GeoSTAR (Geostationary Synthetic Thinned Aperture Radiometer)
GeoSTAR is a new microwave atmospheric sounder technology under development at JPL (Jet Propulsion Laboratory) and at GSFC (Goddard Space Flight Center) within NASA's IIP (Instrument Incubator Program). The overall goals are to complement LEO microwave soundings of the POES series from GEO (on next-generation GOES satellites, considered for GOES-R, -S with a first launch in 2014, and thereafter) and to provide full hemispheric observations with the same accuracy and spatial resolutions of×≤ 50/25 km (temperature/moisture) as are currently (2005/6) available from LEO observations. 1) 2) 3) 4) 5) 6) 7)
As of 2007, the GeoSTAR concept is also considered for a possible selection on the future PATH (Precipitation and All-weather Temperature and Humidity) mission of NASA and/or NOAA. PATH is one of the 17 mission concepts which was recommended in the "decadal survey" report of NRC (National Research Council) released in January 2007. The primary objective of the PATH mission is to fly a microwave array spectrometer to monitor hurricanes and severe storms and improve existing models. The GeoSTAR concept happens to be the only sensor that can meet the PATH mission objectives. 8)
Microwave sounding from GEO (Geostationary Orbit) represents the new frontier in microwave technology. The interest of the meteorological community in microwave GEO observations is based on the capability to continuously view the same portion of the Earth (an area of interest which can be of considerable size - a continent), during day and night and at all weather conditions; microwave observations make it possible to determine the vertical distribution of temperature and humidity in the troposphere. Hence, microwave observations from GEO provide very high temporal resolutions suitable not only for NWP (Numerical Weather Prediction), but in particular for nowcasting and very short range weather forecasts.
GeoSTAR is an atmospheric microwave sounder concept with rain mapping capabilities which employs a 2-D spatial interferometric system also known as sparse array synthetic aperture radiometry. The synthetic aperture approach from GEO has until recently not been feasible, due to the high power needed to operate the onboard high-speed massively parallel processing system required for 2-D synthesis, as well as a number of system and calibration obstacles.
The notional operational GeoSTAR system will provide temperature soundings in the 50-60 GHz band range with a horizontal spatial resolution of 50 km and water vapor soundings and rain mapping in the 183 GHz band with a spatial resolution of 25 km. A possible third band would operate in the 90 GHz window region (and would possibly also cover the 118 GHz oxygen line for additional information about clouds). The radiometric sensitivity will be < 1 K in all channels (minimum requirements). - The approach used with current LEO-observing rain radiometers like TMI (TRMM Microwave Imager) of TRMM (launch Nov. 27, 1997) and GMI (GPM Microwave Imager) of the planned GPM spacecraft (launch in 2010) is primarily based on measuring the scattering effects associated with precipitation.
The greatest advantage of the high frequency GeoSTAR approach is that high spatial resolution is easily achieved. This is because the antenna size required to achieve a certain spatial resolution is inversely proportional to the frequency. For a given spatial resolution, the aperture of a 200 GHz radiometer is 20 times smaller than that of a 10 GHz radiometer. That is what makes it feasible to deploy GeoSTAR as a rain mapper in GEO, where the great advantage of continuous spatio-temporal coverage is also realized. Because of size and weight and torque effects, it is very difficult and high-risk to implement the conventional scanning antenna approach in GEO. GeoSTAR for the first time makes it feasible to directly measure rain from GEO.
Background: The GeoSTAR instrument concept was proposed in response to a 2002 NASA Research Announcement calling for proposals to develop technology to enable new observational capabilities from geostationary orbits (GEO). In particular, GeoSTAR was proposed as a potential solution to the GOES microwave sounder problem (so far there is simply no microwave sounder in GEO).
