Lecture 14:Data input 1: GPS

By Austin TroyUniversity of Vermont

Many materials for this lecture adapted from Trimble Navigation Ltd’s GPS Web tutorial at http://trimble.com/gps/index.html

------Using GIS--Introduction to GIS

Data input•Over the next several lectures we’ll talk about data input sources and methods for GIS. This will include:

•GPS (today)

•Scanning and digitizing

•Geocoding

•Publicly available GIS data sources

•Remote Sensing

Introduction to GIS

GPS•Stands for Global Positioning System

•GPS is used to get an exact location on the surface of the earth, in three dimensions.

•GPS is a very important data input source, used for surveying, military operations, engineering, vehicle tracking, flight navigation, car navigation, ship navigation, unmanned vehicle guidance, agriculture, and of course, mapping

•For mapping, a GPS tells us “where” and allows us to input “what”

Introduction to GIS

GPS•GPS is a worldwide radio-navigation system formed from 24 satellites and their ground stations.

•Uses satellites in space as reference points for locations here on earth

•Ground stations help satellites determine their exact location in space. There are five monitor stations: Hawaii, Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs.

Introduction to GIS

How does GPS work?•GPS derives position relative to satellite “reference points,” using triangulation

• The GPS unit on the ground figures its out distance to each of several satellites using the time it takes for a radio signal to travel to the satellite

•To do this, the exact position of the satellites at a given time, must be known; otherwise they can’t serve as reference points

Introduction to GIS

How does GPS work?

Introduction to GIS

11,500 km

12,500 km

11,200 km

How does GPS work?•We need at least 3 satellites as reference points to “triangulate” our position.

•Based on the principle that where we know our exact distance from a satellite in space, we know we are somewhere on the surface of an imaginary sphere with radius equal to the distance to the satellite.

•With two satellites we know we are in the plane where the two intersect. With three or more, we can get two possible points, and one of those is usually impossible from a practical standpoint and can be discarded

Introduction to GIS

How does GPS work?•Here’s how the sphere concept works

• A fourth satellite narrows it from 2 possible points to 1 point

Introduction to GIS

Source: Trimble Navigation Ltd.

How does GPS work?•This method assumes we can find exact distance from our GPS receiver to a satellite. How does that work?

•Simple answer: see how long it takes for a radio signal to get from the satellite to the receiver.

•Since we know speed of light, we can answer this

•This gets complicated when you think about the need to perfectly synchronize satellite and receiver.

•A tiny error in synchronization can result in hundreds of meters of positional error

Introduction to GIS

How does GPS work?•The difficult part is measuring travel time, because the amount of time elapsed is tiny (about .06 seconds for an overhead satellite), and we require a way to know precisely WHEN the signal left the satellite

•To do this requires comparing lag in exactly similar patterns, one from satellite and one from receiver.

•Analogy, going to a stadium, sitting 1000 feet from the speaker and pressing “play” on your handheld tape player containing REO Speedwagon at exactly the same time as the guy in the sound booth presses play for that same song.

Introduction to GIS

How does GPS work?

•Only, instead of using cheesy eighties rock power ballads, GPS uses something called “pseudo-random code.”

•This code has to be extremely complex (hence almost random), so that patterns are not linked up at the wrong place on the code—that would generate the wrong time delay and hence the wrong distance

Introduction to GIS

Local: “I can’t fight this feeling any more,”

delayed:“I can’t fight this feeling any more,”

Source: Trimble Navigation Ltd.

How does GPS work?•So how do we know that the two Speedwagon fans are pressing “play” at exactly the same time? Do Speedwagon fans all think alike? Hardly.

•We must assume that satellite and receiver generate signal at exactly the same time; if they’re off by 1/1000th of a second, that means 200 m of error

•The satellites have expensive atomic clocks that keep perfect time—that takes care of their end.

•But what about the ground receiver?

Introduction to GIS

How does GPS work?•Here is where the fourth satellite signal comes in.

•While 3 perfect satellite signals can give a perfect location, 3 imperfect signals can’t, but 4 can

•Imagine time to receiver as distance, with each distance from each satellite defining a circle around each satellite of that radius

•If receiver clock is correct, 4 circles should meet at one point. If they don’t meet, the computer knows there is an error in the clock: “ They don’t add up”

Introduction to GIS

How does GPS work?•Dotted lines represent real distance, and solid lines represent erroneous distance, based on clock error—they don’t meet. Notice here we used three circles, because we’re looking in 2D, but in reality (3D) this represents four satellites, or four circles

Introduction to GIS

Source: Trimble Navigation Ltd.

•Assuming the clock error affects all measurements equally, the computer can then simply apply a correction factor that makes circles meet in one place

How does GPS work?•The receiver then knows the difference between its clock’s time and universal time and can apply that to future measurements.

•Of course, the receiver clock will have to be resynchronized often , because it will lose or gain time

•This is one reason why a GPS receiver needs at least four channels to get four signals

Introduction to GIS

How does GPS work?•So now we know how far we are from the satellites, but how do we know where the satellites are?? We can’t use them as a reference otherwise.

•Because the satellites are 11,000 ft up, they operate according to the well understood laws of physics, and are subject to few random, unknown forces.

