Dr. David H. AtkinsonDept. Electrical & Computer Engineering

University of Idaho8 April 2005

Measuring the Zonal Winds on Titan:

The Huygens Probe Doppler Wind Experiment

Cassini/Huygens

HST

Voyager 1

Transmitter Ultrastable Oscillator (TUSO)

Radar Altimeter Antennas

GCMSAtmosphericStructure Instrument

Descent Imager

Surface SciencePackageCommand

Data ManagementUnit (CDMU)

Accelerometers

Batteries

PCDU

ACP SSPGCMS

KSC, Cape Canaveral: 15 Oct 1997

Cassini SOI: 1 July at 02:36 UT

Christmas Day 2004

14 January 2005

7 years, 3 months and 4 billion km from launch

Titan: Pre-Cassini/HuygensN2, CH4, andH2 atmosphere cause a mild greenhouseeffect that raisesthe surface temperature by+21K.

High altitude optically thickorganic haze absorbs solarvisible but istransparent to IR, coolingsurface tempby about 9K.(anti-greenhouseeffect).

Huygens’ Scientific Payload

• ACP - Aerosol Collector and Pyrolyser (G. Israel, France)– Collection of aerosols for chemical-composition analysis.

• GCMS - Gas Chromatograph and Mass Spectrometer(H. Niemann, NASA GSFC)

– gas chemical analyser designed to identify and quantify various atmospheric constituents.

• DISR - Descent Imager & Spectral Radiometer (M. Tomasko, U. Arizona)

– images and spectral measurements. Also spectra of the surface material will be acquired.

• DWE - Doppler Wind Experiment (M. Bird, U. Bonn)– Wind measurements using Probe/Orbiter radio signal

• HASI - Huygens Atmosphere Instrument (M. Fulchignoni, Italy)

– physical and electrical properties of the Atmosphere

• SSP - Surface Science Package (J. Zarnecki, UK)– physical properties of the surface at the impact site

DWE Concept• Goal: In situ determination of zonal wind

speed along Huygens descent path

• Approach: Record Doppler effect on Huygens carrier signal => radial velocity

• Input parameters:1. Huygens descent speed, meridional speed

2. Huygens starting position

3. Cassini position & velocity

Ultra-stable Oscillators (USOs)• TUSO on Huygens Probe

• RUSO in Huygens Receiver on Cassini Orbiter

DWE Hardware

No power!

Channel B Temperature Compensated Crystal Oscillator

DWE from Earth: Geometry

0°

15°

30°

45°

60°

75°

-15°

-30°

-45°

-60°

-75°

Track ofSub-Earth

Point

Track of OrbiterProbe

180°210°240° 150° 120°

• Huygens antenna pointed only ~30° from Earth

• Better SNR than for Galileo Probe detection at Jupiter

• Probe velocity projection in direction to Earth nearly antiparallel to projection in direction to Orbiter

Huygens VLBI Network

Facility Start UTC Stop UTC Comment

GBT 09:31:10 12:15:00 DWE: Real time RSR detection

VLBA Fort Davis 09:31:10 13:45:00

VLBA North Liberty 09:31:10 13:15:00

VLBA Owens Valley 09:30:09 14:49:14 DWE: No nodding – on Titan continuously

VLBA Pie Town 09:30:11 14:15:04 DWE: No nodding – on Titan continuously

VLBA Los Alamos 09:31:10 14:00:00

VLBA Brewster 09:31:10 14:48:00

VLBA Kitt Peak 09:31:10 14:15:00 DWE

VLBA Mauna Kea 09:31:10 16:00:00 DWE

Parkes 12:26:23 16:00:00 DWE: Real time RSR detection

Hobart 11:13:10 16:00:00

Ceduna 10:13:10 16:00:00

Mopra 10:10:10 16:00:00

Kashima 09:31:10 16:00:00

Shanghai 10:01:10 16:00:00

Urumqi 11:31:10 16:00:00 Calibration only, no Huygens signal at 2040 MHz

ATCA 10:01:10 16:00:00 Calibration only, no Huygens signal at 2040 MHz

Onsala 19:01:10 22:15:00

Wettzell 21:46:10 22:15:00

Parkes TelescopeAustralia

Green Bank TelescopeWest Virginia

Huygens Radio Tracking Net

10:18 UTC-ERT 12:30 UTC-ERT

Huygens!(~ -171 dBm)

10:18:15 UTC-GRT10:18:10 UTC-GRT

Green Bank Telescope

Huygens Frequency: GBT + Parkes

DWE: Preliminary Results

• Zonal winds above boundary layer are PROGRADE

• Low-velocity layer between 60-80 km – Stong positive/negative wind shear– ‘Unexpected’, but evident in some GCMs

• Considerable turbulence above 100 km

• Near surface winds are weak (~ 1-2 m/s)

Adieu, Huygens