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Calibration of GNSS Antenna Arrays. Staffan Backén, LTU Dennis M. Akos, LTU. Presentation Overview. GPS satellite orbits Constellation Ground tracks Array processing Interference mitigation Multipath mitigation Increased SNR Orientation Calibration Front end performance Antenna

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Calibration of gnss antenna arrays

Calibration of GNSS Antenna Arrays

Staffan Backén, LTU

Dennis M. Akos, LTU


Presentation overview
Presentation Overview

  • GPS satellite orbits

    • Constellation

    • Ground tracks

  • Array processing

    • Interference mitigation

    • Multipath mitigation

    • Increased SNR

    • Orientation

  • Calibration

    • Front end performance

    • Antenna

  • Future work


Gps satellite orbits
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


Ground tracks of 24h
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


Galileo
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


Advantages of array processing of gnss signals
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


Receiver chain
Receiver chain

  • Traditional GPS receiver architecture

    • Front end, correlator chip, microcontroller

  • Software defined radio approach

    • Front end, data bridge, PC (FPGA)

      • Postprocessing


A post processing evaluation system
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


Extension of tracking algorithms
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


Gain and phase estimation
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


Crosstalk results
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


Antenna calibration
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


Live calibration
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


Live calibration preliminary
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


Future work
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


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