UAVSAR (Unmanned Aerial Vehicle Synthetic Aperture Radar)
UAVSAR is a NASA L-band SAR (Synthetic Aperture) compact pod-mounted polarimetric instrument for interferometric repeat-track observations that is being developed at JPL and at the NASA/DFRC (Dryden Flight Research Center) in Edwards, CA. 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 aircraft (C-20A/G-III). The system will nominally operate at altitudes of ~ 13,800 m. The program was initiated in the timeframe 2003/4 as an Instrument Incubator Project (IIP) funded by ESTO (Earth Science and Technology Office) of NASA. 1) 2) 3) 4) 5) 6)
The primary objective of the side-looking UAVSAR instrument is to accurately map crustal deformations associated with natural hazards, such as volcanoes and earthquakes. Topographic information is derived from phase measurements that, in turn, are obtained from two or more passes over a given target region. The frequency of operation, approximately 1.26 GHz, results in radar images that are well-correlated from pass to pass. Polarization agility facilitates terrain and land-use classification.
The design of the UAVSAR focuses on two key challenges:
• First, repeat pass measurements need to be taken from flight paths that are nearly identical. This instrument utilizes real-time GPS that interfaces with the platform flight management system (FMS) to confine the repeat flight path to within a 10 m tube over a 200 km course in conditions of calm to light turbulence. The FMS is also referred to as the PPA (Platform Precision Autopilot).
• Secondly, the radar vector from the aircraft to the ground target area must be similar from pass to pass. This is accomplished with an actively scanned antenna designed to support electronic steering of the antenna beam with a minimum of 1º increments over a range to exceed ±15º in the flight direction.
The UAVSAR radar is designed from the beginning as a miniaturized polarimetric L-band radar for repeat-pass and single-pass interferometry with options for along-track interferometry and additional frequencies of operation. By designing the radar to be housed in an external unpressurized pod, it has the potential to be readily ported to other platforms such as the Predator or Global Hawk UAVs. 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.
The UAVSAR project will also serve as a technology test bed. As a modular instrument with numerous plug-and-play components, it will be possible to test new technologies for airborne and spaceborne applications.
Figure 1: UAVSAR radar pod attached to GulfStream III, showing rectangular antenna radome (image credit: NASA/JPL)
Status of the UAVSAR implementation and campaign services:
• February 8, 2017: The Louisiana coastline is sinking under the Gulf of Mexico at the rate of about one football field of land every hour (about 18 square miles of land lost in a year). But within this sinking region, two river deltas are growing. The Atchafalaya River and its diversion channel, Wax Lake Outlet, are gaining about one football field of new land every 11 and 8 hours, respectively (1.5 and 2 square miles per year). Last fall, a team from NASA/JPL (Jet Propulsion Laboratory) in Pasadena, California, showed that radar, lidar and spectral instruments mounted on aircraft can be used to study the growing deltas, collecting data that can help scientists better understand how coastal wetlands will respond to global sea level rise. 7)
- The basics of delta building are understood, but many questions remain about how specific characteristics, such as vegetation types, tides, currents and the shape of the riverbed, affect a delta's growth or demise. That's partly because it's hard to do research in a swamp. "These factors are usually studied using boats and instruments that have to be transported through marshy and difficult terrain," said Christine Rains of JPL, an assistant flight coordinator for the program. "This campaign was designed to show that wetlands can also be measured with airborne remote sensing over a large area."
- JPL researchers fly over the Louisiana coastline at least once a year to keep track of subsidence (sinking) and changes in levees. The most recent airborne flights, however, focused on the growing deltas — specifically, flowing water and vegetation.
- JPL's Marc Simard, principal investigator for the campaign, explained that on a delta, water flows in every direction, including uphill. "Water flows not only through the main channels of the rivers but also through the marshes," he explained. "There is also the incoming tide, which pushes water back uphill. The tide enhances the flow of water out of the main channels into the marshes."
