TRADEOFF STUDIESImages can be obtained from an orbiting spacecraft either by taking full pictures aimed at the target area or by sweeping through it using a push broom technique relying on the spacecraft motion. Although the latter option enables to scan larger areas with simpler components (available technology limits the size of CCD matrices and the speed of reading them), it requires an elaborate AD&C subsystem and complex processing and timing, and is therefore excluded. The amounts of data produced by imagery satellites create severe memory storage and transmission rate problems. The use of digital imagery -as opposed to analog imagery- enables to apply compression algorithms and use the CPU digital storage (in addition to other features such as encryption). Storage devices' considerations make the use of delayed video imagery difficult. Live video is however possible, although it requires a large transmitter power, and is considered for future SQUIRTs. The other option is to take and store single digital pictures. In order obtain digital pictures, Previous microsatellites have typically used regular video cameras (NTSC) with a flash digitizer to capture one "frame" or picture. Large and costly digital cameras have also been available in the past. The main drawback of this approach is that the video camera had mostly analog circuitry which were less susceptible to radiation effects, and the frame digitizers were custom built with space qualified parts, which resulted in a more reliable, though much more costly package. We are aware of two recently released low cost digital cameras which implement efficient image compression algorithms, and are already designed to interface with external computers, making them particularly cost effective to integrate: the Logitech Fotoman Plus and the Apple Quicktake. In addition to JPEG compression, the Fotoman implements a thumbnail system, allowing pictures to be previewed prior to downloading. Availability of the Fotoman Plus, the second generation Fotoman, is better than the Apple Quicktake. Inexpensive Fotoman are also available, allowing for testing of the modifcations to the Fotoman Plus, before modifying the more expensive Fotoman Plus. Use of the Apple Quicktake in the SAPPHIRE project is therefore excluded. Examination of the Fotoman Plus reveals the use of an EPROM and the DRAM, both of which are not exceptionally radiation resistant. Avenues to improve radiation resistance are replacing the radiation resistant chips or to power down the Fotoman Plus when it is not in operation. The chips are all surface-mounted, which does not lend itself to replacement. The Fotoman Plus has two batteries, one lithium and two Ni-Cad AA. The Ni-Cad batteries are used to maintain the 32 files and the Fotoware in DRAM, while the lithium is used to maintain only the Fotoware. To power down the system, the batteries must be removed, so that the Fotoman Plus does not remain in a stand-by state, ready to take a picture. This also results in the loss of the Fotoware. Powering down the Fotoman Plus, yields better radiation resistance. Replacing the chips with radiation resistant analogs is ideal, but due to the difficulty in replacing the chips, powering down the Fotoman Plus is chosen. The angle of view must be rotated 20 degrees away from the axis of the antenna on the top of the SAPPHIRE, so that the antenna is not in the field of view of the camera. This can be done by rotating the entire board or by removing the lens and rotating the detached lens. Removing the lens yields a larger possible rotation, but rotation of the entire board yields the necessary 20 degrees, and is much simpler to accomplish, by building a suitable aluminum chassis, rather than removing a lens attached to the CCD attached to a flexible circuit film, which is surface mounted to the printed circuit board (PCB). ________________________________________________________ |