The GeoSTAR scattering approach derives from observations made in recent years with 183 GHz radiometers operated on high altitude aircraft [NAST-M (NPOESS Aircraft Sounder Testbed - Microwave Sounder) of MIT]. When passing over rain cells, a pronounced apparent cooling due to scattering is observed. This cooling can exceed 100 K over intense convective cells, a very large signal that can be used to detect and track hurricanes and other severe storms without the need for high radiometric sensitivity (i.e. dwell time).
Since these radiometers are sounders, they have channels with a varying degree of opacity (so that they can sound different depths of the atmosphere). Some are so opaque that the surface is normally invisible. When rain is observed through an opaque channel, the scattering, which dominates over absorption at these frequencies, causes a severe apparent cooling over a background that represents the underlying atmospheric state (i.e. its temperature, water vapor, and cloud liquid water). This background is easily measured with a sounder. A new method has recently been developed to retrieve rain rates from such high frequency observations and is now being used with the AMSU (Advanced Microwave Sounding Unit) flown on the NOAA-15, -16, and -17 satellites (launched in 1998, 2000, and 2002, respectively), while the AMSU/HSB (Advanced Microwave Sounding Unit/Humidity Sounder for Brazil) is flown on NASA's Aqua satellite (launch May 4, 2002) and provides a nearly identical set of channels. It is expected that the retrieval accuracy will be substantially improved during the NOAA and Aqua missions. GeoSTAR will utilize the same established approach as is being used with the AMSU instrument. 9) 10) 11) 12)
A first effort, GeoSTAR-I, demonstrated the feasibility and calibration potential of such an instrument, operating at 60 GHz. A second instrument GeoSTAR-II, clearly demonstrated the ability to obtain similar performance at 180 GHz using compact "IC" style receivers using state-of-the-art InP monolithic microwave integrated circuit (MMIC) technology, but fell short of demonstrating the final technological piece of the puzzle; a low power, high bandwidth, correlator subsystem capable of processing the PATH intermediate frequency (IF signals). GeoSTAR-III set out to close this final gap and elevate the PATH mission technologies to TRL 6, paving the way for the development of this long-elusive observational gap. We report on the successful development of the GeoSTAR-III system, describe the subsystem development, subsystem environmental testing and radiometric performance, clearly demonstrating the maturity of this instrument technology at a scale approximating the final mission requirements. 13)
In developing the GeoSTAR technology and prototype, a notional space system performing at the same functional level as the AMSU (Advanced Microwave Sounding Unit) system now operating on NASA and NOAA polar-orbiting LEO satellites (POES series) was used for design and sizing purposes. The objective of the notional GeoSTAR system is to produce temperature soundings within the troposphere for most of the visible Earth disk (out to an incidence angle of 60º or more) with a 2-4 km vertical resolution every 30 minutes and humidity soundings with a vertical resolution of 3-4 km every 5-10 minutes. 14) 15) 16)
• GeoSTAR is a non-scanning 2-D imaging system. These soundings are obtained everywhere at the same time - i.e., there is no time lag between different portions of the scene as there is in a mechanically scanned system. That also makes this system ideal for derivation of wind profiles through tracking of water vapor features - although the vertical resolution is limited.
• GeoSTAR produces several 2-D "snapshots" every few seconds. These images are combined over longer time periods to produce low-noise radiometric images that are then used for geophysical "retrievals" or directly assimilated into forecast models.
• It is also possible to recover temporal information at a much higher resolution than the "averaging window" of 30 minutes (in the case of temperature soundings), and this can be exploited when rapidly evolving processes need to be resolved more precisely.
• The retrieval of vertical profiles of temperature, water vapor density and liquid density from spectrally sampled brightness temperatures is well established (AMSU, etc.).
As schematically shown in Figure 1, GeoSTAR consists of a Y-array of microwave receivers, where three densely packed linear arrays are offset 120º from each other. Each receiver is operated in I/Q heterodyne mode (i.e. each receiver generates both a real and an imaginary IF signal). All of the antennas are pointed in the same direction. A digital subsystem computes cross-correlations between the IF signals of all receivers simultaneously, and complex cross-correlations are formed between all possible pairs of antennas in the array.