- When the tide goes out, water drains from the marshes, carrying sediment and carbon. The JPL instruments took measurements during both rising and falling tides to capture these flows. They also made the first complete measurement of the slope of the water surface and topography of the river bottom for both rivers from their origin at the Mississippi River to the ocean — necessary information for understanding the rivers' flow speeds.
- Some types of marsh vegetation resist flowing water better than others, as the new measurements have documented. Simard said, "We were really surprised and impressed by how the water level changes within the marshes. In some places, the water changes by 10 cm in an hour or two. In others, it's only 3 or 4 cm. You can see amazing patterns in the remote sensing measurements."
- Three JPL airborne instruments, flying on three planes, were needed to observe the flows and the movement of carbon with the water. The team measured rising and falling water in vegetated areas using the UAVSAR (Uninhabited Aerial Vehicle Synthetic Aperture Radar) instrument in October 2016. They measured the same changes in open water with the ASO (Airborne Snow Observatory ) lidar. The AVIRIS-NG (Airborne Visible/Infrared Imaging Spectrometer-Next Generation) was used to estimate the sediment, carbon and nitrogen concentrations in the water.
- Now that the team has demonstrated that these airborne instruments can make precise and detailed measurements in this difficult environment, the researchers plan to use the new data to improve models of how water flows through marshes. Scientists use these models to study how coastal marshes will cope with rising sea levels. With so many measurements available as a reality check, Simard said, "Our models will have to catch up with the observations now."
- In this context, view the growth of the Atchafalaya River and its diversion channel, the Wax Lake Outlet - satellite imagery over the last 30 years - presented in the Landsat-7 file at the entry: "October 12, 2015" under Mission status.
Figure 2: Aerial photograph of Atchafalaya Delta (image credit: photo by Arthur Belala, U.S. Army Corps of Engineers)
• May 25, 2016: New Orleans and its surrounding areas continue to sink at highly variable rates due to a combination of natural geologic processes and human activity, according to a new study using NASA airborne radar (UAVSAR). The observed rates of sinking, also known as subsidence, were generally consistent with, but somewhat higher than, previous studies conducted with radar data. 8) 9)
- The maps of Figures 3 and 4 show how much the ground in the New Orleans area sank (subsidence) or rose (uplift) relative to its 2009 elevation, plotted in millimeters per year. Shades of green depict lands that sank, while areas in purple rose. The maps depict data collected from June 2009 to July 2012 and analyzed by a team from NASA/JPL (Jet Propulsion Laboratory), the UCLA (University of California at Los Angeles), and LSU (Louisiana State University).
- The highest rates of sinking were observed upriver along the Mississippi River around major industrial areas in Norco and Michoud (Figure 3)—with up to 50 mm/year of sinking. The team also observed notable subsidence in New Orleans' Upper and Lower 9th Ward, and in Metairie, where the ground movement could be related to water levels in the Mississippi. At the Bonnet Carré Spillway—the city's last line of protection against springtime river floods—research showed as much as 40 mm/year of sinking behind the structure and at nearby industrial facilities.
- While the study cites many factors for the regional subsidence, the primary contributors are groundwater pumping and dewatering—surface water pumping to lower the water table, which prevents standing water and soggy ground. "Agencies can use these data to more effectively implement actions to remediate and reverse the effects of subsidence, improving the long-term coastal resiliency and sustainability of New Orleans," said JPL scientist and lead author Cathleen Jones. "The more recent land elevation change rates from this study will be used to inform flood modeling and response strategies."
- A key component of the study was data from NASA's UAVSAR (Uninhabited Aerial Vehicle Synthetic Aperture Radar), which uses a technique known as In SAR (Interferometric Synthetic Aperture Radar). InSAR compares radar images of Earth's surface acquired over the months and years to map surface deformation with centimeter-scale precision. The technique measures total surface elevation changes from all sources—human and natural, deep seated and shallow. UAVSAR's spatial resolution makes it ideal for measuring change in New Orleans, where human-caused subsidence can be large and often localized. "We were able to identify single structures or clusters of structures subsiding or deforming relative to the surrounding area," Jones said.
