RAFT RF SAFETY AND POWER INHIBITS 15 SEP 2004 ------------------------------------------------------------------------ The RAFT transmitters operate at 4 Watts and are at least 9 dB over the emission level considered to be a catastrophic hazard to the Space Shuttle if they begin transmitting in the payload bay with their normal operational antennas. Using the unpowered bus exception to the safety requirements, this would require four inhibits to prevent unsafe transmission. PRE-DEPLOY CONFIGURATION: ------------------------- The two RAFT satellites, however, are fully enclosed in the SSDL 5510 launcher assembly and in addition, the deployable antennas of each of the two RAFT spacecraft are housed in conducting, grounded sleves on the opposite spacecraft thus shorting the antennas and preventing any radiation from the antennas and thirdly, being spring-plunger deployed there is no positive contact of the antenna rod to its drive cable until they are fully deployed. The only emissions possible in this pre- deployment configuration would then be leakage and these levels should be orders of magnitude below the hazard level. This has been verified by test (below). Thus the RAFT transmitters in their pre-deploy configuration are safe. What keeps the RAFT satellites in this "safe" condition is the SSDL 5510 Launcher Deployment circuits which satisfy the 3 inhibit rule with monitoring. RF EMISSIONS TEST PROCEDURE: A +6 dBm transmitter (4mW) was connected to a model RAFT antenna on a model spacecraft. The RF power was measured at 1 meter distance in the direction of maximum signal to be - 20 dBm. Next, the second spacecraft was placed down over the antenna in the pre-deploy config- uration and the signal was measured again to be -60 dBm. Finally, the sense antenna was moved around in all directions to capture any variation in the antenna pattern and all measurements were within +/- 6 dB of the - 60 dBm nominal value. | | Calibration: | | | +-----------+ | +6dBm | | | | Spectrum | +----++ |--|--|--|-------| Analyzer | +--+ |RAFT | | | | +-----------+ |TX|----|-1- | | | Received -20 dBm +--+ +-----+ | +6 dBm +-----+ | Leakage Test: |RAFT|| | | +-----------+ |-2- || | | | | Spectrum | +----++ |--|--|--|-------| Analyzer | +--+ |RAFT | | | | +-----------+ |TX|----|-1- | | | Received -60 dBm +--+ +-----+ | +6 dBm The effective shielding ot the enclosed RAFT antennas in the PRE-Deploy configuration is thus about 40 dB with a margin of error of about +/- 6 dB. When scaled to the full 4 Watt transmit power, this reduces the effective radiated power (ERP) of RAFT in this configuration to less than 2 mW which is 25 dB below the hazard threshold. POST-DEPLOY CONFIGURATION: -------------------------- After the three monitored inhibits are removed and the ejection command is given, the RAFT satellites are accelerated from the SSDL 5510 launcher at about 1.5 meter per second. After one minute, their one second per minute telemetry beacon will commence. The following table details the sequence of events. Distances are from the top of the SSDL Launcher. TIME DISTANCE ACTION SECS RAFT MARS ----- --------- ------------------ 0.0 -.2m -.1m Ejection Command. Satellites begin to accelerate 0.2 0.0m 0.1m Satellites achieve 1.5 m/s and clear launcher Separation switches enable RAFT batteries Control System begins to cold-boot Satellites internal separation springs begin to separate 0.4 0.3m 0.4m Satellites are fully separated with 0.7 and 1.8 m/s Antennas make connection with their matching circuits 0.7 0.5m 0.9m Control system completes booting sequence and begins 1 minute timer to first transmission 4.3 3.0m 7.3m Satellites clear the RF hazard zone 8.7 6.1m 15m HF antennas separate. making them effective. 60 36m 110m First once-per-minute one second Beacon transmissions begins Thus, RAFT1 and MARScom satellites are more than 30 meters from Space Shuttle prior to first transmission. Transmission consists of a single 1 second beacon once a minute until changed by ground command. Looking at it in the time domain, in under 5 seconds, the RAFT transmitters have exited the critical hazard area of the Payload Bay. FURTHER MITIGATION: Since the SSDL Launcher is at least 2 meters from any other payload bay electronics other than the CAPE system to begin with, the RAFT satellites actually have to travel only 1 additional meter to be at least 3 meters from any Space Shuttle or other payload electronics or cabling. This travel will occur in under 1 second and still within the boot time of the RAFT control system. Thus even an immediate software failure that would prematurely initiate transmission would not occur until beyond the safe distance. Bob Bruninga US Naval Academy Satellite Lab 410-293-6417