Radarsat-2 Mission: Overview And Development Status
Table 1, RADARSAT-2 Beam Modes.
RADARSAT-1 modes
with single and dual
polarization
Transmit H or V
Receive H or V or
(H and V) |
BEAM MODE |
Nominal Swath Width |
Swath coverage to left or right of ground track |
Approximate Resolution Rng X Az |
| Standard |
100 km |
250km-750km |
25m x 28m |
| Wide |
150 km |
250km-650km |
25m x 28m |
| Low Incidence |
170 km |
125km-300km |
40m x 28m |
| High Incidence |
70 km |
750km-1000km |
20m x 28m |
| Fine |
50 km |
525km-750km |
10m x 9m |
| ScanSAR Wide |
500 km |
250km-750km |
100m x 100m |
| ScanSAR Narrow |
300 km |
300km-720km |
50m x 50m |
| Fully -Polarimetric modes Transmit H and V on alternate
Pulse Receive H and V on every pulse |
Standard QP |
25 km |
250km-600km |
25m x 28m |
| Fine QP |
25 km |
400km-600km |
11m x 9m |
| Selective Single |
Triple Fine |
50 km |
400km-750km |
11m x 9m |
| Polarization Transmit H or V Receive H or V |
Ulter-Fine Wide |
20 km |
400km-550km |
3m x 3m |
| Ultra-Fine Wide |
10 km |
400km-550km |
3m x 3m |
4 Spacecraft Characteristics
4.1 SAR Sensor
The RADARSAT-2 SAR antenna will consist of a distributed network of transmit/receive modules arranged on two wings. Some of the SAR characteristics are:
-
Linear FM pulse generation with bandwidths from 12MHz to 100MHz;
- Active phased array antenna,16 rows of 32 T/R modules, each feeding a subarray of 20 elements;
- T/R modules with two power settings and temperature compensation of phase and amplitude;
- Each T/R module can transmit either on H or V polarization, and can receive on both polarizations;
- Weighting: phase only on transmit; amplitude and phase on receive;
- Any subset of columns can be switched off on transmit or receive;
- Two independent receive channels used either for H and V polarizations or for separate reception on the leading and trailing antenna wings.
4.2 Data Handling
All image data is stored in high data rate (400Mbps) solid-state recorders (SSR). These have higher reliability than tape systems used in the past and will permit random image file access. The SSRs have a capacity of 2 x 128 Gbit at the beginning of the mission, decreasing to 2 x 100 Gbit at the end. They can accept data at rates up to 400 Mbps. Block Adaptive Quantization (BAQ) is used to encode signal data with a selectable wordlength (normally 4 bits I + 4 bits Q). The downlink occurs on two 105 Mbps X-band links. The high-power X-Band transmitter will be able to downlink images to ground stations having a minimum 3-metre receiving antenna.
This smaller antenna size will allow a lower "cost of entry" for new ground stations. Encryption is available for command & control as well as for the downlink of signal data.
4.3 Position Determination
-
RADARSAT-2 includes on-board GPS receivers for position determination.
-
The absolute position will be determined to within 60m x, y, z 3s in real time on board the spacecraft, to be downlinked with image data. This accuracy will be achieved with the onboard GPS receiver and onboard orbit models.
-
The RADARSAT-2 absolute position will be determined to within 15m x, y, z 3s within 24 hours of the latest available GPS orbit data. The GPS receiver onboard will collect position measurements that will be processed on the ground to improve the accuracy.
-
RADARSAT-2 is yaw-steered (unlike RADARSAT-1). The yaw steering, combined with the improved attitude control of RADARSAT-2, will simplify image processing and improve image quality.
5 SPECIAL MODES
5.1 UltraFine Imaging Mode
Ultra-Fine mode is designed to provide approximately 3m x 3m resolution images covering swaths of 20km at incidence angles from 30º to 40º.
A pulse bandwidth of 100MHz is used to achieve the required resolution in range. In azimuth a "Dual-Receive" imaging technique is used to achieve the 3 m resolution.
In the Dual-Receive mode of operation, two echoes, one from each wing, are recorded for every pulse transmitted. Because the phase centres of the two receive antennas are in different positions, the two-way path length for the two simultaneous returns are different and so the two returns effectively provide separate samples along the synthetic aperture. With an appropriate choice of PRF, known as the "ideal PRF", the resulting samples will be equally spaced, and standard SAR processing techniques can be employed.
To provide the Doppler bandwidth required to achieve 3m-resolution, an azimuth beamwidth of approximately 0.5º is needed. This is achieved by using a reduced aperture or defocused beam on transmit
In order to improve sensitivity, the T/R modules can be operated at higher transmit power than in the other modes. In addition, the pulse length may be increased. Figure 2 illustrates the Dual-Receive mode of operation. The solid line indicates one pulse transmission with two echoes; the dashed line indicates the pulse and echoes from the next pulse repetition interval.
The Dual-Receive mode is the baseline for Ultra-Fine imaging. However, conventional imaging with a broadened beam (' beam spoiling') was considered as an alternative for Ultra-Fine mode. Dual-Receive mode has the following advantages over beam spoiling:
-
better sensitivity for the same power; and
- increased access at high angles because of low PRF.
Disadvantages of the Dual-Receive mode are:
-
some constraints on PRFs; and
- it is unconventional.
