R/C Simulation and Hardware Proof of Concept Development

1. R/C Simulation and Hardware Proof of Concept Development Dr. Philip A. Dafesh, Dr. R. T. Bow, Mr. G. Fan and Mr. M. Partridge Communication Systems Subdivision The Aerospace Corporation

2. Outline • Background • Simulation Overview and Results • Hardware Overview and Results • Summary and Conclusions

3. Background • As part of the R/C code Development, Constellation Simulations and Hardware Proof of Concept (POC) Demonstrations Were Completed • Signal Processing Worksystem (SPW) Simulation of L5 and Newly Proposed TDMA Code • HW POC was Developed as a Modification to The Aerospace Corporation's "FlexGPS" FPGA-Based Prototyping System • IIRM Modulator Implementation of C/A, P and M-code on L2 • Single Channel Receiver Hardware (Code Corr. and Tracking Loops) • Implemented R/C Transmitter and Receiver as Modification to C/A Hardware in 2 Weeks from Receipt of the R/C Specification • Simulated and Measured Correlation Response and DLL Tracking Loop Response for R/C Codes Alternatives

4. Sample Correlation Data Taken with R/C Code POC Hardware Measured Correlation is In Agreement With Theoretical Reduction in Non-Prompt Correlation for a Single Satellite

5. Simulation • Implements Multiple R/C Code Satellites Using the Signal Processing WorkSystem (SPW) • Simulates the Effect of Random Code Phases and Carrier Doppler for Each Satellite • 10 Satellites in View at Varying Power Levels • Signal Parameters are Adjusted to Mimic the IIRM Implementation • Interplex Modulation of C/A or TDMA, P(Y) and M-Code • Relative Power Adjusted to IIRM Levels

6. Caveats • M-Code Implemented as a 20 TAP M-Sequence • Actual Code is Nonrepeating • Used Actual P-Code • Used Actual C/A and TDMA Codes • Simulation Sample Rate = 4.092x107 • 40 Samples/TDMA Chip (80 Samples/Bit) 1 Bit ~ 2 s 1Chip~ 1 s M0L0M1L1M2L2M3L3M4L4

7. Normalized Correlation of SV1 with Constellation (with P and M)

8. Zoom in to TDMA/CA Results

9. Summary of Observations • R/C Code Correlation Results in Fewer and Smaller Size Non-Prompt Code-Cross Correlations with other Satellites than Does C/A Code • Results Indicate that R/C Code Should Result in Smaller CDMA Noise Contributions During Tracking and Fewer False Alarms During Acquisition • Even when Constellation of Satellites is Considered Including Effects of Doppler, Amplitude and Phase Distribution

10. Hardware Proof of Concept Description • Implementation of a Single IIRM Satellite with Switches to Select Different R/C Code Options on the L2 Carrier • C/A, L5, TDMA • Receiver Can Select Between Correlation of Entire TDMA or Individual 2CM and 2CL Codes • Selectable DLL Early-Late Code Tracking at Spacing from +/- 0.05 chips to +/- 0.5 chips • SVi to SVi Autocorrelation and SVi to SVk Cross-Correlation • New TDMA Code was Implemented in FlexGPS Transmitter and Receiver Correlator within 2 Weeks • Relatively Straightforward Modification to C/A Receiver

11. Hardware Description (Cont.) • All Tests Conducted at L2 Frequency • Correlation Sweep and DLL Tracking at Selectable Early-Late Spacing, Loop Parameters • No Navigation Processing • I and Q Sample Rate = 40.92 MHz • Receiver Bandwidth = 24 MHz • Data Measurement and Diagnostic Capability • Time-Domain, Frequency-Domain & Logic-Level • Oscilloscope, Spectrum Analyzer, Logic Analyzer • PR Residual and Code Correlation Data • Software GUI Control and Data Acquisition Interface

