Future GeoSAR missions are expected to provide higher resolution radar images featuring shorter revisit times by locating a radar payload on-board of a geostationary satellite One of the main challenges in GeoSAR processing is accurately determining the satellite orbit to obtain a precise phase history, in order to properly focus the retrieved data To tackle this challenge, a multiple baseline ground-based interferometer is proposed as a compact and reliable method to achieve an unprecedented accuracy As a proof of concept, this paper presents the results obtained from a single baseline prototype, whose results can be extrapolated to a larger system, able to be used in future missions

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Interferometric orbit determination for geostationary satellites

This paper presents a detailed discussion of calibration procedures used to analyze data recorded from a two-element atmospheric phase interferometer (API) deployed at Goldstone, California. In addition, we describe the data products derived from those measurements that can be used for site intercomparison and atmospheric modeling. Simulated data is used to demonstrate the effectiveness of the proposed algorithm and as a means for validating our procedure. A study of the effect of block size filtering is presented to justify our process for isolating atmospheric fluctuation phenomena from other system-induced effects (e.g., satellite motion, thermal drift). A simulated 24 hr interferometer phase data time series is analyzed to illustrate the step-by-step calibration procedure and desired data products.

https://ntrs.nasa.gov/api/citations/20090020381/downloads/20090020381.pdf

Data Processing for Atmospheric Phase Interferometers

[1] Site test interferometers (STIs) have been deployed at two locations within the NASA Deep Space Network tracking complex in Goldstone, California. An STI measures the difference of atmospheric delay fluctuations over a distance comparable to the separations of microwave antennas that could be combined as phased arrays for communication and navigation. The purpose of the Goldstone STIs is to assess the suitability of Goldstone as an uplink array site and to statistically characterize atmosphere-induced phase delay fluctuations for application to future arrays. Each instrument consists of two ~1 m diameter antennas and associated electronics separated by ~200 m. The antennas continuously observe signals emitted by geostationary satellites and produce measurements of the phase difference between the received signals. The two locations at Goldstone are separated by 12.5 km and differ in elevation by 119 m. We find that their delay fluctuations are statistically similar but do not appear as shifted versions of each other, suggesting that the length scale for evolution of the turbulence pattern is shorter than the separation between instruments. We also find that the fluctuations are slightly weaker at the higher altitude site.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2013RS005268

Tropospheric delay statistics measured by two site test interferometers at Goldstone, California

An observer-based dynamic output feedback H∞ controller is proposed for a class of two-dimensional (2D) uncertain discrete systems described by the Roesser model with actuator saturation, time-varying state delay and external disturbances. First, a delay-dependent Lyapunov stability condition is derived in linear matrix inequality (LMI) form which uses the reciprocal convex approach and H∞ disturbance attenuation performance is also analysed. Secondly, a convex hull is adopted to represent the saturation nonlinearity. The H∞ control synthesis for uncertain 2D discrete systems is described by a Roesser model subjected to actuator saturation and external disturbances using an observer-based dynamic output feedback approach. Some practical examples are provided to highlight the usefulness of the presented results.

Robust output feedback control of 2D discrete systems with actuator saturation and time-varying delay

The capacity of a multiple-input multiple-output (MIMO) satellite communication system heavily depends on the phase relations inside the channel transfer matrix Beneficial phase conditions to achieve the maximum MIMO gain can be realized by sophisticated geometrical setups of the transmit and receive antennas It has been shown that the maximum MIMO gain is maintained, if the antennas are moved within a certain limit around the optimum position Contrarily, variations of the path length induced by the troposphere are in some cases quite harmful for the channel capacity of a satellited-based MIMO system To measure the intensity of these distortions we present an interferometer located near Munich, Germany Here shown observations are transferred to a model of MIMO systems with two satellites The results indicate no remarkable capacity losses for small inter-antenna spacings on Earth

Measurements of phase fluctuations for reliable MIMO space communications

A uniform linear array (ULA) digital beamformer with enhanced selectivity (interference rejection) over conventional true-time delay-sum (TTDS) beamforming is proposed for scanned aperture applications. Conventional TTDS transfer function, which contains only zero-manifolds, is modified by introducing complex pole-manifolds based on 2-D infinite impulse response (IIR) digital beam filters, thereby achieving enhanced selectivity for the same number of antennas in the ULA. For a ULA of 64 antennas, desired direction of arrival (DOA) 30° and interference DOA -60° from array broadside, a relative improvement in signal-to-interference ratio (SIR) of 7 dB is verified. For 128-element ULA, interference DOA -10°, a relative improvement in SIR of 5-10 dB is obtained for the desired DOA in the range 10°-90°. The projected improvements are independent of the weights of the conventional phased-array, TTDS, or filter-sum beamformer used because the method does not change the zero-manifolds of the original array pattern.

