7--SAR System Technology

To support the scientific applications utilizing spaceborne imaging radar systems, a set of radar technologies has been identified which can dramatically lower the weight, volume, power and data rates of the radar systems. These smaller and lighter SAR systems can be readily accommodated in small spacecraft and launch vehicles enabling significantly reduced total mission costs. To prioritize the technology needs, a strawman mission scenario is adopted. It includes global topography mapping missions using interferometric SARs and dual frequency, polarimetric SAR mapping missions that will be flown starting in 2000. Specific areas of radar technology include the antenna, RF electronics, digital electronics and data processing. A core radar technology development plan is recommended to develop and demonstrate these technologies and integrate them into the radar missions in a timely manner. It is envisioned that these technology advances can revolutionize the approach to SAR missions leading to higher performance systems at significantly reduced mission costs.

NASA has flown several spaceborne imaging radar missions for Earth observation, starting with the pioneering L-band SAR system that flew on the SEASAT mission in 1978 (Jordan, 1980). This series of spaceborne SARs has provided an increasing level of system capability, culminating in the SIR-C/X-SAR system (IEEE, 1991). This latest system is the first multi-frequency, polarimetric SAR system designed for Earth observations from space. It flew successfully on two shuttle missions in April and October of 1994. An extensive data set was collected over numerous experiment sites around the globe. The multi-frequency, polarimetric radar measurements will be used to address scientific investigations in the areas of geology, hydrology, ecology, oceanography and other disciplines.

Figure 7-1 summarizes the key features of the series of four spaceborne SAR systems developed and flown by NASA for Earth observations. It also summarizes the key radar system technology features, such as frequency, polarization, transmitter/receiver approach and beam steering capability. In addition to these NASA systems, several other spaceborne SAR missions are being conducted by the international community. Examples of three such systems developed by ESA, Japan, and Canada are shown in Figure 7-2a. The key system technology features of these international SAR systems are shown in Figure 7-2b (IEEE, 1991). The potential applications of measurements from all these radar missions in a wide range of Earth science disciplines are given in previous segments of this report.

In support of the spaceborne SAR missions throughout the past two decades, NASA has also been conducting airborne SAR experiments to develop geophysical algorithms to convert the radar measurements to quantitative geophysical parameters. The airborne SAR system was also used to demonstrate advanced radar system concepts such as the interferometric SAR technique (Zebker et al., 1986). The feasibility of obtaining high resolution digital topography data using this technique has been thoroughly demonstrated on numerous airborne experiments. Furthermore, this technique has recently been extended to measure minute topography changes by the differences in the interferometric SAR measurements obtained in multiple passes over the experiment sites (Gabriel et al., 1989). The error sources associated with these techniques are now well characterized, and a logical next step is to apply them to global measurements from space (Zebker et al., 1994).

To assist the planning of the next phase of activities for spaceborne SARs, especially in view of the fact that there are several other SAR programs that are ongoing in the international community, NASA has requested the National Research Council to conduct a review of the future SAR program direction. A key concern for any future SAR mission is that the mission complexity and cost are often driven by the mass, power, volume and data rate requirements of the radars. Typically, these radars demand large resources from the spacecraft as well as the launch vehicles, leading to high mission costs. An aggressive program to develop key radar technologies for smaller, lighter SAR systems that are more readily accommodated in small spacecraft/launch vehicles can lead to significantly reduced total mission costs. This portion of the review report addresses the specific questions concerning SAR technology and a plan for technology development for SAR program needs. An ad hoc SAR Technology Working Group to support this review was formed. The specific question that this segment of the report addresses is: "What are the priorities in SAR technology development which are critical not only to NASA's maintaining leadership in spaceborne SAR technology but to providing societally relevant geophysical parameters?" This segment summarizes the findings of the SAR Technology Working Group and recommends specific, prioritized technology steps for the NASA SAR program.

The scientific application drivers for SAR measurements have been described in detail in previous segments of this report and are summarized below:

(1) Geology: topography and topographic changes; hazard assessment such as flood potential, volcanoes, earthquakes, etc.

(2) Oceanography: ocean currents, winds, ocean surface features, sea ice thickness and coastal processes;

(3) Hydrology: soil moisture and snow water equivalence;

(4) Ecology: land cover classification, inundation mapping and biomass measurements;

(5) Ice sheets: snow facies, seasonal melt, icebergs, surface morphology, ice velocity and surface topography.