12. Aerospace’s FlexGPS Prototyping System

13. Rapid-Prototype BoardClose-Up of Modulator and Receiver Tx I/Q DACs Rx 40.92 MHz I/Q ADCs

14. Tx/Rx Signal Probe and Control Logic Analyzer Probes GUI Interface Control and I/O

15. What's Different from Prior C/A-Code Prototype • What was Implemented • SV Selectable C/A, L5 or TDMA Code at Transmitter • Selectable 2CM, 2CL, L5-I or C/A Codes in Receiver • 511.5 kHz Square-Wave Gated Integrate and Dump Clock ("0" of square-wave corresponds to 2CM Accumulator Clock and "1" Corresponds to 2CL Accumulator Clock) • Square-wave Gated Correlator With Standard Bi-Phase (+1,- 1) Reference Used Instead of Three-Level (+1,0,-1) Reference Code • No Significant Changes to C/A Code DLL Tracking Channel • Additional Details to be Published at ION GPS 2001 • What was Not Implemented (Planned) • Independent 2CM and 2CL Tracking Channels • Viterbi decoding

16. R/C TDMA Specific Additions • Difference Between C/A and TDMA Receiver Hardware • Requires 2 Code Generators • Same Hardware Initialized Differently and Short Cycled at Different Points to Generate 2CM and 2CL Codes • Requires Different Correlation Method • Implemented as Time Gating or 3-Level Reference • Requires Min of 1 and Max of Two New Tracking Channels for L2 Reception • Additional Changes Include Standard Viterbi Decoding and Carrier PLL Carrier Tracking as would be Implemented for the L5 Signal • Associated Data Demodulation at 25 Hz Rate (50 Hz Coded)

17. Hardware Measured Power Spectral Density TDMA Signal PSD Follows Same Sinc2 Response as C/A Without Observable Spectral Lines

18. Measured Spectral Lines Line Structure of TDMA R/C Code is Within Resolution of Spectrum Analyzer < 1 Hz Line Spacing TDMA R/C Code Has P-Code Like Line Structure: Better Interference Rejection Properties

19. Measured R/C vs C/A Correlation and Pseudorange Measured Mean PR Residual of 803 PR I&D Samples (in Chips):

20. Summary of Hardware Measurement Observations • TDMA Code Has an Undetectable Spectral Line Structure • More P-Code Like and Potentially Better Immunity to Co-Channel Interference than C/A or L5 • TDMA Correlation Response • Comparable to C/A Code but 1/2 the Magnitude for Correlation Over Fixed Integration Period • In agreement with Theoretically Expected Multiplexing Ratio • Significantly Smaller Amplitude Non-Prompt Peaks • Better Acquisition Performance, Smaller CDMA Noise • TDMA Code Pseudorange Response Does Not Exhibit any Biases Relative to C/A Code • Agreement to 0.0001 C/A Chip (within Experimental Error)

21. Planed Hardware Additions • Implementation of Code and Carrier Tracking for Various Combinations • Code and Carrier Tracking 2CM Code with Data • Early-Late Code Tracking, AFC and PLL Carrier Tracking with Data removal • Code and Carrier Tracking 2CL Code without Data • Early-Late Code Tracking, AFC and PLL Carrier Tracking without Data removal • Acquisition Performance of C/A vs 2CM vs 2CL • Industry Inputs Welcome • To Address Specific Concerns with TDMA Receivers

22. Summary and Conclusions • C/A, L5 (1.023 MCPS) and TDMA Code R/C Options were Compared Using a Constellation Simulation and a FPGA Hardware Proof of Concept • New L2 TDMA R/C Code Exhibited Superior Characteristics • A Proof of Concept was Developed for Both Transmitter and Receiver Implementations of the L2 R/C Code • Small Change to Aerospace's FlexGPS Receiver Implemented by Time Gated Correlator with 511.5 kHz Square-wave • Simulation and Hardware Measurements are Constant with Theoretical Expectations for TDMA R/C Codes • Detailed Results to be Published at ION GPS 2001

23. Backup

24. Current Prototype Development Approach SPW/C Floating Point Simulation HDS (Fixed Pt. SPW ) Model Development Integrated SPW w/ Hardware Development System (HDS) Simulation: Cycle Correct Fixed Point (Integer) Simulation Concept to Hardware HDS Subsystem Optimization (Digital Design) HDS: Optimized Prototype Design VHDL Code Generation Sun Workstation Synthesize VHDL to EDIF/XNF Compile FPGA File LAN APTIX Software: Place and Route FPGA Components, Interconnect Routing, Test Points Selection FPGA APTIX Programmable Brassboard HP Logic Analyzer