1. 1 GPS Signal (Module 3) Why is the GPS signal so complex? GPS is a receive-only system for a user and number of users is unlimited. Since there are many functions that must be performed, the GPS signal has a rather complex structure. This module will describe the properties of signal transmitted from the satellite and received by the GPS receiver antenna.

2. 2 GPS Satellite Signal Each GPS satellite transmits a unique navigational signal centered on two L-band frequencies of the EM spectrum: L 1 at 1575.42 MHz and L 2 at 1227.60 MHz. At these microwave frequencies the signals are highly directional and hence are easily blocked, as well as reflected, by solid objects and water surfaces. However, clouds are easily penetrated, but the signals can be blocked by dense or wet foliage. The satellite signals basically consist of: The two L-band carrier waves. The ranging codes modulated on the carrier waves. The navigation messages.

3. 3 L-Band Carrier Frequencies As the name implies, the carrier waves provide the means by which the ranging codes and navigation message is transmitted to Earth (and hence to the user). The primary function of the ranging codes is to permit the signal transit time (from satellite to receiver) to be determined. This quantity is also sometimes referred to in the navigation literature as the time-of-arrival-TOA. The transit time when multiplied by the speed of EM radiation (= 299792458 m/s in a vacuum) gives the receiver-satellite range. The navigation message is modulated on both carrier frequencies and contains the satellite ephemeris, satellite clock parameters, and other pertinent information such as general system status messages and an ionospheric delay model, necessary for real-time navigation to be performed.

4. 4 Choice of the Carrier Frequency To transport data signals, a suitable carrier frequency is required. The choice of the carrier frequency relies on: –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 EM waves in media like air deviates from the speed of light (in vacuum) the more, the lower the frequency is. –The PRN-codes 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.

5. 5 Selection of Two Frequencies 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 100 and 200 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.

6. 6 GPS Signal Characteristics The satellites transmit on two L band frequencies based on a common frequency f o = 10.23 MHz. These are referred as: –f L1 = 154 f o = 1575.42 MHz –f L2 = 120 f o = 1227.6 MHz. The signal can be split into the following three parts: –Carrier: f L1 and f L2 –Navigation data: The navigation data contains information about satellite orbits. This information is transmitted to all satellites from the ground stations in the GPS control segment. –Spreading sequence: Each satellite has two spreading sequences or codes. The first is the coarse acquisition code (C/A) and the other one is the encrypted precision code (P(Y)).

7. 7 Mathematical Model of L 1 Waveform

8. 8 Structure of In-phase Component of L 1 Signal d(t) 50 bps dataTransmitted signal c(t) C/A code Multiply

9. 9 Mathematical Model of L 2 Signal In contrast to L 1 signal, the L 2 signal is modulated with only the 50-bps data and the P-code. The mathematical model of the L 2 waveform is

10. 10 Structure of Quadrature-phase Component of the L 1 Signal d(t) 50 bps data Transmitted signal p(t) P(Y) code Multiply

11. 11 Modulation of the Carrier Signals The (C/A) code is a sequence of 1023 chips. The code is repeated each millisecond giving a chipping rate of 1.023 MHz. The (P) code is a longer code (2.35  10 4 ) with a 10.23 MHz chip rate. It repeats itself each week starting at the beginning of GPS week. The Y-code is used in place of the P-code whenever the anti- spoofing (A-S) mode of operation is activated. The C/A code is available on the L1 frequency and the P-code is available on both L1 and L2. The various satellites all transmit on the same frequencies, L1 and L2, but with individual code assignments. Due to the spread spectrum characteristic of the signals, the system provides a large margin of resistance to interference. Each satellite transmits a navigation message containing its orbital elements, clock behavior, system time and status messages.

12. 12 Signal Generation and Ranging Codes

13. 13 C/A Code The C/A code has the following functions: –To enable accurate range measurements and resistance to errors caused by multipath: Using the C/A code to increase the signal bandwidth reduces errors in measuring signal delay caused by multipath. –To permit simultaneous range measurement from several satellites: The use of a distinct C/A code for each satellite permits all satellites to use the same L 1 and L 2 frequencies without interfering with each other. –To provide protection from jamming: The correlation process that despreads the desired signal has the property of spreading any other signal.

