Lecture 17

FOR 220 Aerial Photo Interpretation andForest MeasurementsLecture 18

Global PositioningSystemsFOR 220 Aerial Photo Interpretation and Forest MeasurementsAvery and Burkhart (2002) pp. 89-98Paine and Kiser (2003) pp. 188-211

A system of satellites, receivers, ground stations, and software for ......

navigatingaround the landscape

and

mapping landscapefeatures

Global Positioning SystemsDefinedFOR 220 Aerial Photo Interpretation and Forest Measurements

Global Positioning SystemsDefinedFOR 220 Aerial Photo Interpretation and Forest Measurements

GPS consists of 3 main segments:

1. Space (Satellite) Segment

2. Ground Control Segment

3. User (Receiver) Segment

24+ satellites (normal constellation)

Extra satellites are typically deployed toback up satellites thatgo down for one reason or another. Currently, there are about 31 satellitesin orbit.

6 orbits around the Earth11,000 mile altitude12 hour orbitGlobal Positioning SystemsSatellite SystemFOR 220 Aerial Photo Interpretation and Forest Measurements

Peter H. Dana 1998Global Positioning SystemsGround Control SystemFOR 220 Aerial Photo Interpretation and Forest MeasurementsPeter H. Dana 1998 Satellites are monitored and maintained by a network of ground stations.

Ground stations located around the world.

Master Control Station located in Colorado

Global Positioning SystemsUsers - Receivers FOR 220 Aerial Photo Interpretation and Forest Measurements

Numerous! Navigation Tract boundary mapping and location Fire boundary mapping Location of inventory or permanent plots Mapping roads, trails, streams Mapping wildlife nest or observation locationsGlobal Positioning SystemsUsesFOR 220 Aerial Photo Interpretation and Forest Measurements

Knowing your horizontal position requires triangulation:

To find your horizontal position, you need the distance from where you are to a least 3 fixed objects.

Even though satellites orbit around the Earth, we know exactly where they are at all times, so they might be considered fixed.

Knowing your vertical position (elevation) requires knowing the position of a 4th fixed object (satellite).Global Positioning SystemsHow it worksFOR 220 Aerial Photo Interpretation and Forest MeasurementsSomething to ponder: Triangulation with only 3 satellites will result in 2 possible solutions for your position. So if youre in Corvallis, where would the other position be?11,000 milesGlobal Positioning SystemsHow it worksFOR 220 Aerial Photo Interpretation and Forest Measurements

11,000 miles11,000 miles11,000 miles3 or more satellites:You can determine your positionGlobal Positioning SystemsHow it worksFOR 220 Aerial Photo Interpretation and Forest Measurements

Only 2 satellites:You cannot determine your positionSatellites broadcast signals in the L-band of the microwave spectrum (between microwave and radio wavelengths: 1 cm to 1 m wavelengths)

These wavelengths can pass through some forest canopies, but may be blocked by tree trunks. Especially if the signal comes in at a low angle on the horizon.

The speed of each signal emitted by a satellite is 186,000 miles/second (speed of light).

If a satellite is 11,000 miles away, it takes about 1/17 of a second for the signal to leave the satellite and reach a receiver.Global Positioning SystemsSpectral resolutionFOR 220 Aerial Photo Interpretation and Forest Measurements An ephemeris (its own location).

An almanac (the location of all other satellites).

A signal that indicates the time being sent (to enable you to triangulate your position on the landscape).

Global Positioning SystemsWhat does each satellite send out?FOR 220 Aerial Photo Interpretation and Forest Measurements Precise clocks in each satellite, and relatively precise clocks in GPS receivers

Receiver calculates distance to each satellite based on time required for signal to arriveThe secret to GPS is time. Coarse Acquisition code (CA)

Carrier Phase Code (L1 and L2)

Military (P Code)Types of GPS SignalsFOR 220 Aerial Photo Interpretation and Forest Measurements3 different signals are emitted by GPS satellites Most widely used GPS signal

Least accurate GPS signal

Simple and cost-effective to use

Since satellites and receivers may not be set exactly on the same time, determining how long it has been since the satellite sent the signal may result in some inaccuracies.Types of GPS SignalsC/A code - Coarse Acquisition codeFOR 220 Aerial Photo Interpretation and Forest Measurements More detailed analysis of each signal within the receiver

More accurate GPS signal than C/A code signal

Time consuming and expensive to use

Used for surveying and benchmark location.Types of GPS SignalsCarrier Phase code (L-1, L-2)FOR 220 Aerial Photo Interpretation and Forest Measurements Reserved for military useTypes of GPS SignalsP code FOR 220 Aerial Photo Interpretation and Forest Measurements

Sources of Error in GPS SignalsFOR 220 Aerial Photo Interpretation and Forest MeasurementsSources of error in GPS signals (in order of significance): Multi-path Error

Selective Availability

Receiver Clock

Atmosphere

Satellite Clock

Ephemeris Error Ephemeris errors

Satellite may not be exactly where it says it is in its ephemerisSources of Error in GPS SignalsSatellite location FOR 220 Aerial Photo Interpretation and Forest Measurements Satellites carry atomic clocks

These clocks are highly precise, and frequently adjusted to the clocks used at ground stations, however, there still may be some slight differences in clock times.

A very slight difference may result in an inaccurate location determination.Sources of Error in GPS SignalsSatellite clocks FOR 220 Aerial Photo Interpretation and Forest Measurements The clock on a GPS receiver must be synchronized exactly with the satellite's clock.