In each of these areas, the contributions of SAR data to the geophysical measurements are also addressed by other segments in this report. In some cases, the SAR systems provide a unique capability for the measurements of the geophysical parameters. In other cases, the radar results are combined with other sensor data for the geophysical studies. All the review panels recommend the collection of a long-term, calibrated time series of SAR data for studies of environmental changes. The next section focuses on translating these science drivers and their associated requirements of the SAR systems into a strawman mission scenario. Based on these SAR system requirements and mission needs, recommendations on a technology program are given in the Radar System Technology Discussions sub-section below.

In order to focus the science application drivers summarized in the Science Panel Needs Summary section into SAR system requirements, we have constructed a strawman spaceborne SAR mission scenario. This strawman scenario addresses the key scientific needs, as advocated in the science panel reports, and the data to address those needs which are not presently available from the planned international radar programs. The scenario includes a global topography mapping mission and a dual frequency, multi-polarization global mapping SAR mission. The global topography mission utilizes the interferometric SAR technique for high resolution, digital topography mapping. The dual- frequency, polarimetric SAR mission focuses on medium-resolution mapping of geophysical parameters such as biomass and soil moisture, and provides high resolution regional-scale measurements for selected experiment studies. Data from this mission scenario, together with data from other international SARs, will provide the comprehensive, long-term data set that is required to address all the science application issues discussed above. This strawman scenario clearly identifies the NASA contribution to the international SAR programs and provides key measurements that will otherwise be unavailable. It is recognized that the mission scenario described is, indeed, a strawman. Significant interactions with the science community, the international SAR mission program planners and NASA are required to properly define the mission scenario. However, for the purpose of this report, this strawman scenario and an assumed schedule for the execution of this scenario will serve as guides for the identification of radar technology needs.

Figure 7-3 shows a schematic drawing of a strawman concept for the global topography mapping mission. This concept utilizes two L-band SAR spacecraft that fly in formation. The formation flying will generate the required interferometric baseline separation between the two SAR antennas. The physical separation of the interferometric baseline needs to be measured to an accuracy of a few mm; the strawman approach is to utilize differential GPS signals. The performance characteristics of the SAR is also shown in Figure 7-3. Radar echoes obtained from the two systems will be processed into SAR imagery. The imagery will then be coherently combined and the interferometric phase measurements will be extracted. These phase measurements are then converted into digital topography information based on the baseline knowledge and other geometric parameters. Data from multiple orbits will be mosaicked to form a global topography model. It is expected that this global digital elevation model will have horizontal posting of about 30 m and vertical height accuracy of 2 to 5 m. This global data set will represent a major improvement in resolution beyond topography information that is currently available. It should be noted that this mission concept can be readily extended to perform measurements of topography changes using multiple passes over selected regions with nearly repeat ground tracks. As mentioned above, this technique has been demonstrated before using similar data sets (Zebker et al., 1994). It is envisioned that topography change detection and measurements need to be conducted on a continuing basis for assessment of hazards and related environmental changes. The key technology driver for a set of low-cost topography mapping missions is to develop a small, light SAR system that can be accommodated in small spacecraft and launch vehicles.

Figure 7-4 shows a schematic drawing for a dual-frequency, polarimetric strawman SAR mission for global vegetation and soil moisture studies. An L-band SAR with quad-linear polarization, together with an X-band SAR with dual polarization, is assumed for this mission. The focus of this mission is on obtaining medium resolution (200 to 300 m) global maps at the L- and X-bands for ecological and hydrological system studies. The specific geophysical measurements include biomass estimates, vegetation classification, plus soil and snow moisture. The mission goal is to provide a global map of these measurements once every several days for assessments of changes in these geophysical quantities. It is also envisioned that the SAR systems can support observations at higher resolution (less than 20 m) over selected areas for regional-scale experiment studies. This system employs advanced on-board SAR signal processing to generate the medium-resolution, accurately calibrated polarimetric SAR imagery over the global land mass. This technology will reduce the on-board data storage and downlink requirements. In addition, a small, lightweight power-efficient SAR system is also required.

