the vehicle is stationary. Same thing for speed because it is calculated it might say 1mph or so, even when the vehicle is stationary. This is normal, and not evidence of problems in the technology.

Legality of installation: Hopefully before you ever get to court, you will have closely examined the laws regarding the installation of surveillance equipment to your target. The laws regarding installation of parasitic and non-parasitic devices can vary widely from region to region. The best approach is to check with your local legal experts and ensure you follow all the rules and regulations. You dont want to be in a situation where your entire case gets thrown out because you failed to procure the appropriate warrants.

Conclusion: GPS is a popular, and if used correctly, an incredibly useful tool for most any investigation. While most of us prefer not to have GPS data discussed in a court of law, the chances are it may happen. If it does happen, dont forget the three items discussed earlier:

1. GPS data is a generally accurate description of the general movements of the target

2. Errors do occur but do not invalidate all the recorded information, and fi nally

3. Make sure all your installations comply with your local laws.

1.9 GPS HAS ITS DAY IN COURT



C h a p te r 2

L aw

E n fo rce m e n t

Tra c k i n g &

Te c h n o l o g y Us a g e




This chapter discusses the various methods in which GPS can be used to benefi t law Enforcement. Now that we know what GPS is and how it works, we will further explore the applications of this technology for Law Enforcement agencies.



The information downloaded will typically include a history of the targets location, time, date and speed. Units from different manufacturers might also have additional information stored so be sure to ask them if you are looking for specifi c data functions for your tracking device.

One of the most exciting innovations of the past several years has been the miniaturization of short-range communication modules. This has allowed the creation of Data Loggers with a short-range communication system to retrieve data remotely. This process is most often referred to as a Walk - by Download because you can walk by the target and get your data without

needing to retrieve the unit - meaning less chance of blowing the case by being spotted.

Memory tracking, with orwithout a short range download, is a great tool for all kinds of investigations including arson, break and enter, parole and restraining order violations, background surveillance and other low profi le cases.

GPS tracking hit the law enforcement scene about 8 years ago. GPS devices at that time were clunky, short on features, and usually required 2 computers to use. Now there are almost as many uses and varieties as you can imagine. This section will outline a few of the more common types of GPS tracking. Keep in mind that technology is always progressing and new techniques are always appearing.

Data logging, or memory tracking, is the foundation of GPS tracking systems. The name says it all. A GPS unit is placed on a target and proceeds to store positional information in memory. When the investigator wants to get the stored information, they simply remove the tracking device, and download the information for analysis.

Figure 11 Install, retrieve, download and analyze

Strengths

Usually have good battery life Historical positions of targets movements stored in

memory Easy installation Cost effective usually cheapest form of GPS

tracking Low maintenance No additional cost after purchase

Weaknesses

Need to access the target twice once to install and once more to remove

If you lose the target vehicle, you lose your data and your tracking unit

No way to access information before removing the unit

Retrieve unit

manually

Figure 12Short range communication

allows remote access to your data

2.1 DATA LOGGING / MEMORY TRACKING



This type of tracking was born from a desire to have the remote communication and memory storage advantages of the real time tracker, and the long battery life of the data logger. Near real time tracking units operate as data loggers most of the time. They have the added advantage of having a communication system that allows them to make calls in certain situations.

For example, you could program the unit to call you every time it stops moving and download all the records it took since it last contacted you. That way, the investigator can look at his / her computer and easily see where the suspect has stopped and when.

Strengths

Serves as a reliable data logger Allows the investigator to receive live updates when

pre-determined switches are triggered Frees up investigators time to simultaneously work

on other cases, thus saving money Lower battery consumption than pure real time

tracking units

Weaknesses

May not always be able to communicate with unit for live updates if not currently within an applicable communication network

May not always be able to call in the unit to get immediate live updates - unit may only be programmed to call in once every day or only when specifi c events have occurred

2.2 NEAR REAL TIME TRACKING

Figure 13An example of how near real time tracking can be

set up

Receive a call or page from the unit

The investigator downloads and processes the data

An offi cer is dispatched,if required



Step 1 Dial TargetStep 2 Phone System Contacts Nearest Cell TowerStep 3 Tower Contacts Remote UnitStep 4 Remote Unit Talks to the Computer

Strengths

Real time information on your target Historical positions of targets movements stored in

memory Can provide tracking anywhere in the world,

depending on communication Improved safety for surveillance offi cers Decreased risk of blown surveillances Frees up offi cers time to work on other cases

Weaknesses

Usually harder to install than Data Loggers Battery life in most units is relatively short More expensive systems Reliance on communication infrastructure; no cell

signal, no tracking! Added expense of communication bills

Live, or Real Time tracking, is one of the most popular uses of GPS equipment there is. By making use of an on-board communication system, the investigator is able to watch a targets movements as they occur. While not a replacement of actual visual surveillance, real time tracking can certainly supplement your physical surveillance efforts.

