Physical Scintillation Simulation Diffraction GratingBased GPS Physical Scintillation
Physical Scintillation Simulation Diffraction Grating-Based GPS Physical Scintillation Simulation Results Kirtland AFB, NM 19 June 2012 Dr. Todd Pedersen Capt. Melanie Huffman Mr. R. Todd Parris AFRL, Space Vehicles Directorate, Kirtland AFB, NM Integrity Service Excellence 1
Experiment Setup • Attempt to simulate ionospheric scintillation and nav outages by physically altering live GPS signal • 2 -D L-band diffraction gratings created out of metallic wire “field fence” with holes 12” wide by 4”-12” high (graduated spacing, wide at top, narrow at bottom) – One curved panel approx 6’ long – One cylinder approx 3’ diameter x 4. 5’ high – Waved over antenna on rooftop • 2 Ashtech Z-12 receivers connected to same antenna Courtesy of C. Mitchell via splitter – Test of receiver-specific effects or power level discrepancies 2
Implementation Cylinder only Topped with panel Foam board added • 5 variations: panel only, cylinder topped with panel, cylinder+panel+foam board with metallic tape, 4 metallic tape foam boards in box configuration 3
Receivers • 2 Ashtech Z-12 receivers – S/N 04299 Firmware 1 L 00 -1 D 04 – S/N 03789 Firmware 1 L 00 -1 D 04 – Data collection by GPS-SCINDA Version 1. 77 on both • Splitter with DC bias to pre-amp powered by RX # 03789 4
Test Timeline 19: 31 UT Antenna connected 19: 36 UT “Scintillation” commencement (single flat grating) 19: 40 UT Cylindrical grating 19: 44 UT Cylinder plus grating on top 19: 50 UT Break 19: 54 UT Foam board added under top grating 19: 57 UT Added foam “wings” 20: 01 UT Back to baseline 20: 06 UT Foam blockers only 20: 10 UT Foam blockers in “box” formation ~24” above 20: 15 UT Attempt to produce maximum impact (rapid movement) 20: 20 UT End of “scintillation” 20: 25 UT Pack up 5
Positioning Errors First Half of Test Some Errors and Extensive Outages Some Errors but No Outages • Identical receivers on same antenna • One has no outages, the other >6 min at a time 6
Positioning Errors Second Half of Test • More severe portion of test (diffraction+blocking) • Some similarity in error patterns, but total outage length differs by > 10 x 7
Fluctuations on Individual Links 03789 04299 Baseline Mesh Panel Cylinder Panel over Cylinder Baseline Added Foam • More similarity in gaps on individual links 8 • Absolute signal levels in close agreement 8
Comparison of L 1 and L 2 Amplitudes RX 04299 L 1 L 2 Loss of codeless tracking? • L 2 drops out more frequently • Dispersion effects? -- more amplitude loss at L 2? • Grating is more reflective at L 2 than at L 1 9 9
Highest Elevation Satellite—L 1 RX 03789 RX 04299 • Mean signal level appears to be unchanged from background with wire mesh only 10
Highest Elevation Satellite—L 2 RX 03789 RX 04299 • Clear difference in signal levels from cylinder and overhead panel • Much less than amplitude drops from foam board 11
Summary • Transmission grating-based scintillation simulation successful – Clear difference between L 1 and L 2 effects, significant loss-of-lock and navigation outages – Only minor reduction in amplitude compared to foil tape – Greater distance from antenna may provide even better performance (far field instead of near field) • Positioning outages extremely receiver-dependent – Identical receivers, identical firmware, same antenna, 10 X difference in outage length • Should use as many receivers as practical with simulator to get a feel for unit-specific characteristics and statistical range for each receiver type – Test with single unit may be very misleading/atypical 12
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