ASER RANGING
Tracking liftoff improves safety
In October 1997, as a Titan missile carrying the Cassini mission to Saturn rose from its launch pad at the Kennedy Space Center (Cape Canaveral, FL), its orientation was precisely tracked by a trio of Nd:YAG lasers. The lasers were an important new resource for engineers to ensure that, in the early seconds of liftoff, the rocket was rising on course. If it showed signs of veering, the rocket would have been ordered to self-destruct in flight to avoid having it hit the earth, which could have produced sufficient explosive overpressure to disperse Cassini`s hardened payload of plutonium into the environment.
These extensive safety measures were necessary because of the unique danger posed by the plutonium in case of a crash. The plutonium was used as fuel for three on-board radioisotope thermoelectric generators (see Laser Focus World, Oct. 1997, p. 15). According to Joe Salg, staff member for Nichols Research Corp. (Huntsville, AL), the standard radars used to track launches do not provide enough information to establish that the attitude of the rocket is correct. And standard video cameras are overpowered by the optical radiation from the missile plume early in the launch, until their automatic gain-control systems compensate for the glare. And when the systems do compensate, the glare is all that can be detected.
Instead, laser imaging can be used; it is effective at night and can see better than a video camera through the glare, heat, and smoke of a liftoff. Three laser systems were positioned approximately 5 km from the launch pad with roughly 90° between them (see photo). Each system consisted of a Big Sky Laser Technologies (Bozeman, MT) pulsed Nd:YAG laser with a second-harmonic generator producing green output at 532 nm. Each laser illuminated the rocket at a repetition rate of 30 Hz, and each was synchronized to the others with a preset delay.
Accurate timing was provided through satellite global-positioning-system receivers. The 10-ns pulsewidth of the laser operating at approximately 100-mJ/pulse output energy allowed accurate ranging and illumination of the rocket. The return light was focused through a 10-in. Cassegrain telescope to a gated charge-coupled-device (CCD) camera with a filter that selects only the green laser light. Video from the CCD cameras was then sent to the Eastern Range Operations Control Center at Kennedy for real-time display.
This imaging system has been tested on several Titan and shuttle launches and may become mandatory for Range Safety flight-control officers use in the future. The Ballistic Missile Defense Organization`s Innovative Technical Experimentation Facility (operated by Nichols Research) uses launches from Kennedy Space Center as targets of opportunity to conduct research in missile plume phenomenology with both active and passive optical sensors. The laser ranging system was developed at the request of the US Air Force, which operates the Eastern Range.
W. Conard Holton