Status of a UAV SAR Designed for Repeat Pass Interferometry for Deformation Measurements Scott Hensley, Kevin Wheeler, Jim Hoffman, Tim Miller, Yunling Lou, Ron Muellerschoen, Howard Zebkert, Sgren Madsen and Paul Rosen Jet Propulsion Laboratory 'stanford University California Institute of Technology MC 9515 4800 Oak Grove Drive 334 Packard Electrical Engineering Pasadena, California 91 109 Palo Alto, California, 94305 Abstract - Under the NASA EST0 sponsored Instrument Incubator Program we have designed a light- weight, reconfigurable polarimetric L-band SAR designed for repeat pass deformation measurements of rapidly deforming surfaces of geophysical interest such as volcanoes or earthquakes. This radar will be installed on an unmanned airborne vehicle (UAV) or a lightweight, high-altitude, and long endurance platform such as the Proteus. After a study of suitable available platforms we selected the Proteus for initial development and testing of the system. We want to control the repeat track capability of the aircraft to be within a 10 m tube to support the repeat deformation capability. We conducted tests with the Proteus using real-time GPS with sub-meter accuracy to see if pilots could fly the aircraft within the desired tube. Our results show that pilots are unable to fly the aircraft with the desired accuracy and therefore an augmented autopilot will be required to meet these objectives. Based on the Proteus flying altitude of 13.7 km (45,000 ft), we are designing a fully polarimetric L-band radar with 80 MHz bandwidth and 16 km range swath. This radar will have an active electronic beam steering antenna to achieve Doppler centroid stability that is necessary for repeat-pass interferometry (RPI). This paper will present are design criteria, current design and expected science applications. I. INTRODUCTION The solid earth science community is seeking earth deformation measurements at a variety of scales, from seconds to decades. The NASA Solid Earth Science Working Group has recommended an observational program that includes both airborne and spaceborne capabilities and this is reflected in the NASA Earth Science Enterprise strategic plant. Ultimately, scientists would like to have earth deformation measurements on an hourly basis with global access, objectives best supported by a spaceborne high-orbit (e.g. geosynchronous) constellation of repeat-pass interferometric SAR satellites. The recommended first step in this observational program is a low-earth-orbit deformation satellite with a repeat period of roughly one week. The sub- orbital radar program enters the Earth Science Enterprise plan as a key supplemental capability, providing repeat-pass measurements at time scales much smaller than one week, potentially as short as twenty minutes. Understanding the time varying nature of rapidly deforming features such as some volcanoes and glaciers or deformation from post seismic transients requires observational sampling intervals of a day or less to capture and model such events. In addition to providing unprecedented temporal detail of deformation of dynamic processes, the suborbital radar will be a testbed for understanding the observational needs for how rapid repeat observations would be acquired. This is a capability that the currently operational NASA AIRSAR system has demonstrated but cannot practically support for science experiments in its current configuration due to lack of track repeatability and beam pointing limitations. A proposal was submitted to the NASA 2002 Instrument Incubator Program (IIP) to develop a repeat pass measurement capability as an augmentation to the existing AIRSAR system. NASA accepted the proposal but directed that the proposed capability be fielded on a UAV or MPV platform to support the long term interests of the airborne science community and that the first year effort be devoted to developing a radar system design and implementation plan. This paper presents the results of the first year of study and the resulting radar design. 11. PLATFORM SELECTION Reliable collection and processing of airborne repeat pass radar interferometric data for deformation measurements imposes additional platform and radar instrument constraints on a UAV platform not normally required by standard SAR