GPS systems are intended to provide their users with certain services. These services are defined in system documentation.
NAVSTAR
The NAVSTAR system is guaranteed by the Department of Defense to provide two sets of services.
Standard Positioning Service (SPS)
SPS is the standard specified level of positioning and timing accuracy that is available, without qualification or restrictions, to any user on a continuous worldwide basis. The accuracy of this service is established by the U.S. Department of Defense based on U.S. security interests. Navstar currently provides horizontal positioning accuracy within 100 meters (2 drms) and 300 meters with 99.99 percent probability. The signals providing standard positioning service are inherently capable of greater accuracy than this. The accuracy of the system is limited through the application of a process called Selective Availability.
Precise Positioning Service (PPS)
PPS is the most accurate positioning, velocity, and timing information continuously available, worldwide, from the basic GPS. This service is limited to authorized U.S. and allied Federal Governments; authorized foreign and military users; and eligible civil users. PPS information is encrypted to prevent use by unauthorized users. The encryption process is known as Anti-Spoofing. Once encrypted, P code is known as Y code. P code capable military user equipment provides a predictable positioning accuracy of at least 22 meters (2 drms) horizontally and 27.7 meters (2 sigma) vertically and timing/time interval accuracy within 90 nanoseconds (95 percent probability). This improved accuracy is provided in two ways. First, P-code users are not subjected to Selective Availability. Second, access to the L2 channel allows the user to correct for atmospheric propagation errors. Access to P-code equipment is tightly controlled
GLONASS
GLONASS also maintains two separate services.
Psuedoranging relies on reading the coded information being broadcast by the satellites to determine how far the receiver is from them. By the way, everybody, this is a draft and I don't have all the theory down.If anybody wishes to correct any mistakes I make, please do!It works something like this.
A GPS receiver has a set of computer-controlled radio receivers and a clock. The radio receivers listen for the GPS satellite transmissions under control of the computer. Among the pieces of information a satellite broadcasts are the current time and it's orbital parameters. From this information, the GPS receiver is able to determine exactly where in space the satellite is.
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By reading only one satellite, the receiver can only know that it is at some point on a sphere-of-position some fixed (but unknown) distance away from the satellite. Obviously, we want to know our position a little better than that, so we use another satellite. Now we have refined our position a little better because we have to be located somewhere in the intersection between the spheres-of-position surrounding the two satellites.
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This is still only a one dimensional fix, however. We want better than that. By using the spheres-of-position from three satellites, we can almost get a two dimensional fix.
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We aren't done yet though. One more variable still has to constrained. The receiver does not know how big the spheres-of-position are because we don't know how far it is to the satellites. Thus, the intersection of the spheres-of-position is still an unknown volume rather than a point. We can constrain time, or we can constrain the known altitude of the receiver.
Constraining Time
WARNING: My theory is really shaky on this subject. Treat this section as a draft that is possibly error prone.
The next thing the receiver wants to know is the exact distance from the satellites to the receiver. Now, as we should all know from basic physics, radio waves, a form of electromagnetic radiation, travel at the speed of light which is constant (well, almost). The distance travelled by a radio signal can be determined if you know how long it has travelled. The problem is, the GPS receiver does not know how long it took for the signals to get to it.
If the receiver uses the time signals from an additional satellite as a stable reference, the receiver can determine when the other satellites must have transmitted their signals so that it heard them when it heard them. This narrows the possible positions of the receiver down to two points. One of the points will be towards the surface of the earth. The other point will be in deep space, probably behind the satellites. This solution is obviously ridiculous and is discarded. The end result is that the receiver can calculate a 3 dimension position.
Constraining Altitude
Remember we said that the receiver was able to determine where in space the satellites were? Well, those satellites are in orbit around a big huge rock called the earth. The receiver must therefore know where the earth is. If the operator knows the local elevation, the receiver can use that to constrain where the receiver must be in relation to the satellites to hear the signals it's hearing. Since the receiver is given the third dimension, height, it will not attempt to calculate it. Thus, with only three satellites and known elevation, the receiver can calculate a 2 dimensional position. The quality of the position will obviously be related to the accuracy with which the current elevation is known.
User Range Estimate
The satellites broadcast health and status messages. One of the status messages transmitted is the user range estimate. This number is the estimate of the combined error due to the broadcast ephemeris and Selective Availability (S/A) and is measured in meters. It is a measure of the best possible accuracy that could be obtained when using that satellite for a position calculation. When S/A is on, URE is commonly 32 meters. With S/A off, URE can be 2 to 5 meters. Thus, when using four satellites to calculate a position, if one of those satellites has a URE of 32 meters and the rest are 5 meters, the best accuracy you can assume for your reading is 32 meters.Dilution of Precision
The position calculation is basic geometry. When the satellites are in specific configurations with respect to the observer, it is possible for small errors to be magnified, creating really horrific errors at the observer. The dilution of precision (DOP) is a dimensionless number indicating how much the geometry-induced is magnifying the error. The best DOP is 1 while the worst DOP is infinite. DOP can be broken into categories:The most common DOP value is called position dop or PDOP. It is the combination of all dops that affect position. It is (or should be) available on all common GPS receivers. URE is multiplied by PDOP to determine possible position accuracy. For example, a position fix calculated using satellites with a URE of 32 and a PDOP of 2 results in a best assumable accuracy of 64 meters. As you can see, bad PDOP can make error mount up pretty quick.
