The Scan Mirror Assembly (SMA) provides the crosstrack scanning motion to develop the 185 km long scene swath. The SMA consists of a flat mirror supported by flex pivots on each side (which have compensators to equalize pivot reaction torque), a torquer, a scan angle monitor (SAM) , 2 leaf spring bumpers and scan mirror electronics (SME). The motion of the mirror in each direction is stopped by the bumper, and is boosted by precision torque pulses during the turnaround period. The amount of torque applied is controlled by the SME microprocessor as determined from the SAM mirror angle pulses. The resulting active scan in each direction is closely controlled to 60,743 µs. The SMA derives timing from a 10 MHz clock routed by buffers in the multiplexer. The 10 MHz is counted down from the 74.9 MHz system clock by circuits in whichever multiplexer is providing the system timing. SAM mirror angle pulses are used by the multiplexer to synchronize the detected scene data. There are redundant sets of scan mirror electronics, SME 1 and SME 2. SME 2 is an identical back up electronic package to SME 1. Additionally, both SMEs have a primary SAM mode of operation and a back up Bumper Mode of operation. SMA characteristics are listed in Table 3.1.
| Table 3.1 | |
|---|---|
| Swath width at 0 degrees North | 185 km |
| Swath width at 40 degrees North | 187 km |
| Scan length at 0 degrees North | 480 m |
| Scan length at 40 degrees North | 484 m |
| Active scan amplitude | 7.695 degrees |
| Scan period | 142.925 milliseconds |
| Scan Frequency | 6.997 Hz |
| Active scan time | 60.743 milliseconds |
| Turn around time | 10.719 milliseconds |
| Object plane scan rate | 4.42191 rad/sec |
| Mirror scan rate | 2.21095 |
| 30 m IFOV dwell time | 9.611 microseconds |
| Scan line length (excludes turn around) | 6320 IFOVs |
| Inertia | < 9.4 in-lb-(sec*sec) |
| Clear aperture | 21.050 in x 16.250 in |
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Last Updated on July 13, 1998