IMPACTS OF THE DECEMBER 2006 SOLAR RADIO BURSTS ON GPS OPERATIONS AMS Fifth Symposium on Space Weather New Orleans, LA January 20-21, 2008 Dr. Charles Carrano 1 Chris Bridgwood 1 Dr. Keith Groves 2 (1)Atmospheric and Environmental Research, Inc. (2)Air Force Research Laboratory

Dec ‘06 Solar Flares from Active Region X9.0 Dec 5 10:18 10:35 10:45 Strength Date Start Peak Stop X6.5 Dec 6 18:29 18:47 19:00 X3.4 Dec 13 02:14 02:40 02:52 X1.5 Dec 14 21:07 22:15 22:26 GOES-13 SXI detects Dec 5 flare Credit: NOAA OSPAN H-alpha (656.3nm) images of Dec 6 “solar tsunami”. Credit: NSO/AURA/NSF and AFRL

Figure source: Dale Gary, NJIT, radio astronomy online lecture notes Spectral Response of a Solar Radio Burst

Slide 4 Ground Stations of the AFRL-SCINDA Network Stations with dual frequency GPS receivers (as of Dec 2006) are shown in yellow Depending on the receiver model, amplitude and phase are reported at Hz

December 6, 2006 X6.5 Flare and Solar Radio Burst

OVSA Solar Radio Burst RHCP Power Figures from Cerruti, Kintner, and Gary: briefing from Space Weather Week 2007 L1 L2 SRB RHCP Power exceeded 10 6 SFU at L band Sub-solar point at 19:34 UT was (22.5S, 115.7W)

Slide 7 GPS Signal Strengths at Ancon on Dec 6 L1  C/No dB-Hz18:3018:4519:0019:1519:3019:4520:00 UT PRN 02 PRN 04 PRN 08 PRN 27 PRN 28 L2  C/No dB-Hz18:3018:4519:0019:1519:3019:4520:00 UT PRN 02 PRN 04 PRN 08 PRN 27 PRN 28 L1 tracking loss L2 tracking loss Deepest L1 fade: 25 dB Deepest L2 fade: 30 dB Longest L1 fade: 4 min GPS receiver model: Ashtech Z-12 34° solar incidence angle at 19:15 UT Identical fades on all links (weak dependence on satellite elevation because SIA is the same) Longest L2 fade: 9 min L1 L2

Slide 8 GPS Observables at Ancon on Dec 6 Ranging errors (not ionospheric delay) SITEC (enhanced ionization from flare) of 5 TECU in 3 min, starting just before peak of flare (18:47) Cycle slip (TEC becomes difficult to measure) GPS receiver model: Ashtech Z-12 34° solar incidence angle at 19:15 UT Ranging errors (on L1, L2 or both) exceed 100 m. These will contribute to net GPS positioning error 18:3018:4519:0019:1519:3019:4520:00 UT Meters PRN 02 PRN 04 PRN 08 PRN 27 PRN 28 Differential Pseudorange Differential Carrier Phase 18:3018:4519:0019:1519:3019:4520:00 UT TECU PRN 02 PRN 04 PRN 08 PRN 27 PRN 28

Slide 9 GPS Position Solution at Ancon on Dec 6 Position Errors: 20 m horizontal 60 m vertical Position Outages (5 min) Degraded tracking 18:3018:4519:0019:1519:3019:4520:00 UT Satellites Used in Position Solution No. Satellites 18:3018:4519:0019:1519:3019:4520:00 UT 18:3018:4519:0019:1519:3019:4520:00 UT GPS Positioning Error Horizontal (m) Vertical (m) GPS receiver model: Ashtech Z-12 34° solar incidence angle at 19:15 UT

Accounting for the Local Solar Incidence Angle

Slide 11 SRB Power and the Solar Incidence Angle C/No without SRB: C/No with SRB: SRB power (satellite elevation cancels): Vertical equivalent (zenith) C/No with SRB (system noise cancels): Ashtech Choke Ring Antenna Gain, g(  ) system noise broadcast power Atmospheric attenuation neglected (<1 dB at L band)

Slide 12 L1 Signal Fades from Different Stations Maximum vertical equivalent L1 fade: 27 dB Fades are nearly identical for any sunlit location once solar incidence angle correction applied 18:3018:4519:0019:1519:3019:4520:00 UT dB-Hz L1  C/No PRN 08 PRN 29 Ancon Antofagasta Sao Luis Kwajalein 18:3018:4519:0019:1519:3019:4520:00 UT dB-Hz Vertical Equivalent L1  C/No PRN 08 PRN 29 Ancon Antofagasta Sao Luis Kwajalein Solar incidence angles at 19:15 UT Ancon:34° Antofagasta:37° Sao Luis:68° Kwajalein:86°

Slide 13 L2 Signal Fades from Different Stations Maximum vertical equivalent L2 fade: 30 dB Fades are nearly identical for any sunlit location once solar incidence angle correction applied 18:3018:4519:0019:1519:3019:4520:00 UT L2  C/No PRN 08 PRN 29 dB-Hz Ancon Antofagasta Sao Luis Kwajalein 18:3018:4519:0019:1519:3019:4520:00 UT Vertical Equivalent L2  C/No PRN 08 PRN 29 dB-Hz Ancon Antofagasta Sao Luis Kwajalein Solar incidence angles at 19:15 UT Ancon:34° Antofagasta:37° Sao Luis:68° Kwajalein:86°

Slide 14 Conclusions The solar radio bursts in December 2006 resulted in unprecedented levels of wideband RHCP power at the GPS L1 and L2 frequencies. The solar radio burst on December 6 (1,000,000 SFU!) significantly degraded overall GPS performance on a global basis. Nearly identical patterns of intermittent signal fading, lasting up to an hour, were observed on all satellite links on the sunlit side of Earth (even those measured by receivers separated by thousands of miles) during these bursts. The GPS signal fades, some as deep as 25 dB, were modulated by the local solar incidence angle and antenna gain pattern. The maximum vertical equivalent fade at L1 was 27 dB. SITEC events were observed shortly after the associated solar flares, at rates approaching TEC/min. These rates appeared to vary according to the solar incidence angle. Under these conditions, GPS receivers experienced difficulty tracking and also incurred ranging errors associated with loss of code lock. These factors led to elevated GPS positioning errors of up to 20/60 meters in the horizontal/vertical directions. These solar radio bursts came as a surprise during solar minimum, and suggest that loss of GPS operations during solar maximum could be more common than previously anticipated.