US‐Egypt Workshop on Space Technology and Geo‐information for sustainable development,  Cairo, Egypt 14‐17 June, 2010 

NAVSTAR Simulator for space-born receivers By Hany Bekhit Ottafy Research Engineer, NARSS E-mail: [email protected] Abstract In the process of developing a GPS receiver, working on-board a satellite, the testing of receiver dynamics is a vital process. The GPS system dynamics is a direct result for mutual motion between NAVSTAR constellation satellites and GPS receivers' antennas. The problem for testing space-born GPS receivers is to put the receiver in its real working conditions including high speed and the resulting Doppler effect, and noise. Most of the currently existed simulators for NAVSTAR satellites don't provide testing for Doppler shift that is necessary for testing the dynamics of the software of space-born GPS receivers. Moreover, not all simulators provide testing with internal noise source. This paper presents the structure of our NAVSTAR simulator. This simulator provides Doppler frequency shift that can be used to test space-born receivers. In addition, the simulator has the capability to generate the required noise level at the receiver according to receiver's environment factors. Keywords GPS, receiver, NAVSTAR, Doppler, L1, C/A, satellite, simulator, space-born Introduction The block diagram of the GPS navigation system is shown below:

Figure 1: GPS system blocks The NAVSTAR simulator provides generation the GPS signals plus channel effects (Doppler and noise). These generated signals of 32 satellites are used to assess the performance of spaceborn GPS receivers. The structure of this simulator has, for each NAVSTAR satellite, signal generator and Doppler shift calculator. The signals generated from all satellites are then summed up (with their respected power level) and the generated noise is then added. The resulted signal is then frequency up-converted to nominal frequency L1 of 1575.42 MHz. The detailed NAVSTAR simulator structure is given below. NAVSTAR simulator project motivation This project is one of the currently developing projects of the Egyptian Space Program (ESP). This simulator will participate in testing of currently developing and other coming satellites. The simulator The NAVSTAR simulator functional block diagram is shown below:

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US‐Egypt Workshop on Space Technology and Geo‐information for sustainable development,  Cairo, Egypt 14‐17 June, 2010 

GPS Data frames generators

GPS C/A code generator

Noise generator XOR

Mixer Mod IF

Receiver power level calculator

Summing circuit

BPF

GPS signal

DAC -120 dB

LO

RF Section

Doppler calcuator

GPS Satellites orbit propagator (position, velocity)

Range determination

GPS receiver location determination (position, velocity)

PC program

One NAVSTAR satellite

Figure 2: GPS simulator functional block diagram The NAVSTAR simulator consists of (mainly) four parts, namely, the satellite signal simulator in intermediate frequency (IF), the super high frequency (SHF) or RF section, computer program (Graphical User Interface, GUI), and the channel effects. In this section, brief description for each functional block is given. The satellite signal simulator in IF This section generates the GPS 32 satellites’ signal in the intermediate frequency where the C/A codes and data are specific for each satellite .This section performs the following functions: 1. Initialization The initialization function performs the following tasks: reception of almanac data from PC and store them in memory, reception of receiver’s orbit parameters from PC and store them in memory, reception of noise parameters from PC and pass them to the noise generation function, and confirm upon reception of correct data to initiate the simulator other functions. 2. GPS satellite orbit propagation This function takes as inputs the GPS orbit parameters (Almanac) for each satellite from the computer program and calculates the position and velocity for each satellite independently according to the input data. 3. GPS receiver location determination This function calculates the GPS receiver position and velocity at any given time depending on the input data about receiver’s orbital parameters, hence, this function always consider the receiver to be mounted on-board a satellite.

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US‐Egypt Workshop on Space Technology and Geo‐information for sustainable development,  Cairo, Egypt 14‐17 June, 2010  4. GPS data frames generators These generators generate the data to be transmitted by the GPS satellite in the preset data frame structure. The update of data fields’ values are taken from the GPS satellite orbit propagator for that current satellite and from input data. 5. GPS C/A code generator The C/A code of that GPS satellite is generated according to its polynomial with the C/A code bit rate of 1023 kbit/s. 6. Range determination This function calculates the range between the GPS satellite and the GPS receiver according to their respect positions. This function takes as inputs the positions of GPS satellite and receiver, and, accordingly, selects the GPS satellites that are in the visibility zone of receiver’s antenna. This function limits the number of satellites that will be visible to receiver’s antenna by controlling the presence of satellites’ signals in the resultant signal. 7. Receiver power level calculator This function calculates the signal power level at the receiver input taking into account the losses in signal power, where the attenuator in the RF section is included in this calculation. The output of this function is the GPS signal of that satellite scaled to power level equivalent to that calculated by the range determination function. 8. The intermediate frequency generator (IF) This function generates the intermediate frequency taking into account the Doppler shift in carrier. This Doppler shift is an input to this function where it is calculate by Doppler calculator function. The nominal frequency (without Doppler shift) will be about 75.42 MHz. 9. The modulator This modulator is a BPSK modulator which modulates the IF carrier phase according to the sign of the resultant signal after xor-ing the C/A code bit with GPS data bit. 10. The summing circuit The function of this summing circuit is to sum up all 32 satellites signals and then add the generated noise to the resultant signal to generate the complete GPS NAVSTAR simulator signal. 11. The Digital-to-Analog Converter (DAC) The function of DAC circuit is to convert the simulator IF signal from its digital form to its analog equivalent form. The output of this function is the analog IF signal from all satellites having all channel effects. The channel effects 1. The Doppler frequency shift calculator This function takes as inputs the velocities of both the GPS receiver and the GPS satellite and calculates the Doppler frequency shift in carrier frequency. This Doppler shift is the input to the IF generator function. 2. Noise generator This function generates the channel accumulated-noise in the band of GPS signals and does not consider the receiver noise itself. The effect of channel noise is minor with relative to the effect of Doppler shift on the signal.

