Bjorn H. Lambrigsten

Initial Results of the Geostationary Synthetic Thinned Array Radiometer (GeoSTAR) Demonstrator Instrument

The design, error budget, and preliminary test results of a 50-56-GHz synthetic aperture radiometer demonstration system are presented. The instrument consists of a fixed 24-element array of correlation interferometers and is capable of producing calibrated images with 1deg spatial resolution within a 17deg wide field of view. This system has been built to demonstrate a performance and a design which can be scaled to a much larger geostationary Earth imager. As a baseline, such a system would consist of about 300 elements and would be capable of providing contiguous full hemispheric images of the Earth with 1 K of radiometric precision and 50-km spatial resolution. An error budget is developed around this goal and then tested with the demonstrator system. Errors are categorized as either scaling (i.e., complex gain) or additive (noise and bias) errors. Sensitivity to gain and/or phase error is generally proportional to the magnitude of the expected visibility, which is high only in the shortest baselines of the array, based on model simulations of the Earth as viewed from geostationary Earth orbit. Requirements range from approximately 0.5% and 0.3deg of amplitude and phase uncertainty, respectively, for the closest spacings at the center of the array, to about 4% and 2.5deg for the majority of the array. The latter requirements are demonstrated with our instrument using relatively simple references and antenna models, and by relying on the intrinsic stability and efficiency of the system. The 0.5% requirement (for the short baselines) is met by measuring the detailed spatial response (e.g., on the antenna range) and by using an internal noise diode reference to stabilize the response. This result suggests a hybrid image synthesis algorithm in which long baselines are processed by a fast Fourier transform and the short baselines are processed by a more precise (G-matrix) algorithm which can handle small anomalies among antenna and receiver responses. Visibility biases and other additive errors must be below about 1.5 mK on average, regardless of baseline. The bias requirement is largely met with a phase-shifting scheme applied to the local oscillator distribution of our demonstration system. Low mutual coupling among the horn antennas of our design is also critical to minimize the biases caused by crosstalk of receiver noise. Performance is validated by a three-way comparison between interference fringes measured on the antenna range, solar transit observations, and the system model.

Near Field Characterization of the GeoSTAR Demonstrator

The GeoSTAR proof of concept demonstrator can be characterized at close range by means of a simple near-to-far- field phase correction. This reduces the test set-up configuration to reasonable dimensions. In order to simulate the Earth as seen from GEO, the target consists of a disc of absorbent material at ambient temperature placed against the sky. This work presents the details of the near-to-far-field correction as well as some preliminary results that confirm its suitability to characterize the demonstrator.

Analysis of Array Distortion in a Microwave Interferometric Radiometer: Application to the GeoSTAR Project

The geostationary synthetic thinned array radiometer represents a promising new approach to microwave atmospheric sounding from geostationary orbit based on passive interferometry. Distortion due to mechanical or thermal constraints produces a displacement of the ideal antenna positions in the array that causes sampling errors. In this paper, the impact of array distortion on radiometric error is analyzed in detail so as to identify the dominant sources of error. A preliminary analysis showing that array distortion can be well corrected by means of an external phase reference is also presented.

Prototype development of a geostationary synthetic thinned aperture radiometer, GeoSTAR

Preliminary details of a 2-D synthetic aperture radiometer prototype operating from 50 to 55 GHz will be presented. The laboratory prototype is being developed to demonstrate the technologies and system design needed to do millimeter-wave atmospheric soundings with high spatial resolution from Geostationary orbit. The concept is to deploy a large thinned aperture Y-array on a geostationary satellite, and to use aperture synthesis to obtain images of the Earth without the need for a large mechanically scanned antenna. The laboratory prototype consists of a Y-array of 24 horn antennas, MMIC receivers, and a digital cross-correlation subsystem

Robust Array Configuration for a Microwave Interferometric Radiometer: Application to the GeoSTAR Project

