Curtis W. Chen

Performance assessment of along-track interferometry for detecting ground moving targets

Along-track interferometry (ATI) is an interferometric synthetic aperture radar technique that can be used to measure Earth-surface velocities. As such, the ATI technique holds promise for the detection of slowly moving ground targets. However, the models often used to characterize ATI performance were developed mainly in the context of mapping ocean currents, and they do not necessarily apply to the case of discrete, moving ground targets amidst clutter. We provide expressions for more accurately modeling the behavior of an ATI system in the context of ground moving target indication. Analysis and design equations are given for topics including target defocus, signal-to-noise and signal-to-clutter ratios, interferometric correlation, interferometric phase bias, target detection, geolocation accuracy, and area coverage rate.

Radar options for global earthquake monitoring

Fine temporal sampling is essential for disaster management, e.g. of flooding, fires, landslides, hurricanes, and earthquakes. A powerful technique for mapping such natural hazards is synthetic aperture radar (SAR) interferometry, providing displacement measurements at the subwavelength scale and decorrelation estimates. Pre-seismic deformation, one of the most elusive aspects of earthquakes, will require much finer temporal sampling than present InSAR capabilities provide. Observations taken every few hours could provide time series data of rapidly evolving phenomena, such as pre-eruptive volcano dynamics, leading to major advances in predictive capability, improving the potential for modeling as well as for civil protection. Such radical performance improvements could be attained through large constellations of conventional low Earth orbit (LEO) satellites or small constellations of geosynchronous SARs. The unique capability of a geosynchronous SAR in terms of instantaneously accessible area is contrasted with the requirements for huge electronically steered array (ESA) antennas. The optimal approach is very much dependant on technological developments, in particular geosynchronous SAR depends on the development of affordable very large ESA antennas, but also other technological developments will be required.

Measurement and mitigation of the ionosphere in L-band Interferometric SAR data

Satellite-based repeat-pass Interferometric Synthetic Aperture Radar (InSAR) provides a synoptic high spatial resolution perspective of Earths changing surface, permitting one to view large areas quickly and efficiently. By measuring relative phase change from one observation to the next on a pixel-by-pixel basis, maps of deformation and change can be derived. Variability of the atmosphere and the ionosphere leads to phase/time delays that are present in the data that can mask many of the subtle deformation signatures of interest, so methods for mitigation of these effects are important. Many of these effects have been observed in existing ALOS PALSAR data [1], and studies are underway to characterize and mitigate the ionosphere using these data. Since the ionosphere is a dispersive medium, it is possible in principle distinguish the ionospheric signatures from the non-dispersive effects of deformation and the atmosphere. In this paper, we describe a method for mapping the ionosphere in InSAR data based on a multi-frequency split-spectrum processing technique. We examine a number of PALSAR data sets, including fully polarimetric and single-polarization 28 MHz bandwidth data, where anomalous effects in phase, amplitude and image registration have been observed. We demonstrate the estimation of the ionosphere by means of the split spectrum technique for estimating differential TEC, whereby a radar waveform is transmitted over the full PALSAR spectral band and widely separated portions of the receive spectrum are processed independently and compared for dispersive effects, and quantify its performance.

System concepts and technologies for high orbit SAR

This paper discusses large aperture, high orbit radar concepts for measuring sub-centimeter-level surface displacements from space. These measurements will enable applications such as earthquake simulation, modeling and forecasting. We explain the need for large aperture, high orbit arrays and discuss the technologies required to achieve these missions.

Impact of the ionosphere on an L-band space based radar

We have quantified the impact that the ionosphere would have on a L-band interferometric synthetic aperture radar (SAR) mission using a combination of simulation, modeling, Global Positioning System (GPS) data collected during the last solar maximum, and existing spaceborne SAR data. Using the Jet Propulsion Laboratorys Global Ionospheric Maps (GIM) total electron content (TEC) estimates derived from the worldwide array of GPS stations, we determined that the sun synchronous orbit which would minimize TEC at the time of imaging has dawn and dusk equator crossings. Such an orbit also avoids the equatorial post-sunset irregularities. We used the GIM data to examine the day-to-day variability in the background ionosphere and to quantify the impact of the background ionosphere on single pass SAR performance. With the exception of Faraday rotation related effects on single polarization systems, degradation due to the background ionosphere can be avoided if a reasonable model for the ionosphere is used during processing. Our studies reveal that Faraday rotation angles rarely exceeded the 10/spl deg/ threshold that impacts biomass retrieval and that repeat pass interferometric SAR decorrelation due to variations in the background ionosphere causing variable Faraday rotations is a negligible effect. Even a dawn-dusk orbit will not avoid high latitude ionospheric irregularities. We evaluated the strength of the ionospheric irregularities using GPS scintillation data collected at Fairbanks, Alaska and modeled the impact of these irregularities on azimuth resolution, azimuth displacement, peak sidelobe ratio (PSLR), and integrated sidelobe ratio (ISLR). Our examination of ionospheric artifacts in InSAR data has revealed that the artifacts occur primarily in the polar cap data, not auroral zone data as was previously thought.