A larger source of error than satellite clock errorSources of Error in GPS SignalsReceiver clockFOR 220 Aerial Photo Interpretation and Forest Measurements Deflection of signals when passing through the ionosphere and troposphere

The least amount of deflection, when the signal passes through the atmosphere, is when the satellite is directly overhead of the receiver.Sources of Error in GPS SignalsAtmospheric Interference FOR 220 Aerial Photo Interpretation and Forest Measurements A delay in receiving a signal from a satellite, due to the signal bouncing off of other objects, such as trees or buildings.Sources of Error in GPS SignalsMultipath error FOR 220 Aerial Photo Interpretation and Forest Measurements

Sources of Error in GPS SignalsMultipath error FOR 220 Aerial Photo Interpretation and Forest Measurements

from Hoar, G.J. 1982. Satellite surveying: Theory, geodesy, map projections. Magnavox Advanced Products and Systems Co. Torrance, CA.Sources of Error in GPS SignalsSelective Availability FOR 220 Aerial Photo Interpretation and Forest MeasurementsIntentional degradation of GPS signals by Department of Defense.

Recap: Sources of Error in GPS SignalsFOR 220 Aerial Photo Interpretation and Forest Measurements

Positional Uncertainty for GPS SolutionsFOR 220 Aerial Photo Interpretation and Forest MeasurementsGDOP - Geometric Dilution of PositionPDOP/LDOP - Positional / Locational Dilution of PositionWe get better solutions for positions when satellites are more widely spaced than when they are all clustered overheadError associated with this dilution of solution is generally given on most receivers as a GDOP or PDOP value. The lower the value, the better the position.

Simultaneous collection of data with two receivers:

(1) a "base station," which can be found in a fixed location within 200 miles of the area where data is being collected.

(2) a "rover," the GPS unit you are using in the field.Differential CorrectionA technique that allows improved accuracy of collected data FOR 220 Aerial Photo Interpretation and Forest Measurements

Some of the differences (errors) between the base station and rover data can be recognized, and removed from the rover's data.

Requires post-processing of field data.

(1) download the rover data

(2) download the base station data

(3) use software to make the corrections

Remove or reduce all errors except multipath.Differential CorrectionA technique that allows improved accuracy of collected data FOR 220 Aerial Photo Interpretation and Forest MeasurementsDifferential CorrectionA technique that allows improved accuracy of collected data FOR 220 Aerial Photo Interpretation and Forest MeasurementsDifferential correction continues to provide significant improvement of positional accuracy even after Selective Availability was turned off in May 2000.

SA OnSA OffSA Off withDifferential CorrectionDifferential CorrectionFOR 220 Aerial Photo Interpretation and Forest MeasurementsWAAS - Wide Area Augmentation System

A different approach to differential correction

25 ground stations, administeredby FAA, collect GPS data

Differentially corrected data sentback up to WAAS satellite in geostationary orbit.

WAAS satellites transmit corrections (currently only covers N. America).

WAAS enabled devices use informationto improve positional accuracy.

Many modern recreational grade receivers are WAAS enabled.Differential CorrectionFOR 220 Aerial Photo Interpretation and Forest MeasurementsRDGPS - Real-Time Differentially Corrected GPS Differential correction is done on-the-fly in the roving receiver.

Base station data are collected and transmitted via radio signals.

Receivers equipped to do RDGPS can pick up radio signals with corrections and do differential correction real time.

US Coast Guard has a network of base stations which transmit correction data for free. Only available along coastlines, great lakes and portions of Mississippi River.

Many subscription services in North America.Each satellite that is tracked requires a "channel" in the receiver.Receivers generally have 6, 8, or 12 channels available.

Recreation-grade receivers: designed for casual users (recreationists). Typically uses CA code, and has the least accurate positioning ability.

Navigating receivers: same as recreation receivers, designed for portability and long battery life.

Mapping-grade receivers: Designed to include more features, allow higher accuracy, and to store more data, as well as attribute data. Typically use L1, L2 code.

Survey-grade receivers: Designed for extreme accuracy. Usually larger and heavier than other receiver types. Very expensive. Typically use L1, L2 code.GPS EquipmentReceivers FOR 220 Aerial Photo Interpretation and Forest MeasurementsGPS EquipmentReceivers FOR 220 Aerial Photo Interpretation and Forest Measurements

How much for that receiver in the window?

Recreation Grade:$75 - $600dramatic drop in prices over last 2 years

Mapping Grade:$600 - $10,000

Survey Grade:$10,000 - $50,000

GPS EquipmentReceivers FOR 220 Aerial Photo Interpretation and Forest Measurements

What kind of positional accuracy can I get? (Horizontal accuracy)

Recreation Grade:1 - 3 meters possible, typically requires post processing or realtime correction.

Mapping Grade:1m - sub-meter possible

Survey Grade:1-5 cm (yes, centimeters!)

Mounted on a roof, a pole, a truck, or a person.GPS EquipmentAntennas FOR 220 Aerial Photo Interpretation and Forest Measurements

Types of Data CollectedFOR 220 Aerial Photo Interpretation and Forest Measurements

Generally, points or lines GPS Project PlanningWhat to keep in mind when considering using GPS FOR 220 Aerial Photo Interpretation and Forest Measurements1) When is the best time to collect the data?

2) How much time will I need to collect the data?

3) Do I need to differentially correct the data?Where is the base station?Will the base station have data covering my time period?

4) Do I need to remove multipath error?

5) Will the GPS data be integrated with other GIS data?Do the features need to connect?Do the features need to be attributed?GPS Project PlanningMission Planning Software FOR 220 Aerial Photo Interpretation and Forest Measurements

FOR 220 Aerial Photo Interpretation and Forest MeasurementsWhat is the name of this landscape feature?

Why are these trees red? Why are some of them yellow?

What are these red areas?

What are these dots?

Whats going on here?