Department of Agricultural Engineering

 
GPS/GIS Technology

Global Positioning System Technology (GPS) and its Use in Farming
 
What is a GPS?
The Global Positioning System (GPS) determines the accurate location of a point (the latitude and longitude) on earth. GPS is based on 24 space vehicles (SV's), or satellites, orbiting the earth. It uses the following concepts:
 
Triangulation from satellites:
  To triangulate, the GPS measures the time it takes a radio signal from a satellite to reach a point on earth. Using this time, the distance from satellite to a point on earth is computed.
 

Distance = time (sec) x 186,000 miles/second
  To measure time, GPS requires very accurate clocks. The clocks on satellites are based on atomic oscillations. There are four clocks on each satellite.  Each clock is worth $100,000. We need to know where the satellite is in space to determine distance.
 
As the GPS signal travels through the ionosphere (80-120 miles) and troposphere
(water vapors), it is delayed
.
 
Triangulation
GPS is based on satellite ranging where we determine our position on the earth by measuring our distance from a group of satellites, about 10,900 miles away from earth. In order to understand triangulation, let us assume that a satellite is in the center of an imaginary sphere and is transmitting signals in all directions. Two imaginary spheres (of two satellites) will intersect at several points. Three imaginary spheres will intersect at two points, one of the points will be on earth and the other point will be in space. Four imaginary spheres of four satellites will intersect on only one point on earth. According to trigonometry, we really need four satellite ranges to unambiguously locate a point.
 
Measuring Distance From a Satellite
To measure the distance from the satellite, we multiply the velocity of light with the time it takes for the signal to reach earth.  For example, it will take 0.06 seconds for a signal to reach earth from a satellite at 12,000 miles away from earth. Very precise electronic clocks are nowadays relatively inexpensive. Most receivers can measure time with nanosecond accuracy. In order to measure the length of time a signal would take to reach earth, we synchronize the satellites and receivers such that they generate the same code at the same time. Once we receive the code, then we look back and see when the receiver generated the same code. The time difference will tell how long it took for the signal to reach earth. The signals are complicated pseudo-random codes that are repeated every millisecond.
 
Clocks in Satellites and Receivers
The clocks in satellites are atomic clocks which run on the oscillation of an atom. They are extremely precise. The clocks in receivers are not as precise. In order to offset this inaccuracy, we get an extra satellite range measurement.  According to trigonometry, three perfect range measurements locate a point in three-dimensional space while four imperfect measurements can eliminate any timing offset (as long as the offset is consistent). The ranges which contain timing errors are called pseudo ranges.  If the four spheres do not intersect at a single point, then the computer in the receiver pursues a series of trims from the ranges until they intersect at a single point. This is how the time offset is corrected.  For real-time position measurements, we need a four-channel receiver (at least) so that one channel is assigned each satellite.
 
Knowing Satellite Position in Space
High altitude (about 11,000 miles) of satellites keep them clear from earth's atmosphere.  Therefore, the predictions of the orbits of these satellites will be very accurate. The Air Force injects each satellite into a very precise orbit and some receivers have almanacs that precisely tell where the satellites will be. Each satellite goes around the earth once every 12 hours.  The Department of Defense (DoD) measures their altitude, position, and speed when they pass over specifically-designated locations on earth. The DOD looks for variations in position, speed, and altitude.  These variations are called ephemeral errors which are caused by gravitational pulls from the moon, sun, and by the pressure of solar radiation.  These variations (errors) which are minor are constantly relayed to satellites.  In turn, the satellites relay these back to earth along with their timing information.  GPS satellites also transmit data about their health and orbital locations.
 
Ionosphere and Troposphere
The earth's atmosphere delays radio signals from satellites.  The ions in the outer atmosphere (ionosphere) and the water vapor in the troposphere cause these delays.  The ionosphere is a blanket of charged particles 80-120 miles above earth. When light travels through the ionosphere, its velocity decreases at a rate inversely proportional to its frequency squared.  Only the most advanced dual-frequency receivers have the ability to correct this type of error.  They are called the "ionosphere-free solution." Similarly, water vapor decrease the velocity of radio signal that travels at the speed of light.  However, there is no way to correct this error which is relatively small in magnitude (in most cases resulting in inaccuracies less than the width of an average street).
 
Receiver Errors
The receivers sometimes may round off a mathematical operation or an electrical interference might cause an error in correlating the pseudo-random codes.
 
Multipath Errors
When the signals from satellites bounce around from adjacent objects and then reach our antenna, they cause errors called multipath errors.
 
Geometric Dilution of Precision (GDOP)
The accuracy of data depends on which satellites we use. If two satellites are close to each other, then we get dilution of precision and the error is magnified. The wider the angle between the satellites, the better the measurement. Good receivers have the ability to choose the four best satellites.
 
Selective Availability
Using an operational mode, SA (selective availability), the department of defense purposely degrades the accuracy of GPS.  It is generally the largest component of error, if implemented.
 
Pseudo-Random Codes
The pseudo-random codes allow a receiver to figure out a time difference between itself and the satellites.  GPS signals are very low power but can be received by antennas a few inches above earth. We know the patterns of pseudo-random codes. If we divide the signals into time periods (chips) and compare our satellite signals with the inherent earth's background radio signals, or receivers signals with earth's background radio signals, only 50% of the time the chips will match and 50% of the time they would not. However, if we slide the satellite's codes back until they match with our receiver's codes, we will get a lot more matches. Further, the matches will amplify over a longer time span. There is very little information in GPS signals.  They are simple timing marks.
 
C/A (Course Acquisition) Code and P (Precise or Protected) Code
There are two types of pseudo-random codes:
  C/A code:  This code is used by civilians and has lower frequency than P code.
 
  P code:  This code can be encrypted such that only military receivers will have access to it. In addition, it is impossible to jam P code.  Traditionally, the P code is superimposed on a carrier that is ten times the frequency of a C/A carrier.  Using the Selective Availability (S/A) operational mode the DOD can even degrade the accuracy of C/A code. In the case of S/A, the DOD creates artificial clock errors in the satellites. It is the largest source of error in the GPS system.  However, starting from 2 May 2000, the S/A operational mode was suspended by the U.S. Government thereby eliminating the errors in GPS measurements associated with this tactical mode.
 
Differential GPS
A receiver placed at a given (known) location (base station) calculates the total error in satellite range. This error is then used to correct the locations measured by the other receivers (rover units) in the same locations. Since satellites are at altitudes of about 11,000 miles, the error of the base station will be identical to the errors of rover units.
 
Receivers
There are two types of receivers:
  Sequencing receivers:  These are one-or two-channel receivers and are generally used for recreational purposes.
 
  Continuous receivers:  These receivers are often used for surveying and scientific purposes.  They generally have 4, 6, 8, 10, or 12 channels. They eliminate the inaccuracies associated with geometric dilution of precision.
 
 

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Department web site designed and maintained by Dr. Walid Shayya.  Last modified: July 27, 2009.