14. 14 P Code The P-code, which is used primarily for military applications, has the following functions: –Increased jamming protection: Since the bandwidth of the P- code is 10 times greater than that of the C/A code, it offers approximately 10 dB more protection from narrow band interference. –Provision for antispoofing: The receiver rejects the false signal and decrypt the desired one. –Denial of P-code use: Encrypted code. –Increased code range measurement accuracy: Accuracy improves as the bandwidth improves.

15. 15 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. 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 measurement 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.

16. 16 PRN Sequences Only the C/A code will be considered. Spreading sequences used as C/A codes in GPS are called Gold codes as R. Gold discovered them 1967. They are also referred as pseudo-random noise sequences or simply PRN sequences. Generation of Gold codes is shown below.

17. 17 The C/A code generator contains two shift registers known as G 1 and G 2. Each shift register has ten states generating sequences of length 1023. The two resulting 1023 chip long sequences are modulo-2 added to generate a 1023 chip long C/A code. Every 1023 rd period, the shift registers are reset with all ones, make the code start over. To make different C/A codes for the satellites, the output of the two shift registers are combined in a very special manner. The selection of states for the modulo-2 adder is called the phase selection.

18. 18 C/A Code Phase Assignment

19. 19 Code Division Multiplexing Code Division Multiplexing (CDM) is a technique in which each channel transmits its bits as a coded channel-specific sequence of pulses. This coded transmission typically is accomplished by transmitting a unique time-dependent series of short pulses, which are placed within chip times within the larger bit time. All channels, each with a different code, can be transmitted on the same fiber and asynchronously demultiplxed. Other widely used multiple access techniques are Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA). Code Division Multiplex techniques are used as an access technology, namely Code Division Multiple Access (CDMA), in Universal Mobile Telecommunications System (UMTS) standard for the third generation (3G) mobile communication identified by the ITU. Another important application of the CDMA is the GPS.

20. 20 CDM and Spread Spectrum CDM allows signals from a series of independent sources to be transmitted at the same time over the same frequency band. This is accomplished by using orthogonal codes to spread each signal over a large, common frequency band. At the receiver, the appropriate orthogonal code is then used again to recover the particular signal intended for a particular user. The key principle of CDM is spread spectrum. Spread spectrum is a means of communication with the following features: –Each information-bearing signal is transmitted with a bandwidth in excess of the minimum bandwidth necessary to send the information. –The bandwidth is increased by using a spreading code that is independent of the information. –The receiver has advance knowledge of the spreading code and uses this knowledge to recover the information from the received, spread-out signal.

21. 21 Information Flow through the Spreading and Despreading Circuits Source Spreading code generator Exclusive OR Modulator User Exclusive OR Antennas 110110 011101010000100101 Channel 3000 bit/s 1000 bit/s Spreading increases the bandwidth by 3000 / 1000

22. 22 Although any sequence of “1”s and “0”s can be used as a spreading code, practical spreading codes must look like a sequence of random, independent, equiprobable bits. These codes are called pseudo-random (PN)—rather than truly random—because the transmitter and receiver must generate the same sequence (otherwise despreading will not work). The bits in a PN code are essentially uncorrelated. Thus, when a PN spreading code is applied to additive white Gaussian noise, the power spectral density of the noise remains flat and unchanged.

23. 23 Data Sequence for Spreader and Despreader

24. 24 The GPS Navigation Signal In order for a GPS navigator to derive real-time position (and to make the task of the GPS surveyor easier when he comes to reduce his data), a navigation message is transmitted on both L-band frequencies, containing the following information: – Predicted satellite ephemerides. – Predicted satellite clock correction model coefficients. – GPS system status information. –The GPS system ionospheric model.

25. 25 Data Stream The data stream conveys the navigation message, which includes, but not limited to, the following information: –Satellite Almanac Data: Each satellite transmits orbital data called almanac, which enables the user to calculate the approximate location of every satellite in the GPS constellation in any given time. –Satellite Ephermeris Data: This signal enables a much accurate determination of satellite positioned to convert signal propagation delay into an estimate of user position. It is valid only for few hours. –Signal Timing Data: The data stream includes time tagging, which is used to establish the transmission time of specific points on the GPS signal. This information is needed to determine the satellite-to-user propagation delay used for ranging. –Ionospheric Delay Data: Ranging errors due to ionospheric effects be partially canceled by using estimates of ionospheric delay that are broadcast in the data stream. –Satellite Health Message: This is data regarding current health of the satellite, so the receiver may ignore the satellite if it is not functioning well.

