Radio Occultation

Atmospheric Profiling with Global Navigation

Satellite Systems (GNSS)

Overview• The Idea: A first look at planetary atmospheres

• Next step: Applying the technique to Earth• The principles

– The GPS system and the GPS measurement– How RO works– Unique characteristics of the observations

• Satellite missions• Science Applications

– Meteorology– Climate– Space Weather

Question: How can we learn if planets have an atmosphere?

Send a space probe from Earth to the far side of the planet in question and send a known radio frequency back to Earth.

If the planet has no atmosphere the radio signal received on Earth will travel on a straight line

As the signal grazes the planet’s Limb it’s radio signal is occulted (thus radio occultation)

…. but if there is an atmosphere the ray will be bent!

Measure the Doppler frequency shift of the received radio signal on Earth.

Question: But how do we know if the ray is bent?

For a straight ray the Doppler shift is caused only by the relative motion of the transmitter relative to the receiver - and can be predicted based on orbital mechanics For a bent signal the Doppler shift will noticeably different than

predicted based on orbital mechanics only!

Mariner IV at MarsJuly 1965

Planetary Radio Occultation

Radio occultation was first applied to Planetary atmospheres by teams at Stanford U. and NASA/JPL

Mariner V at Venus19 October 1967

Subsequently RO was used to study the atmospheres of many planets

The same measurement principle can also be used to observe Earth’s atmosphere

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Low-Earth Orbiter LEO

TransmitterThe signal is received on the LEOAnd atmospheric properties can be obtained

There are some key advantages for radio occultation on Earth

Signals Abundant

GPS GlonassGalileo---------------60–90

sourcesin space

GPS Signal CoverageTwo L-band frequencies:

L1: 1.58 GHzL2: 1.23 GHz

~3000 km

GPS Signal Structure

The GPS Signal Spectrum

Carrier+

Code

Carrier

• A GPS receiver in LEO can track GPS radio signals that are refracted in the atmosphere

GPS Satellite

LEO Satellite

Radio Signal

LEO Orbit

Atmosphere

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

Determining Bending from observed Doppler (a)

Earth

Bending angle

Transmittedwave fronts Wave

vector of receivedwave fronts

€

Δx

€

ψ€

Φ

€

rv

€

rk

From orbit determination we know the location of source and We know the receiver orbit Thus we also know

€

rv

€

Φ

We measure the Doppler frequency shift:And compute the bending angle

€

fD = 1Δt = v

Δx = vλ cosψ = fT

vc cosψ

€

=Φ−ψ

Deriving Bending Angles from Doppler• The projections of satellite orbital motion of transmitter and

receiver along the ray path produces a Doppler frequency shift• After correction for clock and relativistic effects, the Doppler

shift, fd, of the transmitter frequency, fT, is given as

€

fd =fT

cVT • ˆ e T + VR • ˆ e R( ) ( )RRR

rRTTT

rT

T VVVVc

fφφφφ θθ sincossincos −++−=

• where: c is the speed of light and the other variables are defined in the figure with VT

r and VT representing the radial and

azimuthal components of the transmitting spacecraft velocity.

vT

€

sin(φR) = a /rR

€

sin(φT) = a /rT

From Doppler + orbits we obtain bending as a function of impact parameter

€

(a) = 2adn

ndxa

∞

∫ dx

x 2 − a2

€

n x( ) = exp −1

π

α (a) da

a2 − x 2x

∞

∫ ⎡

⎣

⎢ ⎢

⎤

⎦

⎥ ⎥

Define the refractional radius x=nr, where n=1+N*10-6

Now we have a profile of refractivity as a function of “x”We compute the “mean sea level height” of the observation: hmsl=x-Rc-G (where Rc is radius of curvature, and G is the geoid height)

Steps taken in determining “MSL” altitude z• Determine the lat/lon of the ray path perigee at the‘occultation point’ (that point where the

excess phase exceeds 500 meters)• Compute the center of sphericity (C) and radius of curvature (Rc) of the intersection of the

occultation plane and the reference ellipsoid at the assigned lat/lon. • Do the Abel inversion in the reference frame defined by the occultation plane and C.• Now height r is defined as the distance from the perigee point of the ray path to C.• G is the geoid correction. We currently use the JGM2 geoid.

