Navigation Accuracy and Interference Rejection for an Adaptive GPS Antenna Array David S. De Lorenzo, Jason Rife, Per Enge Stanford University GPS Lab Dennis Akos University of Colorado 05 June 2006 The authors gratefully acknowledge the support of the JPALS Program Office, and the Naval Air Warfare Center Aircraft Division through contract N00421-01-C-0022.
Constrained Adaptive Processing • JPALS has stringent limits on pseudorange & carrier-phase errors – Required to meet carrier-landing accuracy limits. • GPS antennas introduce distortion on received signals – Apparent as deterministic pseudorange and carrier phase biases – Dependent on the incoming signal line-of-sight. • JPALS will likely require multi-element STAP algorithms to improve C/No under jamming & RFI conditions. • STAP algorithms can introduce additional pseudorange and carrier phase biases. Goal: Through temporal & spatial constraints on a STAP algorithm, move down & left in the trade space, to a point where deterministic pseudorange & carrier phase corrections based on signal LOS can be applied. 2 6/13/2006 Stanford University JPALS Research
Outline • Software tools for signal simulation & tracking – Signal simulator • C/A or P-code signal generator that includes antenna distortion effects – Software receiver • Tracks a wide range of GNSS signals – GPS C/A & P-code, Galileo L1 & E6 • Includes multi-antenna and space-time adaptive signal processing • Verification of software receiver & multi-signal tracking performance • Research methodology: – Characterize biases vs. RFI rejection – Evaluate different compensation schemes • Preliminary results – FRPA vs. CRPA biases and interference rejection 3 6/13/2006 Stanford University JPALS Research
Stanford HW & SW Development 7-element array rectangular patch antennas manufactured at Stanford University Software Receiver all-in view, multi-signal HP Vector Data collection computer GNSS receiver Signal Analog front-end (2) ICS-650 cards Analyzer with STAP array 4-channel GP2015 with sampling at 5-65 MHz processing 10 MHz common clock and 12-bit A/D resolution * Possibility of including data from OSU’s multi-element antenna array. 2m high-gain dish 4 6/13/2006 Stanford University JPALS Research
Software-Based Signal Simulator • Gain/phase data for each look direction (10 S/V C/A & “P” code constellation) and antenna (7 rectangular patches) 1.023 & 10.23 Mchip/sec – Data courtesy Ung-Suok Kim Bandlimited WGN A/D & CW interference … Front-end … A/D Front-end Gain/phase response for PRN #1 [Azimuth = 0°, Elevation = 40°] Antenna Gain Response for PRN #1 [Az = 0 deg, El = 40 deg] Antenna Phase Response for PRN #1 [Az = 0 deg, El = 40 deg] 250 1.2 Antenna #1 Antenna #1 Antenna #2 Antenna #2 Antenna #3 200 Antenna #3 1 Antenna #4 Antenna #4 Antenna #5 Antenna #5 Front-end BW ~19MHz Antenna #6 150 Antenna #6 0.8 Antenna #7 Antenna #7 Phase Gain 100 0.6 50 0.4 Front-end BW ~19MHz 0 0.2 0 -50 1480 1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 1450 1500 1550 1600 1650 1700 Frequency (MHz) Frequency (MHz) 5 6/13/2006 Stanford University JPALS Research
Signal Generation at Intermediate Freq. • Received signal at master element: [ ] ( ) ( ) ( ) ⋅ − τ ⋅ − τ ⋅ π + + θ 2 cos 2 P D t x t f f t 1 S L D • Mixing signal: [ ( ) ] ⋅ π L − 2 cos 2 f f t 1 IF • Signal after LPF: [ ] ( ) ( ) ( ) ⋅ − τ ⋅ − τ ⋅ π + + θ cos 2 P D t x t f f t S IF D • Received signal at subsidiary element: [ ] ( ) ( ) ( )( ) ⋅ + Δ − τ ⋅ + Δ − τ ⋅ π + + Δ + θ 2 cos 2 P D t t x t t f f t t 1 S L D • Signal after mixing & LPF: [ ] ( ) ( ) ( ) ( ) ⋅ + Δ − τ ⋅ + Δ − τ ⋅ π + + θ + π + Δ cos 2 2 P D t t x t t f f t f f t 1 S IF D L D 6 6/13/2006 Stanford University JPALS Research
