Observations of the surface of Titan by the Radar Altimeters on the Huygens Probe
Introduction
The Radar Altimeters on the Huygens Probe measured the distance to the surface of Titan during its descent through the atmosphere. Originally foreseen purely as an engineering subsystem for the real time provision of instantaneous altitude data in order to trigger near-surface measurements by the camera and other instruments, it was soon realized that several scientifically useful parameters could be derived from the radar surface return. Indeed, in principle the probe radar altimeter offers some of the highest-spatial-resolution radar data on Titan’s surface, owing to its operation at ranges ∼20 to ∼500 times closer than the Cassini orbiter’s instrument. Previous space probes have carried radar altimeters which have also yielded planetary surface information, for example a topography profile indicated during the entry and descent of Viking (Withers et al., 2002) and the lunar radar reflectivity estimated from the altimeter on Surveyor 1 (Parkes, 1966, Brown, 1967). Such measurements are of particular interest on worlds with thick atmospheres where the surface is obscured from remote optical observation – e.g. the radar altimeter on Venera 8 was used to estimate a terrain profile and ground reflectivity (Bashmashnikov et al., 1976).
Three principal datasets were recorded by the Huygens altimeter. First is the instantaneous altitude determined in the instrument itself used for on-board sequencing. Second, an internal parameter, the voltage of the Automatic Gain Control (AGC) was recorded: this is in effect a measure of echo signal strength and thus, indirectly, of the reflectivity of the Titan surface beneath the probe. These two quantities were recorded as housekeeping telemetry by the Huygens probe Command and Data Management Unit (CDMU) system. Third, the radar Intermediate Frequency (IF) signal within the altimeter was sampled by the Huygens Atmospheric Structure Instrument’s Permittivity and Wave Analyzer (HASI/PWA) where both hardware processing and digital signal processing has been performed. This processing (e.g. Fig. 9 of Grard et al., 2006) allows the measured altitude to be calculated to a higher precision, post flight, than possible by the radar unit itself via its real time interface to the probe and allows recovery of other information.
Although a brief review of the altimeter operation was described a ‘lessons learned’ paper (Trautner et al., 2006) and in the industrial flight operation report (Couzin, 2006) and some cursory plots of the radar altimeter data have been presented with minimal commentary in a few papers (e.g. Fig. 8 of Fulchignoni et al., 2005; Fig. 8 of Grard et al., 2006), a systematic close study of the data have not been published until now, in part because certain nonideal aspects of the altimeter demanded on-ground calibration measurements, and in part because observations of Titan’s surface by the Cassini orbiter in 2005/2006 were too sparse to properly interpret the Huygens altimeter results in context. This paper examines the altimeter data with the benefit of a decade of Cassini observations, and documents the data in the archive: we refer to the NASA Planetary Data System (PDS) records, but note that the data are mirrored on the ESA Planetary Science Archive (PSA).
Section snippets
Huygens radar altimeter operation
The Radar Altimeter system consists of two separate and independent units (Radar Altimeter Units A and B – also referred to in some documentation as ‘Proximity Sensor’ A and B), described in some detail in Hughes (1994). The probe command and data handling systems, including the altimeters, were fully redundant. Each system uses independent 15 cm by 15 cm slotted wave-guide antennas, one to receive and one to transmit – using separate antennas (Fig. 1) for transmit and receive functions
Flight data
Although the original requirement for altitude triggering was only for detection of the 10 km altitude mark, the altimeters were turned on early (at an altitude of about 60 km) since it was recognized the results might be of scientific interest. In flight, the first signals from the Titan surface were received at an altitude of about 45 km (see Fig. 3), although detections were intermittent. Below an altitude of 28 km both radars were continuously locked on the surface return. Due to an omitted
Altitude history
If the Titan atmosphere were unknown, then the descent rate of the probe and parachute with known aerodynamic characteristics could be used to infer an atmospheric density profile. However, this is more accurately determined with direct measurements of temperature and pressure, as performed on Huygens.
In a smoothly hydrostatic atmosphere (without vertical winds) the only small-scale variation in the range history to the surface will be that due to surface topography. Subtracting a smoothed
Comparison with Cassini orbital data
The physiography and spectral properties of the landing site observed in the near-infrared have been reported previously (e.g. Rodriguez et al., 2006, Soderblom et al., 2007); Synthetic Aperture Radar (SAR) observations by the Cassini radar were discussed by Lunine et al. (2008) and Soderblom et al. (2007).
The Cassini Saturn orbiter generally obtains most of its data in a side-looking mode, but short (∼300 km) and occasionally long (>1000 km) tracks of nadir-pointed altimetry data are obtained
Conclusions
The data from the Huygens probe radar altimeters have been reviewed. Interpretation of the altitude history seems consistent with a ∼100 m high bright highland at the beginning of the altimeter-observed part of descent, and with a small (few tens of m) variation within the dark area the probe landed in. Another ∼50 m variation over the dark region may be terrain, or may be related to vertical winds. The terrain height variations are consistent with topographic profiles recorded in this area by
Acknowledgments
The authors would like to thank J. Ylinen and N. Hughes and the staff of Ylinen electronics Ltd. for their cooperation during the development of the radar instrument and during the balloon and helicopter tests performed. Dr. M. Tomasko and the DISR team are thanked for valuable discussions on the data interpretation. M. Fluchignoni is thanked for accommodating the IF processing in the HASI experiment. The Huygens archive stands as a lasting monument to the efforts of those involved, and the PDS
References (36)
Huygens Probe descent dynamics inferred from Channel B signal level measurements
Planet. Space Sci.
(2007)A stratospheric balloon experiment to test the Huygens atmospheric structure instrument (HASI)
Planet. Space Sci.
(2004)Electric properties and related physical characteristics of the atmosphere and surface of Titan
Planet. Space Sci.
(2006)Titan’s surface at 2.2-cm wavelength imaged by the Cassini RADAR Radiometer: Calibration and first results
Icarus
(2009)DISR imaging and the geometry of the descent of the Huygens probe within Titan’s atmosphere
Planet. Space Sci.
(2007)TRMM precipitation radar
Adv. Space Res.
(2000)Radar-bright channels on Titan
Icarus
(2010)Attitude and angular rates of planetary probes during atmospheric descent
Planet. Space Sci.
(2010)Dunes on planet Tatooine: Observation of barchan migration at the Star Wars film set in Tunisia
Geomorphology
(2013)Titan dune heights retrieval by using Cassini Radar Altimeter
Icarus
(2014)
Titan’s surface from Cassini RADAR SAR and high resolution radiometry data of the first five flybys
Icarus
Cassini/VIMS hyperspectral observations of the HUYGENS landing site on Titan
Planet. Space Sci.
Correlations between Cassini VIMS spectra and RADAR SAR images: Implications for Titan’s surface composition and the character of the Huygens Probe Landing Site
Planet. Space Sci.
Determining Titan surface topography from Cassini SAR data
Icarus
Comparison of viking lander descent data and MOLA topography reveals kilometre-scale offset in Mars atmosphere profiles
Icarus
Analysis and interpretation of cassini Titan radar altimeter echoes
Icarus
Characteristics of the surface of Venus measured by radioaltimeter in the landing area of the Venera 8 descent vehicle
Cosm. Res.
Lunar surface surveyor radar response
J. Geophys. Res.
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