STS-51 was a Space Shuttle Discovery mission that launched the Advanced Communications Technology Satellite ACTS in September 1993. The flight also featured the deployment and retrieval of the SPAS-ORFEUS satellite and its IMAX camera, which captured spectacular footage of Discovery in space. A spacewalk was also performed during the mission to evaluate tools and techniques for the STS-61 Hubble Space Telescope servicing mission later that year. STS-51 was the first shuttle mission to fly a GPS receiver, a Trimble TANS Quadrex. It was mounted in an overhead window where limited field of view and signal attenuation from the glass severely impacted receiver performance.