In addition to providing you with constant updates, the majority of systems on the market also continually store the information in memory. This means you can watch events in real time and rest assured all the data is being stored for later analysis.

Real time systems have also been the back upon which new features have ridden. Some examples include audio, video and switching capabilities. Imagine sitting in your offi ce watching your suspect drive on street level maps, listening to what the occupants are saying, while also taking pictures of those same occupants.

Many real time units use cell phones as the link to provide the best coverage across North America, allowing the investigator the freedom to sit back anywhere in the world, and track the remote unit live while it drives around North America. Think of using a real time tracking system anytime you need to have real time information on your suspect.

2.3 REAL TIME / LIVE TRACKING

Figure 14Sample set up for live GPS tracking



With new digital cellular systems also comes a new method of transmitting data, commonly called IP (Internet Protocol) based communication. This new mode transmits data via the Internet. The transmitted data is broken up into little packages of information, and then reassembled on the receiving end. The result is a communication method that is opening new doors for those involved in covert tracking!

GPS technology has reached a stage where most people are very comfortable with the concept of covert remote tracking. Law enforcement agencies are now starting to push the limits, looking for ways to harness this tremendous technology in bigger and better ways.

IP-based GPS units do not need permanent cellular connections. With just your tracking software and access to the internet, you can now view your targets movements from the safety of your offi ce, from your surveillance vehicle, or even from your home!

Cellular communication has undergone a dramatic change over the last few years. From the days when analog was the only choice, new and more advanced technologies dot the market. Whether its GSM or CDMA, Investigators in North America have tremendous choices at their fi ngertips.

2.4 MULTI / TEAM TRACKING

Why IP Based Technology?

Figure 15Retrieve data from multiple vehicles

through IP based tracking.

From One to Many

The majority of investigations today have multiple parties interested in the data from a covertly installed GPS device. Traditional communication methods have restricted connection to a unit to one user. Using the Marathon Server, multiple users can be connected to the same unit simultaneously. Imagine sitting at your desk tracking the unit and calling your team members to connect at the same time to watch important events, and logging in via their map enabled handheld devices to receive real time position updates. The most important aspect of any case is the information collected. And now with IP based tracking, information from one can be distributed to many.

From Many to One

With a renewed focus on public safety and large scale investigations, the need to track multiple targets, and multiple types of targets simultaneously has never been greater. It is in these types of investigations that IP based systems really shine. As discussed, IP based systems operate using the internet for communication. This means that the GPS units dont need permanent cellular connections. The number of IP based connections is a function of the software, and your internet connection capacity. Some systems are even capable of handling hundreds of simultaneous connections. This translates into a ready made system able to track multiple units simultaneously on 1 computer screen. Perfect for command and control or situational awareness operations centers. Make sure your information from many, gets to one.



As the use of GPS based technology spreads, were fi nding new ways to apply it for the benefi t of our friends in law enforcement. Using a combination of multi-tracking capable software (such as Orions SkyTrack Software), and IP-capable tracking units, users now have the ability to track multiple units at the same time, on one computer screen. With a renewed focus on public safety, and large scale investigations, the need to team track or track multiple targets simultaneously has never been greater. It is in these types of investigations that IP based systems really shine. The perfect solution to keep command and control centers updated on everyones location! The benefi ts of total team tracking solutions can be applied to a wide array of law enforcement applications.

Most surveillance operations involve tracking a subject with a combination of technical and physical assets. For example, a suspects vehicle will have a covert GPS system installed, and a team of surveillance agents following along to observe the activities of that subject. These operations can be dangerous for the surveillance agents and for the case itself. If just one of the surveillance agents is burned, the entire case could be blown! By installing a covert GPS system in the target vehicle, and a non-covert GPS system in each of the surveillance team vehicles, operators can now watch the movements of everyone involved, in real time.