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US‐Egypt Workshop on Space Technology and Geo‐information for sustainable development,  Cairo, Egypt 14‐17 June, 2010  The radio frequency (RF) section 1. The RF local oscillator (LO) This function generates the RF frequency needed to up-convert the GPS signal to be centered at 1575.42 MHz (if Doppler shift is zero). The LO frequency will be 1.5 GHz. 2. The mixer The mixer functions here as up-converter to heal up the signal at IF to the band of GPS signal of 1575.42 ± 1 MHz. 3. The band pass filter (BPF) The BPF selects the GPS signal at 1575.42 MHz with bandwidth of 150 MHz (3dB) and attenuates the out-of-band signals as much as possible. The design parameter for attenuation value at frequency of 1424.58 MHz should be not less than 30 dB. 4. The attenuator The attenuator attenuates the GPS signal by 120 dB. The output from this attenuator is the GPS simulated signal. The PC program The purpose for this computer program is to provide the running simulator, at the beginning of simulator work, with the receiver’s orbital parameters, NAVSTAR satellites almanac, and noise parameters. In addition, this computer program should read receiver position and velocity from the GPS receiver locator module and register these data in it hard disk for further analysis for the sake of this simulator assessment (post session assessment). NAVSTAR simulator design specifications The design parameters for this simulator are listed below: Table 1: NAVSTAR simulator design parameters Parameter Output Frequency Number of simulated satellites Number of simulated channels Channels Type Max Velocity (velocity of GPS receiver in orbit) Pseudorange accuracy Pseudorange Rate accuracy Delta Pseudorange accuracy Inter-channel Bias Signal level Phase Noise (Max) Number of RF Outputs User interfaces • Control interface •

RF output

Value L1: 1575.42 MHz 32 12 (max) GPS C/A code Up to 10 km/s 5m 1 m/s 1m 0 -145 dBm ± 15 dB -88 dB/Hz 1 •

RS232

•

SMA

The project prototype realization The simulator specified above has many deferent technologies such as PC programming, HDL programming, embedded programming of µ-processor, and SHF circuit design using µ-strip technology. To have the simulator as physical product, this requires building a prototype first for the sake of testing and debugging.

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US‐Egypt Workshop on Space Technology and Geo‐information for sustainable development,  Cairo, Egypt 14‐17 June, 2010  The problem to have FPGA and µ-processor in a single board is already solved. The Xilinx University Program (XUP) provides many developing kits for software/hardware development. Here, in NARSS, we have such development boards. One of these boards is the Vertex II Pro (FPGA) kit, which has the Vertex II Pro XC2VP30 chip installed with many peripherals. The V2P chip has two µ-processors types installed inside; one is a hardware core, which is PowerPC® µprocessor, and the other one is a software core µ-processor, which is MicroPlaze™. The simulator is programmed using FPGA and PowerPC µ-processor. Xilinx provides a developing IDE tool for development embedded software for PowerPC µprocessor, which is the Embedded Development Kit (EDK) tool. This tool uses the C language as its programming language with its own complier and depends on the µ-processor architecture. The block diagram shown above contains the functions to be fulfilled during the project period. The project task is to build up the simulator for NAVSTAR 32 satellites and a receiver mounted on a satellite. The channel effects will be the Doppler shift and noises. The RF section will be only one for all satellites with power attenuator with constant gain of 120 dB. Calculations for link budget and maximum Doppler frequency shift were carried out for satellites with altitudes up to 2000 km. Current project status  According to the project schedule, the prototype will be manufactured in July 2010, and the process of testing the prototype will start in July/August 2010. The algorithms and codes for generation of GPS frames and C/A code are written, the RF section and DAC circuits are in the process of manufacturing, and the PC program is written and in process of testing/debugging. The functions of receiver power level calculation, noise generator, summing circuit, range determination, and Doppler frequency shift calculator are in development phase. Conclusion The under development NAVSTAR simulator provides generation the 32 GPS satellites’ signals at L1 frequency with C/A code plus channel effect for space-born receivers. This simulator will be used to assess the performance of space-born GPS receivers both installed in EgyptSat-2 satellite or other coming satellites. The structure, functionally, of the simulator was given, and briefly described. The design parameters of this simulator show a similar performance for the currently used simulators worldwide. References

[1] “Global Positioning System Standard, Positioning Service, Signal Specification”, 2nd Edition, June 2, 1995 [2] “GPS SPS Signal Specification - Annex A thru C”, 2nd Edition, June 2, 1995 [3] James Bao, Yen Tsui, “Fundamentals of Global Positioning System Receiver; a Software Approach,” chapter 5, John Wiley & Sons, Inc., Publication, New Jersey, 2005 [4] Guochang Xu, “GPS Theory, Algorithm and Applications,” Chapter 5, Springer Berlin Heidelberg New York, 2nd edition [5] Name,… Title, Journal.

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