1. Radio Occultation
From GPS/MET to COSMIC

2. Background: Global Positioning System (GPS) Satellites Low-Earth Orbit (LEO) Satellites
A GPS receiver in LEO can track GPS radio signals that are refracted in the atmosphere
GPS Satellite
LEO Orbit
Atmosphere
Radio Signal
LEO Satellite

3. Occultation Geometry Occultation geometry
During an GPS occultation a LEO ‘sees’ the GPS rise or set behind Earth limb while the signal slices through the atmosphere
Occultation
geometry
The GPS receiver on the LEO observes the change in the delay of the signal path between the GPS SV and LEO
This change in the delay includes the effect of the atmosphere which delays and bends the signal

4. Signals Abundant Glonass Galileo --------------- 60–90 sources
GPS
Glonass
Galileo
60–90
sources
in space
T. Yunck, JPL

5. Unique Attractions of GPS Radio Occultation
1. High accuracy: Averaged profiles to < 0.1 K
2. Assured long-term stability
3. All-weather operation
4. Global 3D coverage: stratopause to surface
5. Vertical resolution: ~100 m in lower trop
6. Independent height & pressure/temp data
7. Compact, low-power, low-cost sensor
T. Yunck, JPL

6. Radio Occultation Mission Overview
JPL + Stanford
Use Radio
Occultation (RO) to
Explore planetary
atmospheres
COSMIC
Operational Demonstration
METOP
(COSMIC II)
Operations RO Missions
UCAR manages GPS-MET
RO Mission - Proof of Concept
CHAMP and SAC-C Missions
Improved Proof of Concept

7. COSMIC at a Glance
Constellation Observing System for Meteorology Ionosphere and Climate (ROCSAT-3)
6 Satellites launched in late 2005
Orbits: alt=800km, Inc=72deg, ecc=0
Weather + Space Weather data
Global observations of:
Pressure, Temperature, Humidity
Refractivity
TEC, Ionospheric Electron Density
Ionospheric Scintillation
Demonstrate quasi-operational GPS limb sounding with global coverage in near-real time
Climate Monitoring
Geodetic Research

8. COSMIC Status

9. Payloads GPS Occultation receiver Tiny Ionosphere Photometer (TIP)
High-resolution (1 Hz) absolute total electron content (TEC) to all GPS satellites in view at all times (useful for global ionospheric tomography and assimilation into space weather models)
Occultation TEC and derived electron density profiles (1 Hz below the satellite altitude and 50 Hz below ~140 km), in-situ electron density
Scintillation parameters for the GPS transmitter–LEO receiver links
Data products available within minutes of on-orbit collection
Tiny Ionosphere Photometer (TIP)
Nadir intensity on the night-side (along the sub-satellite track) from radiative recombination emission at 1356 Å
Derived F layer peak density
Location and intensity of ionospheric anomalies (Auroral Oval)
Tri-band Beacon (TBB)
Phase and amplitude of radio signals at 150, 400, and 1067 MHz transmitted from the COSMIC satellites and received by chains of ground receivers.
TEC between transmitter and receivers
Scintillation parameters for LEO transmitter - receiver links

10. 6 Satellite COSMIC Microsat Constellation
COSMIC System
6 Satellite COSMIC Microsat Constellation
S band
S band
GPS s/c
L band
S band
Taiwan OPS
R.O. RT
Data E/S (Fairbanks)
R.O. RT
Data E/S (Kiruna)
RT Fiducal
Network
TT&C
TT&C
NSPO
MOC, MCC, SCC, FDF T1 T1
Payload Commands and All Real-Time Data Products
LAN
Real Time CDAAC (Boulder)
S/C Telemetry
C W B T1
TACC
vBNS STARTAP Tanet, I2
Internet
U.S. Universities & Mission
Teams
NESDIS
Other
Users
Other Customers
University
Science
Centers
Operational Centers

13. NESDIS CDAAC Getting COSMIC Results to Weather Centers NCEP Input
Data
NESDIS
CDAAC
ECMWF
CWB
GTS
BUFR Files
WMO standard
1 file / sounding
UKMO
JMA
Canada Met.
This system is currently under development by UCAR, NESDIS, + UKMO
Data available to weather centers within < 180 minutes of on-orbit collection