With this system in place surveillance teams can allow the traditional surveillance box to grow a little bigger, reducing the likelihood of being detected by the target. Remote operators can track the movements of all participants and warn investigators if the target displays counter surveillance techniques. Team Tracking operators can help the team recover lost targets, and tell agents when to move in if the target stops. When tracking in areas that are not familiar to the surveillance team, operators can provide directions to get agents in or out of critical position. They can also provide other information about the surrounding areas. Many of these benefi ts can also be delivered to personnel protection teams using Team Tracking technology.

The use of IP based communication means that information derived from GPS units can be distributed to many different users, and information from multiple units can be funneled to one user. While GPS equipment will never solve all the problems encountered in an investigation, the use of IP based systems can increase the safety and effectiveness of surveillance and special operations teams.

Excellent Coverage: Continuing to grow in its popularity, the reach of this new technology has spanned the far corners of North America. IP based technology ranges from GPRS on the GSM network, to 1XRTT on the CDMA network, and is expected to become the most popular technology among cellular networks to date.

Multi-tracking: When using a combination of Multi-Tracking enabled software, and IP-capable tracking units, users now have the ability to track multiple units at the same time, on one computer screen. With a renewed focus on public safety, and large scale investigations, the need to team track or track multiple targets simultaneously has never been greater. It is in these types of investigations that IP based systems really shine. The perfect solution to keep command and control centers updated on everyones location!

Enhanced Data Security: All Orion IP-based Guardians have incorporated a new enhanced security option that will allow up to 5 users to log into the tracking software to view the targets movements, but only the assigned administrator will have the ability to alter any of the units settings. This gives multiple agents the freedom to log on and retrieve information on their targets whereabouts, without compromising any of the units previously programmed settings or data.

More Economic: IP based tracking solutions can offer users a much more cost effective airtime package. Based only on data sent, users can pay only for the services they need, and are no longer being forced into buying large bundle packages.

How can Multi / Team Tracking Benefi t Law Enforcement?



Dynamic IPA dynamic IP address changes, at the point of login, each time you connect to your Internet Service Provider (ISP). This allows ISPs to keep a pool of addresses available to subscribers. If you disconnect from the ISP, your address is returned to the pool, becoming available to the next computer that connects.

Static IPEach computer on the Internet has an address, an example of such is 194.69.121.3 The IP address distinguishes between each and every computer on the Internet. This IP

address usually changes every time you make an Internet connection, so if you come offl ine and then connect a little later your IP address will have changed. A static IP address is fi xed, much like a telephone number.

If your ISP gives you a static address, you will always use the same address. Servers usually have static addresses, so they can always be found at the same location.You would need a Static IP Address if you wish to turn your computer into some sort of server. If you have decided to store downloadable documents or software on your server, a Static IP is the solution.

Comparing Dynamic and Static IP

Excellent Coverage Fast Connection Times

Allows Multiple Agents to Track Multiple Targets

Increased Officer Safety Affordable Airtime Packages

Enhanced Data Security



C h a p te r 3

W i re l e s s

Co m m u n i c a t i o n s




This chapter discusses some of the more popular wireless communications that GPS can be used in conjunction with. It will review a variety of the most commonly used wireless data technologies, what the fundamentals are for each, and highlight some of their advantages and disadvantages, in relevance for the Law Enforcement Community.



3.1 WIRELESS RADIO FREQUENCY (RF)

Spread Spectrum - Unlicensed Radio

Another popular solution to the licensing problem is the ISM (Industrial, Scientifi c, & Medical) bands. As long as radios meet the FCC requirements for the use of these bands, they are open to anyone. Ordinary radios are limited to 100 mW of radiated power in these bands. However, Spread Spectrum radios are allowed a maximum of 1 Watt, because the radiated energy is spread across a larger spectrum, and is lower at any one frequency. In addition to all the other good reasons for using spread spectrum radios, the rules that allow them to transmit at ten times the power means that they are a natural choice in the unlicensed bands.

The term Spread Spectrum (SS) describes a communications technique, typically using the frequency band of 902 to 928 MHz, whereby a radio frequency signal is modulated (spread) a second time so as to generate an expanded bandwidth wideband signal. Sophisticated hopping sequences and forward error correction reduces interference from unwanted sources. These radio enhancements provide very low bit-error rates and greater range of use. Spread Spectrum is usually used for data transmission. The two most popular types of Spread Spectrum modes are Frequency Hopping and Direct Sequence.

