Testing CRPA Systems

1. Testing CRPA Systems

2. Agenda • Overview of CRPA Systems • Challenges of Testing CRPA Systems • CRPA Test Solutions using Simulators • Testing with the Antenna in an Anechoic Chamber • Testing without the Antenna

3. Agenda • Overview of CRPA Systems • Challenges of Testing CRPA Systems • CRPA Test Solutions using Simulators • Testing with the Antenna in an Anechoic Chamber • Testing without the Antenna

4. Interfering Threats Like other radio frequency (RF) communication systems, Global Navigation Satellite System (GNSS) signals, such as Global Positioning System (GPS) are prone to interference This is of particular significance for GNSS/GPS which has very low received powers, well below the noise floor There are many sources of interference, some intentional and some unintentional that can affect acquisition and tracking such as the following: AM and FM modulated signals from radio, mobile and other communication systems that varies in received signal power from proximity to the broadcasting antenna / region Pulsed continuous wave (CW) or noise sources from a radar system for example CW or swept CW for a generic signal coming in and out of the GPS band Electromagnetic interference (EMI) from other equipment Various spread-spectrum noise signals, such as band limited Gaussian noise Proprietary & Confidential—Page 4

5. Interference Mitigation Techniques • As a result of the growing threat of interference and jamming from unintentional (radio, mobile, EMI, etc.) and intentional (electronic warfare) sources, interference mitigation is important • Anti-interference can be performed with aiding sources or by the antenna using antenna electronics or by the receiver using various signal conditioning methods • Some examples of anti-interference techniques are: • Using a Controlled Reception Pattern Antenna (CRPA) and associative Antenna Electronics (AE) for nulling out interference sources • The receiver can increase the analog-to-digital (A/D) bits for narrowband mitigation or adjust the correlation spacing (Ref. Misra and Enge Global Positioning System) • The receiver can integrate aiding sources from other GNSS constellations, inertial, air data, etc. to augment and aid the navigation solution Proprietary & Confidential—Page 5

6. Controlled Reception Pattern Antenna (CRPA) systems are an effective anti-interference method CRPA systems have been primarily used on military platforms and vehicles but are seeing a growing use in commercial applications The hardware is comprised of a multi-element antenna and the corresponding antenna electronics (AE) for processing the received data from each antenna An example is shown to the right for the GAS-1, which uses a 7-element, 14” diam. antenna and AE module The current trend is that CRPA antennas are getting smaller and the electronics are getting smarter such as the Advanced Digital Antenna Production (ADAP) for better protection Overview of CRPA Systems 7-element Antenna Antenna Electronics (AE) Proprietary & Confidential—Page 6

7. Overview of CRPA Systems (cont.) • The overall concept of the CRPA system is to dynamically ‘control’ the antenna pattern to maximize the reception gain in the direction of the satellites while reducing the reception gain (i.e. causing a null) in the direction of the interference • The method of performing this anti-interference ‘control’ is by using nulling and beamsteering techniques in the antenna electronics (AE) • Receivers use serial and discrete control, monitor and test (CMT) interfaces to communicate to the AE in the CRPA system • For example the Rockwell GEM-V receiver using the GAS-1 CRPA system shown below Proprietary & Confidential—Page 7

8. A Reduced Size CRPA (R-CRPA) for GNSS Receivers Inder Gupta, John Volakis and Chi-Chih Chen, ION NTM 2007 Overview of CRPA Systems (cont.) • Nulling (described to the right) reduces the antenna gain by creating ‘nulls’ in the relative antenna azimuth and elevation direction of the interference using various Finite Impulse Response (FIR) filters • Another example is shown below for measured CRPA antenna pattern data from 1 and 2 interferers GPS Signals (received OK) Jammer (nulled) ReceptionAntenna Gain N GPS Signals (nulled and not received) Proprietary & Confidential—Page 8

9. Overview of CRPA Systems (cont.) • Beamsteering is another technique used by CRPA systems which increases the gain in the direction of the satellites • If supported by the CRPA system, this aids the receiver in initial satellite acquisition which is the weakest link in hostile environments and important for military receivers • This is also used by cell phone networks for increasing integrity and throughput • Using knowledge from the receiver to where the satellites are relative to the antenna, the steered boresight beam can increase the signal-to-noise ration by at least 6dB • An example of a boresight beam for antenna zenith is shown below from an ADAP CRPA system ADAP: Enhancing GPS Protection for NAVWAR. Anna Vo, GPS Wing, Los Angeles AFB Charles R. Falchetti, .S. Navy SPAWAR Systems Center Allen W. Morrison, SAIC. ION NTM 2007. Proprietary & Confidential—Page 9

