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 high accuracy 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 repeat beam 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 short-pulse 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.
This Brief includes a Technical Support Package (TSP).
Ka-Band Radar Terminal Descent Sensor
(reference NPO-44462) is currently available for download from the TSP library.
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Overview
The document discusses the development and capabilities of the Ka-band Radar Terminal Descent Sensor (TDS), designated as NPO 44462, created by NASA's Jet Propulsion Laboratory (JPL) for the Mars Science Laboratory (MSL) mission. The motivation behind this innovation stems from the need for precise measurements during the landing phase of planetary missions, particularly for the MSL's sky-crane landing technique. This method requires high accuracy in velocity and slant range measurements across multiple beams, as well as robust performance under various dynamic conditions, including high vehicle velocities and attitude changes.
Existing altimeters and velocimeters used in previous planetary landers and aircraft were found inadequate to meet these stringent requirements. The JPL's TDS addresses these challenges by utilizing a Ka-band center frequency and employing unique antennas for each of its 5-8 beams. The sensor operates with short, independent measurement dwells on each beam and incorporates pulse-Doppler processing, which enhances its performance.
A significant advancement of the TDS is its ability to improve velocity sensing accuracy by more than an order of magnitude compared to previous sensors. The use of a millimeter-wave center frequency helps eliminate angle-of-arrival errors that can lead to substantial velocity inaccuracies over uneven terrain. Additionally, the TDS employs a memory-less approach to altimetry, allowing it to re-acquire targets for each unique measurement, thus overcoming issues related to ambiguous measurements and high dynamics that have historically affected altimeter designs.
The document also references an upcoming publication that will provide further details on the radar terminal descent sensor's development and its application in the MSL mission. This innovation not only enhances the safety and reliability of planetary landings but also represents a significant leap forward in radar technology for aerospace applications.
Overall, the Ka-band Radar Terminal Descent Sensor exemplifies JPL's commitment to advancing space exploration technologies, ensuring that future missions can achieve their objectives with greater precision and safety. The document serves as a technical support package, highlighting the sensor's capabilities and the potential for broader technological, scientific, and commercial applications stemming from this research.
