Concept Development for Advanced Spaceborne Synthetic Aperture Radar

NASA Goddard Space Flight Center (GSFC) has developed a new approach that uses a single phased array antenna and a single pass configuration to generate interferograms, known as Digital Beamforming Interferometry. A digital beamforming radar system allows the implementation of non-conventional radar techniques, known as Digital Beamforming Synthetic Aperture Radar Multi-mode Operation (DBSAR). DBSAR is an L-Band airborne radar that combines advanced radar technology with the ability to implement multimode remote sensing techniques, including several variations of SAR, scatterometry over multiple beams, and an altimeter mode. The Multiple channel data acquired with a digital beamformer systems allows the synthesis of beams over separate areas of the antenna, effectively dividing the single antenna into two antennas. The InSAR technique is then achieved by generating interferograms from images collected with each of the antennas. Since the technique is performed on the data, it allows for synthesizing beams in different directions (or look angles) and performs interferometry over large areas. Digital Beamforming Interferometry has potential in many areas of radar applications. For example, NASA GSFC innovators developed the first P-Band Digital Beamforming Polarimetric Interferometric SAR Instrument to measure ecosystem structure, biomass, and surface water.

NASA's SDR uses Field-Programmable Gate Array (FPGA) technology to enable flexible performance on orbit. A first-generation FM-modulated transceiver is capable of operating at up to 1 Mbps downlink and 50 kbps uplink, full duplex. An FPGA performs Reed-Solomon (255,223) encoding, decoding, and bit synchronization, providing Consultative Committee for Space Data Systems (CCSDS) and Near Earth Network (NEN) telemetry protocol compatibility. The transceiver accepts data from the onboard flight computer via a source synchronous RS422 interface. NASA's second-generation full duplex SDR, known as PULSAR (programmable ultra-lightweight system-adaptable radio, Figures 1 and 2 below) incorporates command receiver and telemetry transmitters, as well as updated processing and power capabilities. An S-band command receiver offers a max uplink data rate of 300 Kbps and built-in QPSK demodulation. X- and S-Band telemetry transmitters offer a max downlink data rate of 150 Mbps and flexible forward-error correction (FEC) using Reed-Solomon encoding (LDPC rate 7/8 and 1/2 convolution in development), and it uses QPSK modulation. The use of FEC adds an order of magnitude increase in telemetry throughput due to an improved coding gain. An onboard FPGA uses high-speed logic for uplink/downlink and encoding/decoding processes. Balloon flight testing has been conducted and is ongoing for PULSAR.

Langley has developed various technologies to enable the portable detection system, including: - 3-inch electret condenser microphone - unprecedented sensitivity of -45 dB/Hz - compact nonporous windscreen - suitable for replacing spatially demanding soaker hoses in current use - infrasonic calibrator for field use - piston phone with a test signal of 110 dB at 14Hz. - laboratory calibration apparatus - to very low frequencies - vacuum isolation vessel - sufficiently anechoic to permit measurement of background noise in microphones at frequencies down to a few Hz - mobile source for reference - a Helmholtz resonator that provides pure tone at 19 Hz The NASA system uses a three-element array in the field to locate sources of infrasound and their direction. This information has been correlated with PIREPs available in real time via the Internet, with 10 examples of good correlation.

This method begins with the optical seeking and ranging of a target satellite using LiDAR. Upon approach, the tumble rate of the target satellite is measured and matched by the approaching spacecraft. As rendezvous occurs the spacecraft deploys a robotic grappling arm or berthing pins to provide a secure attachment to the satellite. A series of robotic arms perform servicing autonomously, either executing a pre-programmed sequence of instructions or a sequence generated by Artificial Intelligence (AI) logic onboard the robot. Should it become necessary or desirable, a remote operator maintains the ability to abort an instruction or utilize a built-in override to teleoperate the robot.

The goal of COTSAT as a technology demonstration unit is to demonstrate the ability for drastic cost reduction in spacecraft design and to develop methods and technologies for maximizing reuse of developed spacecraft hardware, software and related technology on future missions. This approach will enable for rapid response capabilities given advances in rapid prototyping. COTSAT consists of the following sub-systems: - An artificial environment container, which comprises much of the satellite structure, is used to contain the single atmosphere environment. The artificial atmosphere container is used to replicate an Earth-like atmosphere, allowing the use of Commercial-Off-The Shelf (COTS) hardware and electronics which were not necessarily originally designed to operate in the vacuum environment of space. - A key design element in the bus structure of COTSAT is the modular platform upon which the bus is assembled. This structure allows for a logic-flow integration of components leading to ideal placement of electronics. - The Electrical Power System (EPS) architecture utilizes a distributed power and self-monitor approach. - The Command and Data Handling (C&DH) subsystem provides a number of critical capabilities, including spacecraft health and status monitoring, communication, payload science data management and subsystem management. - The COTSAT communications architecture incorporates four independent communications paths. - The software architecture consists of modular, independent software daemons for each subsystem or capability such as the star tracker, the Inertial Measurement Unit (IMU), the reaction wheels, the main executive, the communications system, the control system and the payload. - The COTSAT has a three-axis Attitude Determination And Control System (ADACS), using four reaction wheels and three magnetic torque coils. - To aid in technology development and testing, the COTSAT hardware and technology performance has been verified by a number of prototype test-beds. There have been three major test platforms during the development cycle.