10. Overview of CRPA Systems (cont.) • The ability to steer nulls and beams from the AE is capable through the use of spatial diversity from a multi-element antenna in the CRPA system • The maximum number of nulls (i.e. interference sources) supported by the antenna array is equal to one less than the number of antenna elements (N-1) • The more elements in an array, the greater the degree of directionality for nulls and beams • To prevent spatial correlation, the antenna elements are typically placed ½ the L1 carrier wavelength apart in order to measure the relative phase shift as a function of the elevation and azimuth angles • For instance, 0° phase shift is received if perpendicular to array and 180° phase shift if horizontal • An example of a typical 7 element CRPA antenna is shown below, which is similar to the GAS-1 antenna • Modern techniques are creatively producing multi-element CRPA’s much smaller however 6-Element CRPA (4.25” diam) 7-Element CRPA (7” diam) ½ L1 wavelength = 9.5cm http://www.navsys.com/Papers/0201001.pdf A Reduced Size CRPA (R-CRPA) for GNSS Receivers Inder Gupta, John Volakis and Chi-Chih Chen, ION NTM 2007 Proprietary & Confidential—Page 10

11. Agenda • Overview of CRPA Systems • Challenges of Testing CRPA Systems • CRPA Test Solutions using Simulators • Testing with the Antenna in an Anechoic Chamber • Testing without the Antenna

12. Challenges of Testing CRPA Systems • To help protect the war fighter, CRPA systems are increasingly being used by military vehicles and personnel • Communications systems, such as cell phone networks, are beginning to take advantage of beamsteering to achieve higher density cells, with higher throughput • Unlike Fixed Reception Pattern Antennas (FRPA) which use 1-element connected directly to the receiver, CRPA systems have multiple antenna elements and the antenna electronics (AE) which perform various Digital Signal Processing (DSP) techniques • As a result, CRPA testing presents several challenges for the system developer and integrator for verifying the CRPA system performance under the changing threat environments Proprietary & Confidential—Page 12

13. Challenges of Testing CRPA Systems (cont.) • Testing CRPA systems requires testing at a minimum with the antenna electronics (AE) in order to quantify the nulling and beamsteering performance • This requires modeling multiple antenna inputs to the AE depending upon the number of elements used in the corresponding CRPA system • These models however require each elements antenna (gain and phase) to be modeled in addition to the corresponding GPS satellites in view and directivity of the interferers • The timing and phase is essential for each antenna input in order for the AE to effectively model the directivity of the signals received at each antenna • Some test requirements however require that the antenna and possibly the vehicle be used as well for testing the total system performance • Using the antenna in the loop provides several benefits, but the test environment now has to use transmitted GPS satellites and interferers which increases the complexity and size of the test environment • The timing and phase is also essential for each of the satellites and interferers in the environment Proprietary & Confidential—Page 13

14. Traditional Test Approach • Traditionally CRPA systems are verified on a test range shown below which provides the benefit that the test environment uses the actual antenna, AE, receiver and vehicle in addition to the real world interferers • Using this approach presents additional challenges however such as: • Access to a test range may not be feasible because of cost or location • The test environment is susceptible to unintentional interference from FM transmitters, radar, etc. • Can potentially interfere with legitimate GPS users in the area • Competitors or other organizations can eavesdrop on the testing • Testing may be prevented due to weather conditions • Test conditions are not repeatable (e.g. GPS constellation changes) • High cost and human resources • Supports limited dynamics because restricted to test range Proprietary & Confidential—Page 14

15. Laboratory Test Approach • Another approach is testing CRPA systems in a laboratory environment which also provides similar benefits to the traditional approach but it is also repeatable, supports various dynamics and protects against eavesdropping and unintentional interference • Several challenges also exist for testing in the laboratory which include: • Access to an anechoic chamber for testing the antenna, AE and receiver • Requires a specialized simulator for testing in an anechoic chamber or multi-element AE system • In the anechoic chamber the satellite signal arrival angles are fixed, limiting the test duration • Results require more justification when compared to the traditional test results Proprietary & Confidential—Page 15

16. Agenda • Overview of CRPA Systems • Challenges of Testing CRPA Systems • CRPA Test Solutions using Simulators • Testing with the Antenna in an Anechoic Chamber • Testing without the Antenna

17. Simulators can be used for testing with the vehicle, antenna and antenna electronics (AE) in the loop Using an anechoic chamber, a multi-output wavefront simulator can be used for modeling each of the satellites Various interferers can be placed around the anechoic chamber as necessary for modeling the interference environment Several methods support testing without the antenna in the loop This requires modeling of the antenna and signal/interferer phase relationships The multi-output wavefront simulator can be integrated into custom Digital Signal Processing (DSP) phase matrix systems to independently manipulate the satellite and interferer phase relationships Another test configuration utilizes separate single output simulators modeling each antenna input to the AE combining both the satellite and interferer signals CRPA Test Solutions using Simulators Proprietary & Confidential—Page 17

