Technical appendix 

Description of configuration of radars
mounted on board the Shuttle

Simple presentation

The SRTM mission will be carried out using the US Shuttle Transportation System (STS) as space vehicle, with the on-board instrumentation (Synthetic Aperture Radars) coming from the previous SIR-C/X-SAR missions. This instrumentation has been modified in order to attain the specific objectives of this mission.

Inside the American STS (Shuttle Transportation System) two radars, complete with antennas, will be operating. The first, supplied by NASA/Jet Propulsion Laboratory, is in band C (5.3 GHz), and the second, supplied by the Italian and German Space Agencies, in band X (9.6 GHz).

Both radars will be controlled from the ground through a command and control system resident at NASA's Johnson Space Center operations center in Houston.

For this mission, NASA/JPL have developed a special highly sophisticated structural and mechanical system enabling a mechanical arm, carrying two other antennas at its end, to be extended to a length of sixty meters. These antennas at the end of the arm will enable reception of the echo from the radars located on board the STS. This system of two antennas per frequency band will enable the third dimension to be reconstructed and topographical output to be obtained in both bands (C and X). A further highly sophisticated system, known as AODA, has been designed to control deployment of the mechanical arm, verify alignment of the antennas and supply all data needed for end-of-mission processing of the data recorded during the mission aboard the Shuttle.

Technical presentation

The SRTM flight segment consists of four major subsystems; the Structural and Mechanical System (SMS), the C-band (5.3 GHz) Aperture Radar (C-Radar) system, the Attitude and Orbit Determination Avionics (AODA) system, and the X-band (9.6 GHz) Radar (X-Radar) system. Of the above systems, the SMS, C-Radar, and X-Radar use existing SIR-C/X-SAR hardware with modification and augmentation, whereas the AODA is a new system. Refer to Figures 1-2 and 1-3 for an overview of the SRTM experiments.

SIR-C, which was developed by the Jet Propulsion Laboratory (JPL), is comprised of two active aperture phased array radar systems that can operate in the L-band and C-band (the L-band capability will nominally not be used). The antenna panels are supported by a 4 x 12 meter rigid structure that attaches to the orbiter at four sill trunnions and two keel trunnions. Pallet-mounted electronic assemblies provide RF excitation, command, timing, and data handling functions. The C-band (and L-band) radar operates with vertical and horizontal polarization. C-Radar uses the C-band part of the SIR-C hardware with addition of a new Outboard Antenna Subassembly (OAS) to provide a 60-meter baseline interferometric receive capability, as well as other hardware to be outlined in Section 2 of this manual. The C-Radar inboard antenna beams can be electronically steered to any angle between 20o and 60o off-nadir, while the outboard can be electronically steered plus and minus TBSo. C-Radar data will be stored on high rate digital cassette recorders.

The X-SAR system was developed as a joint effort by the German space agency (DARA) and the Italian space agency (Agenzia Spaziale Italiana, ASI), and is a conventional SAR that uses a vertically polarized slotted waveguide antenna. Pallet-mounted electronics provide RF excitation, command, timing, and data handling functions. X-Radar is comprised of the basic X-SAR flight equipment and an additional (receive only) antenna (mounted in a fixed position at the end of the mast) with associated electronics. The X-Radar inboard antenna will be mechanically tilted in elevation (tilt range of 15o to 60o) from its launch stowed position to the same point angle as that of the X-band outboard antenna. Once the inboard beam has been positioned, it will nominally remain at the same angle throughout the mission. Beam alignment will subsequently be accomplished through electronic steering of the outboard antenna beam. X-Radar data will be stored on high-rate cassette recorders.

The AODA will provide mast deployment verification and antenna alignment verification, as well as provide the data necessary to reconstruct the SRTM interferometric baseline vector and antenna attitudes and orbital positions. It will also provide limited attitude control of the Outboard Antenna Structure (OAS). AODA is comprised of several systems, including star and target tracker assemblies, electronic distance meters, inertial reference units, and global positioning systems.

Fig. 1

Figure 1

Fig. 2

Figure 2

Figures 1 and 2 show the STS configuration with arm not yet deployed (fig. 1), and at maximum extension during the operational phase of the mission (fig. 2).