SNR problems for COSMIC

Doug Hunt, Bill Schreiner, Stig Syndergaard (UCAR); Brian Giesinger

(Broad Reach); Penina Axelrad (UCAR, CU)

Overview

• Background information• The problem• Study methodology• Possible causes• Observations• Conclusion

Background

• The main COSMIC payload is a GPS occultation receiver (GOX/IGOR)

• This receiver tracks GPS satellites at a one-second data rate on two zenith-mounted patch antennas, used for Precise Orbit Determination (POD)

• Two other patch array antennas (the 'OCC' antennas) with a higher gain but a narrower pattern are used to sample data at 50Hz to track occultations

• All of these antennas receive signals on L1 (1575.42 MHz) and L2 (1246.4375 MHz) bands

Antenna 0 (POD)

Antenna 1 (POD)

Antenna 2 (Occultation)

Antenna 3 (Occultation)

GOX

Description of problem

• SNRs on various antennas are suddenly depressed or restored with no obvious reason

• This example shows L2 rising and fading over one day

SNR drop/rise can be for only L2:

Example: POD antenna

...or both L1 and L2:

POD antenna

• SNR drop/rise can be on a POD (Precise Orbit Determination) antenna (see last two slides), or on a high-gain occultation antenna

• Can be L2 and L1, or

L2 L1

OCC antenna

...or L2 only

OCC antenna

Here is a case in which L2 SNR on a POD antenna is restored over 20 seconds

Days or hours lost

• An outage can last for hours • ...Or days...

• ...and can have peculiar structure

Sometimes SNR loss is permanent

• L2 SNR on antenna 0 for FM5 dropped on day 2007.146 and has not recovered

What operations problems does this cause?

• This can cause the dreaded 'reboot loop' when it happens on the default POD antenna:

SNR drops too low PCM Current drop every 15 minutes showing GOX reboot

No more SNRs, no SVs tracked!

What problems does this cause?• Causes complete loss of occultations on the antenna failing

Possible causes

• Build-up of static charge on antennas

– Could be associated with satellite environment (space weather)

• Deformation of antennas due to heating

– Could be associated with solar exposure

• Interference with other instruments on the satellite

– Spacecraft GPS

• Larry Young, JPL

– Tri-Band Beacon

– Spacecraft transmitter or receiver

• Temperature effects on the GOX

– Thermal aging in ceramic filters in GOX front-end

• Brian Giesinger, BroadReach Engineering

• GOX firmware bugs

Study methodology• A list of ~100 SNR problems was assembled from over 18000 daily SNR plots

– A list of 50 of the most interesting were categorized

• A large database (over 1/2 Billion records!) of correlative information on the COSMIC spacecraft and the GOX was assembled, all indexed and time-tagged for easy access

• This database includes

– Simulation data for direct sun exposure to each antenna, including shading by the solar panels

– Time series of over 50 satellite State Of Health (SOH) values, including:

• Instrument and satellite temperatures

• Instrument and satellite currents and voltages

• Sun beta angle

• Attitude

• Solar array drive angle

• Battery charge

• Spacecraft GPS status (not the GOX)

• Spacecraft transmitter and receiver status

Study methodology (cont.)• This database also includes

– Geomagnetic indices

• Kp and sunspot number

• GOES electron and proton fluence and Xray flux

– A history of all commands ever sent to or executed by a spacecraft (67+ million of them!)

– GOX state-of-health data

• Temperatures and voltages

– GOX reboot times

– Spacecraft positions

– SNR values

• Daily averages (top 10%)

• 60 second averages

• Individual SNRs for specific cases

• Custom software was written to allow quick querying and plotting of variables from this database

Observations

Cases studied

• Problem categories

– POD antenna, L2-only: 9 cases

– POD antenna, L1 and L2: 12 cases

– OCC antenna, L2-only: 12 cases

– OCC antenna, L1 and L2: 17 cases

• For each comparison variable– Tally the number of correlating events and non-correlating

events

Distribution by S/C

• This problem occurs mostly on FM6, with some occurrences on FM1, 2, 3, 5 and 6. None on FM4.

99 SNR events recorded, from ~18000 antenna-FM-days(many more happened but not recorded)

Count by FM

Distribution by antenna

• Most all SNR problems occurred on two antennas: ANT0 and ANT3

ANT0 (POD)

ANT3 (OCC)

GOX

GOX temperature correlation• For some antennas, frequencies and time periods, SNRs correlate

inversely with GOX temperature. This was first noticed by Stig Syndergaard in 2007.

This one cures itself (FM3, ANT3)

This one persists thewhole mission (FM5, ANT1)

Spacecraft GPS

• The one S/C with no GOX SNR problems is also the only S/C with the bus GPS completely inoperative

FM Average # SVNs tracked % of time in 'Continuous mode'1 2.45 47%2 4.29 89%3 0.29 0%4 0.00 0%5 4.45 93%6 0.59 2%

Spacecraft GPS tracking per FM

Little correlation with GOX reboots

• Only 17 of 67 SNR events examined correlated with a reboot

SNR drop upon reboot

SNR rise with no reboot

Blue lines show reboots

Reboot signaled by dropin PCM current

No correlation with orbit position

• There was no noticeable relation between SNR events and position in orbit

Little correlation with spacecraft transmitter operation

• Only 16 of 54 events checked correlate with spacecraft transmitter operation

SNR event durings/c downlink

No SNR event durings/c downlink

Custom 3D spacecraft model for sunlight illumination studies

• Accurate OpenGL spacecraft model taken from satellite blueprints

• Takes into account satellite position, attitude, solar position and solar array drive angle

• Includes antenna shading by solar panels

• Color assignments:

– ANT0 = purple

– ANT1 = blue

– ANT2 = red

– ANT3 = green

• Sun is aligned with viewer, sun exposure for each antenna is computed as the number of pixels of the antenna color visible divided by the total pixels for the antenna when viewed straight on.

Some correlation between sunlight on antennas and SNR events

SNR events often line up with strong solar illumination on the antenna

60% of SNR events studied correlate with high (>50%) solar illumination

An interesting event

Shown earlier on temperature correlation slide

FM3, ANT3 shows temperature correlation with SNR until day 2007.074. On that day, at a time of high sun exposure and magnetometer reading, the SNR issue was resolved.

No obvious correlation between space weather indices and SNR events

Examined GOES proton and electron fluence, XRAY flux, kindex and sunspot numberHere is a plot of electron fluence and SNR events

• Build-up of static charge on antennas

– Unlikely: No correlation of SNR events with on-orbit locations or space weather events

• Deformation of antennas due to heating, solar exposure

– Possible: Correlation observed between solar exposure to antennas and SNR events. Antenna deformation could cause durable SNR depression.

• Interference with other instruments on the satellite

– Spacecraft GPS

• Possible: FM4 is the only s/c with no SNR problems and has the bus GPS completely disabled

– Tri-Band Beacon -- Unlikely: No correlation found despite long search

– Spacecraft transmitter or receiver -- Unlikely: No correlation found

• Temperature effects on the GOX

– Thermal aging in ceramic filters in GOX front-end

• Likely: There are definite SNR/temperature correlation on some s/c and antennas

• GOX firmware bugs -- Unlikely: Little correlation with reboots

• Question: Why are failures only for antennas with long cables? Still no good answer.

Conclusion

• SNR problems seem to stem from (at least) 2 sources– Antenna problems– GOX temperature sensitivity

• More attention to these issues must be given when planning follow-on missions

• I would be happy to share the database I've developed with those interested in studying this problem