- In addition to the UAVSAR data, researchers from the Center for GeoInformatics (C4G) at LSU provided up-to-date GPS positioning information for industrial and urban locations in southeast Louisiana. This information helped establish the rate of ground movement at specific points. C4G maintains the most comprehensive network of GPS reference stations in the state, including more than 50 Continuously Operating Reference Stations, or CORS sites, that acquire the horizontal and vertical coordinates at each station every second of every day. CORS data help pin InSAR data down to specific, local points on Earth.
Figure 3: This map shows how much the ground in the New Orleans area sank (subsidence) or rose (uplift) relative to its 2009 elevation, plotted in millimeters per year. Shades of green depict lands that sank, while areas in purple rose (image credit: NASA Earth Observatory)
Figure 4: Detail map of New Orleans (image credit: NASA Earth Observatory)
• April 2013: A NASA C-20A piloted aircraft carrying the UAVSAR is wrapping up studies over the U.S. Gulf Coast, Arizona, and Central and South America. The plane left NASA's Dryden Aircraft Operations Facility in Palmdale, CA, on March 7. NASA's Jet Propulsion Laboratory in Pasadena built and manages UAVSAR.
- The campaign is addressing a broad range of science questions, from the dynamics of Earth's crust and glaciers to the carbon cycle and the lives of ancient Peruvian civilizations. Flights are being conducted over Argentina, Bolivia, Chile, Colombia, Costa Rica, El Salvador, Ecuador, Guatemala, Honduras, Nicaragua and Peru. 10)
• October 25, 2012: NASA/JPL researchers have developed a method to use a specialized NASA 3-D imaging radar to characterize the oil in oil spills, such as the 2010 BP Deepwater Horizon spill in the Gulf of Mexico. The research can be used to improve response operations during future marine oil spills. 11)
UAVSAR characterizes an oil spill by detecting variations in the roughness of its surface and, for thick slicks, changes in the electrical conductivity of its surface layer. Just as an airport runway looks smooth compared to surrounding fields, UAVSAR "sees" an oil spill at sea as a smoother (radar-dark) area against the rougher (radar-bright) ocean surface because most of the radar energy that hits the smoother surface is deflected away from the radar antenna. UAVSAR's high sensitivity and other capabilities enabled the team to separate thick and thin oil for the first time using a radar system.
Legend to Figure 5: The oil appears much darker than the surrounding seawater in the greyscale image. This is because the oil smoothes the sea surface and reduces its electrical conductivity, causing less radar energy to bounce back to the UAVSAR antenna. Additional processing of the data by the UAVSAR team produced the two inset color images, which reveal the variability of the oil spill's characteristics, from thicker, concentrated emulsions (shown in reds and yellows) to minimal oil contamination (shown in greens and blues). Dark blues correspond to areas of clear seawater bordering the oil slick.
• October 2, 2012: The modified NASA C-20A (G-III) aircraft, with JPL's UAVSAR has left California for a 10-day campaign to study active volcanoes in Alaska and Japan. 12)
• Summer 2011: Since commencing operational science observations in January 2009, the UAVSAR project has conducted over 150 flights acquiring 1700 flight lines of data in 12 countries. So far, the project delivered over 15 TB of POLSAR (Polarimetric SAR) and RPI (Repeat-Pass Interferometry) data products to the science investigators. 13) 14)
- With the current radar configuration onboard the NASA Gulfstream-III operated by NASA Dryden, the project has been conducting science observations including semi-annual observations of the San Andreas fault, monthly observations of the Sacramento Delta levees, annual observations of the Cascades, Aleutian, Hawaiian volcanoes, and science campaigns to Greenland, Iceland, Central America, and Canada to study the polar ice, volcanoes, earthquakes, tropical and temperate forests, and soil moisture.