In the small-scale prototype of Figure 2 there are 24 antennas and 276 complex correlations (=24 x 23/2). Accounting for conjugate symmetry and redundant spacings, there are 384 unique so-called u-v samples in this case. Each correlator and antenna pair forms an interferometer, which measures a particular spatial harmonic of the brightness temperature image across the field of view (FOV). By sampling the visibility over a range of spacings and azimuth directions one can reconstruct, or "synthesize" an image in a computer by discrete Fourier transform.
Figure 1: Sparse array (left) and u-v sampling pattern (right), as implemented in the GeoSTAR prototype (image credit: NASA)
As illustrated in Figure 1, the spacings between the various antenna pairs yield a uniform hexagonal grid of visibility samples. By radio astronomy convention, the spacings are called the "baselines" with the dimensions "u" and "v". The primary advantage of the sparse array is that it uses far less physical antenna aperture space than the comparable real aperture.
Note: The smallest spacing of the sample grid in Figure 1 determines the unambiguous FOV, which for GeoSTAR must be larger than the Earth disk diameter of 17.5º when viewed from GEO. This sets both the antenna spacing and diameter at about 3.5 wavelengths (λ), or 2.1 cm at 50 GHz (6 mm at 183 GHz). The longest baseline determines the smallest spatial scale that can be resolved, which for the prototype Y-array is about 0.9º [i.e. 17.5º (384)-1/2]. To achieve a 50 km spatial resolution at 50 GHz, a baseline of about 4 m is required for GeoSTAR. This corresponds to approximately 100 receiving elements per array arm, or a total of about 300 elements.
Array integration: The design is based on commercially available Si Ge (Silicon Germanium) MMICs (Microwave Monolithic Integrated Circuit) that are surface mountable devices (SMD). The functionality includes a direct to baseband conversion at 165 to 183 GHz frequency range. The LO can be tuned to the required channels within that range to sample the 200 MHz total bandwidth (upper and lower sidebands). 17)
The concept design of the radiometer modules enables to integrate receivers in the array with the 6 mm spacing required for GeoSTAR (Figure 2). The high receiver density requires population of both sides of the waveguide plate and staggering the receivers. Compensation for the difference is being done in the RF path lengths in the IF circuits.
Figure 2: Integration of the MIMRAM miniature modules in an array with 6 mm spacing (image credit: NASA/JPL)
The assembly of the GeoSTAR prototype was essentially completed in May 2005. As of 2006, most of the required technology and feasibility of the synthetic aperture approach has been demonstrated at JPL in the form of a small ground-based prototype instrument. The GeoSTAR prototype consists of a small Y-array configuration of 24 horn antennas, MMIC receivers, and a digital cross-correlation subsystem. The prototype system operates with 4 channels between 50 and 54 GHz.
Figure 3: Schematic of the prototype GeoSTAR configuration (image credit: NASA/JPL)
Synthesis arrays are new and untested in atmospheric remote sensing applications, and the calibration poses many new problems, including those of stabilizing and/or characterizing the phase and amplitude response of the antenna patterns, of the receivers and correlators. The prototype was built with the same receiver technology, antenna design, calibration circuitry, and signal processing schemes as are envisioned for the spaceborne system. Only the number of antenna elements differ.
Figure 4: Photo of the GeoSTAR 50-55 GHz prototype instrument (image credit: NASA/JPL)
As illustrated in Figures 1 and 4, there is no receiver at the exact center of the array as there would be in a fully symmetric system. However, a center feedhorn poses a number of unnecessary complications to the system, related to the physical package (there is not enough room) and the electrical design. The solution is to remove the one horn from the center of the array, stagger the three arms counter clockwise, and then bring them together so that the three innermost horns form an equilateral triangle. This staggered-Y configuration eliminates the need for an odd receiver at the center, which simplifies both mechanical and electronic design. The only penalty is a slight and negligible loss of visibility coverage.
