The terminal descent sensor (TDS) is a radar altimeter/velocimeter that improves the accuracy of velocity sensing by more than an order of magnitude when compared to existing sensors. The TDS is designed for the safe planetary landing of payloads, and may be used in helicopters and fixed-wing aircraft requiring highaccuracy velocity sensing.
The TDS uses 35.75-GHz frequency to optimize accuracy without requiring new technology, and incorporates a millimeter- wave center frequency to eliminate angle-of-arrival errors that can result in large velocity errors over non-homogeneous terrain. A memory-less approach to altimetry reacquires the target on each beam for each unique measurement, overcoming problems of ambiguous measurements or high dynamics that have plagued previous altimeter designs. The independent beam-to-beam and repeatbeam performance avoids “loss of lock” problems, as well as any issue where the heat shield, or an anomaly of some sort, might put the radar in a false state.
The “sky-crane” concept developed for the 2009 Mars Science Laboratory (MSL) mission allows the delivery of much larger payloads than the previously developed airbag landing methods, and it overcomes the problems of egress that pallet landers traditionally have faced. The system requires high-accuracy velocity on a minimum of three independent beams, high-accuracy slant range measurements on all velocimeter beams, and performance over an aggressive range of vehicle dynamics, including high attitude excursions, high attitude rates, and high attitude vehicle velocities. Also necessary are knowledge and control of the touchdown vehicle velocity: the MSL rover requires less than 1.5-m/s vertical and 0.75-m/s horizontal velocities at touchdown. This altimeter/velocimeter innovation can meet these needs, enabling the skycrane concept.
At the time of this reporting, the TDS was in breadboard form, and was a single-channel, Ka-band model created with a commercial- off-the shelf (COTS) antenna, connectorized RF components, miniature Ka-band RF hybrids in small, connectorized packages for the T/R module, and a LabVIEW/laptop interface. The RF design is shown in the figure. The equipment has been verified with bench testing that included shortpulse generation, Doppler/velocity product generation, FPGA (field-programmable-gate-array) timing, RF power levels, and RF passband response.
This work was done by Brian Pollard, Andrew Berkun, Michael Tope, Constantine Andricos, Joseph Okonek, and Yunling Lou of Caltech for NASA’s Jet Propulsion Laboratory. NPO-44462.