- In 2011, the project received funding to add P-band polarimetry capability to UAVSAR to study subcanopy and subsurface soil moisture over a 3-year period. The plan is to replace the L-band active array antenna and frequency up/down-conversion electronics with a P-band passive antenna, high power amplifier, and corresponding frequency up/down-conversion electronics. Flight testing of the P-band configuration is planned for the spring of 2012.
- In addition, the two-year AITT (Airborne Instrument Technology Transition) GLISTIN-A (Airborne Glacier and Land Ice Surface Topography Interferometer) project of NASA is funding the development and integration of the Ka-band VV-polarization single-pass interferometry capability to UAVSAR. This involves the development and mounting of the Ka-band front-end electronics to the backplane of a newly developed Ka-band antenna. Flight testing of the Ka-band is also planned for the spring of 2012. Both existing UAVSAR electronics pods will be able to support any of the 3 radar frequency configurations to provide maximum flexibility to the airborne radar test-bed.
• On April 4, 2010, a major earthquake of magnitude 7.2 rocked Mexico's state of Baja California and parts of the American Southwest. The "El Mayor-Cucapah quake" was centered 52 km south-southeast of Calexico, CA, in northern Baja California. It occurred along a geologically complex segment of the boundary between the North American and Pacific tectonic plates. The quake, the region's largest in nearly 120 years, was also felt in southern California and parts of Nevada and Arizona. It killed two, injured hundreds and caused substantial damage. There have been thousands of aftershocks, extending from near the northern tip of the Gulf of California to a few miles northwest of the U.S. border. 15)
The UAVSAR aircraft provided SAR imagery of the earthquake region from overflights on Oct. 21, 2009 and April 13, 2010. A JPL science team used the data from the two overflights and generated interferograms of the earthquake region. Each UAVSAR flight serves as a baseline for subsequent quake activity. The team estimates displacement for each region, with the goal of determining how strain is partitioned between faults.
Figure 6: Overview of the UAVSAR interferogram of the magnitude 7.2 Baja California earthquake of April 4, 2010, overlaid atop a Google Earth image of the region (image credit: NASA/JPL)
• A nearly identical UAVSAR pod will be attached to a Global Hawk UAV platform for uninhabited operational flight tests. An additional pod containing another L-band antenna and GPS/INU unit will also be attached to the Global Hawk platform providing dual L-band data collection capability. 16) 17)
• In response to the Earthquake disaster in Haiti on Jan. 12, 2010, NASA has added a series of science overflights of earthquake faults in Haiti and the Dominican Republic on the island of Hispaniola to a previously scheduled three-week airborne radar campaign to Central America. 18)
Legend to Figure 7: The city is denoted by the yellow arrow; the black arrow points to the fault responsible for the Jan. 12, 2010 earthquake.
• The UAVSAR platform has been flown throughout most of California. Since November 2009, JPL scientists have collected data gathered on a number of Gulfstream III flights over California's San Andreas fault and other major California earthquake faults, a process that will be repeated about every six months for the next several years. From such data, scientists will create 3-D maps for regions of interest (Ref. 18).
