S. Vetrella

The TOPSAR interferometric radar topographic mapping instrument

The authors have augmented the NASA DC-8 AIRSAR instrument with a pair of C-band antennas displaced across track to form an interferometer sensitive to topographic variations of the Earths surface. During the 1991 DC-8 flight campaign, data were acquired over several sites in the US and Europe, and topographic maps were produced from several of these flight lines. Analysis of the results indicate that statistical errors are in the 2-4-m range, while systematic effects due to aircraft motion are in the 10-20-m range. The initial results from development of a second-generation processor show that aircraft motion compensation algorithms reduce the systematic variations to 2 m, while the statistical errors are reduced to 2-3 m. >

A tethered interferometric synthetic aperture radar (SAR) for a topographic mission

Analyzes a spaceborne interferometric synthetic aperture radar (SAR) for high-resolution, topographic applications. Two physical antennas are vertically spaced and are carried along parallel paths by two different platforms connected by a tether. Tethered space systems have been proposed by several authors for different applications and a joint US-Italian program exists to deploy in 1992 a small satellite (Tethered Satellite System) from a Space Shuttle. A system performance analysis is carried out, considering a theoretical study and a numerical simulation. By evaluating the tethered interferometric SAR impulse response, the height of various point scatterers is computed and an error budget is obtained. Results show that this system is capable of achieving a root-mean-square (RMS) error in height measurement adequate for several applications. >

Dynamics and control of the tether elevator/crawler system

This paper investigates the dynamics and acceleration levels of a new tethered system for micro- and variable-gravity applications. The system consists of two platforms tethered on opposite sides to the Space Station. A fourth platform, the elevator, is placed in between the Space Station and the upper platform. Variable-g levels on board the elevator are obtained by moving this facility along the upper tether, while microgravity experiments are carried out on board the Space Station. By controlling the length of the lower tether the position of the system center of mass can be maintained on board the Space Station despite variations of the systems distribution of mass. The paper illustrates the mathematical model, the environmental perturbations and the control techniques which have been adopted for the simulation and control of the system dynamics. Two sets of results from two different simulation runs are shown. The first set shows the system dynamics and the acceleration spectra on board the Space Station and the elevator during station-keeping. The second set of results demonstrates the capability of the elevator to attain a preselected g-level.

The global topography mission gains momentum

An accurate description of the surface elevation of the Earth is of fundamental importance to many branches of Earth science. Continental topographic data are required for studies of hydrology, ecology, glaciology, geomorphology, and atmospheric circulation. For example, in hydrologic and terrestrial ecosystem studies, topography exerts significant control on intercepted solar radiation, water runoff and subsurface water inventory, microclimate, vegetation type and distribution, and soil development. The topography of the polar ice caps and mountain glaciers directly reflects ice-flow dynamics and is closely linked to global climate and sea level change.

A bistatic SAR mission for earth observation based on a small satellite

Abstract The authors present a new scientific space mission consisting of a satellite carrying a receiving- only SAR which receives the signal transmitted by the ENVISAT-1 SAR. The integration of ENVISAT-1 SAR and bistatic radar data offers an improved potentiality of surface classification, three-dimensional observation, and the opportunity of advanced scientific experiments in the field of bistatic scattering. The small satellite nominal orbit and the attitude manoeuvres are designed in order to maintain an adequate overlap between the two radar swaths along the whole orbit, taking into account the ENVISAT-1 attitude and pointing. A preliminary satellite design (2-year lifetime) is then performed to evaluate the orbit decay and to determine the appropriate orbit manoeuvres (every 4 days) to control the satellites relative phase. The numerical simulation shows that a spacecraft of about 584kg is able to meet the mission requirements.

Mission analysis and design of a bistatic synthetic aperture radar on board a small satellite

Abstract This paper is aimed at a feasibility study of a new space-based observation technique: bistatic synthetic aperture radar, e.g. an active microwave sensor operating with separated transmitting and receiving antennae. Assuming ESA ENVISAT ASAR as main mission, a small satellite (BISSAT) equipped with a receiving-only antenna and flying in formation with ENVISAT is studied in details. In addition to conventional ASAR images, echoes gathered by BISSAT form an additional data set. They offer an added value to ENVISAT scientific exploitation, as they could be used for novel mapping applications, taking into account their unique pointing characteristics and differences in terrain-scattering properties as a function of antennae separation. A numerical simulation is conducted to perform a mission study. The antennae separation along the orbit is selected according to both applicative and ENVISAT safety requirements. The BISSAT attitude and pointing requested for swath overlap is then presented. Finally, a preliminary design of microwave payload and platform is outlined.

Attitude dynamics and control of a vertical interferometric radar tethered altimeter

This paper investigates the attitude dynamics and control of a vertical interferometric synthetic aperture radar tethered altimeter: a system based on two space platforms of comparable mass connected by 1-km tether. Two vertically spaced physical antennas on these platforms supply the terrain height measurement data by making use of radar interferometry. Since the height accuracy is strongly affected by the combined dynamics of the two platforms, this paper will focus on the analysis and control of high-frequency attitude oscillations. The results show that the proposed control system can limit the attitude angle differences and rates to 10 ~ 3 and 10 ~ deg/s, respectively, as required by the proposed application.

Passive and active calibrator characterization using a spaceborne SAR system simulator

Presents a spaceborne SAR system simulator based on a coherent approach. After a description of the original aspects of the computer simulator (i.e., digital terrain model, Earth ellipsoid rotation, space platform orbital and attitude dynamics, antenna pattern, chirp sampling, and two-dimensional compression), the characterization of passive and active calibrators embedded in different backgrounds is discussed in terms of the SAR impulse response function. An assessment of the SAR processor performance is derived by computing the one-dimensional ground range and azimuth resolutions of the simulated calibrators. Finally, various plots and numerical results are shown in order to illustrate impulse responses under clutter and no clutter assumptions. >

Advanced stepped-frequency GPR development

In the framework of ARCHEO, a national research project funded by the Italian Ministry for Universities and Scientific and Technological Research, a new ground penetrating radar has been developed by the Italian Consortium for Research on Advanced Remote Sensing Systems. The system has been specially designed to meet archaeological requirements and it will be used to identify and characterize buried finds. The paper summarizes the main guidelines followed during the design phase and presents the radar architecture.

SPACE STATION BASED TETHERED INTERFEROMETER FOR NATURAL DISASTER MONITORING

A synthetic aperture radar interferometer based on two vertically spaced physical antennas carried along parallel paths by the Space Station and by a small tethered subsatellite is presented. This remote sensing system is aimed at producing and updating topographic maps. In particular, it offers geometric, radiometric, and temporal resolutions adequate for natural-disaster monitoring. The system performance is identified by means of a computer simulation, taking account of the tethered platform dynamics and of the Space Station orbital decay. Numerical results show the capability of observing any point of the Earth surface in the range of latitudes ±51.6 deg with a repetitivity of about 2 days, by using range steering of the antenna beams (15-45 deg). Then, the attitude pointing accuracy requirements are derived by evaluating the signal-to-noise ratio and the swath overlap decrease due to the antenna misalignment (yaw 0.15 deg, pitch 0.10 deg, roll 0.20 deg).