- Horizontal DOP (HDOP)
- Vertical DOP (VDOP)
- Geometric DOP (GDOP)
- Time DOP (TDOP)
On a good day, you will probably not see PDOP much better than 1.5. PDOP tends to get really bad as satellites are moving below the horizon. In my experience, when this happens PDOP tends to get very bad very quickly. The receiver then switches to a new satellite, improving PDOP dramatically. Many receivers have a programmable cutoff, where it just ceases to function after PDOP gets too bad. Usually you will want to set it for a cutoff of about 6. Really high PDOP can affect DGPS sessions too, so you won't want to do precision work during those times.
Horizontal Accuracy
Horizontal accuracy is best when the satellites being used are fairly low to the horizon (15 to 30 degrees) and are equally spaced around the horizon. The geometry of that configuration is such that the system errors aren't magnified too much. Accuracy is worst when all the satellites used are directly overhead. The observation angles are so small that slight errors are magnified enormously.Vertical Accuracy
Remember we said that position accuracy was affected by the geometry of the satellites around the receiver? Well, it just isn't possible to put a GPS satellite underground. Which means that all the satellites are on one side of the receiver (above it). Thus, the geometry of the configuration is never all that good. The accuracy of vertical positions is usually 1.5 to 2 times worse than that of the horizontal solution.This may not seem that bad; however, we as users are many times much more sensitive to vertical error than horizontal error. A horizontal error of 100 meters in a GPS receiver on an aircraft may make the difference between my putting the aircraft on the runway or in a field. A 100 meter vertical error may plow me into the ground!
Clock Errors
Satellite Orbit Error
The GPS receiver relies on knowing exactly where the satellites are in space with respect to the center of the earth. If there is an error in that position, the receiever will calculate an error in it's position. Generally, the error seen at the receiver will be the same as the error in the satellite position. That is, a 32 meter error in the satellite position will result in a 32 meter error on the ground.The ephemeris broadcast by the satellites is fairly inaccurate. It has to be this way because the signal has to be short enough to transmit in a reasonable period of time. The broadcast ephemeris limits GPS accuracy to approximately 5 meters at best.
Radio Wave Propagation
Radio waves do not always travel in a straight path. Radio waves can be bent by the atmosphere and can be reflected by surfaces. Most of the radio energy transmitted by a GPS satellite travels in a fairly straight path from the satellite to the receiver. Some of the energy travels a different path but ultimately gets directed into the antenna. Radio signals that travel a different path from the main beam are delayed in time. This is called multipath. Radio signals which are delayed from the main signal but are still received by the GPS receiver can confuse the receiver, resulting incorrect positions.
Antenna Phase Center
The GPS concept relies on radio signals travelling in a straight line from the antennas on the satellites to the antenna on the receiver. In theory, the position calculated by the receiver is at the center of the antenna where all the singals "enter" the receiver. But where is the "center" of the antenna? Radio signals affect an antenna at the "electrical" or "phase" center. The elecrical center of the antenna is not necessarily the physical center of the antenna. In fact, the phase center of the antenna can change if you look at it from different directions! This is usually only a problem for differential GPS applications requiring high precision in the sub-centimeter range.Antennas that are manufactured the same way will usually have the phase center in the same place. In applications requiring extreme accuracy, the same model of antenna will be used on both the base station and the rover. Each antenna is then aimed north to insure that the phase center of each antenna is located in the same place with respect to each other every time.
Carrier Phase Ambiguity
Selective Availability
The biggest source of error is manmade. Called Selective Availability, or S/A, this is a procedure used by the Department of Defense used to intentionally limit the accuracy of the civilian-useable GPS signal. The intent of S/A is to limit the uses of the civilian signal by potentially hostile forces while leaving it accurate enough for many legitimate uses.The major mechanisms for Selective Availability is time error and satellite position error. The GPS receiver calculates the position of the satellite from a broadcast ephemeris. The position of a satellite is only valid at any particular instant. If the receiver clock is wrong, the calculated position will be wrong. The Selective Availability process includes "jittering" the satellite clock, inducing small psuedorandom errors. Since the receiver clock syncs to the satellite time, it inherits this instability. The end result is that the satellite position isn't calculated very accurately. Selective Availability, combined with the error inherent in the broadcast ephemeris, usually limits the accuracy of a civilian GPS receiver to 32 meters at best.