Whilst GPS is everywhere and is literally rocket science what are the nuts and bolts of how it works. Mike Forbes at
Electric Compass explains more...
In standard GPS navigation, each GPS receiver produces “replicas” of the code transmitted by the various satellites. These replicas represent the code the receiver expects to receive from any given satellite. A satellite is located and verified when a replica of the code matches up with the code received(a process known as code phase tracking). At the beginning of each individual signal is a sort of time stamp.
When the two signals are examined by the receiver (called “correlation”), the time stamps of the two signals are compared and a time difference is ascertained. Given this time difference and the rate of propagation of the signal, the GPS receiver uses the simple formula of Rate times Time equals Distance (R*T=D) to compute the distance to each satellite. Due to the uncertainties introduced by the many variables this distance to each satellite is only an estimate, and is known as the pseudo-range.
Pseudo-Range Navigation
The pseudo-range from each satellite can be seen as a radius
of a large sphere, and the location of the GPS receiver is one point on that sphere. When several pseudo-ranges from several satellites are used in conjunction, the position of the receiver is simply the intersection of these spheres at a given time. The position is first determined in what is known as the Earth-Centered, Earth-Fixed (
ECEF) coordinate system, which describes the receiver’s position relative to the center of the earth. From this
ECEF location the receiver then easily deduces the latitude, longitude, and altitude, which of course describes the receiver’s position on the surface of the earth.In solving for the
ECEF position the receiver needs to examine four variables (three dimensions and time), and a minimum of four satellites is required. In the event that only three satellites are available, a two-dimensional fix can be calculated by assuming a certain altitude. The greater the number of satellites visible to the receiver the greater the level of GPS accuracy, as five or more satellites can provide position, time and redundancy.
Types of GPS Errors
GPS accuracy is diluted by errors that can be introduced by a number of sources. GPS errors can be any combination of noise, bias, and blunders.Noise errors combine the electronic noise from the space segment and the noise generated by the user’s device.Bias errors were historically a result of the intentional degradation of GPS accuracy by the DOD known as Selective Availability, but this source of bias error is no longer active.
There are many means of improving the accuracy and precision of Global Positioning System data. The most common method of improving position information is known as Differential GPS, or
DGPS.
DGPS is predicated on the concept that for two receivers positioned reasonably close to each other, several of the errors will be common to both devices, and can therefore be subtracted from the navigation solution. Errors common to both receivers are known as common mode errors, and do not include
multipath errors or errors due to noise in the user segment. Specifically,
DGPS requires that one receiver is stationary at a known location, and that it sends corrected signals to a roving station; the roving station then incorporates the new information into the range corrections for each satellite.The best
DGPS corrections for the roving station occur when the common-mode errors are most similar, or when the receivers are 100 km or closer to each other.
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