• Observation flights on a campaign basis started in August 2009 in temperate and boreal forests. Three sites were located in New Hampshire, Maine and Québec. The UAVSAR sites covered Bartlett, Hubbard Brook, Penobscot, Howland and Mont-Morency experimental forests in addition to covering part of White Mountain National Forest, Laurentides Wildlife Reserve, Jacques-Cartier and Grand-Jardins National Parks. UAVSAR collected data over the 3 sites on 4 different days spread throughout the 11-day campaign. 20)
Figure 8: UAVSAR composite radar image around Québec City, Canada, during an 11-day campaign in August 2009 (image credit: NASA/JPL) 21)
• In early June 2009, NASA's UAVSAR team had completed all the objectives of the Arctic Ice Radar Mission in Greenland and flew to Keflavik International Airport (Iceland) to measure the topography and 3D surface velocities of the temperate ice caps of Iceland. Between June 10-14, 2009, NASA's Gulfstream III science research aircraft flew five repeat data flights over Iceland. 22)
• First engineering test flights on the Gulfstream III aircraft of the UAVSAR instrument started in September 2007 at Dryden. The sensor is undergoing a one-year development and test period to improve robustness and validate its ability to meet the science objectives. 23)
Legend to Figure 9: Within the caldera, the rising dome can be seen to the lower left. Two glaciers that are being pushed together as the dome rises are clearly visible to the upper right within the caldera. Although the UAVSAR data was collected when the peak was covered in snow, much detail is visible in the L-band radar imagery. The volcanic caldera dominates the image, with the dome visible to the lower left within the caldera. Two glaciers within the caldera, to the upper right of the dome, are being pushed together as the dome expands. The edge of Spirit Lake is at the upper center of the image and the tree line is visible in green. - Repeat pass images of Mt. St. Helens will be used to measure the dome deformation and the glacier movement.
Figure 10: Modified NASA Gulfstream III in early flight tests with the UAVSAR pod attached to the underside of the aircraft (image credit: NASA/DFRC)
The design features an AESA (Active Electronically Scanned Array) antenna which is electronically steered along-track to assure that the antenna beam can be directed independently, regardless of speed and wind direction. Other features supported by the antenna include elevation monopulse and pulse-to-pulse re-steering capabilities that will enable some novel modes of operation. 24)
From an implementation point of view, the objectives of the UAVSAR project are to:
- Develop a miniaturized polarimetric L-band SAR for use on an UAV
- Develop the associated processing algorithms for repeat-pass differential interferometric measurements
- Conduct measurements of geophysical interest, particularly changes of rapidly deforming surfaces such as volcanoes or earthquakes.
Figure 11: Illustration of the overall system configuration (image credit: NASA/DFRC)
Table 1: Key parameters of the UAVSAR instrument
Figures 11 and 12 provide an overview of the UAVSAR system elements and its main interfaces. The radar has been designed to minimize the number of interfaces with the aircraft for improved portability. The aircraft provides 28 V DC power to the radar via the PDU (Power Distribution Unit), which is also responsible for maintaining the thermal environment in the pod, and the radar provides its real-time DGPS position data to the aircraft for use by the Precision Autopilot. Waypoints for the desired flight paths are generated prior to flight by the G-III FMS (Flight Management Subsystem) and loaded into the PPA (Platform Precision Autopilot) as well as into the radar's ARC (Automatic Radar Controller) along with radar command information for each waypoint.
The ARC (Automatic Radar Controller) is the main control computer for the radar and controls all major functions of the radar during flight. It is designed to operate in a fully autonomous mode or to accept commands from the ROW (Radar Operator Workstation) either through an ethernet connection on crewed platforms or through an Iridium modem for unpiloted platforms.
The CTU (Control and Timing Unit) controls the timing of all the transmit and receive events in the radar timeline and thus interacts with many of the radar digital and radio frequency (RF) electronics. The active array antenna consists of 24 130 W L-band Transmit/Receive (TR) modules that feed 48 radiating elements within the 0.5 m by 1.5 m array. Figure 13 illustrates how the various electronic subsystems of UAVSAR are arranged within the pod.
Figure 12: Block diagram of the UAVSAR system including aircraft, real-time differential GPS, and ground system interfaces (image credit: NASA/JPL)
The UAVSAR system requires just under 2 kW of DC power when the radar is transmitting L-band polarimetric RTI (Repeat Track Interferometric) SAR measurements. This is well within the capacity of the Gulfstream III aircraft and many other platforms considered for hosting the UAVSAR radar. The standby DC power is on the order of 150 W. The active array antenna has a mass of < 50 kg; the mass of each T/R module is ~ 0.5 kg. The remainder of the radar electronics in the payload bay has a mass of < 100 kg (approximately 20 kg for the RFES (Radio Frequency Electronic Subsystem), 30 kg for the DES (Digital Electronic Subsystem), and 30 kg for cabling, power distribution, etc.). The combined instrument mass is < 230 kg.