Frequency Hopping Spread SpectrumFrequency Hopping SS involves the application of a pseudorandom code, which causes the transmitter

to periodically hop or jump to a new frequency, transmit information on the frequency for a defi ned period of time, then hop to the next frequency and repeat the process. In order for the receiver to recover the transmitted information, it must hop to the same frequencies as the transmitter. Thus, the pseudorandom code and some synchronization information must be known at the receivers end of the link. Frequency Hopping Spread Spectrum usually uses narrow-band transmit and receive techniques, thus providing long distance communications with excellent noise immunity and interference rejection capabilities.

Direct Sequence Spread SpectrumDirect Sequence SS also involves the application of pseudorandom codes known to both ends of the link, but the codes are used to cause a fi xed frequency transmitter to spread its power more or less evenly across a wide band of RF spectrum. Pseudorandom codes are selected to give the spread signal a noise-like character, which when detected by a conventional receiving device, looks very much like random noise. The receiver must be wide enough to recover all of this bandwidth in order to recover the transmitted signal, and then, using the same pseudorandom code as the transmitter, de-spread the signal to its original data component. Direct Sequence systems also have good immunity to noise and interference when used with highly directional parabolic antennas in relatively short-range applications.

In the past, customers needing wireless communication systems would be required to obtain an FCC

license. In some localities, licenses are becoming di cult, or even impossible to obtain. A popular solution

to the licensing problem is provided by the emergence of Spread Spectrum (SS) technology. This new

digital technology provides most of the capabilities and performance of a licensed radio system with a

license-free approach. For long distance data communications in a control or data gathering application,

Frequency Hopping Spread Spectrum (FHSS) radios provide a robust solution.



How do they Work?RF systems operate with 2 separate pieces; a transmitter and a receiver. As you can guess, the transmitters job is to transmit a signal, and the receivers is to receive it. Not too hard to do really. What makes RF tracking receivers great is their ability to tell what direction the signal is coming from.

The receiver is able to do this by using what is called a Directional Antenna Array. The fi rst thing the receiver does is lock onto the frequency that your unit is transmitting on. Refer at Figure 31 to see how it works. The signal hits antenna #1 fi rst, then #3 and #2 almost at the same time, and fi nished off with #4, indicating that the signal came from the left. Its able to fi gure this out by timing when the signal hits the individual antennas.

This information is then translated into a graphical display (as seen in fi gure 32), so we can get a visual indication of where the signal is coming from. Most new systems have a 360-degree display though older models just show left or right. After you have a rough idea of direction you can watch the signal strength and follow the signal all the way to the target.

What are they used for?RF Tracking systems have been used for a wide variety of cases. While GPS technology has taken over some of the workload, RF beacons are still the ideal technology for some applications.

Sample Applications Package tracking RF systems dont need to See

the Sky Tracking situations where extremely small size is

the pivotal factor.Now that weve covered the way that RF tracking systems work, lets talk about another way of tracking thats not as widely known. The past several years have seen a tremendous rise in the number of cellular phones. What used to be the hallmark of the rich businessperson has now become just another tool in everybodys hands. As a result, cell phones have also become very popular with the criminal element, thinking that wiretaps would be unable to access their conversations. Enter the combination of Cellular Intercept Receivers (CIR) and Radio Frequency (RF) Direction Finder.

Radio Frequency Direction Finding

RF Tracking

Figure 16Directional antenna array

Figure 17Sample 360o display

RF Tracking a term many investigators have surely heard being tossed around. Since the end of World

War II, and up until the early 1990s, Radio Frequency Direction Finding (RF Tracking) was the tracking

equipment of choice. In fact, its still used extensively for certain tracking cases. Most of the James Bond

movies exaggerated the capabilities of the system, but had the basic premise right.

1 2

3 4

Radio Signal

Antenna Array



Radio Frequency 101

What is RF?RF = Radio FrequencyElectromagnetic (EM) carrier waves upon which audio, video, or data signals can be superimposed for transmission. Generally considered from approx. 300kHz to 1Thz.

Wavelength and FrequencyHigher frequency = Shorter wavelengthOne wavelength = The time it takes for one complete cycle of the EM wave. Expressed in Hertz (hz). Equals the number of wave cycles per second. One complete cycle is considered 360 degrees of the wave

Typical RF Link ComponentsTransmitter + AntennaIntervening TerrainFresnel ZoneReceiver + Antenna

Transmitter + Receiver = TransceiverA device that has the capacity to both receive and transmit data in a two way link is called a transceiver.

