A Millimeter-wave Phased Array Radar for Hazard Detection and Avoidance on Planetary Landers Brian D. Pollard, Gregory Sadowy, Delwyn Moller, and Emesto Rodriguez Jet Propulsion Laboratory California Institute of Technology 4800 Oak Grove Drive Pasadena, CA 91 109 818.354.7718 pollard@jpl.nasa.gov Abstract- Safe, precise landing in difficult planetary terrain, including areas that are rocky, heavily sloped, or both, requires remote sensing of the surface in order to choose an appropriate landing site. While active or passive optical sensors might seem appropriate for this application, we are developing an alternate approach based on a millimeter-wave electrically scanned phased array radar. A radar offers several advantages over present optical sensors, including a substantial reduction in dust or engine plume susceptibility, a larger range of operating altitudes, and a coherent measurement of the platform velocity. In this paper, we describe the overall system design for a radar being developed for the NASA Mars Science Laboratory, set to launch in 2009. We discuss the terminal descent scenario, and the requirements imposed by the terminal guidance and landing vehicle hazard tolerance. We present the digital elevation and roughness map generation performance of our candidate sensor as derived from terminal descent simulations over synthetic Mars terrain, where roughness is derived from a optimal threshold crossing algorithm. We also present a novel, fast velocity vector retrieval algorithm that avoids the necessity of inverting the entire phase matrix, making it ideal for onboard computation. Finally, we describe our upcoming hardware development efforts, including the development of a system test bed that will be used in the prototype sensor. This system is capable of the required 1 GHz bandwidth and five hops in center frequency. Our future development and testing plans will use this test bed to develop a full millimeter-wave phased array, leading to a unique hazard detection sensor for this exciting mission. TABLE OF CONTENTS 1. INTRODUCTION ................................ 1 2. TERMINAL DESCENT SCENARIO .......... 1 3. SYSTEM REOUIREMENTS .................... 2 4. BASIC SYSTEM ARCHITECTURE .......... 3 REFERENCES ....................................... 8 '0-7803-723 1 -X/01/$10.00/@3 2002 IEEE IEEEAC PAPER #I 188, UPDATED OCT 14,2002 I 1. INTRODUCTION Past lunar and martian landing missions have actively sensed the landing surface during the terminal descent phase, but have uniformly done so with respect to obtaining altitude and / or velocity information. Nearly all recent landing missions, including the Mars Viking Mission (1 976), Mars Pathfinder (1997), Mars Polar Lander (1999), and the upcoming twin Mars Exploration Rover (set to launch in 2003) made use of radar altimetry; Viking and Mars Polar Lander were further distinguished by the use of radar for sensing of the lander velocity vector. While sensing of the lander altitude and velocity ~IC necessary portions of a descent and landing system, safe landing in difficult terrain may also require imaging of the landing surface and detection of hazards. The upcoming Mars Science Laboratory (MSL) mission, set to launch in 2009, is currently expected to include such an imaging sensor as a part of an active hazard avoidance system. Proposed imagers for that system include active and passive optical systems, and the altemative described in this paper, an active millimeter wave phsaed array radar. In the following sections we describe a nominal terminal descent scenario for the MSL mission, and discuss the appropriate requirements imposed by the terminal guidance and landing vehicle. We then present a basic radar system architecture designed to meet those requirements, and describe the necessary algorithms and available products, including the sensor altitude, digital terrain maps of the landing surface, digital roughness maps of the landing surface, and the spacecraft velocity vector. We conclude with a discussion of the key technologies required for sensor development, and discuss present and planned hardware prototype and system verification efforts. 2. TERMINAL DESCENT SCENARIO The basic terminal descent scenario is discussed fully in [l] (and the references therein), and summarized in Figure 1. We briefly summarize the scenario here in order to establish context for the radar, and to motivate the requirements discussion below. 1