A recent interesting SAR technique development is the use of an along-track interferometric SAR system for measurements of ocean phenomena, especially ocean currents. We note that a possible mission concept for along-track interferometric SAR ocean studies is similar to Figure 7-3, except that, instead of flying side-by-side, the spacecraft will fly along-track relative to each other. This approach does not require significant technology changes relative to the strawman mission scenario.

As a basis for planning and prioritizing the SAR technology development, we have arbitrarily selected an assumed schedule for the two missions mentioned. We have assumed that the global topography mapping mission will be developed in the 1997 to 2000 time frame with a launch in 2000 and the dual-frequency, polarimetric SAR mission for global mapping will be developed in the 2000 to 2003 time frame with a launch in 2003. Of course, this schedule needs to be reexamined in the future, but it will serve to set the requirements for the technology development.

In this section, the state of the art in spaceborne SAR system technology, as well as the technology needed to address future potential missions are discussed. The five main areas of discussions are: antenna, RF electronics, digital electronics, data processors and system-level technology concepts. For each of these areas, the projected technology needs and their impact on the SAR missions are described. As discussed in the Recommended SAR Technology Program Approach section below, an advanced radar test-bed should be developed. Each of the technology items discussed should be fully tested in a modular fashion within this test-bed.

Antenna Technology Discussions

It is recognized that a key technology challenge in SAR systems is the antenna system. Due to the desire to limit the level of azimuth and range ambiguities in the SAR signal, the physical size of the antenna used cannot be smaller than certain prescribed limits. A typical size for a spaceborne L-band SAR antenna is 10 x 2 m. An antenna of similar size will be required for the strawman missions. Table 7-1 summarizes the size and weight of the antenna systems used in the SAR systems on SEASAT, ERS-1, JERS-1, RADARSAT and SIR-C (Gibbons, et al., 1994; Jordan, 1980). Although the physical size requirements do change with frequency, orbit altitude, swath width and required antenna gain, the antennas listed in Table 7-1 are all large and represent challenges in accommodation for the spacecraft and launch vehicle. These antenna systems also use different technology approaches. The SIR-C/X-SAR system utilizes a distributed phased array antenna at the L- and C-bands, and a slotted waveguide antenna at the X-band (Figure 7-5). The L- and C-band phased arrays employ multiple transmit/receive (T/R) modules that are distributed across the physical apertures of the antennas. These distributed T/R modules also provide electronic beam-steering capability. The ERS-1 and RADARSAT SAR systems utilize slotted waveguide antennas at the C-band, and the JERS-1 SAR system utilizes a microstrip planar array at the L-band. For the strawman mission scenario described in the Strawman SAR Mission Scenario section above, the key technology challenges for the antennas required are: reduction in the antenna weight, accommodation of a large antenna within the envelope of the launch vehicle shroud, reliability of the deployment mechanism, and reduction in the loss through the antenna feed network.

For the mission to be launched in 2000, a lightweight antenna with a minimal stowed volume to fit within a small launch vehicle shroud is required. The use of light weight composite material to reduce the antenna weight and maintain the required surface flatness is a key technology. Compact deployable antenna structures, with highly reliable deployment mechanisms, need further development and demonstration. Improvements in low loss feed network material technology are desired. As a target for this phase, a passive, L-band planar antenna system with a size of 12 x 2 m that is suitable for the global topography mapping mission should weigh less than 70 kg. In addition, it is also possible to consider using the antenna structure in the SAR system as an additional resource for the spacecraft. For example, the antenna structure can serve as an integral part of the spacecraft structure, which can be used for the mounting of other spacecraft subsystems. This aspect should be investigated in the detailed mission design.

For the missions to be launched in 2003 and beyond, the use of inflatable antenna technology, in the form of planar phased arrays or other suitable reflector shapes in SAR antennas, must be developed. This technology can provide significant advantages in reducing the volume of the stowed antenna at launch, which will allow SAR systems to fly in smaller launch vehicles. It can also reduce the mass of the antenna as well (Freeland et al., 1992). Figure 7-6 shows an example of such an inflatable reflector that is under development for a technology demonstration flight on the Space Shuttle. However, several key technological challenges must be addressed before this approach can be readily adopted for SAR applications: (1) an appropriate antenna feed approach (if the radiative elements are on the inflated antenna surface), (2) the design concept to provide electronic beam-steering, (3) lifetime of the material used, (4) mechanical control of the antenna, and (5) total system reliability. In particular, the incorporation of electronic beam-steering capability with the required fast beam switching time and low side-lobe level, into an inflatable structure must be demonstrated. Another key concern for the global SAR mapping mission is that the inflatable antenna has to support dual-frequency operation and polarimetric measurements.