Figure 5: Block diagram of the receiver system of one arm (image credit: NASA/JPL)
Figure 5 shows a simplified block diagram of the prototype system. A number of tests have already been done in a laboratory setting, and the results are very encouraging - no serious problems have been identified so far, and the system is working exactly as expected. Further testing continues in various environments.
As of 2006, the GeoSTAR system concept is being recognized as the most likely candidate to provide the missing functionality of microwave sounding for the future NOAA GOES spacecraft series - starting with GOES-R. Hence, a full-fledged instrument development can only be the answer.
New array geometry:
To extend the geometry of Figure 1 to a larger system with 183 GHz and 55 GHz arrays, there are three problems: 1) of nesting the two large arrays without creating a gap in at least one array arm; 2) of providing redundancy - as a safeguard against failures - in the critical short baselines of the array; and 3) of improving the antenna gain. 18)
The demonstrator instrument has a sample grid spacing of about 3.8 wavelengths. This produces an alias-free field of view (FOV) that matches the 17º Earth diameter (as viewed from GEO). With the geometry of Figure 1, this spacing also limits the elemental antenna diameter to 3.8 wavelengths, and hence antenna gain, such that only about 42% of the received energy originates within the Earth disk in a practical design. This 42% `Earth-disk efficiency' represents a direct loss in sensitivity in GeoSTAR which should be avoided.
Figure 6: New array layout with the 183 GHz array nested within the 55 GHz array (image credit: NASA/JPL)
Figure 6 presents a design which addresses the above problems while preserving the benefits of the original system. The 183 GHz array has been nested within the 55 GHz array in a manner that leaves room for the receiver packages as before, and the above problems have been addressed with the following design features:
1) In Figure 2, the three primary arms of the 55 GHz array have been spread apart by 8 times the basic 3.8 wavelength sample-grid spacing. This creates room for the 183 GHz array, but it creates an 8-sample-wide gap in the visibility coverage on the u-v plane. This gap is re-filled by adding the 8-element segment, as shown, to the inside-end of each array arm.
2) Redundancy is provided by two schemes: The 55 GHz, array adds a second 8-element segment to the side of each arm (making an "F" shape with the above segment), whereas the 183 GHz array arms have been shifted to create the center triangle. These two schemes yield somewhat different degrees of redundancy at the various baselines, but both arrays achieve the goal of providing redundant coverage near the center of the u-v plane so that the failure of any one receiver does not eliminate a visibility sample. There are 192 elements in each arm of the 183 GHz array, and 104 elements per arm at 55 GHz. Both arrays switch to the two-row configuration about 16 elements away from the center.
3) Increased antenna gain is made possible through the outer regions of the array in Figure 6 by shifting the even numbered elements (i.e. every other element) in each arm diagonally and inward by 2 grid positions (2 x 3.8 wavelengths) to form a second row which is offset to one side by SQRT(3) times the original grid-spacing. This allows for an antenna which is SQRT(3) times larger, and therefore 3 times higher in gain than before.
Following the successful prototype development and consequent proof of concept and the NRC decadal survey recommendation of a PATH mission, various mission studies and related development efforts are now under way. Two focus areas have been identified:
• Weather applications, with emphasis on tropical cyclones and severe storms
• Climate research, with emphasis on atmospheric processes and climate variability on intra-seasonal to inter-annual time scales and mesoscale to continental/ocean-basin spatial scales.
It is anticipated that the concept will be mature enough that a space mission can be implemented in the 2014-2016 time frame.
