Calibration of GNSS Antenna Arrays

1. Calibration of GNSS Antenna Arrays Staffan Backén, LTU Dennis M. Akos, LTU

2. Presentation Overview • GPS satellite orbits • Constellation • Ground tracks • Array processing • Interference mitigation • Multipath mitigation • Increased SNR • Orientation • Calibration • Front end performance • Antenna • Future work

3. GPS Satellite Orbits • Designed for 24 satellites • 6 planes • 4 satellites per plane • Uneven phasing of satellites in the planes • Orbital parameters • Medium Earth Orbit • Semi-major axis 26559.8 km • Eccentricity 0° • Inclination 55° • Spin Period 11h 58m • Spin period of the earth is 23h 56m • ~10km accuracy from day to day • Direction very similar

4. Ground Tracks of 24h • Satellite positions during 24hr, inertial coordinate system • Individual ground tracks • Uneven phasing to improve availability in case of satellite failure • No satellites straight up for high or low latitudes (57°) • However, they are visible from across the poles

5. Galileo • Three orbital planes • 30 satellites • 56° inclination • A slightly more optimal constellation compared to GPS • Main advantage – more satellites in total Photo: ESA-J.Huart

6. Advantages of Array Processing of GNSS signals • Advantage of directivity • Increased gain • Multipath mitigation • Interference mitigation • Angular positioning • Difficulties • Antenna array – mutual coupling • Multiple front ends – synchronization • Signal processing algorithms – complexity / performance trade off

7. Receiver chain • Traditional GPS receiver architecture • Front end, correlator chip, microcontroller • Software defined radio approach • Front end, data bridge, PC (FPGA) • Postprocessing

8. A post processing evaluation system • Eight antenna / front end pairs • Synchronized sampling • A common clock is of utmost importance! • 16MHz, 2bits, 8 channels • High datarate • USB2 • Front ends • Evaluation boards • Cables

9. Extension of tracking algorithms • A delay lock loop • Tracking the correlation peak • A phase lock loop • Tracking frequency and phase • When synchronized – all signal energy should be in the inphase component • Store the vectors generated in the tracking loops for the channel we track • Multiply with the IF data from the other channels

10. Gain and Phase Estimation • Gain estimation • The SNR should be the same across channels • Good results • Phase shift estimation • Compensate for data bit shifts before calculating phase shift • Ambiguous – could be more than one wavelength • Phase consistency • Good results

11. Crosstalk - results • Good agreement in general • Correlates well with distance between front ends • Worst case ~-33dB • Shielding might need to be improved Correlator Power Ratio Spectral Estimate

12. Antenna Calibration • When antennas are in close proximity, they influence each other • Arrays of antennas need to be calibrated • Traditionally done in anechoic RF chambers • This antenna array was calibrated by FOI in Linköping

13. Live calibration? • A skyplot is an alternative way to illustrate satellite positions • As we know where the satellites are at all times, and they move across the sky, they can be used to calibrate an antenna array

14. Live calibration! (preliminary) • Two 12h data sets were recorded • 90° rotation of the array between the sets • The gain difference looks reasonably smooth • And so does the phase shifts of the different elements • Ok, is the array ”good enough” • TBD

15. Future work • Construction of a new array • Eight element in a circular configuration • Multipath characterization • Based on the error envelope of the phase for reflected signals • Implement adaptive beamforming algorithms • New front end hardware for interference mitigation (8+ bits per front end) Photo: ESA-J.Huart