The geometric height in the atmosphere is computed :z = r - Rc - G

Center of curvature C

r

Rc - radius oflocal curvatureof ref. ellipsoid

G - geoid height z - geometricheight

Definition of Altitude in Radio Occultation

referenceellipsoid

€

N = 77.6P

T+ 3.73e5 Pw

T 2− 40.3 ×106 ne

f 2

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

Observed Atmospheric Volume

L~300 kmZ~1 km

1. High accuracy: Averaged profiles to < 0.1 K

Unique Attractions of GPS Radio Occultation

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

CHAMP

SAC-C

GRACE

Ørsted

Sunsat

IOX

GPS/MET

The first RO profile from Earth

CHAMP in orbit since July 15, 2000

COSMIC/FormoSat3 (6)

EQUARSC/NOFSMETOP

The next wave…

COSMIC at a Glance Constellation Observing System for Meteorology Ionosphere

and Climate (ROCSAT-3) 6 Satellites launched in 2006 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

COSMIC Status

Location of Profiles

1.5 months after launch

Final constellation

Mission science payloads

•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 15 - 120 minutes of on-orbit collection

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

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)

GPS Occultation receiver

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COSMIC EQUARS

Radiosondes

COSMIC + EQUARS Soundings in 1 Day

Occultation locations for COSMIC (6 s/c, 3 planes) and EQUARS, 24 hrs

Science Applications

Weather Climate

Space Weather

Evolution of forecast skill for northern and southern hemispheres

Courtesy, Simmons 2004

Evolution of forecast skill for the northern and southern hemispheres: 1980-2001. Anomaly correlation coefficients of 3, 5, and 7-day ECMWF 500-mb height forecasts for the extratropical northern and southern hemispheres, plotted in the form of running means for the period of January 1980-August 2001. Shading shows differences in scores between hemispheres at the forecast ranges indicated (from Holingsworth, et al. 2002).  

The GPS-MET Experiment on MicroLab-I

1995 - ?

1

10

100

1000200 220 240 260 280 300

Temperature profiles near England

Occultation at 52.6N. 355 E.

Radiosonde at 54.5 N. 353.9 E.

Radiosonde at 53.5 N. 357 E.

Temperature, K

At about 95-4-25:00:00 UTC

Figure from the paper by Nishida et al., J. Met. Soc. Japan, 78(6), p.693, 2000.

RO provides best results between 8-30 km (effects of moistureand ionosphere are negligible). Is capable of resolving the structure of thetropopause and gravity waves above the tropopause.

“dry temperature”computed from refractivity assuming no water vapor

Case 1: Hurricane Isabel (2003)

• Developed in the lower Atlantic ocean, tracked northwest and landed at North Carolina coast on Sept 18, 2003

• The hurricane was category 4 or 5 for a period of 6 days.

• The WRF simulation covered a period when the hurricane was category 2.

• 24-h forecast from 4-km WRF simulation, valid at 0000 UTC 17 September 2003.

A

B

A B

Equivalentpotential temperature

Radarreflectivity

Temp, K ΔTemp, K

Hei

ght,

km

Hei

ght,

km

CHAMP-SACC Profile Comparison

Full Profiles

Hajj et al., 2004

Avg Delta Profiles

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From Healey et al. GRL, 2004

GPS RO Data Impact on Weather Prediction

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Vertical cross sections of zonally-averaged model temperature changes averaged over 20 years (years 60-79) in NCAR Climate System Model in which carbon dioxide alone is increased by 1% per year (Meehl etal., 1998).

Effects of CO2 increase on climate change simulated by

NCAR Climate System Model (CSM)

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Global Temperatures from 1995- 200550 mb and 100 mb levels

Polar temperatures at 50 mb from 1995-2005

North Pole South Pole

Equatorial temperatures at 50 mb from 1995-2005

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GPS - NCEP/NCAR reanalysis refractivity difference at 300 mb

Southern Hemisphere

GPS - radiosonde refractivity difference at

300 mb Southern Hemisphere

Height of 300 mb Surface, Summer 1995

8.2 km Geopotential Height (gpkm) 9.7 km

S. S. Leroy

Importance of Space Weather

CHAMP Electron Density profiles

GPS/MET Ionospheric Climatology