A Radar Terminal Descent Sensor for the Mars Science Laboratory mission

The soft-touchdown, “sky-crane” concept employed by the 2009 NASA Mars Science Laboratory mission requires an order-of-magnitude improvement from previous missions in the sensing of vehicle velocity and altitude. This paper describes the development of a new radar “Terminal Descent Sensor” that provides decimeterper-second velocity accuracy while also providing better than 2% range accuracy on six unique beams. This sensor design uses a millimeter-wave center frequency (Ka-band) and pencil beam antennas to achieve the required velocity precision and to overcome the problems that angle-of-arrival errors can cause in velocity reconstruction. Included are discussions of the design concept, driving requirements, hardware architecture, and results from a high fidelity performance simulation.

SMAP L-Band Microwave Radiometer: Instrument Design and First Year on Orbit

The Soil Moisture Active–Passive (SMAP) L-band microwave radiometer is a conical scanning instrument designed to measure soil moisture with 4% volumetric accuracy at 40-km spatial resolution. SMAP is NASA’s first Earth Systematic Mission developed in response to its first Earth science decadal survey. Here, the design is reviewed and the results of its first year on orbit are presented. Unique features of the radiometer include a large 6-m rotating reflector, fully polarimetric radiometer receiver with internal calibration, and radio-frequency interference detection and filtering hardware. The radiometer electronics are thermally controlled to achieve good radiometric stability. Analyses of on-orbit results indicate that the electrical and thermal characteristics of the electronics and internal calibration sources are very stable and promote excellent gain stability. Radiometer NEDT < 1 K for 17-ms samples. The gain spectrum exhibits low noise at frequencies >1 MHz and 1/f noise rising at longer time scales fully captured by the internal calibration scheme. Results from sky observations and global swath imagery of all four Stokes antenna temperatures indicate that the instrument is operating as expected.

RFI Characterization and Mitigation for the SMAP Radar

The Soil Moisture Active-Passive (SMAP) mission will launch in late 2014 and will carry a combined L-band radiometer/radar instrument for the retrieval of global soil moisture and surface freeze-thaw state. Radio frequency interference (RFI) is a known challenge for Earth remote sensing in the L-band portion of the spectrum. This paper addresses efforts to characterize and mitigate RFI for the SMAP radar. A model for the RFI environment due to surface-based emitters is developed, and is shown to agree well with the observations of currently operating L-band radar systems. An analysis of the environment due to space-based emitters is also presented. Techniques to mitigate RFI in the radar band are described, and are shown to perform sufficiently well to meet the stringent SMAP measurement requirements. A companion paper addresses the different issues encountered with RFI in the radiometer band.

MEO SAR System Concepts and Technologies for Earth Remote Sensing

Next-generation interferometric synthetic aperture radar (InSAR) systems may provide the basis for establishing an earthquake-forecasting capability within a twenty-year time frame. Such systems would need to provide data with fine temporal resolution, so the system architecture would need to allow for wide-area coverage in order to minimize the effective interferometric repeat time. This paper discusses the coverage advantages associated with medium-Earth orbit (MEO) InSAR systems for observing geophysical phenomena. As MEO architectures dictate the need for large radar antennas, this paper also presents a discussion of advanced antenna technologies—and associated challenges—that might provide revolutionary decreases in the mass densities of large radar aperture antennas.

Phoenix Landing Radar Heatshield Anomaly

This paper details a specific and unexpected problem faced by the Entry, Descent, and Landing team on the Phoenix Mars Mission that demonstrates two problems in mission architecture: 1) the difficulties inherent in adapting commercial-off-the-shelf technology to a system different from the product’s intended scope; and 2) the design of test environments sufficientlyflightlike as to reveal systemic problems in newarchitectures. Specifically, the interaction of the entry heat shield with the landing radar could not be tested in a flightlike manner; hence, detailed simulation of the landing radarwas conductedwith the expectation that the radar could lock on echoes from the heat shield and report an erroneous altitude, and that phenomenon could be handled with appropriate flight software. Surprisingly, it was discovered that the heat shield spoofed the radar’s internal logic that was designed to prevent locking on ambiguous radar returns. Mitigating the unexpected problem proved to be much more challenging than the anticipated heatshield lock scenario, ultimately requiring last-minute changes to both the radarfirmware and the flight software.