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Huygens Ground Track Determination A Comparison
of DWE and DISR Results R. Dutta-Roy1, B. Rizk2,
S.W. Asmar3, D.H. Atkinson4 , M.K. Bird1, M.W.
Bushroe2, E. Karkoschka2, E.A. McFarlane2, C.
See2, M.G. Tomasko2, and the DWE and DISR
Teams 1Radioastronomisches Institut, Univ. Bonn,
53121 Bonn, Germany, 2Lunar and Planetary
Laboratory, Univ. of Arizona, Tucson, AZ 85721,
3Jet Propulsion Laboratory, Caltech, Pasadena, CA
91109, 4Dept. Elec. Engineering, Univ. of Idaho,
Moscow, ID 83843
Abstract Determination of the descent trajectory
of the Huygens Probe during its descent through
the atmosphere of Titan on 14 January 2005 is
currently under construction. Toward this
purpose, measurements from various on-board
experiments and system instruments are combined
to retrieve the altitude, descent speed and
horizontal motion. This task is coordinated by
the Huygens Descent Trajectory Working Group
(DTWG). The Doppler Wind Experiment (DWE) was
designed to retrieve the zonal (east-west)
velocity of Huygens. No other on-board experiment
can provide this parameter with comparable
accuracy. The ground track can also be retrieved
from images of the Descent Imager/Spectral
Radiometer Experiment (DISR), albeit with less
accuracy. Here we present a comparison of the
ground tracks retrieved by DWE and DISR. We show
that these two independent retrievals of the
horizontal motion of Huygens are remarkably
consistent.
1. DWE Ground Track Determination
2. DISR Ground Track Determination The
DISR-derived trajectory from t0 37 min until
impact (lowest 55 km altitude, 46 km horizontal
travel) is an image-based reconstruction of the
ground track, derived by assembling
gnomonically-projected mosaics by overlaying
separate image frames like jigsaw pieces. The
probe position at the time of the highest
altitude image was adopted from the DTWG and DWE
reconstructions. Using the DISR sun sensor, it is
possible to determine the direction of the
Huygens flight. As the direction towards the sun
is known, the horizontal velocity can be
separated into meridional and zonal speed.
DWE measures carrier Doppler shift range rate
between Huygens and receiving antenna.

• Input parameters
• Position and velocity of receiving antenna.
• Huygens altitude and descent speed, obtained by
  DTWG from evaluation of temperature, pressure and
  molecular weight measurements.
• Huygens initial longitude and latitude, obtained
  by DTWG from integration of the probe motion
  using measured probe acceleration during entry
  phase and initial entry state from Cassini
  Navigation Team at NASA/ESA interface (1270 km
  altitude).
• AssumptionNo meridional (north-south) motion
  (based on theoretical considerations).
• Output parameters
• Huygens zonal (east-west) speed.
• Longitude profile (integrated zonal speed).

Zonal wind speed as retrieved by DWE.Each
marks a frequency/range rate measurement.
Huygens latitude/longitude during descent.Blue
dots mark individual position measurements
green lines are fitted polynomials.
Huygens ground track from DISR for the last 16.5
km altitude.The positions are projected onto a
DISR panorama mosaic.
Several 70-s gaps (VLBI calibration)
25-min gap between Green Bank (GBT) and Parkes
3. Comparison of Results Zonal Speed and
longitude drift
4. Comparison of Results Meridional Speed and
latitude drift
For DISR, the longitude drift is the prime
measurement, and the zonal wind speed is its
derivative.For DWE, the zonal speed is the prime
measurement, and longitude drift is its
integration.The initial position for the DISR
longitude profile determination was taken from
the DWE result, but the longitude profile
thereafter was derived independently.
The DISR derived ground track can also be used to
test of the DWE assumption of zero meridional
winds.
Generally very good agreement. An initial
discrepancy is due to the algorithm used to fit
the polynomial to the DISR data
points. Statistical error for DISR polynomial
3-5 m/s. Statistical error for DWE measurements
5 cm/s. The DWE measurement may contain a small
systematic error (80 cm/s in the upper
atmosphere, decreasing to 10 cm/s near the
surface) due to the initial Huygens delivery
error. This error is roughly proportional to the
measured zonal wind speed.
DISR error bars for meridional speed3-5 m/s
This jump in the residuals is probably due to a
slightly inaccurate drift estimate by DWE during
the 25-min data gap between the GBT and Parkes
tracks.
An error of 0.1 deg in latitudinal position
mimics 0.04 m/s zonal wind.
A meridional speed of 1 m/s mimics 0.42 m/s zonal
wind.
0.01 deg ? 450 m
Zoom into near surface phase
DISR ground track near surface based on only two
measurements.Shape consistent with DWE
measurement.

• Conclusions
• The latitude drift inferred by DISR has no
  significant impact on theDWE result.
• Meridional speed has a greater impact on the DWE
  zonal wind retrieval than a latitudinal
  displacement. However, considering the relatively
  large error bars of the DISR inferred meridional
  speed, the DWE assumption of vanishing meridional
  motion is consistent with the DISR result.

Phases of nearly constant residuals indicate
almost perfect agreement.
Conclusion The result shows very good agreement
with the DWE result.Maximum deviations do not
exceed 500 m over a distance of approximately 46
km.
ReferencesM. K. Bird et al.,The Vertical
Profile of Winds on Titan. Nature, 2005 (in
press)M. G. Tomasko et al., Rain, Winds and Haze
during the Huygens Probes Descent to Titans
Surface. Nature, 2005 (in press) Contact
duttaroy_at_astro.uni-bonn.de