International Telemetering Conference Proceedings / October 30-November 02, 1995 / Riviera Hotel, Las Vegas, Nevada / Since June of 1993, an experimental GPS receiver system has been orbiting the earth aboard a small, low-altitude, polar-orbiting satellite called RADCAL. The purpose of the experiment was to prove the concept of using GPS for satellite navigation. If successful, the system would also provide a backup to the satellite's primary navigation beacon. The goal: provide position and velocity data to an accuracy of three to five meters, and provide attitude data to within a degree. The configuration of the RADCAL GPS experiment precluded realtime feedback loops for navigation; the data was stored and downloaded after a designated collection period. On the ground, a lengthy process was used to yield the position and attitude data days after the collection event. The GPS receivers and ground equipment were configured in several modes; they ultimately yielded a position accuracy of five meters, and attitude of two degrees. This was the original goal, and the experiment was considered successful. However, one of the receivers failed in November 1993, and the other failed in January 1995. The GPS receivers were commercially available and not spaceflight proven; they were suspected of being vulnerable to single-event upsets and latchups. This turned out to be the cause of the failure of both receivers. The interface between the GPS receivers and RADCAL's other subsystems proved to be the area which could not tolerate corrupt data. The single-event latchups problems would ultimately lead to the failure of the receivers. These difficulties, as well as other lesser obstacles, provide a host of lessons learned for future satellite navigation systems.

International Telemetering Conference Proceedings / October 17-20, 1994 / Town & Country Hotel and Conference Center, San Diego, California / An increasing number of satellite users and manufacturers are looking to lightweight, inexpensive satellites as substitutes to traditional large, expensive satellites with multiple payloads. Neither the Department of Defense nor the commercial sector can bear the financial or reputational consequences associated with massive program failures. With the low cost and weight of these new satellites, users can achieve mission success without great risk. One example of this new class of inexpensive spacecraft is the RADCAL (RADar CALibration) satellite. Detachment 2, Space & Missile Systems Center at Sunnyvale, CA operates the satellite. RADCAL is a 200-pound polar orbiting satellite with an average altitude of 450 miles. It is primarily used by 77 worldwide radars to calibrate their systems to within five meter accuracy. Also flying on board RADCAL is a communication payload for remote field users with small radios. The RADCAL program has satisfied all mission requirements. However, with the limited size and cost come certain challenges, both in the satellite and on the ground. Pre-launch testing was not as comprehensive as with more expensive programs; anomalies have arisen that require extensive workarounds. Data management is not a straightforward task, and it is sometimes difficult and inexact to track satellite performance. These challenges are presented with their solutions in the following discussion; this paper addresses the functional, operational, and testing aspects associated with the RADCAL satellite.