The Geostationary Synthetic Thinned Array Radiometer represents a promising new approach to microwave atmospheric sounding from geostationary orbit based on passive interferometry. One of the major concerns about the feasibility of this new concept is related to the ability of the sensor to cope with the failure of one or several of its single receivers/antennas. This letter shows that the inclusion of a small percentage of additional antennas significantly reduces the degradation of radiometric resolution caused by such receiver failure. Impact of antenna failure is analyzed, taking into account two test images with very different spatial harmonic content. A tradeoff analysis of several array topologies is performed so as to minimize the number of additional antennas while keeping worst case radiometric error within a reasonable level

Initial Results of the Geosynchronous Synthetic Thinned Array Radiometer (GeoSTAR)

An error budget is presented to meet 1 Kelvin radiometric accuracy in a geostationary atmospheric sounder with 50 km spatial resolution on the earth. The gain and phase errors are weighted by the magnitude of visibility versus antenna separation, and requirements range between approx.0.5% and 0.3 degrees of amplitude and phase, respectively, for the closest spacings at the center of the array, and about 5% and 3 degrees for the majority of the array. The latter requirement is met by our design without any special testing or stabilizations by reference signals. The former is met using an internal noise diode reference and by measuring the detailed antenna patterns on the antenna range. Biases and other additive errors in the raw visibility samples must be below about 2 mK on average, and this requirement is met by a phase shifting scheme applied to the local oscillator distribution. An outline of the data processing is presented, along with the first images from this system.

A dual-gain antenna option for GeoSTAR.

GeoSTAR is a radiometer concept to provide high resolution microwave images of the Earth from geostationary Earth orbit (GEO) in bands from 50 to 183 GHz. The system consists of a Y-array of correlation interferometers, and uses aperture synthesis to achieve high resolution hemispheric coverage of the Earth. A ground-based 50 GHz demonstration instrument has been built and tested at the Jet Propulsion Laboratory which has now validated the calibration approach and error analysis. These analysis show that the antenna gain of the original design is marginal, since only about 42 percent of the received energy originates in the Earth disk as viewed from GEO. This degrades signal-to-noise (delta-T), and poses a problem for the 183 GHz bands where receiver noise and resolution requirements are greatest. This paper presents a new approach to the array geometry which solves this problem by arranging the majority of elemental antennas along two rows within each of the three array arms. The new geometry provides a factor of SQRT(3) times more distance between adjacent elements, and therefore enough physical space to raise the gain of the antenna elements by a factor of 3. The visibility sample grid and number of elements are unchanged. Only the shortest baselines retain the original design.

Field tests of the GeoSTAR demonstrator instrument

Ground based tests of the GeoSTAR (Geostationary Synthetic Thinned Array Radiometer) demonstrator instrument are reported which simulate the view of the Earth from geosynchronous Earth orbit (GEO). The test used a 4-meter target disk mounted on a tower above the instrument to simulate the brightness of the Earth with a contrasting cold background. Continuous observations at 50.3 GHz for over 100 hours, along with simultaneous atmospheric measurements from independent radiometers, yielded an excellent data set with which to test all aspects of the GeoSTAR calibration. This paper presents a preliminary look at these data, and presents an algorithm to remove the aliased background from the synthesized image.

Prototype development of a geo stationary synthetic thinned aperture radiometer (GeoSTAR)

Weather prediction and hurricane tracking would greatly benefit of a continuous imaging capability of a hemisphere at millimeter wave frequencies. We are developing a synthetic thinned aperture radiometer (STAR) prototype operating from 50 to 56 GHz as a ground-based testbed to demonstrate the technologies needed to do full earth disk atmospheric temperature soundings from geostationary orbit with very high spatial resolution. The prototype consists of a Y-array of 24 MMIC receivers that are compact units implemented with low noise InP MMIC LNAs, second harmonic I-Q mixers, low power IF amplifiers and include internal digital bias control with serial line communication to enable low cost testing and system integration. Furthermore, this prototype STAR includes independent LO and noise calibration signal phase switching circuitry for each arm of the Y-array to verify the operation and calibration of the system.