Design & Testing of Redundant Radar Altimeters for Reentry Vehicle Using Commercial Hardware

This paper describes the adaptation and testing of a commercial Radar Altimeter (RALT) to provide fault tolerant altitude data on the Sierra Space (formerly Sierra Nevada Corporation, Space Systems Group) Dream Chaser® spaceplane. Failure to measure altitude above ground on an autonomous vehicle during the landing phase of the mission could result in loss of vehicle; therefore, a redundant RALT system design is needed. A design solution using three time synchronized Ultra 4503 model RALTs was implemented to allow simultaneous operation while preventing radio interference between units. The design solution was flight tested on two different atmospheric flight platforms, demonstrating that multiple RALTs could successfully operate simultaneously. The flight tests, however, also exposed other unique implementation issues that were addressed through modification of the RALT programming. A review of the aerospace environment impacts on the commercial hardware is also presented. The Dream Chaser vehicle is designed for prolonged on-orbit operations followed by entry, descent, and landing (EDL) in a manner similar to the National Aeronautics and Space Administration's (NASA) Space Shuttle program. While the RALTs will only be operated within the pressurized volume of the Dream Chaser during EDL, the ability for the commercial hardware to survive the low-Earth orbit (LEO) radiation environment, lightning strike effects, and other environments is reviewed and mitigation approaches discussed.