•This allows us to know where a satellite should be at any given moment.

Introduction to GIS

How does GPS work?•There is a digital “almanac” on each GPS receiver that tells it where a given satellite is supposed to be at any given moment.

•While the positions can be predicted very accurately based on simple mathematics, the DOD does monitor them using precise radar, just to make sure.

•These errors are called “ephemeris” and are caused by gravitational pull of other celestial bodies

•That info is relayed to the satellite, which transmits the info when it sends its pseudo random code.

Introduction to GIS

How does GPS work?•Even after all this, there are still many factors that can generate errors and reduce positional accuracy

•One of the biggest error sources is the fact that the radio signal does not travel at the exact speed of light in different parts if the atmosphere as it does in the vacuum of space.

•This can be partly dealt with using predictive models of known atmospheric behavior

Introduction to GIS

Source: Trimble Navigation Ltd.

How does GPS work?•Signals also can bounce off features, like tall buildings, cliffs and mountains, resulting in “multipath error,” where a direct signal hits, followed by a bunch of “bounced” signals which can confuse the receiver.

•Good receivers have algorithms that can deal with this by determining what counts as a multi-path signal and choosing the first one as the signal to use

•There are other errors as well, resulting from things like ionospheric distortions and satellite inaccuracies

Introduction to GIS

How does GPS work?•Until May of 2000, the DoD intentionally introduced a small amount of error into the signal for all civilian users, calling it “selective availability,” so non- US military users would not have the same positional accuracy as the US military.

•SA resulted in about 100 m error most of the time

•Turning off SA reduced error to about 30 m radius

•Here is Clinton’s letter: http://www.ngs.noaa.gov/FGCS/info/sans_SA/docs/statement.html

Introduction to GIS

Differential GPS•This is a way to dramatically increase the accuracy of GPS positioning to a matter of a few meters, using basic concepts of geometry

•This was used in the past to overcome SA, but with that gone, is now used for reducing the 30m error

•DGPS uses one stationary and one moving receiver to help overcome the various errors in the signal

•By using two receivers that are nearby each other, within a few dozen km, they are getting essentially the same errors (except receiver errors)

Introduction to GIS

Differential GPS•DGPS improves accuracy much more than disabling of SA does

•This table shows typical error—these may vary

Introduction to GIS

Source: http://www.furuno.com/news/saoff.html

How does DGPS work?•The stationary receiver must be located on a control point whose position has been accurately surveyed: eg. USGS benchmarks

•The stationary unit works backwards—instead of using timing to calculate position, it uses its position to calculate timing

•It determines what the GPS signal travel time should be and compares it with what it actually is

•Can do this because, precise location of stationary receiver is known, and hence, so is location of satellite

Introduction to GIS

How does DGPS work?•Can do this because, precise location of stationary receiver is known, and hence, so is location of satellite

•Once it knows error, it determines a correction factor and sends it to the other receiver.

Introduction to GIS

How does DGPS work?•Since the reference receiver does not know which satellites the mobile receiver is using, it sends a message to it telling the correction factor for all

•It used to be that only big companies and governments could use DGPS because they had to set up their own reference receiver station

•Now there are many public agencies that maintain them, especially the Coast guard; these stations broadcast on a radio frequency, which GPS receivers with a radio receiver can pick up

Introduction to GIS

How does GPS work?•DGPS: now or later?

•If you don’t need the exact DGPS measurement at the moment you take a GPS measurement in the field, you can correct your data later.

•All you need to know is the roving receiver’s measured positions and the exact times they were taken

•At a later time, the reference receiver’s corrections can then be integrated with the roving GPS data based on the time and location; no radio link needed for this

Introduction to GIS

Surveyor DGPS•There are even more accurate types of DGPS that surveyors use

•These are accurate to a matter of millimeters

•This uses a very involved method that won’t be discussed here

•One of the techniques they use though, carrier-phase GPS” is beginning to make its way into consumer GPS

•Use carrier-phase signal, which is much smaller cycle widths than the standard code phase signal

Introduction to GIS

Aviation DGPS•FAA is implementing DGPS for the continent, so all planes can get extremely accurate GPS navigation, called Wide Area Augmentation System (WAAS)

•They have installed 25 ground reference stations as well as a master ground station that almost instantaneously processes and sends out satellite errors

•Improves error to 7 m and, when finished, will allow GPS to be used as primary navigational tool for Category I landings, where there is some visibility.

•Soon, it will allow zero-visibility landing navigation

Introduction to GIS

GPS Uses•Trimble Navigation Ltd., breaks GPS uses into five categories:

•Location – positioning things in space

•Navigation – getting from point a to point b

•Tracking - monitoring movements

•Mapping – creating maps based on those positions

•Timing – precision global timing

•You can learn about all these applications at these web links, but we mainly care about mapping

Introduction to GIS

GPS Uses•The uses for GPS mapping are enormous. Here are just a few examples:

•Centerlines of roads

•Hydrologic features (over time)

•Bird nest/colony locations (over time)

•Fire perimeters

•Trail maps

•Geologic/mining maps

•Vegetation and habitat

Introduction to GIS