1.1.3 Principles of Measurement

Figure 1.9 ERS-1 image June 21 1992, Mt. St. Augustine Volcano, Cook Inlet (Copyright ESA, 1992)

The antenna beam of a side-looking radar is directed perpendicular to the flight path and illuminates a swath parallel to the platform ground track. Due to the motion of the satellite, each target element is illuminated by the beam for a period of time (integration time). As part of the on-ground processing, the complex echo signals received during this period are added coherently. This process is equivalent to synthetically forming a long antenna (called Synthetic Aperture).

Assuming a constant angular beam width along-track (azimuth) for the entire swath, the achievable synthetic aperture increases with the distance(slant range) between radar and target.

The range resolution of a pulsed radar system is limited fundamentally by the bandwidth of the transmitted pulse (slant range resolution = c/2B hence the wider the bandwidth, the better the range resolution). A wide bandwidth can be achieved by a short duration pulse. However, the shorter the pulse, the lower the transmitted energy (for a fixed-peak power limitation) and the poorer the signal-to-noise ratio, hence the radiometric resolution. To preserve the radiometric resolution, the technique adopted by ASAR is to generate a long pulse with a linear frequency modulation (or chirp). The length of the pulse is defined to be consistent with the requirement for the signal-to-noise ratio. The chirp bandwidth is defined by the required range resolution. After the received signal has been compressed, the range resolution will be optimised without loss of radiometric resolution.

The azimuth resolution of a real aperture radar system is a function of the antenna length (the larger the antenna, the better the azimuth resolution). It can be shown that a spaceborne real aperture radar, giving a useful azimuth resolution for points on the Earth's surface, will require an impractically large antenna. Aperture synthesis, therefore, offers a means of greatly improving the azimuth resolution for a given antenna length.

The measurement principle of ASAR depends on the use of coherent radiation, together with precise knowledge of the transmit and receive point of the radar pulse. For a given target, as the platform moves, the distance from the radar to the target (i.e., the slant range) changes continuously, hence the phase of the reflected signal changes according to a law given by the geometry of observation. As this law is deterministic, it is therefore possible to correctly phase the return signals with respect to each other so that the net effect is equivalent to them all having been received simultaneously by an antenna of length equal to the path length over which the radar signals were collected (i.e., the synthetic aperture). In this way, the synthesised antenna can be thought of as a number of independently radiating elements (i.e., the real aperture), whose separation is established by the Pulse Repetition Frequency (PRF) and the platform velocity. The change of phase with respect to time is the Doppler angular frequency. The azimuth resolution is determined by the Doppler bandwidth of the received signal. For ASAR, the bandwidth of target returns, in azimuth, is defined by the Doppler bandwidth covered between the half-power points of the one-way azimuth pattern. This implies that pulses must be transmitted with a repetition frequency greater than the azimuth bandwidth in order to satisfy the Nyquist sampling criterion.

There is an upper limit on the PRF imposed by the geometry. If the PRF is so high that return signals from two consecutively transmitted pulses arrive simultaneously at the receiver, there will be ambiguities in the response. This will, therefore, define a set of unambiguous intervals for a given geometry and PRF, which corresponds to constraining the ground range extent of the region illuminated within the elevation beamwidth of the antenna footprint (i.e., the swath width).

As a consequence of the ASAR antenna being used for pulse transmission and echo reception, there will be echoes that are not received due to periods when the antenna is transmitting pulses and hence not receiving echo returns. For a given geometry and PRF, these "blind" intervals will lie at constant ground range positions.

The return from nadir (the ground point vertically below the satellite) will be significantly larger than the returns from the required swath, because of its close range and high reflectivity. To avoid this unwanted signal saturating any other returns arriving at the same time, the PRFs and swaths for ASAR are chosen such that the nadir returns do not occur in the imaging window.

The significant feature of the ASAR instrument is the active phased array antenna, which allows independent control of the phase and amplitude of the transmitted radiators from different regions of the antenna surface. It also provides independent weighting of the received signal to each of these regions. This offers great flexibility in the generation and control of the radar beam, giving the ASAR instrument the capability to operate in a number of different modes. These modes use two principal methods of taking measurements; the ASAR instrument may operate as a conventional stripmap SAR or as a ScanSAR.

ASAR Stripmap Modes (Image, Wave)

When operating as a stripmap SAR, the phased array antenna gives ASAR the flexibility to select an imaging swath by changing the beam incidence angle and the elevation beamwidth. In addition, the appropriate PRF required to ensure acceptable ambiguity performance and to suppress unwanted nadir returns is selected.

In the Image Mode, ASAR operates in one of seven predetermined swaths with either vertically or horizontally polarised radiation; the same polarisation is used for transmit and receive (i.e., HH or VV). See figure1.10 .

Figure 1.10 Image Mode

The Wave Mode uses the same swaths and polarisations as Image Mode. However, a continuous strip of data is not required. Instead, small areas of the ocean are imaged at regular intervals along the swath. This intermittent operation provides a low data rate, such that the data can be stored on board the satellite, rather than being downlinked immediately to the ground station. See figure1.11 .

Figure 1.11 Wave Mode

ASAR ScanSAR Modes (Wide Swath, Global Monitoring, and Alternating Polarisation)

While operating as a stripmap SAR, ASAR is limited to a narrow swath which is imposed by the ambiguity limitation. This constraint can be overcome by utilising the ScanSAR principle, which achieves swath widening by the use of an antenna beam which is electronically steerable in elevation.

Radar images can then be synthesised by scanning the incidence angle and sequentially synthesising images for the different beam positions. The area imaged from each particular beam is said to form a sub-swath. The principle of the ScanSAR is to share the radar operation time between two or more separate sub-swaths in such a way as to obtain full image coverage of each.

The system transmits pulses to, and receives echoes from, a sub-swath for a period long enough to synthesise a radar image of the area within the beam footprint at the required resolution. It then switches beams to illuminate a different sub-swath and continues in this manner until the full-wide swath is covered, at which point it returns to the original sub-swath and the scanning cycle is repeated.

The imaging operation is, therefore, split into a series of bursts of pulses, each burst providing returns from one of the sub-swaths. Each burst will be processed to provide an image of a section of the corresponding sub-swath. The imaging operations must therefore be such that it cycles around the full set of sub-swaths sufficiently rapidly for the imaged sections in any one sub-swath to be adjoining or overlapping.

ASAR operates according to the ScanSAR principle, as described above, in two measurement modes: the Wide Swath Mode and Global Monitoring Mode. These use five predetermined overlapping antenna beams which cover the wide swath.

Figure 1.12 Wide Swath Mode

Figure 1.13 Global Monitoring Mode

An additional ASAR measurement mode, called Alternating Polarisation Mode, has also been defined which employs a modified ScanSAR technique. Instead of scanning between different elevation sub-swaths, the Alternating Polarisation Mode (co-polar) scans between two polarisations, HH and VV, within a single swath (which is preselected, as for Image and Wave Modes). In addition, there are two cross-polar modes, where the transmit pulses are all H or all V polarisation, with the receive chain operating alternatively in H and V, as in the CO-polar mode.

Figure 1.14 Alternating Polarisation Mode

The ASAR Instrument's Capabilities

Table 1.1 summarises the ASAR capabilities.

The use of the ASAR generic processor for near real-time (NRT) and off-line processing in the processing and archiving centres (PACs) and national stations offering ESA services, is a simplification for processing and product validation. This allows full product compatibility between the different processing centres.