TransmittersThe key transmitter specifi cation is its output powerUsually expressed in dBm (decibels, or D B - Ms) or mW (milliwatts). More transmission power means longer range but less battery life.

ReceiversThe key receiver specifi cation is its sensitivity. The lower a receivers sensitivity, the weaker the incoming signal it can detect. Usually expressed in dBm and will be a negative number ie: -106dBm. It is hard to compare receivers since different manufacturers use different criteria to benchmark their products. Some manufactures try to trick and will express their sensitivity in dBWAdd 30 to convert dBW to dBm.ie: -136dBW = -106dBm

Antennas Antennas come in many shapes and sizes. Antennas have gain dependent on the antenna type and are usually expressed in dBi. No matter what antenna you use, the installation is still the most important factor in performance.

Carrier Frequency The frequency or number of cycles per second of the RF energy carrying the data. The data is encoded onto the carrier frequency by modulating it. There are many types of modulation. Frequency and Amplitude modulation are the most common types. Different carrier frequencies behave differently when they interact with the physical world.

Frequency BandsAll transmitters must operate within frequency and power guidelines, unless the user is exempt from regulations. Some bands are reserved for specifi c uses ie: Cellular service, while some bands are not ie: ISM.

Industrial/Scientifi c/Medical (ISM)433.05 - 434.79Mhz (US), 804 - 866Mhz (Europe), 902 - 928 (NA), 2.4 - 2.4835GHz, 5.725 - 5.850 GHz (US) Limited to 1W or less transmitter output power. 900Mhz, 2.4Ghz and 5.8Ghz bands licensed for Spread Spectrum use.

2.4Ghz Good Only world wide licensed band. Widest bandwidth means fastest possible data

transfer rate. Faster data transfer means less on time for the

transmitter and results in power savings Very small antennas are possible Allows the use of spread spectrum to increase

connection rate and resistance to interference Will radiate through small openings2.4Ghz Bad Poor obstruction penetration Short wave length = Shorter range compared to

lower frequencies Does not diffuse as well as lower frequencies

900Mhz Good Good obstruction penetration Good data transfer rate Reasonably sized antennas Good bandwidth will allow spread spectrum

usage Better range then 2.4Ghz systems900Mhz Bad North America only, 850Mhz in Europe Compromised frequency



Radio Frequency 101 cont

433Mhz Good Longer wavelength = Longer range Good obstruction penetration433Mhz Bad Large antennas Narrow bandwidth means low data rates, fewer

channels, less ability to avoid interference USA only Large Freznel Zone may cause reduced ground

to ground range

Other: VHF 220Mhz 150-180Mhz Commonly used in beacon systems Very long ranges, very large antennas, very narrow

bandwidths Very congested frequencies

The Usual Problems Not getting the advertised range Dead zones Picket Fence coverage Unpredictable behaviour

Real World IssuesThe theoretical range and the realistic urban range are usually an order of magnitude apart. Advertised ranges are almost always Free Space. As RF energy strikes an object four things happen.

Absorption Refl ection Refraction Diffraction

AbsorptionAll material has a certain RF permeability and this usually differs for different frequencies. The lower the permeability, the more RF energy is lost when it passes through the object. Metal, mirrored glass have very poor RF permeability. Concrete and earth are also not good. Plastic, Gyproc, glass, wood all have good permeability.

Refl ectionAs the RF energy refl ects from an object, it changes phase, and contributes to multi-path effects. Multiple refl ections off many objects interfering with each other

causes destructive/constructive interference zones.

Interference PatternsWhen two waves that are out of phase with each other, they can either add to each other or subtract from each other depending on their relative phase. Two waves 180 degrees out of phase of equal amplitude will cancel each other completely.

RefractionThe portion of the wave that is not absorbed or refl ected is refracted when it passes through an obstruction. A refracted wave suffers a phase shift and is bent the same way light bends when it passes through water.

Diff ractionDiffraction is the bending of the carrier wave that occurs when it passes through an opening or over an object that is less the one wave length in size. Longer wave lengths are more likely to be diffracted. Diffracted waves suffer a phase shift which is not always a bad thing. It allows an area that might normally be in an RF shadow to h

orion electronics ltd. gps technical reference guide

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