Advancing the technology:
In addition to prototyping this new antenna design, the focus of IIP-07 (Instrument Incubator Program-07) was to develop the remaining most challenging technology, namely low-noise low-power 183 GHz receivers and an ASIC-based correlator. Since the aperture synthesis concept had already been demonstrated in IIP-03, it was not felt to be necessary to construct a new fully functional sounder. Instead, the focus was on developing components and subsystems. 19) 20) 21)
By leveraging MMIC development effort under related programs [ESTO's ACT (Advanced Component Technology) – and the IPP (Industrial Partnership Program)] the development of a new 183 GHz receiver was a tremendous success, deemed enabling of the PATH mission. This receiver technology has now been transferred to the HAMSR (High Altitude MMIC Sounding Radiometer), a 25-channel aircraft-based microwave sounder, originally developed at JPL under the IIP-98 program. The radiometric sensitivity in the 183 GHz band improved by an order of magnitude as a result, as shown in Figure 7, where the right panel shows the thermal noise level of the new receiver (green line) vs. the original receiver (blue curve). HAMSR is now (2011) the most sensitive and accurate microwave sounder in existence. This receiver was flown for the first time in the NASA GRIP (Genesis and Rapid Intensification Processes) hurricane experiment in late summer of 2010 and is now field tested.
Figure 7: HAMSR/GeoSTAR 183 GHz receiver (left); noise level (right), image credit: NASA/JPL
The new antenna design has been prototyped by manufacturing a small number of "tiles", each of which consist of a 4 x 4 array of feedhorns and receivers. A complete array is formed by mounting tiles end to end. This is illustrated in Figure 8 and Figure 9. A notable achievement is the substantial miniaturization of the receivers, which are now assembled on small chip carriers and are substantially smaller than the receivers developed in IIP-03.
Figure 8: Illustration of the 4 x 4 receiver tile (right) and arrays of tiles (left), image credit: NASA/JPL
Figure 9: Receiver "tile" components (left), feedhorns (center), receiver boards (right), image credit: NASA/JPL
The GeoSTAR development sponsored by the IIP program has largely been extremely successful and has already resulted in technology that has seen application in real science missions. The maturity of the key technology and the instrument design is now approaching a level where a space mission can be initiated (Ref. 21).
Table 1: Performance characteristics of GeoSTAR (Ref. 19)
The GeoSTAR-III system (shown in Figure 10) comprises 144 receivers operating between 165 and 183 GHz (Ref. 20). The receivers are arranged in 9 tiles of 16 receivers. Each receiver is fed by a profiled feed antenna, while each tile distributes the local oscillator (LO) signal via waveguide manifold at half the RF response frequency. The receivers forming an in-phase /quadrature-phase (I-Q) superheterodyne network, utilize a miniature multi-chip integrated circuit, which is mounted on a printed circuit board providing IF amplification and DC bias functionality. IF signals are carried via 288 miniature coaxial cables to the correlation subsystem, for high-speed processing of the analog signals into 288 (287) products containing the visibility information of the observed scene (Ref. 13).
Figure 10: The GeoSTAR-III instrument comprising 144 receivers operating at 180 GHz with a CMOS-based correlator subsystem (image credit: NASA/JPL)
183 GHz Receivers: The 183 GHz receivers for GeoSTAR-III have been described in detail in previous publications. The block diagram and photograph of an example of one such receiver is shown in Figure 11. While no design changes were required for the receivers, successful development of 144 receivers operating at F-band required careful attention. For this effort , combination of robotic and manual assemble was employed to demonstrate that this approach is readily scalable to even larger arrays. From previous efforts, it was understood that detailed characterization of the gain and noise of the receivers is essential to the final array performance. Accordingly, an automated test-set was developed to simultaneously characterize amplitude and phase response as well as the noise of each receiver in a non-destructive configuration. The net result was a receiver yield of greater than 80% with performance of 10-15 dB gain and noise between 300-500 K.