Figure 13: Configuration of the L-band radar electronics and antenna within the pod structure (image credit: NASA/JPL)
PPA (Platform Precision Autopilot):
The PPA hardware components and interfaces are outlined in Figure 14. The main element is the AIC (Autopilot Interface Computer), a Phytec MPC565 single board computer operating at 56 MHz.
Figure 14: System architecture of the PPA (image credit: NASA/DFRC)
The PPA software is hosted on AIC providing interfaces to three external data sources, the differential GPS (dGPS), the DCAPS (Data Collection and Processing System), and the Operator Station. The AIC also outputs control signals to the ILS (Instrument Landing System), I2S (Interface System), which is in turn connected to the navigation receiver. The differential GPS receiver (dGPS) , developed at JPL, provides ECEF (Earth Centered Earth Fixed) positions in meters. It achieves high accuracy using two sources of GPS correction (Inmarsat and Iridium) and two differential GPS units. The dGPS position accuracy is estimated at 10 cm horizontally and 20 cm vertically (1σ) at 1 Hz.
For an observation flight, an experimenter may only select waypoints and the desired flight altitude - all other functions of platform control and navigation are being provided by the PPA.
DES (Digital Electronic Subsystem):
DES provides the overall timing and control signals for the radar as well as the telemetry and data acquisition functions. The ARC is providing coordination and control of all the radar subsystems. ARC can be operated in in two primary modes, manual mode and automatic mode. In automatic mode the ARC tracks the aircraft trajectory using data from the INU (Inertial Navigation Unit) and initiates power state transitions, data take start and end and other housekeeping functions based on proximity to predefined waypoints uploaded from the ROW prior to flight. The ARC flight software uses VxWorks as its operating system.
In addition to commanding the radar timing unit during science data collection, the ARC flight software handles the embedded GPS/INU telemetry collection, tracks the aircraft flight path, performs electronic beam steering calculations, updates attenuation parameters and monitors temperatures inside the pod.
The UAVSAR radar can operate in a number of complex modes including multi-polarization, multi-frequency and multi-antenna modes of operation.
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 ±20º 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 (Ref. 5).
The antenna aperture comprises 48 patch antenna elements arranged as an array of 4 elements in elevation by 12 elements in azimuth. The elevation spacing of the elements is 10 cm and the azimuth spacing is 12.5 cm. The corresponding aperture size is 0.4 m x 1.5 m, but the antenna groundplane is larger (0.6 m x 1.75 m) to accommodate the various antenna electronics subassemblies, and also to facilitate operation with existing P-band equipment (Ref. 5).
The antenna elements are single-layer microstrip patches (~ 8 cm x 8 cm in size) on a low-permittivity dielectric, fabricated from fiberglass honeycomb. The patch elements are fed with two probes for each polarization to provide the required bandwidth over scan. The elements are capable of radiating both horizontal (H) and vertical (V) polarization (Ref. 5).
There are 24 T/R modules feeding elements pair-wise in elevation. This architecture facilitates beam scanning in azimuth also enables short-baseline cross-track interferometry between the antenna upper and lower halves, while using the minimum number of T/R modules. A bank of four T/R modules is fed from a single ESS (Energy Storage Subsystem) - essentially a custom DC/DC converter that provides 32 V DC power at up to 47 A pulsed. The T/R modules are configured to transmit either an H-pulse or a V-pulse, and to receive both an H-pulse and V-pulse simultaneously. The peak power of the T/R modules is 100 W, and the maximum duty cycle is 5%. The average and peak RF powers radiated by the antenna are 93 W and 1.86 kW, respectively (assuming losses of 1.1dB). The T/R modules are cooled by means of an air duct that runs along the length of the antenna. The air velocity in the duct is controlled to maintain a relatively low thermal gradient across the T/R modules (Ref. 5).