26. 26 GPS Navigation Data

27. 27 Three parts forming the signal in the L1 frequency. C/A code repeats itself every millisecond. Navigation bits last 20 ms. For each navigation bit the signal contains 20 complete C/A codes.

28. 28 See how the L1 signal for a satellite is created with BPSK modulation of the C/A code and data on the carrier wave. The resulting signal is the product of all the three signals.

29. 29 Signal Power Levels The L 1 C/A code signal is transmitted at a minimum level of 478.63 W (26.8 dBW) effective isotropic radiated power (EIRP). Future satellites may radiate higher power. As the signal propagates toward the earth, it losses power density due to spherical spreading. The loss is accounted by a quantity called the free-space loss factor (FSLF) given by

30. 30 The GPS Receiver There are two range-type measurements that can be made on the GPS signals: Pseudo ranges, and Carrier phase observations. Both are a product of the operation of the GPS receiver (that is, the acquisition and maintenance of signal tracking), both are used for GPS navigation (position, velocity and time (PVT) determination), and both have a role in the specialized data processing that characterizes GPS surveying. Before studying these measurements, it is useful to consider the overall GPS hardware tracking operation.

31. 31 Received Satellite Signal The received satellite signal level is actually less than the background noise level, hence correlation techniques are used to obtain the satellite signals. A typical satellite tracking sequence begins with the receiver determining which satellites are visible above the horizon. Satellite visibility is estimated from predictions of present PVT, and on the stored satellite almanac information residing within the receiver. (If no stored almanac information exists, or only a very poor estimate of PVT is available, the receiver will carry out a “sky search”, attempting to randomly locate and lock onto a signal. The receiver will then decode the Navigation Message and read the almanac information about all the other satellites in the constellation.) A carrier-tracking loop is used to track the carrier frequency while a code-tracking loop is used to track the C/A and/or P code signals. The two tracking loops have to work together in an iterative manner, aiding each other in order to acquire and track the satellite signals.

32. 32 Receiver Carrier Tracking Loop The receiver’s carrier-tracking loop will locally generate an L 1 carrier frequency (or L 2 if the receiver is capable of tracking this frequency) which differs from the received carrier signal due to a Doppler offset of the carrier frequency. This Doppler offset is proportional to the relative velocity along the line-of-sight to the satellite. In order to maintain lock on the carrier, the carrier-tracking loop must, in effect, adjust the frequency of the receiver-generated carrier until it matches the incoming carrier frequency. The amount of this offset is the “beat” frequency which can be processed to give a periodic carrier phase measurement. The derivative of this phase measurement is the “Doppler” measurement, which is used to determine the receiver’s velocity.

33. 33 Correlation Properties The most important characteristics of the C/A codes are their correlation properties. There are two correlation properties: –Nearly no cross correlation properties. For example, for two codes C i and C k for satellite i and k, the cross correlation can be written as –Nearly no correlation except for zero lag. This property is useful to find out when two similar codes are perfectly aligned.

34. 34 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. 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. 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.

35. 35 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.

36. 36 Signal Acquisition and Tracking When a GPS receiver is tuned on, a sequence of operations must ensuer before information in a GPS signal can be accessed and used to provide a navigation solution. The operations are: –Determine which satellites are visible to the antenna. –Determine the approximate Doppler of each visible satellite. –Search for the signal both in frequency and C/A code phase. –Detect the presence of a signal and confirm detection. –Look onto and track the C/A code. –Look onto and track the carrier. –Perform data bit synchronization. –Demodulate the navigation data.

37. 37 References Global Positioning Systems by MS Grewal, LR Weill, AP Andrews, Wiley, 2001. Peter Rinder and Nicolaj Bertelsen, Design of a Single GPS Software Receiver, Aalborg University 2004. The Internet