Signal Generation w/ Antenna Distortion ( ) ( ) fft S f s t 4 x 10 P-code Correlation Peak w/o Distortion - PRN #1 Power Spectral Density Estimate via Welch 4 3.5 3 Correlation Output Power/frequency (dB/Hz) 2.5 2 1.5 1 0.5 0 -1.5 -1 -0.5 0 0.5 1 1.5 Code Phase (chips) Autocorrelation of s(t) 0 5 10 15 20 25 30 35 40 Frequency (MHz) ( ) − ⊗ 1 s out t fft Antenna Gain Response for PRN #1 [Az = 0 deg, El = 40 deg] Antenna #1 1.2 Antenna #2 Antenna #3 4 P-code Correlation Peak Distortion - PRN #1 1 Antenna #4 4 x 10 Antenna #5 Front-end BW ~40MHz Antenna #6 0.8 Antenna #7 3.5 Gain 3 0.6 Correlator Output 2.5 0.4 2 0.2 1.5 Gain/phase distortion data 1 0 1480 1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 Frequency (MHz) 0.5 Antenna Phase Response for PRN #1 [Az = 0 deg, El = 40 deg] 250 available from HFSS simulation Antenna #1 0 Antenna #2 -1.5 -1 -0.5 0 0.5 1 1.5 Code Offset (chips 200 Antenna #3 Autocorrelation of s out (t) Antenna #4 – Verified by comparison to Antenna #5 150 Antenna #6 Antenna #7 anechoic chamber testing Phase 100 – Data courtesy Ung-Suok Kim 50 Front-end BW ~40MHz 0 -50 1450 1500 1550 1600 1650 1700 Frequency (MHz) s(t) and gain/phase data are antenna & S/V specific 7 6/13/2006 Stanford University JPALS Research
Standard Satellite Constellation Antenna Gain Response for PRN #5 [Az = 120 deg, El = 20 deg] Antenna Gain Response for PRN #4 [Az = 90 deg, El = 50 deg] 1.2 Antenna #1 1.2 PRN #5 Antenna #1 Antenna #2 PRN #4 Antenna #2 Antenna #3 1 Antenna #3 Antenna #4 1 Antenna #4 Antenna #5 Front-end BW ~40MHz Antenna #5 Antenna #6 Front-end BW ~40MHz 0.8 Antenna #6 Antenna #7 0.8 Antenna #7 Gain 0.6 Gain 0.6 Antenna Gain Response for PRN #3 [Az = 60 deg, El = 40 deg] 1.2 Antenna #1 0.4 PRN #3 Antenna Gain Response for PRN #6 [Az = 150 deg, El = 60 deg] 0.4 Antenna #2 Antenna #3 1.2 Antenna #1 1 0.2 Antenna #4 PRN #6 Antenna #2 0.2 Antenna #5 Front-end BW ~40MHz Antenna #3 Antenna #6 1 Antenna #4 0.8 0 Antenna #7 Antenna #5 1480 1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 0 Front-end BW ~40MHz Frequency (MHz) 1480 1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 Antenna #6 Gain 0.8 Frequency (MHz) 0.6 Antenna #7 Gain 0.6 0.4 0.4 0.2 0.2 0 1480 1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 Frequency (MHz) 0 1480 1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 Frequency (MHz) Antenna Gain Response for PRN #7 [Az = 210 deg, El = 70 deg] Antenna Gain Response for PRN #2 [Az = 30 deg, El = 30 deg] 1.2 Antenna #1 1.2 Antenna #1 PRN #7 PRN #2 Antenna #2 Antenna #2 Antenna #3 Antenna #3 1 Antenna #4 1 Antenna #4 Antenna #5 Front-end BW ~40MHz Antenna #5 Front-end BW ~40MHz Antenna #6 Antenna #6 0.8 Antenna #7 0.8 Antenna #7 Gain Gain 0.6 0.6 0.4 0.4 0.2 0.2 0 0 1480 1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 1480 1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 Frequency (MHz) Frequency (MHz) Antenna Gain Response for PRN #1 [Az = 0 deg, El = 40 deg] 1.2 Antenna #1 PRN #1 Antenna Gain Response for PRN #8 [Az = 240 deg, El = 20 deg] Antenna #2 Antenna Gain Response for PRN #10 [Az = 300 deg, El = 80 deg] Antenna #3 1.2 Antenna #1 Antenna Gain Response for PRN #9 [Az = 270 deg, El = 30 deg] 1 PRN #8 1.2 Antenna #4 Antenna #1 Antenna #2 1.2 Antenna #5 Antenna #1 Front-end BW ~40MHz PRN #10 Antenna #2 Antenna #3 PRN #9 Antenna #6 1 Antenna #2 Antenna #3 0.8 Antenna #4 1 Antenna #7 Antenna #3 Antenna #5 Antenna #4 1 Front-end BW ~40MHz Antenna #4 Antenna #5 Antenna #6 Front-end BW ~40MHz Gain 0.8 Antenna #5 0.6 Antenna #7 Front-end BW ~40MHz Antenna #6 0.8 Antenna #6 Antenna #7 0.8 Antenna #7 Gain 0.6 Gain 0.4 Gain 0.6 0.6 0.4 0.2 0.4 0.4 0.2 0 0.2 1480 1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 0.2 Frequency (MHz) 0 1480 1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 0 Frequency (MHz) 0 1480 1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 1480 1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 Frequency (MHz) Frequency (MHz) 8 6/13/2006 Stanford University JPALS Research
Wideband Jammer Signal Generation at IF ( ) ( ) fft J f j t Power Spectral Density Estimate via Welch • For each jammer – six swept Power/frequency (dB/Hz) sinusoids cover frequency band from ~0 Hz to ~f s /2 ( ) • Amplitude scaled as needed 0 5 10 15 20 25 30 35 40 − Frequency (MHz) ⊗ 1 j out t fft to achieve desired J/S ratio Antenna Gain Response for PRN #1 [Az = 0 deg, El = 40 deg] Antenna #1 1.2 Antenna #2 Antenna #3 1 Antenna #4 Antenna #5 Front-end BW ~40MHz Antenna #6 0.8 Antenna #7 Gain 0.6 ( ) ( ) 0.4 ≡ Gain/phase distortion data j t j t 0.2 are currently not available out 0 1480 1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 Frequency (MHz) [ ] ( ) ( ) ( ) ∑ ′ 2 ⋅ π + + + Δ + π + Δ 2 cos 2 2 P f f t f t t f f t 1 J IF J J L J i i i 9 6/13/2006 Stanford University JPALS Research
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