Beamforming is implemented by a combination of phase shifters and attenuators in the T/R modules, and by means of a network of printed circuit manifolds. The T/R module vendor, Remec Defense and Space Inc., is designing custom L-band MMICs to implement the phase shifter and LNA functions of the T/R module. There are four RF manifolds: one for transmit, two for receive (one for H and one for V), and one for calibration. Separate manifolds are provided for the upper and lower halves of the antenna to facilitate short baseline cross-track interferometry. The manifolds consist of two 12-way corporate dividers fabricated as stripline transmission lines in multi-layer printed circuit boards that are located between the two rows of T/R modules. The common ports of the 12-ways are connected to a switching network that routes receive, transmit, and calibrations signals to and from the RF electronics, as required by the radar operating mode (Ref. 5).
Figure 15 is a block diagram of the phased antenna array which was designed, built and integrated at JPL. The figure shows the top side and bottom side of the antenna and depicts the electronics assemblies and radiating aperture, respectively. An aluminum honeycomb panel forms the mechanical backbone of the antenna (Ref. 6).
The T/R modules and antenna switch network are controlled through a digital interface called the T/R antenna controller (TRAC). The TRAC receives command and timing information from flight software and a hardware interface called the central timing unit (CTU), and in turn feeds LVDS serial data to the T/R modules through twisted-pair cables (Ref. 6).
Table 2: Requirements of the T/R modules (Ref. 6)
The T/R module block diagram is shown in Fig. 16. The module comprises two sides. The top side is populated with the power amplifiers of the transmit chain, polarization switch, calibration coupler, and circulators. The bottom side has receiver sub-assemblies (comprising LNA, attenuator and phase shifter), the transmit phase shifter, CPLD controller, memory, drivers, receive protection switches, and voltage regulators (Ref. 6).
Calibration: In order to form a beam that is scanned to the proper direction, it is necessary to calibrate the T/R modules and associated beamforming network. This calibration procedure is essentially a process of equalizing the phase and amplitude of each RF path to and from a particular antenna port. In transmit mode, compressed operation of the amplifiers prevents amplitude trimming with attenuators, so modules are positioned in the array to as to form a power taper that is symmetric about the azimuth and elevation directions. In receive mode, amplitude is trimmed to within 0.5 dB using a 5-bit digital attenuator (Ref. 6).
Considerable simplification of the calibration process is obtained as a result of the accuracy, linearity, isolation, and predictable temperature dependence of the beamforming networks. However, even with these simplifying attributes, calibration of this 24-element active array (as currently configured) is both labor-intensive and time-consuming (Ref. 6).
Planned UAVSAR onboard processing capabilities:
A future extension of the UAVSAR system calls for an autonomous disturbance detection and monitoring system with imaging radar that combines the unique capabilities of the imaging radar with high throughput onboard processing technology and onboard automated response capability based on specific science algorithms. This smart sensor development leverages off recently developed technologies in real-time onboard synthetic aperture radar (SAR) processor and onboard automated response software as well as science algorithms previously developed for radar remote sensing applications. 25)
The challenges are the radar's high raw data rate, requiring large onboard data storage and high downlink capability, and low data latency, requiring delivery of perishable information in time to be of use. Recent onboard SAR processor development (for the DoD Space Based Radar program) is the first step towards reducing the downlink data rate. High fidelity polarimetric and interferometric SAR (InSAR) processing technology will reduce the downlink data rate by hundreds of orders of magnitude. In particular, the onboard processing capability will contribute to several radar-based mission concepts for monitoring natural hazards and the global carbon cycle. Forest fire and hurricane-induced damages on coastal landscapes and forests are considered the two most important disturbances of natural ecosystems and threats to human habitats.
