The Aalborg GPS Software Defined Radio Receiver

A receiver for the Global Positioning System (GPS) signals provides information on its position and time. The position is given in an Earth-Centered and Earth-Fixed coordinate system. This means that a static receiver keeps its coordinates over time, apart from the influence of measurement errors. The system time (GPST) counts in weeks and seconds of week starting on January 6, 1980. Each week has its own number. Time within a week is counted in seconds from the beginning at midnight between Saturday and Sunday (day 1 of the week). GPST is maintained within the system itself. Universal Time Coordinated (UTC) goes at a different rate which is connected to the actual speed of the rotation of the Earth. At present 14 seconds have to be added to UTC to get GPST. The GPS has 6 orbital planes with at least 4 satellites. At the moment GPS consists of 29 active satellites. They complete about 2 orbits/day. 1 The Transmitted GPS Signals Satellite positioning systems exploit Spread Spectrum (SS) techniques. SS came alive in 1980s and is popular for applications involving radio links in hostile environments. SS is an RF communications system in which the baseband signal bandwidth is intentionally spread over a larger bandwidth by injecting a higher-frequency signal. As a direct consequence, energy used in transmitting the signal is spread over a wider bandwidth and appears as noise. The ratio (in dB) between the spread baseband and the original signal is called processing gain. Typical SS processing gains run from 10 dB to 60 dB, see [1]. To apply an SS technique, simply inject the corresponding SS code somewhere in the transmitting chain before the antenna. That injection is called the spreading operation. The effect is to diffuse the information in a larger bandwidth. Conversely, you can remove the SS code by a despreading operation, at a point in the receive chain before data retrieval. The effect of a despreading operation is to reconstitute the information in its original bandwidth. Obviously, the same code must be known in advance at both ends of the transmission channel. In GPS, SS modulation is applied on top of a BPSK modulation, see below. Intentional or un-intentional interference and jamming signals are rejected because they do not contain the SS code. This characteristic is the real beauty of SS. Only the desired signal, which has the code, will be seen at the receiver when the despreading operation is exercised. In GPS the codes are digital sequences that must be as long and as random as possible to appear as “noise-like” as possible. But in any case, they must remain reproducible. Otherwise, the receiver will be unable to extract the message that has been sent. Thus, the sequence is “nearly random”. Such a code is called a pseudorandom number (PRN) or sequence. The PRN sequences applied in GPS are Gold sequences. These sequences are generated by feedback shift registers, and they are inserted at the data level. This is the direct sequence form of spread spectrum (DSSS). The PRN is applied directly to data entering the carrier modulator. All GPS satellites use the same carrier frequencies: On L1 1575.42 MHz and L2 1227.60 MHz. In a modernized GPS there will be a new civilian frequency L5 (then the military might remove L2 from civilian use). Each satellite has two unique spreading sequences or codes. The first one is the coarse acquisition code (C/A) and the other one is the encrypted precision code (P(Y)). The C/A code is a sequence of 1 023 chips. (A chip corresponds to a bit. It is simply called a chip to emphasize that it does not hold any information.) The code is repeated each ms giving a chipping rate of 1.023 MHz. The P code is a longer code ( . 2 35 10 : . chips) with a chipping rate of 10.23 MHz. It repeats itself each week starting at the beginning of the GPS week. The C/A code is only modulated onto the L1 carrier while the P(Y) code is modulated onto both the L1 and the L2 carrier. The purpose of PRN codes is twofold: They spread the signals and they provide for measuring the travel time between satellite and receiver. The system keeps all C/A code starts aligned in all active satellites. In the rest of this presentation we focus on the L1 signal. Each satellite transmits a continuous signal with at least three components: – a carrier wave with frequency f1 = 1575.42 = 154 × 10.23 MHz – an individual PRN code which is a sequence of −1 and +1 each of length 1 millisecond – a data bit sequence which carries information from which the satellite’s position can be computed. The length of one navigation bit is 20 milliseconds. The PRN code and the data bits are combined through modulo-2 adders. The result is modulated onto the carrier signal using the binary phase shift keying (BPSK) method: The carrier is instantaneously phase shifted by 180° at the time of a chip change. When a navigation data bit transition occurs, the phase of the resulting signal is also phase shifted 180°. So the signal transmitted from satellite k is ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) . s t P C t D t f t P P t D t f t P P t D t f t cos sin sin 2 2 2 2 2 2 k C k k L