Greg Sadowy

UAVSAR: a new NASA airborne SAR system for science and technology research

NASAs Jet Propulsion Laboratory is currently building a reconfigurable, polarimetric L-band synthetic aperture radar (SAR), specifically designed to acquire airborne repeat track SAR data for differential interferometric measurements. Differential interferometry can provide key deformation measurements, important for studies of earthquakes, volcanoes and other dynamically changing phenomena. Using precision real-time GPS and a sensor controlled flight management system, the system will be able to fly predefined paths with great precision. The expected performance of the flight control system will constrain the flight path to be within a 10 m diameter tube about the desired flight track. The radar will be designed to be operable on a UAV (unpiloted aerial vehicle) but will initially be demonstrated on a on a NASA Gulfstream III. The radar will be fully polarimetric, with a range bandwidth of 80 MHz (2 m range resolution), and will support a 16 km range swath. The antenna will be electronically steered along track to assure that the antenna beam can be directed independently, regardless of the wind direction and speed. Other features supported by the antenna include elevation monopulse and pulse-to-pulse re-steering capabilities that will enable some novel modes of operation. The system will nominally operate at 45,000 ft (13800 m). The program began as an Instrument Incubator Project (IIP) funded by NASA Earth Science and Technology Office (ESTO).

The UAVSAR instrument: Description and first results

The UAVSAR instrument, employing an L-band actively electronically scanned antenna, had its genesis in the ESTO Instrument Incubator Program and after 3 years of development has begun collecting engineering and science data. System design was motivated by solid Earth applications where repeat pass radar interferometry can be used to measure subtle deformation of the surface, however flexibility and extensibility to support other applications were also major design drivers. In fact a Ka-band single-pass radar interferometer for making high precision topographic maps of ice sheets is being developed based to a large extent on components of the UAVSAR L-band radar. By designing the radar to be housed in an external unpressurized pod, it has the potential to be readily ported to many platforms. Initial testing is being carried out with the NASA Gulfstream III aircraft, which has been modified to accommodate the radar pod and has been equipped with precision autopilot capability developed by NASA Dryden Flight Research Center. With this the aircraft can fly within a 10 m diameter tube on any specified trajectory necessary for repeat-pass radar interferometric applications. To maintain the required pointing for repeat-pass interferometric applications we have employed an actively scanned antenna steered using INU measurement data. This paper presents a brief overview of the radar instrument and some of the first imagery obtained from the system.

Status of a UAVSAR designed for repeat pass interferometry for deformation measurements

NASAs Jet Propulsion Laboratory is currently implementing a reconfigurable polarimetric L-band synthetic aperture radar (SAR), specifically designed to acquire airborne repeat track interferometric (RTI) SAR data, also known as differential interferometric measurements. Differential interferometry can provide key deformation measurements, important for the scientific studies of earthquakes and volcanoes. Using precision real-time GPS and a sensor controlled flight management system, the system will be able to fly predefined paths with great precision. The expected performance of the flight control system will constrain the flight path to be within a 10 m diameter tube about the desired flight track. The radar will be designed to operate on a UAV (unpiloted aerial vehicle) but will initially be demonstrated on a minimally piloted vehicle (MPV), such as the Proteus built by scaled composites or on a NASA Gulfstream III. The radar design is a fully polarimetric with an 80 MHz bandwidth (2 m range resolution) and 16 km range swath. The antenna is an electronically steered along track to assure that the actual antenna pointing can be controlled independent of the wind direction and speed. Other features supported by the antenna include an elevation monopulse option and a pulse-to-pulse resteering capability that will enable some novel modes of operation. The system will nominally operate at 45,000 ft (13800 m). The program began out as an Instrument Incubator Project (IIP) funded by NASA Earth Science and Technology Office (ESTO).

UAVSAR: New NASA Airborne SAR System for Research

NASAs Jet Propulsion Laboratory is currently building a reconfigurable, polarimetric L-band synthetic aperture radar (SAR), specifically designed to acquire airborne repeat track SAR data for differential interferometric measurements. Differential interferometry can provide key deformation measurements, important for studies of earthquakes, volcanoes, and other dynamically changing phenomena. Using precision real-time GPS and a sensor controlled flight management system, the system will be able to fly pre-defined paths with great precision. The expected performance of the flight control system will constrain the flight path to be within a 10 m diameter tube about the desired flight track. The radar will be designed to be operable on a Unpiloted Arial Vehicle (UAV) but will initially be demonstrated on a NASA Gulfstream III. The radar will be fully polarimetric, with a range bandwidth of 80 MHz (2 m range resolution), and will support a 16 km range swath. The antenna will be electronically steered along track to assure that the antenna beam can be directed independently, regardless of the wind direction and speed. Other features supported by the antenna include elevation monopulse and pulse-to-pulse re-steering capabilities that will enable some novel modes of operation. The system will nominally operate at 45,000 feet (13,800 m). The program began as an Instrument Incubator Project (IIP) funded by NASA Earth Science and Technology Office (ESTO).

The UAVSAR phased array aperture

The development of a microstrip patch antenna array for an L-band repeat-pass interferometric synthetic aperture radar (InSAR) is discussed in this paper. The instrument will be flown on an unmanned aerial vehicle (UAV) and will provide accurate topographic maps for Earth science by 2007. The antenna operates at a center frequency of 1.2575 GHz and with a bandwidth of 80 MHz, consistent with a number of radar instruments that JPL has previously flown. The antenna is designed to radiate orthogonal linear polarizations for fully-polarimetric measurements. Beam-pointing requirements for repeat-pass SAR interferometry necessitate electronic scanning in azimuth over a range of plusmn20degrees in order to compensate for aircraft yaw. Beam-steering is accomplished by transmit/receive (T/R) modules and a beamforming network implemented in a stripline circuit board. This paper focuses on the electromagnetic design of the antenna tiles and associated interconnects. An important aspect of the design of this antenna is that it has an amplitude taper of 10dB in the elevation direction. This is to reduce multipath reflections from the wing that would otherwise be detrimental to interferometric radar measurements. The amplitude taper is provided by coupling networks in the interconnect circuits as opposed to using attenuators in the T/R modules. Details are given of material choices and fabrication techniques that meet the demanding environmental conditions that the antenna must operate in. Predicted array performance is reported in terms of co-polarized and cross-polarized far-field antenna patterns, and also in terms of active reflection coefficient. Measured performance of a 4-element by 2-element antenna tile is presented

An active membrane phased array radar

We have developed the first membrane-based active phased array in L-band (1.26GHz). The array uses membrane-compatible transmit/receive (T/R) modules (membrane T/R) for each antenna element. We use phase shifters within each T/R module for electronic beam steering. We discuss the T/R module design and integration with the membrane. We also present transmit and receive beam-steering results for the array.

Aperture-coupled thin-membrane microstrip array antenna for beam scanning application

A microstrip array using an aperture-slot-coupling technique with very thin membranes has been developed at the L-band frequency for a beam scanning application. This technology-demonstration array with 4/spl times/2 elements achieved a relatively wide bandwidth of 100 MHz (8%) and /spl plusmn/45/spl deg/ beam scan. Very narrow coupling slots were used with each having an aspect ratio of 160 (conventional slot aspect ratio is between 10 to 30) for coupling through a very thin membrane (0.05 mm thickness). This thin-membrane aperture-coupling technique allows the array antenna elements to be more easily integrated with transmit/receive amplifier (T/R) and phase shifter modules. The paper addresses only the radiator portion of the array. The array and active components will be presented in a separate paper.