2-D-IIR Time-Delay-Sum Linear Aperture Arrays

A two-element site test interferometer has been deployed at the NASA Deep Space Network (DSN) tracking complex in Goldstone, California, since May 2007. The interferometer system consists of two offset-fed 1.2 m parabolic reflectors which monitor atmospheric-induced amplitude and phase fluctuations on an unmodulated beacon signal (20.199 GHz) broadcast from a geostationary satellite (Anik F2). The geometry of the satellite and the ground-based infrastructure imposes a 48.5 elevation angle with a separation distance of 256 m along an east-west baseline. The interferometer has been recording phase fluctuation data, to date, for 1 yr with an overall system availability of 95 percent. In this paper, we provide the cumulative distribution functions (CDFs) for 1 year of recorded data, including phase rms, spatial structure function exponent, and surface meteorological measurements: surface wind speed, relative humidity, temperature, barometric pressure, and rain rate. Correlation between surface measurements, phase rms, and amplitude rms at different time scales are discussed. For 1 year, phase fluctuations at the DSN site in Goldstone, are better than 23 for 90 percent of the time (at 48.5 elevation). This data will be used to determine the suitability of the Goldstone site as a location for the Next Generation Deep Space Network.

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Phase Fluctuations at Goldstone Derived from 1-Year Site Testing Interferometer Data

A modification to the well-known Applebaum adaptive beamformer is proposed employing a low complexity space-time network-resonant IIR beamfiler. The network-resonant filter is a multiple input multiple output (MIMO) linear multidimensional recursive discrete system. It is applied as a pre-filter to an Applebaum adaptive beamformer in order to perform highly directional receive mode beamforming with improved noise and interference rejection. The spatial selectivity (directivity) of the Applebaum beamfomer is enhanced by introducing complex-manifolds from the 2-D IIR beamfilter to the zero-manifold-only transfer function of the adaptive beamformer. This leads to the proposed network-resonant adaptive Applebaum array, which shows angle-dependent levels of additional improvement of output SINR (best case, up to 12 dB improvements, near the end-fire direction). The ability to increase the improvement of the output SINR by 0–12 dB compared the best available adaptive Applebaum beamformer without using additional antenna elements in the array is a useful feature of the scheme. The proposed network-resonant Applebaum adaptive beamformer can be implemented using an upgrade to the digital signal processor without change to the array or RF electronics.

Applebaum adaptive array apertures with 2-D IIR space-time circuit-network resonant pre-filters

Widely distributed (sparse) ground-based arrays have been utilized for decades in the radio science community for imaging celestial objects, but have only recently become an option for deep space communications applications with the advent of the proposed Next Generation Deep Space Network (DSN) array. But whereas in astronomical imaging, observations (receive-mode only) are made on the order of minutes to hours and atmospheric-induced aberrations can be mostly corrected for in post-processing, communications applications require transmit capabilities and real-time corrections over time scales as short as fractions of a second. This presents an unavoidable problem with the use of sparse arrays for deep space communications at Ka-band which has yet to be successfully resolved, particularly for uplink arraying. In this paper, an analysis of the performance of a sparse antenna array, in terms of its directivity, is performed to derive a closed form solution to the expected array loss in the presence of atmospheric-induced phase fluctuations. The theoretical derivation for array directivity degradation is validated with interferometric measurements for a two-element array taken at Goldstone, California. With the validity of the model established, an arbitrary 27-element array geometry is defined at Goldstone, California, to ascertain its performance in the presence of phase fluctuations. It is concluded that a combination of compact array geometry and atmospheric compensation is necessary to ensure high levels of availability.

https://ntrs.nasa.gov/api/citations/20100015635/downloads/20100015635.pdf

Roberto J. Acosta

Papers

Tropospheric delay statistics measured by two site test interferometers at Goldstone, California

2-D-IIR Time-Delay-Sum Linear Aperture Arrays

Phase Fluctuations at Goldstone Derived from 1-Year Site Testing Interferometer Data

Applebaum adaptive array apertures with 2-D IIR space-time circuit-network resonant pre-filters

Directivity of a Sparse Array in the Presence of Atmospheric-Induced Phase Fluctuations for Deep Space Communications