Several 70-s gaps (VLBI calibration)

1. 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). • Assumption:No 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 speed:3-5 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. References:M. 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 Probe’s Descent to Titan’s Surface. Nature, 2005 (in press) Contact: duttaroy@astro.uni-bonn.de