1

Integrity Service Excellence

Physical Scintillation Simulation

Diffraction Grating-Based

GPS Physical Scintillation Simulation Results

Kirtland AFB, NM

19 June2012

Dr. Todd Pedersen Capt. Melanie Huffman

Mr. R. Todd Parris

AFRL, Space Vehicles Directorate, Kirtland AFB, NM

2

•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 via splitter– Test of receiver-specific effects or power level

discrepancies

Courtesy of C. Mitchell

Experiment Setup

3

Implementation

•5 variations: panel only, cylinder only, cylinder

topped with panel, cylinder+panel+foam board

with metallic tape, 4 metallic tape foam boards in

box configuration

Cylinder only Topped with panel Foam board added

4

Receivers

•2 Ashtech Z-12 receivers– S/N 04299 Firmware 1L00-1D04– S/N 03789 Firmware 1L00-1D04– Data collection by GPS-SCINDA Version 1.77 on both

•Splitter with DC bias to pre-amp powered by RX #

03789

5

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

6

Positioning ErrorsFirst Half of Test

•Identical receivers on same antenna

•One has no outages, the other >6 min at a time

Some Errors but No Outages

Some Errors and Extensive Outages

7

Positioning ErrorsSecond Half of Test

•More severe portion of test (diffraction+blocking)

•Some similarity in error patterns, but total outage

length differs by > 10x

88

Fluctuations on Individual Links

•More similarity in gaps on individual links

•Absolute signal levels in close agreement

03789

04299

Mesh Panel

Mesh Cylinder

Panel over Cylinder

Added Foam

BaselineBaseline

99

Comparison of L1 and L2 Amplitudes

• L2 drops out more frequently• Dispersion effects?-- more amplitude loss at L2?• Grating is more reflective at L2 than at L1

L1

L2

RX 04299

Loss of codeless tracking?

10

Highest Elevation Satellite—L1

•Mean signal level appears to be unchanged

from background with wire mesh only

RX 03789

RX 04299

11

Highest Elevation Satellite—L2

•Clear difference in signal levels from cylinder and

overhead panel•Much less than amplitude drops from foam board

RX 03789

RX 04299

12

Summary

•Transmission grating-based scintillation simulation

successful– Clear difference between L1 and L2 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, 10X

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