Results from POD, RO profile collection, and processing from Spire's satellite constellationP. Platzer, D. Ector, O. Nogues Correig, and V. Irisov
Prepared for OPAC-IROWG 2016
OUTLINE• Current state of Spire
• Radio occultation scenario
• Data processing algorithm
• Precise orbit determination
• Statistical comparison with COSMIC
• Examples of atmospheric profile obtained by Spire
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POWERFULL SATELLITE CONSTELLATION• 17 satellites launched• 30+ satellites in space by year end• Completing up to 2 satellites per week• 21 ground stations globally• 30 ground stations by year end
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3
RADIO OCCULTATION SCENARIO
POD antenna
Forward ROantenna
Backward ROantenna
V
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DATA PROCESSING ALGORITHMRO phase
Bending angle and impact parameter
TEC and ionosphere compensation
Refractivity profile
Atmospheric profiles
Climatology model
Reference GPS data
RTorb: Precise Orbit Determination (POD)RO phase excess
The code is implemented in C++ with interprocess communication in Python.
Core RO processing code was developed by Michael Gorbunov(Spire contractor).
POD processing (RTorb) was developed by GPS Solutions (Christian Rocken)(Spire contractor) and licensed to Spire.
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PRECISE ORBIT DETERMINATION
LEO satellite orbital positions and velocities are determined by solving the equations of motion given a force model and GPS code and phase measurements
POD allows for precise computation of atmospheric phase delay on L1 and L2 frequencies
Force Model• Earth gravity field model
• Lunisolar perturbations
• Atmospheric drag
• Solar radiation pressure
GPS Observations and Corrections• Dual frequency code and phase
measurements
• GPS clock and orbit corrections with 5 second resolution
• Attitude corrections
Numerical Algorithm• Kalman filter approach to solve
equations of motion
• Backward smoothing
LEO positions/velocities
Excess L1/L2 atmospheric phase delay-Use dual frequency measurements of an occulting and a reference satellite-Remove effects of geometric range, tx/rx clock offsets, ionosphere to compute carrier phase delay due to the atmosphere
Overview of POD/Excess Phase Processing
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COMPARISON TO UCAR PROCESSING
• Spire POD processing results for a COSMIC satellite were compared to UCAR which uses Bernese POD software
• RMS differences of about 200 mm (position) and .2 mm/s (velocity) observed over 10 days
• Minor Spire/UCAR differences are likely due to details in POD processing
OVER 1 DAY OVER 10 DAYS
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Spire processing compared with CDAAC and ECMWF: “dry” temperature, 68000 profiles, 0-90˚lat., COSMIC orbits
Spire-CDAAC Spire-ECMWF CDAAC-ECMWF
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The same but with the Spire POD: 5900 profiles, 0-90˚lat.
Spire-CDAAC Spire-ECMWF CDAAC-ECMWF
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Spire processing compared with CDACC and ECMWF: “dry” temperature, 30000 profiles, 45-90˚lat., CDAAC orbits.
Spire-CDAAC Spire-ECMWF CDAAC-ECMWF
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The same but with the Spire POD: 2700 profiles, 45-90˚lat.
Spire-CDAAC Spire-ECMWF CDAAC-ECMWF
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RAW PHASE AND PHASE EXCESS MEASURED BY SPIRE FM2 (EXAMPLE 1)
550 600 650 700 750 800 850 900-7100
-7000
-6900
-6800
-6700
-6600
-6500
-6400
dPhs
/dt,
m/s
Time, s
L1 ion.L2 ion.L1L2
840 850 860 870 880 890 900-20
-10
0
10
20
30
40
50
60
70
80
Time, s
d/dt
(Pha
se e
xces
s), m
/s
ionosphere atmosphere
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ATMOSPHERIC TEMPERATURE PROFILE FROM SPIRE FM2 OBSERVATION (EXAMPLE 1)
The time difference between GFS profile and occultation event is 2.7 hours.
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150 200 250 3000
5
10
15
20
25
30
35
40
T, K
Z, k
m
NCEP GFSSpire
ATMOSPHERIC TEMPERATURE PROFILE FROM SPIRE FM2 OBSERVATION (EXAMPLE 2)
The time difference between GFS profile and occultation event is 3 hours.
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150 200 250 3000
10
20
30
40
50
60
T, K
Z, k
m
NCEP GFSSpire
10-6 10-4 10-2 100 1020
10
20
30
40
50
60
Water vapor, g/m3
Z,
km
NCEP GFS
• Radio occultation (RO) full chain data processing software was developed at Spire during the last 6 months. Advanced POD algorithm was implemented.
• Reprocessing of the COSMIC data shows the bias and standard deviation of the retrieved temperature profiles comparable with UCAR processing.
• Spire example profiles shown were collected summer 2016. Comparison with GFS data demonstrates the high quality of the retrieved atmospheric profiles.
• Spire objective is processing of large amount of the profiles on a daily basis.
• Algorithm and hardware improvement is going on…
CONCLUSIONS
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Acknowledgements. COSMIC data were provided by COSMIC Data Analysis and Archive Center.