18. Agenda • Overview of CRPA Systems • Challenges of Testing CRPA Systems • CRPA Test Solutions using Simulators • Testing with the Antenna in an Anechoic Chamber • Testing without the Antenna

19. Testing in an Anechoic Chamber Proprietary & Confidential—Page 19 • Instead of traditional testing outside using real satellites, GPS simulators can be used for testing CRPA systems in an anechoic chamber to prevent unintentional interference and interference with other GPS receivers • This test environment enables complete CRPA system testing with the vehicle, antenna, antenna electronics (AE) and receiver to verify system level beamsteering and nulling performance • The general assumptions for anechoic chamber CRPA simulation testing are: • Typically one scenario is tested with a specific date, time, location • Limited scenario duration to an hour or two because of satellite dynamics • Vehicle trajectory supports a full range of dynamic • Satellites are simulated as individual channel outputs from a GPS simulator • Direction to satellites and interferers are important • Position to satellites are fixed to their relative satellite positions in the scenario • Chamber satellites and interferers need to be “calibrated” for phase, timing and path loss

20. +15 min -15 min Testing in an Anechoic Chamber (cont.) • The basic overview of the anechoic chamber CRPA test configuration is described to the right • The CRPA antenna or vehicle, AE, etc is placed in the center of the chamber • The satellite geometry restricts the test to 1 scenario, a fixed date and time and fixed user location • Relative to the mid-point satellite orbital positions during the scenario, transmitters are positioned around the chamber to simulate the satellites • Interferers are positioned in the chamber as desired to demonstrate nulling or beamsteering performance The duration of the scenarios are typically around 30 minutes to minimize errors in the satellite positions relative to the antenna azimuth and elevation beamsteering/nulling directions. Because relative satellite positions change less at higher elevations (e.g. less doppler), higher mask angles may permit longer test durations. Proprietary & Confidential—Page 20

21. Testing in an Anechoic Chamber (cont.) • The satellite transmitters are connected to a multi-output GPS wavefront simulator shown to the right • Scenarios are defined with SimGEN for specifying the desired satellite characteristics and vehicle trajectory Proprietary & Confidential—Page 21

22. Multi-output Wavefront Simulator Spirent provides a unique multi-output wavefront simulator specifically designed for CRPA testing The simulator is configured as a separate Intermediate Frequency (IF) Generator and Radio Frequency (RF) Upconverter The hardware supports up to 24 independent output channels Supports 12-channels of L1, L2 and the legacy and modernized codes (L2C, M-code) for simulating up to 12 satellites for all-in-view testing Individual channels permits the single channel phase shift or delay capability necessary for CRPA testing Each individual channel output possesses higher L1/L2 reference signal levels (-86.76dBm/-88dBm respectively) for signal loss compensation It also provides a combined output for typical receiver testing with a nominal L1/L2 signal level of -130dBm/-136dBm respectively RF Upconverter IF Generator Proprietary & Confidential—Page 22

23. Using SimGEN for CRPA Testing • SimGEN supports all of the same scenario generation capabilities (e.g. using satellite almanacs, defining vehicle motion, etc.) when running in either combined output or individual output modes • Some of the capabilities for antenna modeling however may not be used when using the antenna in the loop, for instance antenna level and phase patterns • The Sky Plot may be useful for easily viewing relative satellite positions to the antenna for transmitter placements Proprietary & Confidential—Page 23

24. Simulator Radiation Considerations Assuming the test-setup is in a controlled anechoic chamber The receiver is likely to be in the ‘near-field’ of radiation if it is closer than 2 wavelengths from the transmitting antenna (for L1 this is 0.38m) If establishment of the correct polarization is required, a Right-Hand Circularly-Polarized (RHCP) transmit antenna is needed, and the transmit to receive separation needs to be >0.38m True path loss must be measured rather than calculated, as the test set-up in a chamber is never ideal The gain of the transmit antenna and loss of the cable connecting this back to the simulator RF output needs to be taken into account Only if careful characterization of the losses is taken into account can meaningful C/No tests be carried out which is why calibration of the system is essential Proprietary & Confidential—Page 24

25. Anechoic Chamber Calibration • Because the received phase and signal power relationships are essential for accurately representing the GPS satellites, calibration must be performed for testing in an anechoic chamber • If calibration is not performed, each transmitted satellite will have different delays and signal powers (shown to the right) • Once calibration is performed, the satellite signals are synchronized and transmitted with the correct power levels at the antenna (shown to the right) Proprietary & Confidential—Page 25