RF Electronics Technology Discussions

The RF subsystems of spaceborne SARs generate high power microwave transmitter pulses and amplify the radar echoes received through the antenna for digitization and processing. A key technology is the high peak power transmitter that is required. The SIR-C system utilizes distributed T/R modules, whereas the X-SAR utilizes a central transmitter/receiver system. In the multiple T/R module approach, the total peak transmit power is shared among all the modules, and each one only needs to transmit at a relatively lower peak power. Furthermore, the overall system reliability is improved because the multiple modules provide inherent redundancy. Figure 7-7 shows an L-band T/R module used on SIR-C. The key technology issues for the RF electronic subsystems for future SAR missions are substantial reduction in their weight and volumes, and significant improvement in their power efficiency. The power efficiency improvement can lead to reduction in the requirements on the spacecraft power subsystem with corresponding savings in mission cost. Another concern is to improve the reliability of these high-power RF components to reduce the risk of sensor failure. It is also important that the RF electronics operate with stable system gains to allow accurate calibration of the radar results.

Specifically, for the global topography mission at 2000, lightweight, highly efficient solid-state transmitters at the L-band, with peak power in excess of 300 W, are required. Silicon bipolar transistors at this power level presently exist for L-band operation, but their power efficiency should be improved beyond 60%. In addition to the high-power transmitters, the low-power portions of the RF subsystems, such as the exciters and receivers, should be miniaturized by the appropriate use of MMIC technology (see schematic diagram in Figure 7-8). This can lead to drastic reduction of system mass and volume.

For the radar missions through 2010, there are several emerging technologies that should be incorporated. Gallium arsenide MESFET and derivatives, such as heterostructure FETs and pseudomorphic HEMTs, should deliver several tens of watts at C-band and perhaps X-band (Figure 7-9 shows the present assessment of the available peak power per single device in various frequencies). Another example is the microwave power module. This is a blending of solid-state and vacuum electronics with performance benefits not attainable by either technology alone. It combines a wideband MMIC driver, an efficient miniaturized vacuum power booster, and integrated power conditioning, into a compact, lightweight package capable of average powers from several tens to several hundreds of watts. It is suitable for operation at frequencies up to the K-band (where solid-state devices are not as efficient as they are at lower frequencies; see Figure 7-10 for an engineering model example). It should also be noted that continuing improvement in high-power electronics tubes are useful for SAR operations at higher frequencies. Again, in all cases, further reductions in weight and improvements in power efficiency are key issues. For the missions beyond 2000, the technology of performing direct synthesis of RF drive signal from clock signals, avoiding any frequency synthesis process, should be pursued. By delivering the required RF drive signals by direct synthesis, significant portions of the exciter electronics will be eliminated, with a corresponding reduction in mass and volume.

An important challenge in the development of miniaturized RF electronics is the mechanical packaging technology. The small size in the circuitry presents significant challenges in minimizing design time, eliminating undesired RF coupling, and allowing for proper thermal dissipation. Furthermore, the small-scale circuitry will present problems in electrical and performance testing. New methodologies for testing by non-intrusive probing techniques should be developed and applied.

Digital Electronics Technology Discussions

A significant cost driver for SAR systems is that they typically generate large volumes of data at high rates. For example, the SIR-C/X-SAR system has five radar channels that operate at 45 Mb/s. For SIR-C, the digital data handling subsystem was 145 kg in weight and consumed about 800 W of power. Specific technology needs in the digital electronics for future SAR systems include substantial reduction in weight and power consumption, and increased automation of system operation.