The change detection concept under development is based on the NASA's AIST-02 (Advanced Information Systems Technology) project to provide innovative on-orbit and ground capabilities for the communication, processing, and management of remotely sensed data and the efficient generation of data products and knowledge. The ASE (Autonomous Sciencecraft Experiment) implementation on the EO-1 (Earth Observing-1) mission, operational as of 2008 and providing automated disturbance detection and a monitoring capability for forest fire and hurricane-induced damages applications, serves as a conceptual starter for the smart sensor implementation on UAVSAR.
In the UAVSAR smart sensor concept (Figure 17), raw data from the radar observation are routed to the onboard processor via a high-speed serial interface. The onboard processor will perform SAR image formation in real time on two raw data streams, which could be data of two different polarization combinations or data from two different interferometric channels. The onboard processor will generate real-time high resolution imagery for both channels. The onboard processor will also execute calibration routines and science algorithms appropriate for the specific radar application.
Autonomous detection is performed by an intelligent software routine designed to detect specific disturbances based on the results of science processing. If no change is detected, the process stops and the results are logged. If "change" due to specific disturbances is detected, the onboard automated response software will plan new observations to continue monitoring the progression of the disturbance. The new observation plan is routed to the spacecraft or aircraft computer to re-target the platform for new radar observations.
The hardware for the prototype autonomous system is a self-contained VME chassis with single board computers and FPGA (Field Programmable Gate Array) processor boards, high-speed serial interfaces for data routing, and Ethernet connection for processor control (Figure 18). The VME (Virtual Machine Environment)-based change detection on-board processor (CDOP) consists of two custom FPGA boards with two Xilinx Virtex II-Pro FPGAs each and large high-speed SRAM (Static Random Access Memory) to perform real-time SAR image formation, a custom fiber-channel-to-RocketIO interface card to handle the data transfer rate in excess of 1 Gbit/s between the UAVSAR, processor components, and onboard memory.
Two identical FPGA boards are utilized in order to perform SAR image formation of two raw data channels concurrently. COTS G4 PowerPC cards are used for preprocessing and polarimetric or interferometric postprocessing. The processor control software consists of realtime, multi-threaded code that is running on the G4 CPU to route data from UAVSAR's data acquisition controller to the two FPGA processors and to generate processor parameters for two processor channels respectively.
Figure 17: Operational scenario of the UAVSAR-based smart sensor (image credit: NASA/JPL)
Figure 18: High level hardware architecture of the UAVSAR smart sensor (image credit: NASA/JPL)
The G4 PowerPC post-processor will perform the science data processing task, whereas autonomous detection and monitoring capability, as well as the CASPER (Continuous Activity Scheduling Planning Execution and Replanning) software, will reside either on UAVSAR's Radar Operator's Workstation (ROW) or a separate laptop computer.
The UAVSAR onboard processor data flow is shown in Figure 19. Live and/or archived raw data are first unpacked and reformatted before being routed to the FPGA processor. The preprocessor generates the phase correction factors for motion compensation and processing parameters for SAR image formation from the ephemeris data. In the FPGA processor, range compression focuses the image in the cross track direction. The presum module resamples the pulses to a user-specified along track location and spacing to reduce the number of pulses to process in the along track (azimuth) direction while reducing the noise on each radar pulse.
Motion compensation is the process where the radar signal data are resampled from the actual path of the antenna to an idealized path called the reference path. This process is necessary to align the phase centers of two data channels to the same reference path to ensure maximum correlation. Azimuth processing focuses the image in the along track direction. Radiometric and phase calibrations are necessary to generate polarimetric or interferometric science data products.
The post-processor geolocates the two SAR images with information from the ephemeris data and generates application-specific science data products such as the biomass and fuel load map. The science data products are routed via Ethernet to a laptop or UAVSAR's ROW for disturbance detection and monitoring.
The UAVSAR-based automated disturbance detection and monitoring system is expected to be operational by 2010.
Figure 19: Data flow in the UAVSAR onboard processor (image credit: NASA/JPL)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).