26. Anechoic Chamber Calibration (cont.) • After positioning the satellite transmitters in the necessary locations, the chamber requires calibration • Calibration is performed using a Network Analyzer and a reference antenna and cable • The Network Analyzer is then used to characterize each path in both delay and power loss • The reference antenna is ‘aimed’ at each satellite transmitter to minimize antenna phase and amplitude errors Proprietary & Confidential—Page 26

27. Anechoic Chamber Calibration (cont.) • The results from the calibration provide signal power loss and delay for each transmit satellite path to the antenna • Include cables, connectors, signal repeaters and the free-space loss • For example, the signal loss and delay for a 30 m cable run, antenna separation of 3 m, the approximate readings would be 50dB and 250,000 pico-seconds • The signal delays are with respect to the first L1 Channel • The calibration procedure is provided in Spirent document DGP00712AAA STR4790 Anechoic Chamber Calibration Procedure • This document also contains calibration spreadsheets for recording the signal loss and delays to be used by the simulator for compensation • Using the measured signal losses and delays throughout each path, the Level Offset (LOFF) and Zero Delay (ZDLY) values can be calculated on the spread-sheets • The LOFF and ZDLY spread-sheet data are then written to the multi-output simulator non-volatile memory for signal compensation to ensure accurate reception of the intended signals by the CRPA system • Verification of the calibration may be performed by processing a receivers nominal performance with the final anechoic chamber antenna configuration and signal compensations in the simulator Proprietary & Confidential—Page 27

28. Anechoic Chamber Calibration (cont.) • Shown below are the example spread-sheets for calculating the necessary Level Offset (LOFF) and Zero Delay (ZDLY) values required for calibrating an anechoic chamber CRPA test environment from the measurements performed • Details regarding this and how to write these to the simulators non-volatile memory (NVRAM) are provided in the calibration procedure No delay for Channel 1. All delays are referenced to this channel. Proprietary & Confidential—Page 28

29. Agenda • Overview of CRPA Systems • Challenges of Testing CRPA Systems • CRPA Test Solutions using Simulators • Testing with the Antenna in an Anechoic Chamber • Testing without the Antenna

30. Testing without the Antenna • Sometimes testing with the antenna in a CRPA system is unnecessary or impossible because of anechoic chamber resources or test requirements • An alternative CRPA test solution shown to the right uses the multi-output wavefront simulator for digitally applying the necessary antenna signal and interferer phase effects for transmission to the antenna electronics (AE) • This is a complex alternative that uses advanced digital signal processing (DSP) techniques for applying the necessary phase-shifted antenna effects Proprietary & Confidential—Page 30

31. Testing without the Antenna (cont.) • This implementation is performed by transmitting each individual RF output through a splitter for phase shifting via a complex coaxial matrix • The complex DSP based hardware solution performs the signal combination and delay in the digital domain before upconversion back to L1 and L2 for transmission to the AE as described below Proprietary & Confidential—Page 31

32. Challenges The Phase Shift Matrix is custom requiring a lot of development which makes it very expensive The complexity requires separate arrays for GPS L1 and L2 and the interferers Requires the multi-output wavefront simulator This design does not test the antenna, thus accurate modeling of the antenna is required in the phase matrix Advantages This design permits a laboratory test system not requiring an anechoic chamber The tests are repeatable and controllable No limitation with dynamic testing and test durations Testing without the Antenna (cont.) Proprietary & Confidential—Page 32

33. Testing without the Antenna (cont.) • Another alternative CRPA simulation test system uses a standard combined RF coaxial output for modeling each antenna element to the antenna electronics as described below RF Inputs Antenna Electronics (AE) Proprietary & Confidential—Page 33

34. Testing without the Antenna (cont.) • This creates a time delayed wavefront for both the GPS and interference signals • All wavefronts are coherent with the intended vehicle motion • Variants of this implementation can exist depending upon how the wavefronts are generated • One example is the variable Butler Matrix implementation described to the right that creates a single interference wavefront that is coherent with the simulated motion • This variation was developed at the 46th Test Group Holloman Air Force Base Proprietary & Confidential—Page 34

35. Challenges Because each simulator output models an antenna element, more simulator hardware is necessary This design does not test the antenna, thus accurate modeling of the antenna is required in each simulator Synchronization of the simulators is essential for modeling a coherent wavefront Since there are more simulators, the SimGEN control bus bandwidth increases Advantages The technology already exists by using commercial GPS simulators This design permits a laboratory test system not requiring an anechoic chamber The tests are repeatable and controllable No limitation with dynamic testing and test durations Testing without the Antenna (cont.) Proprietary & Confidential—Page 35