GPS Radio Occultation Sounding Zhen Zeng (HAO&COSMIC)

GPS Radio Occultation Sounding

Zhen Zeng (HAO&COSMIC)

Occultation is from the Latin word “Occult” meaning “to hide”.

Conditions: both a transmitter and a receiver off the planet of interest;

Value: many “conventional” measurements (comprehensive, continuous, synoptic);

Cost: expansive or cheap?

Establishment of GPS constellation

April, 1995

GPS/MET

Early temperature profile from GPS/MET

CHAMP

SAC-C

GRACE

Ørsted

Sunsat

IOX

T. P. Yunck

Basic measurements: precise measurement of phase delay and amplitude of GPS signals.

Principles of occultation technique

The velocity of GPS relative to LEO must be estimated to ~0.2 mm/sec (velocity of GPS is ~3 km/sec and velocity of LEO is ~7 km/sec) to determine precise temperature profiles

The LEO tracks the GPS phase while the signal is occulted to determine the Doppler

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LEO

vleo

vGPS

Tangent point

Principles of occultation technique

Atmospheric refractive index where is the light velocityin a vacuum and is the light velocity in the atmosphere

Refractivity

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• Hydrostatic dry (1) and wet (2) terms dominate below 70 km

• Wet term (2) becomes important in the troposphere and can constitute up to 30% of refractivity at the surface in the tropics

• In the presence of water vapor, external information information is needed to obtain temperature and water vapor

• Liquid water and aerosols are generally ignored

• Ionospheric term (3) dominates above 70 km

(1) (2) (3)

6 Satellites was launched:01:40 UTC 15 April 2006

Three instruments:GPS receiver, TIP, Tri-band beacon

Weather + Space Weather data Global observations of:

Pressure, Temperature, HumidityRefractivityIonospheric Electron DensityIonospheric Scintillation

Demonstrate quasi-operational GPS limb sounding with global coverage in near-real time

Climate Monitoring

COSMIC (Constellation Observing System for Meteorology, Ionosphere

and Climate)

COSMIC launch picture provided by Orbital Sciences Corporation

• All six satellites stacked and launched on a Minotaur rocket

• Initial orbit altitude ~500 km; inclination ~72°

• Will be maneuvered into six different orbital planes for optimal global coverage (at ~800 km altitude)

• All satellites are in good health and providing initial data

Launch on April 14, 2006Vandenberg AFB, CA

Chris Rocken

COSMIC Ionospheric Data

GPS Receiver

TEC Profiles Scintillation

TIP

UV Radiation

TBB

ScintillationTEC

From presentation by Stig Syndergaard, UCAR/COSMIC

Absolute TEC Processing

• Pseudorange local multipath

• Phase cycle-slip & outliers (Blewitt, 1990)

• Phase-to-pseudorange leveling

• Differential code bias (DCB) calibrationFrom presentation by Stig Syndergaard, UCAR/COSMIC

Pseudorange Local Multipath Calibration

From presentation by Stig Syndergaard, UCAR/COSMIC

Pseudorange Local Multipath Calibration

From presentation by Stig Syndergaard, UCAR/COSMIC

LEO Differential Code Bias Estimation

From presentation by Stig Syndergaard, UCAR/COSMIC

LEO Differential Code Bias Estimation

From presentation by Stig Syndergaard, UCAR/COSMIC

Electron Density Profile Processing

• Subtracting positive elevation angle data from negative elevation angle (Schreiner et al., 1999)

• Model independent estimate of upper boundary electron density (Syndergaard et al., 2006)

• Profile retrieval based on straight-line and spherical symmetry assumptions

First Collocated Ionospheric Profiles

From presentation by Stig Syndergaard, UCAR/COSMIC

COSMIC Occultation Over South Atlantic: 1815 UTC

Black: CosmicBlue: IRIRed: IDA3D

Horizontal LinesRepresentationError

From presentation by Gary S. Bust, Atmospheric & Space Technology Research Associates

COSMIC NmF2 - 1 week

From presentation by Bill Kuo, UCAR/COSMIC

Quiet

Storm

COSMIC #2 GAIM

COSMIC #2 GAIM

Comparisons During Quiet and Disturbed Conditions

From presentation by Ludger Scherliess, Utah State University

Comparison of NmF2 and HmF2 between COSMIC and GAIM during Apr. 21-28, 2006

Good agreement of NmF2 between COSMIC and GAIM;

Higher peak heights from GAIM than those from COSMIC

Comparison of Ne(h) between COSMIC (red), Ionosondes (green)and TIEGCM (black) on Aug. 17 - 21nd

COSMIC agree well with ionosonde obs, especially the HmF2;

Vertical structures from COSMIC coincide well with TIEGCM in the mid-lat, but not in the tropics.

TIEGCM shows a bit higher HmF2 compared with obs.

COSMIC agree well with ionosonde observations;

Global map of NmF2 revealed from COSMIC is well represented by TIEGCM model, though TIEGCM shows higher peak density in the low latitude.

Maps of NmF2 for: COSMIC (dots), Ionosondes (stars), TIEGCM (contour)

RED = COSMIC satBLUE = GPS sat

Formosat-3/COSMIC Observations of Scintillations

From presentation by Chin S. Lin, AFRL

TIP 135.6-nm passes 14 Sep 2006 FM1 FM3 FM6 0-24 UT (2100 LT)

From presentation by Clayton Coker, NRL

COSMIC

Radiosondes

COSMIC Soundings in 1 Day

Chris Rocken

About 90,000 soundings / month Or ~10 soundings / 2.5 x 2.5 pixel / month

All local times sampled every day!

COSMIC Education Module

A joint effort by COMET and COSMIC.

It covers:

- Basics of GPS radio occultation science

- Applications to weather, climate, and ionosphere

- COSMIC Mission description

http://www.meted.ucar.edu/COSMIC/

http://www.cosmic.ucar.edu

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Applications

• Atmospheric science and weather forecasting

• Global change and Climate• Ionospheric monitoring and space we

ather

Using GPS temperatures to study the extratropical tropopause

1) Observational characteristics of double tropopauses

Occurs frequently, related to interaction between tropics and extratropics

acd.ucar.edu/~randel

(see also Schmidt et al., GRL, 2006)

2) The tropopause inversion layer

W. Randel

Equatorial QBO-- T. Schmidt, Atmos. Chem. Phys., 2005

Diurnal tides

Some promising forecast scores in the stratosphere. EG, verification

against radiosonde at 100 hPa. Red = COSMIC experiment.

The results in the NH are new. We had not seen improvements in NH with CHAMP. Note that the improvements are generally statistically significant at the 5% level.

NH

TR

SH

Sean Healy

Shu-peng Ho

Validation of Microwave Lower Stratosphere Temperatue using CHAMP RO data

Study ionospheric Irregularities

Pavelyev, GPS solution, 2005

edTECN dS

edTECN dS

edTECN dS

2

4 2

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: Amplitude scintillations averaged in 40-80

km interval

Questions for the future

• Retrieval Problem

-- Ionosphere residual

-- Spherical symmetry assumption

-- Orbits

-- Multipath

-- Cycle slips, qualtiy control …• What is optimal assimilation variables?• What is optimal processing strategy for climate records?