Figure 11: Block diagram of the MIMRAM module and photo of the assembly. Two low noise amplifier MMICs provide low noise and high gain, and the second harmonic I-Q mixer MMIC enables quadrature conversion to baseband with low LO power (image credit: NASA/JPL)
CMOS ASIC Analog-Digital Converter/Digital Correlator: The sole outstanding technology at the outset of this effort was the correlation subsystem. Previous efforts demonstrated the instrument performance with FPGA and smaller ASIC correlators, but none were able to achieve the simultaneous bandwidth and power requirements for the larger mission. It was recognized early in this effort that integrating the A/D function with the digital correlation processing is necessary to meet the mission power consumption goals. Using 65 nm CMOS, it was possible to realize a 2 bit, 64 x 64 input ASIC chip operating at 1 GHz clock rate (for a 500 MHz bandwidth).22) The chip, shown in Figure 12, meets all of the performance and power requirements for the PATH correlation subsystem. Three chips were integrated into the GeoSTAR-III correlator subsystem (Figure 13) and subsequently integrated with the instrument.
Figure 12: Packaged 64x64 A/D Correlator ASIC chip (image credit: NASA/JPL)
Figure 13: The correlation subsystem comprising 3 ASIC on boards and an FPGA-based readout circuit (image credit: NASA/JPL)
In order to raise the TRL (Technology Readiness Level) of the array subsystems is was necessary to perform environmental testing on representative subassemblies. Thermal vacuum tests were performed on a partially populated tile. The tile was cycled over temperature from -40ºC to +60ºC with no anomalies observed. This test captured the MMICs, receiver modules, IF and bias circuits as well as the LO distribution.
It was also deemed necessary to perform vibration testing. This too was done on a representative manifold (excluding the feed horns which were not mechanically designed for that purpose). The random vibration testing, which captured the receiver modules, mechanical mounting to the PCB IF/Bias boards, PCB components and the manifold, showed no anomalies between before and after testing.
Instrument Integration: An Al honeycomb panel was fabricated to provide a mechanically and thermally stable support for the array. This optical bench, which was designed to provide stability to λ/16, was fabricated from two 0.030" face sheets separated by 1.5". The tiles are each kinematically mounted to the panel with three standoffs. Alignment of the tiles used "jigs" applied to the antenna apertures. LO distribution and phase switching were similar to GeoSTAR-II, but with modified distribution manifolds for each arm of the array. The correlator subsystem was mounted directly to the back of the panel.
Observational results: Tests of the full array proceeded with the setting of the A/D thresholds on the correlator ASICs. This was done to provide a uniform signal distribution with only the receiver noise as an input. Next, the system observed a source in the near-field to confirm that the correlator channels were functional. The system was then moved outdoors to observe the sun. The 'first light' performance (Figure 14) appears to be excellent in that the visibility magnitude measured of the solar disk exhibits very good agreement with the Airy function versus baseline that one expects to observe. The synthesized image of the sun also matched the anticipated image very well—accounting for the fact that there are many known missing baselines which have not yet been filled. Finally, observations of the moon were performed and are shown in Figure 6.
Figure 14: "First light" observations of the sun. The image of the solar disk matches the anticipated image (image credit: NASA/JPL)
Figure 15: Uncalibrated, resolved image of the moon (clipped from time lapsed video), image credit: NASA/JPL
In summary, the project demonstrated the performance of a 144 element, 180 GHz synthetic aperture array receiver. The receiver employs a CMOS 64 x 64 input A/D –correlator chip in the correlation subsystem. The array subsystem underwent environmental testing to demonstrate TRL 6, while the entire system was used to perform observations of physical targets to demonstrate a total system TRL of 5 (6 when environmental tests of the correlation subsystem are completed). This advancement in TRL has prepared the GeoSTAR system to meet the requirements of the PATH mission.
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20) , Pekka Kangaslahti, David Pukala, Alan Tanner, Ian O'Dwyer, Bjorn Lambrigtsen, Todd Gaier, Xiaobing Mei, R. Lai, "Miniature Low Noise G-band I-Q Receiver" Proceedings of IEEE-International Microwave Symposium, Anaheim, CA, USA,May 20-28, 2010
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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 (email@example.com) .