For the global topography mapping mission in 2000, ASIC or FPGA technology needs to be adopted to reduce the digital electronics system size and weight. Examples of modules that need to be miniaturized include the digital chirp generator, data digitizer, the formatter, and the timing and control modules. Operating at clock rates of 50 to 100 MHz, their power consumption needs to be reduced by a factor of 10. Fortunately, the performance of digital synthesizers, analog-to-digital converters, floating point multipliers and accumulators, and high speed memories is benefiting from technology improvements that will lower power consumption and raise clock frequencies. In particular, heterojunction bipolar technology can lead to power consumption reduction by a factor of 5 and can more than double the operation speeds. For example, the sampling rate of 12-bit ADCs are projected to grow from 10 MS/s to over 50 MS/s, and that of 8-bit ADCs from 100 MS/s to 1 GS/s. At the same time, scaled CMOS and complementary gallium arsenide heterostructure FET technology are reducing both the access time and power consumption of SRAMs for high speed applications. In addition to these technology items, use of the multi-chip module packaging approach should also be adopted to improve the packaging volume efficiency and system reliability. To reduce the cost of post-launch mission operation, the radar command and operation functions should be highly automated. The radar system controller must perform the function of detailed system parameter setup; this may vary as a function of the orbital location (e.g., PRF and data window position) as well as the gain setup of the RF and IF portions of the radars in an automated fashion to avoid excessive ground commanding. Onboard selection of proper RF/IF gain based on the radar echo strength, or the use of adaptive data quantization schemes such as a block floating point quantizer is required. These system automation issues will allow significant reduction in mission operation costs.

For the radar missions through 2010, the key digital technology item is on-board SAR processors for missions such as the global vegetation and soil moisture mapping mission. Unfortunately, current programs to develop radiation-hardened processors and memories are lagging behind the commercially available products. A radiation-hardened, 32-bit, 20-Mips processor is not yet available, and the current radiation-hardened SRAM technology is 256K. The onboard SAR data processor should be small, light and consume relatively low power (<50 to 100W). Technology advances in digital signal processing elements should be explored to develop a compact system architecture that is capable of the high throughput rate required(>4 GFLOPS at a sustained rate). This onboard processing technology can lead to dramatic decreases in downlink requirements and subsequent ground data handling. For the radar missions in this time period, the technology of very-high-speed ADC to perform direct data digitization at RFs from 1 to 10 GHz, followed by high-speed digital filters, should be developed. This approach will eliminate portions of the traditional RF subsystem and can potentially lead to more stable system calibration with improved system dynamic range.

Data Processing Systems

At present, nearly all the spaceborne SAR missions utilize ground data processing systems to convert the raw radar data into imagery. The ground processors also perform radiometric calibration, geometric rectification, further processing of the imagery into level 2 products, and the data archive and distribution. There are three major hardware approaches to the development of SAR processing systems: general commercial off-the-shelf (COTS) CPUs that can provide multi-GFLOPS level of computation rate; general COTS CPU augmented by special-purpose, high-computation-rate signal processing boards; and custom-designed, special-purpose SAR processing hardware.

For the radar mission in 2000, the present technology trend is to continue to capitalize on the advancement in COTS hardware that the computer industry is providing, and to utilize highly transportable SAR processing software to reduce the system implementation cost. A key technology driver continues to be the front-end interface to the radar data storage input. Significant investments to increase the throughput, reliability and transportability of this front-end I/O interface are required. Other important technology developments should occur in high-throughput geophysical product generation systems. These systems perform geophysical parameter retrieval based on the SAR imagery and generate higher-level data products. Although the geophysical inversion algorithms are maturing, the use of high-speed computation systems to analyze large data volumes of SAR need to be demonstrated. Significant advances have already been made in the polar ice geophysical processing, and these system approaches need to be extended to other application areas. An issue that should also be addressed is the incorporation of these SAR data sets, both at the imagery level and higher level data products, into the EOS data systems, so that they can be readily accessed by the science community.

For the SAR missions through 2010, as mentioned in the Digital electronics technology discussions above, the use of onboard processors should allow the generation of SAR imagery on the spacecraft. Continuing improvements of onboard processors should be pursued by including the radiometric and geometric calibration functions that require engineering information provided with the radar sensor. In addition to the continuing improvements in the onboard processor systems, further improvements in the higher level product generation system technology and the development of efficient data product verification processes need to be performed.

