In 1973 a project group was formed with representatives of the U.S. armed forces and the Defense Mapping Agency to develop a new navigation and positioning system. This new satellite positioning system, which would replace the old DOPPLER- or TRANSIT-navigation system, became known as the NAVSTAR GPS. Where NAVSTAR stands for “NAVigation by Satellite Timing And Ranging”, and GPS for “Global Positioning System”. The system was put into operation by the U.S. Department of Defense for military applications. Civilian users are given access with some restrictions. The name GPS is most commonly used today. We will use the term GPS from here onwards as a synonym to NAVSTAR GPS. The main characteristic of the GPS is that it is a continuously available global system which allows for instantaneous position determination using simultaneously measured pseudo-ranges to at least four satellites.

GPS Constellation Overview

The presently available full constellation guarantees simultaneous observations of at least four GPS satellites from (almost) every point on the surface of the Earth at (almost) every time of the day. This is accomplished by 24 satellites (21 plus 3 active spares) located in six orbital planes in almost circular orbits with an altitude of about 20 200 km above the surface of the Earth. The orbital plane is inclined by 55 degrees with respect to the equator. The sidereal revolution period of the GPS satellites is 11 hours 58 minutes (approximately half a sidereal day). Consequently, the same satellite configuration is repeated 4 minutes earlier every day for one and the same location. This leads to an almost perfectly repeated “ground-track” (projection of the satellite position on the Earth) of the GPS satellites as shown in the Figure below (TBD). Due to this orbital revolution period the GPS satellites are in deep 2:1 resonance with the rotation of the Earth with respect to inertial space which gives rise to resonance perturbations. The first GPS satellite PRN 4 (PRN: Pseudo-Random Number) was launched on February 22, 1978. PRN 4 was the first in a series of 11 so-called Block I satellites. The Block I satellites had an inclination of about  63 degrees with respect to the equator. At present the Block I satellites are no longer active. They were followed by the Block II, IIA and IIR satellites. The operational constellation is realized by the Block II, Block IIA, and Block IIR satellites.

GPS Signal Overview

All signals transmitted by the satellite are derived from the fundamental frequency (f0) of the satellite oscillator. The two carrier frequencies, f1 and f2 (the corresponding wavelengths are 19 and 24 cm), are modulated with the codes and the navigation message to transmit information such as the readings of the satellite clocks, the orbital parameters, etc. The C/A-code (Coarse-Acquisition, Clear-Access, or Civil-Access) is modulated on the f1 carrier only. The P-code (Precise or Protected) is modulated on both carriers. There are two limitations for civilian users, namely SelectedAvailability and Anti-Spoofing, briefly referred to as SA and AS, respectively. Both deteriorate the achievable accuracy for civilian users significantly. Selective Availability (SA), the denial of full accuracy, is accomplished by “manipulating” navigation message orbit data (epsilon) and/or the satellite clock frequency (dither). So far, only the satellite clock frequency has been manipulated. With this dithering process the GPS satellite clocks are artificially degraded by adding a signal with an unknown frequency and amplitude to the know clock behavior. This is done to degrade the performance of GPS for the “normal” users. Both, the frequency and amplitude of the added signal, change rapidly over time. The amplitude of this “clock dithering” is of the order of 0.3 microseconds (which corresponds to roughly 100 meters) and the frequency is of the order of only a few minutes. This SA clock dithering limits the accuracy of real time position estimates to 25 meters RMS. Selected (military) users possess special “keys” to remove the SA-effect in real time giving them access to the full navigation potential of GPS, i.e., one meter real time absolute point positioning.

Anti-spoofing (AS) is a protection against “fake” transmissions by encrypting the P-code to form the Y-code. This ensures that the GPS signals cannot be disturbed (spoofed) by a GPS-like transmitter on the Earth. The anti-spoofing procedure converts the P-code to the Y-code which is only usable when a secret conversion algorithm is available to the receiver. The Y-code is the “modulo two sum” of the P-code and the encryption code, also referred to as the W-code. Only selected (military) users have access to the conversion algorithm. The effect of AS is that civilian users have only access to the C/A-code and therefore only to one single frequency. This disables the possibility to eliminate ionospheric refraction using observations on two different frequencies. This further limits the accuracy for the civilian users. For real time non-differential applications, the SA effect limits the accuracy to about 100 meters. At this accuracy level the effect of AS is negligible because both, the effects of a more precise code and the ionosphere, are negligible compared to the 100 meter level SA effect. For real time differential GPS AS is the accuracy limiting factor. In differential applications, the SA effect cancels out almost completely. The accuracy is in this case limitedby the degraded accuracy of the code and by ionospheric refraction.