GEOFFREY BLEWITT: BASICS OF THE GPS TECHNIQUE The signals from a GPS satellite are fundamentally driven by an atomic clocks(usually cesium, which has the best long-term stability). The fundamental frequency is 10.23 Mhz Two carrier signals, which can be thought of as sine waves, are created from this signal by multiplying the frequency by 154 for the LI channel (frequency= 1575.42 Mhz; wavelength 19.0 cm), and 120 for the L2 channel (frequency=1227. 60 Mhz; wavelength= 24.4 cm). The reason for the second signal is for self-calibration of the delay of the signal in the Earth's ionosphere Information is encoded in the form of binary bits on the carrier signals by a process known as phase modulation. (This is to be compared with signals from radio stations, which are typically encoded using either frequency modulation, FM, or amplitude modulation, AM) The binary digits 0 and I are actually represented by multiplying the electrical signals by either +l or -l, which is equivalent to leaving the signal unchanged, or flipping the phase of the signal by 180. We come back later to the meaning of phase and the generation of the binary code There are three types of code on the carrier signals The C/A code The p code The Navigation Message The C/A (course acquisition )code can be found on the LI channel. As will be described later, this is a code sequence which repeats every I ms. It is a pseudo-random code, which appears to be random, but is in fact generated by a known algorithm. The carrier can transmit the C/a code at 1.023 Mbps(million bits per second). The"chip length", or physical distance between binary transitions(between digits +I and-1), is 293 metres. The basic information that the C/A code contains is the time according to the satellite clock when the signal was transmitted (with an ambiguity of l ms, which is easily resolved, since this corresponds to 293 km). Each satellite has a different C/A code, so that they can be uniquely identified The P("precise")code is identical on both the LI and L2 channel. Whereas C/A is a courser code appropriate for initially locking onto the signal, the P code is better for more precise positioning. The P code repeats every 267 days. In practice, this code is divided into 7 day segments; each weekly segment is designated a"PRN number, and is designated to one of the GPS satellites. The carrier can transmit the P code at 10.23 Mbps, with a chip length of 29.3 metres. Again, the basic information is the satellite clock time or transmission, which is identical to the C/A information, except that it has ten times the resolu Unlike the C/A code, the p code can be encrypted by a process known as"anti-spoo or“A/S”(see below) The Navigation Message can be found on the ll channel, being transmitted at a very slow rate of 50 bps. It is a 1500 bit sequence, and therefore takes 30 seconds to transmit. The Navigation Message includes information on the broadcast Ephemeris(satellite orbital ) satellite clock corrections, almanac data(a crude ephemeris for ionosphere information, and satellite health status 2.3. 2 Denial ofaccuracyGEOFFREY BLEWITT: BASICS OF THE GPS TECHNIQUE 7 The signals from a GPS satellite are fundamentally driven by an atomic clocks (usually cesium, which has the best long-term stability). The fundamental frequency is 10.23 Mhz. Two carrier signals, which can be thought of as sine waves, are created from this signal by multiplying the frequency by 154 for the L1 channel (frequency = 1575.42 Mhz; wavelength = 19.0 cm), and 120 for the L2 channel (frequency = 1227.60 Mhz; wavelength = 24.4 cm). The reason for the second signal is for self-calibration of the delay of the signal in the Earth’s ionosphere. Information is encoded in the form of binary bits on the carrier signals by a process known as phase modulation. (This is to be compared with signals from radio stations, which are typically encoded using either frequency modulation, FM, or amplitude modulation, AM). The binary digits 0 and 1 are actually represented by multiplying the electrical signals by either +1 or −1, which is equivalent to leaving the signal unchanged, or flipping the phase of the signal by 180 o . We come back later to the meaning of phase and the generation of the binary code. There are three types of code on the carrier signals: • The C/A code • The P code • The Navigation Message The C/A (“course acquisition”) code can be found on the L1 channel. As will be described later, this is a code sequence which repeats every 1 ms. It is a pseudo-random code, which appears to be random, but is in fact generated by a known algorithm. The carrier can transmit the C/A code at 1.023 Mbps (million bits per second). The “chip length”, or physical distance between binary transitions (between digits +1 and −1), is 293 metres. The basic information that the C/A code contains is the time according to the satellite clock when the signal was transmitted (with an ambiguity of 1 ms, which is easily resolved, since this corresponds to 293 km). Each satellite has a different C/A code, so that they can be uniquely identified. The P (“precise”) code is identical on both the L1 and L2 channel. Whereas C/A is a courser code appropriate for initially locking onto the signal, the P code is better for more precise positioning. The P code repeats every 267 days. In practice, this code is divided into 7 day segments; each weekly segment is designated a “PRN” number, and is designated to one of the GPS satellites. The carrier can transmit the P code at 10.23 Mbps, with a chip length of 29.3 metres. Again, the basic information is the satellite clock time or transmission, which is identical to the C/A information, except that it has ten times the resolution. Unlike the C/A code, the P code can be encrypted by a process known as “anti-spoofing” , or “A/S” (see below). The Navigation Message can be found on the L1 channel, being transmitted at a very slow rate of 50 bps. It is a 1500 bit sequence, and therefore takes 30 seconds to transmit. The Navigation Message includes information on the Broadcast Ephemeris (satellite orbital parameters), satellite clock corrections, almanac data (a crude ephemeris for all satellites), ionosphere information, and satellite health status. 2.3.2 Denial of Accuracy