14. Summary
Radio occultation is a new and promising remote sensing technique
Technique was demonstrated and now COSMIC aims to:
Improve data quality in lower troposphere - new technology
Increase number of soundings
Show impact in operational models - work at NCEP, UKMO, ECMF
COSMIC launch is on schedule for December of 2005
COSMIC “operational demonstration” - should be followed by continuous operational missions

16. RO provides best results between 8-30 km (effects of moisture
and ionosphere are negligible). Is capable of resolving the structure of the
tropopause and gravity waves above the tropopause.
“dry temperature”
computed from refractivity
assuming no water vapor
Figure from the paper by Nishida et al., J. Met. Soc. Japan, 78(6), p.693, 2000.

17. Result Improvements at UCAR
Most recent RO processing Statistics
A comprehensive reprocessing of all CHAMP, SAC-C and GPS/MET data has been completed using CDAAC 1.01 software
New

18. RO - ECMWF Comparison Data Poor regions Data Rich regions
Comparison of RO profiles with ECMWF profiles in data rich (most land masses) and data poor regions (mostly oceanic regions). The better agreement from 5-30 km indicates that superior ECMWF performance in data rich regions

19. Observed Atmospheric Volume
L~300 km
Z~1 km
Top: Schematic depiction of tubular volume over which atmosphere contributes information to a single occultation phase (ray) measurement. The intensity of shading in the tube represents the relative weighting of atmospheric properties that contribute to the value retrieved at the center of the tube. For typical atmospheric structures, L and Z are approximately 300 and 1 km respectively.
Bottom: Typical along-track weighting function for a single radio occultation measurement (from Melbourne et al, 1994). Most of the information is contributed by a mesoscale atmospheric volume centered at the ray tangent point.

20. Processing Overview
Processing of profiles takes < 15 minutes after data reception

21. Radio occultation
processing overview
R. Kursinski

22. Deriving Bending Angles from Doppler
The projection of satellite orbital motion along signal ray-path produces a Doppler shift at both the transmitter and the receiver
After correction for relativistic effects, the Doppler shift, fd, of the transmitter frequency, fT, is given as
where: c is the speed of light and the other variables are defined in the figure with VTr and VTq representing the radial and azimuthal components of the transmitting spacecraft velocity. a vT

23. Ionospheric calibration
Is performed by linear combination of L1 and L2 bending angles at the same
impact parameter (by accounting for the separation of ray tangent points).
bending angle
impact parameter
Effect of the small-scale
ionospheric irregularities
with scales comparable
to ray separation is not
eliminated by the linear
combination, thus resulting
in the residual noise on
the ionospheric-free
bending angle.

24. Optimization of the observation bending angle
The magnitude of the
residual noise can be
very different for
different occultations,
but it almost does not
depend on height for
a given occultation.
Above a certain height,
climatology provides
better estimate of the
atmospheric state than
RO observation. The
observed bending angle
is optimally weighted
with climatology. This
does not improve the
value of the bending
angle at large heights,
but results in reduction
of error propagation
downward after the
Abel inversion.
where
The weighting function is calculated individually for each occultation.

25. Define the refractional radius x=nr, where n=1+N*10-6
Now we have a profile of refractivity as a function of height

26. Atmospheric refractivity N=(n-1)*10-6
Ionospheric term dominates above 70 km
Hydrostatic (dry) wet terms dominates at lower altitudes
Wet term becomes important in troposphere (> 240 k) and
Can be 30% of refractivity in tropics
Liquid water and other aerosols are generally ignored

27. Deriving Pressure Temperature Humidity
After converting GPS Doppler phase => a(a) => n(r) and removing effects of ionosphere, from we have a profile of dry refractivity for altitudes from ~150 km down to the 240K level in the troposphere.
We use the hydrostatic equation, dP = -g r dz = -g nd md dz to derive a vertical profile of pressure versus altitude over this altitude interval.
If we start high enough P(ztop) =0 with negligible error
Given P(z) and nd(z), we can solve for T(z) over this altitude interval using the equation of state (ideal gas law): T(z) = P(z) / (nd(z) R)
Below the 240k level we need additional information (usually temperature from a weather model) to obtain water vapor pressure and humidity.