Other SAR System Technology Considerations

In the discussions on the science application drivers and the strawman mission scenario, the radar resolution is typically envisioned to be in the range of tens to hundreds of meters. Indeed, the global mapping of vegetation and soil moisture will be conducted with a resolution of 200 to 300 m. However, for certain applications, a high resolution data set, say, at 2 m, can provide significant advantages. For example, once an area of interest is mapped at low resolution, selected segments of that area can be mapped at higher resolution for more detailed studies. These high-resolution data may have commercial and other civilian and military applications beyond the scientific studies discussed here. The high resolution mapping would not be global in nature, but would focus on selected regions for detailed studies. This type of high resolution mapping should be considered for the global topography mapping mission, as well as for the dual-frequency, polarimetric SAR mapping mission. Continuing improvements in the radar sensor technology, especially in high peak transmit power, large system operation bandwidth and high resolution onboard signal processors, would be required.

Although many of the scientific studies have been using radar measurements obtained over distinct frequencies that are widely separated, a tradeoff on the use of a single wide-band radar system, operating over, say, 40% of the bandwidth for some of the applications, should be examined. This wide-band approach can be readily accommodated in most antenna and RF system technologies. It eliminates the use of multiple hardware sets for the distinct frequencies. Of course, the penalty is that the frequency separation is only 40% or so of the RF frequency, but that may be sufficient for certain applications. The utility of such a system concept should be explored.

It is recommended that an aggressive radar technology development program be incorporated as part of the core NASA radar program activities. Its primary purpose is to reduce the mass, power, volume and data rate of the radar systems in order to reduce the total mission cost. In addition, prudent adoption of these technologies in developing the radar instrument and data processing systems can improve the system performance, increase its reliability, and lower the radar system development cost. This section summarizes the technology development discussions in the Radar System Technology Discussions section into a specific SAR technology program that is designed to address the needs for the radar missions envisioned for the 2000s.

For the radar mission in 2000, an aggressive technology demonstration program is recommended to ensure that the technologies listed in the Radar System Technology Discussions section above are ready and suitable for incorporation into the radar mission. The crucial technology demonstration studies are:

(1) Antenna: a technology demonstration model that includes the use of lightweight composite material for the antenna subarrays, a compact deployable antenna structure with the associated deployment mechanism, and a low loss antenna feed network using low loss material;

(2) RF subsystem: development of high efficiency solid-state L-band transmitter module and MMIC-based lightweight exciter and receiver modules;

(3) Digital electronics: development of miniaturized digital chirp generator, timing and control module, data digitization and handling unit using ASIC or FPGA technology and appropriate multi-chip module packaging approach;

(4) Data processing systems: demonstration of high-throughput, front-end interfaces for the ingestion of radar data into state-of-the-art COTS hardware system for SAR processing.

Table 7-2 shows the projected mass breakdown of such an advanced SAR system for the global topography mission. It is envisioned that the SAR system should weigh less than 100 kg, which represents a factor of 3 or more reduction relative to current, similar systems. A systematic approach to this technology demonstration process is a radar test-bed in which an advanced technology radar system is built up from all technology demonstration items. After each technology item is tested, it will be integrated into the test-bed for an end-to-end system-level test. It should be noted that many of the electronics modules listed above are applicable to radars at different frequencies, and therefore can be used in radar systems beyond the first strawman mission in 2000.

For the radar missions in 2003 and beyond, the technology program should include continuing improvements in the above areas and the following major enabling technology items:

(1) Antenna: technology development for an inflatable antenna system that can support electronic beam-steering and multi-frequency, polarimetric SAR observations;

(2) RF electronics: technology demonstration of microwave power modules for X-band transmitters and advanced solid-state transmitters for L- and C- band SAR systems and development of direct RF synthesis scheme to further reduce the size, weight and power consumption of the RF subsystem;

(3) Digital electronics: technology demonstration for onboard SAR processor to convert the raw SAR data into SAR imagery and development of direct data digitization at RF with high speed ADCs

(4) Data processing system: development of a high-throughput processing system for higher-level data product generation.

All these developments should be an integral part of the core radar technology program. The development schedule should ensure that they are ready to support the missions in the first decade of the 2000s. Again, these technology items should be integrated into the radar test-bed to provide full functional demonstration in an end-to-end fashion. With these advances, radar missions should be sufficiently light and small to be launched in Pegasus or other launch vehicles of similar capabilities. These technology advances will fundamentally revolutionize the approach to SAR missions, leading to higher-performance systems at significantly reduced mission costs.