GPS Frequencies and signals 1 Frequencies Each GPS satellite transmits unique ranging code signals on two frequencies: 1575.42 MHz (L1) and 1227.60 MHz (L2). The Fundamental oscillator frequency of GPS system, Fo is 10,23 MHz. The carrier L1 = 154∙Fo. There are three main digital signals or codes modulated into carrier frequencies. Near time enhancements include new signals implementations L1C, L2C (gives second carrier frequency for civil users), L5 (“Safety Of Life” signal, mainly for civil aviation! The modulation in use for digital signal transmission from space is phase modulation-PM, also known as phase-shift keying (PSK). At each phase shift, the bit is flipped from 0 to 1 or vice versa. This is the method used in GPS. The Coarse Acquisition (C/A) code is transmitted on L1 and can be received by any type of GPS receiver. The C/A code consists of 1023 bits and is repeated every millisecond. The Precision (P-code) code is transmitted on L1 and L2. P-code is encrypted and available only to users with appropriate decryption equipment provided by the USA Department of Defense. The P-code is transmitted at 10.23 MHz and repeats every 267 days. Both codes are synchronized to the satellite’s atomic clocks. C/A and P codes are specific for every single GPS satellite i.e. satellites are distinguished via personal code called pseudo random noise code (PRN code). In view satellites PRN codes or numbers appear on the GPS receiver screen after turning on the receiver. Navigation message (includes Almanac data) with relatively low bit rate - 50 bps. Refer to Figure 2-1 for GPS signals. A GPS Navigation Data Message is combined with each ranging code and transmitted on both L1 and L2 frequencies. 2.1 GPS Navigation Data Message The GPS Navigation Data message consists of 25 frames, each 1500 bits long, transmitted at as a streem of digital data with 50 Hz (50 bps) rate. The complete message requires 12.5 minutes for transmission and contains the transmitting (in view) satellite’s clock correction data and satellite’s predicted path (Ephemeris). Remaining part of Navigation data Message contains information about all satellites in the constellation (Almanac). 2.2 Ephemeris and clock data Clock data contains particular satellite’s on board atomic clock drift information from GPS time. GPS time is kept on Colorado at MCS. Ephemeris data contains precise orbital parameters of SV, taking into account SV drift on theoretical orbit. This information comes from MCS and it permits the receiver to estimate the exact position of the satellite at any time. This is critical for computing the receiver’s position. 2.3 Almanac Data The satellites also transmit almanac data, which contains an indicator of the health of all the satellites and coarse orbital data, atmospheric delay parameters, ionosphere model data and the current GPS time and offset from UTC time. Each satellite transmits almanac data for the entire constellation. The entire almanac is broadcast over a period of 12.5 minutes. When a receiver is new, or has not been operated for a long time, it has to acquire a new almanac before it can begin to compute a position. 2.4 Range Determination The PRN code transmitted by each satellite is also generated in the receiver. The receiver uses code matching techniques to determine the time it took the signal to travel from the satellite to the receiver. Refer to Figure 2-4. Figure 2-4: PRN Code Comparison The speed of the signal is closely approximated by the speed of light, with variations resulting from ionospheric and atmospheric effects modeled from parameters contained in the almanac. The distance from the receiver to the satellite, referred to as a pseudo range, is computed by multiplying the signal travel time and the average speed of the signal. When computing position, the receiver also requires the position of the tracked satellite which is provided by the Navigation message (ephemeris data). 3 GPS signal strength, frequency domain and filtering. The strength of the transmitted GPS signals is very low and cannot be easily viewed on a spectrum analyzer. For this reason, it is susceptible to both intentional and unintentional interference. The minimum received power levels at the surface of the earth are as follows: L1 C/A code -160 dBW or -130 dBmW L1 P code -163 dBW or -133 dBmW L2 P code -166 dBW or -136 dBmW The received signals are at least 16 dB below the noise level of the receiver and require code matching (correlation) technique to recover the PRN code. Spread spectrum techniques are used by satellite transmitters to reduce the effects of noise and improve signal to noise ratio without increasing the transmitter power. C/A code is below noise level. Shown on the drawing on the right side. Signal is multiplied in the receiver by the internally calculated code to allow tracking. C/A-code chip is 1.023 Mhz P-code chip is 10.23 Mhz The calculated power spectrum derives from the Fourier transform of a square wave of width 2π and unit amplitude. Common function in DSP called the “sinc” function. Filtering allows to remove some portion of the frequency spectrum that contains unwanted signal: Low Pass Filter: lets all frequencies below a cutoff frequency through. High Pass Filter: lets all frequencies above a cutoff frequency through. Band Pass Filter: lets all frequencies within a specified frequency window pass through. The window is called the pass band C/A code acquisition is shown on the right side. C/A-code is 1023 chips long and repeats every 1/1000 s, therefore it is inherently ambiguous by 1 msec or ~300 km. Modulo-2 must add the transmitted and received codes after correlation to increase SNR and narrow bandwidth.