The principle of position determination by GPS and the accuracy of the positions strongly depend on the nature of the signals. A variety of criteria was considered in the development of a suitable signal structure. In consequence the GPS signal is quite complex and offers the possibility of determining the following parameters: one-way (passive) position determination, exact distance and direction determination (Doppler effect), transmission of navigation information, simultaneous receiving of several satellite signals, provision of corrections for ionospheric delay of signals and insusceptibility against interferences and multi path effects. In order to fulfil all these requirements, the signal structure described below was developed.
Choice of the carrier frequency
To transport data signals, a suitable carrier frequency is required. The choice of the carrier frequency is submitted to the following requirements:
- Frequencies should be chosen below 2 GHz, as frequencies above 2 GHz would require beam antennae for the signal reception
- Ionospheric delays are enormous for frequency rages below 100 MHz and above 10 GHz
- The speed of propagation of electromagnetic waves in media like air deviates from the speed of light (in vacuum) the more, the lower the frequency is. For low frequencies the runtime is falsified.
- he PRN-codes (explained below) require a high bandwidth for the code modulation on the carrier frequency. Therefore a range of high frequencies with the possibility of a high bandwidth has to be chosen.
- The chosen frequency should be in a range where the signal propagation is not influenced by weather phenomena like, rain, snow or clouds.
Based on these considerations, the choice of two frequencies proved to be advantageous.
Each GPS satellite transmits two carrier signals in the microwave range, designated as L1 and L2 (frequencies located in the L-Band between 1000 and 2000 MHz).
Civil GPS receivers use the L1 frequency with 1575.42 MHz (wavelength 19.05 cm). The L1 frequency carries the navigation data as well as the SPS code (standard positioning code). The L2 frequency (1227.60 MHz, wavelength 24.45 cm) only carries the P code and is only used by receivers which are designed for PPS (precision positioning code). Mostly this can be found in military receivers.
Modulation of the carrier signals
C/A and P-Code
The carrier phases are modulated by three different binary codes: first there is the C/A code (coarse acquisition). This code is a 1023 “chip” long code, being transmitted with a frequency of 1.023 MHz. A “chip” is the same as a “bit”, and is described by the numbers “one” or “zero”. The name “chip” is used instead of “bit” because no information is carried by the signal. By this code the carrier signals are modulated and the bandwidth of the man frequency band is spread from 2 MHz to 20 MHz (spread spectrum). Thus the interference liability is reduced.
The C/A code is a pseudo random code (PRN) which looks like a random code but is clearly defined for each satellite. It is repeated every 1023 bits or every millisecond. Therefore each second 1023000 chips are generated. Taking into account the speed of light the length of one chip can be calculated to be 300 m.
Pseudo Random Numbers (PRNs)
The satellites are identified by the receiver by means of PRN-numbers. Real GPS satellites are numbered from 1 – 32. To WAAS/EGNOS satellites and other pseudolites higher numbers are assigned. These PRN-numbers of the satellites appear on the satellite view screens of many GPS receivers. For simplification of the satellite network 32 different PRN-numbers are available, although only 24 satellites were necessary and planned in the beginning. For a couple of years, now more than 24 satellites are active, which optimizes the availability, reliability and accuracy of the network.The mentioned PRN-codes are only pseudo random. If the codes were actually random, 21023 possibilities would exist. Of these many codes only few are suitable for the auto correlation or cross correlation which is necessary for the measurment of the signal propagation time. The 37 suitable codes are referred to as GOLD-codes (names after a mathematician). For these GOLD-codes the correlation among each other is particularly weak, making an unequivocal identification possible.The C/A code is the base for all civil GPS receivers. The P code (p = precise) modulates the L1 as well as the L2 carrier frequency and is a very long 10.23 MHz pseudo random code. The code would be 266 days long, but only 7 days are used.
For protection against interfering signals transmitted by an possible enemy, the P-code can be transmitted encrypted. During this anti-spoofing (AS) mode the P-code is encrypted in a Y-code. The encrypted code needs a special AS-module for each receiving channel and is only accessible for authorized personnel in possession of a special key.
The P- and Y-code are the base for the precise (military) position determination. Since January 31, 1994 the AS-system is operating continiously and the P-code is only transmitted as Y-code.
Transmission of data
In the GPS system data are modulated onto the carrier signal by means of phase modulations. Phase modulation is a rarely used technique, compared to amplitude modulation (AM) or frequency modulation. In the following, these three modulation techniques shall be explained shortly.
For the amplitude modulation the amplitude, which corresponds to the strength of the signal, is changed in accordance to the data signal that shall be transported. If this principle would be applied to sound waves, the sound level would change in order to transport a signal. With increasing attenuation it becomes more and more difficult to filter the data from the signal. This kind of modulation is known from AM radio (that’s what AM stands for: amplitude modulation).
For the frequency modulation, the carrier frequency itself is changed by modulating the data signal onto it. If we stay with the example of the sound waves, the pitch of the tones would be changed while the volume would be kept constant. Frequency modulated signals are less susceptible for disturbances and provides a higher bandwidth than AM modulation. This kind of modulation is used for FM radio.
When a data signal shall be modulated onto a carrier signal by phase modulation, the sine oscillation of the carrier signal is interrupted and restarted with a phase shift of e.g. 180°. This phase shift can be recognized by a suitable receiver and the data can be restored. Phase modulation leads to an extension of the frequency range of the carrier signal (leading to a spread spectrum) depending on how often the phase is shifted. When the phase changes, wave peaks are followed by wave minimums in a shorter distance than were in the original carrier signal (as can be seen in the graph).
This kind of modulation can only be used for the transmission of digital data.The following graph shows the composition of signals which are transmitted by GPS-satellites. The setup of the NAV/System data is explained in the chapter “data signal composition”.
Remark: Modulo 2 Sum means that sums are formed according to arithmetic rules. If the result is larger than 2, only the rest is kept which can not be divided by 2 (0+0=0; 0+1=1; 1+0=1; 1+1=0).