Dynamic Environment Communications Analysis Testbed (DECAT) and its applications to Space Shuttle Orbiter and Space Station usage of the Global Positioning System (GPS)

The Communication System Simulation Laboratory (CSSL) provides a facility for analyzing and simulating space communication systems. The CSSL has simulation tools that model NASA communication systems and the environment in which they operate. One of these tools is a radio frequency coverage analysis tool called the Dynamic Environment Communications Analysis Testbed (DECAT). DECAT provides the capabilities to model the orbital and structural dynamics of space vehicles and to incorporate user-written control functions to calculate such values as the occurrence of planetary or structural blockage. DECAT can analyze a space vehicle´s usage of the Global Positioning System (GPS) from a communication link point of view by using control functions to calculate which GPS space vehicles (SVs) are available based on Earth blockage, structural blockage, vehicle antenna pattern, and signal margin. DECAT also contains control functions to calculate the geometric dilution of precision (GDOP) and position dilution of precision (PDOP) given combinations of available SVs. The paper focuses on DECAT simulations of Space Shuttle Orbiter and Space Station usage of GPS. For Space Shuttle mission 51, DECAT was used to investigate usage of GPS for relative positioning for both the Space Shuttle and the Orbiting and Retrievable Far and Extreme Ultraviolet Spectrometer (ORFEUS) Shuttle Pallet Satellite (SPAS). How often the Space Shuttle can receive four GPS SVs and how often the Space Shuttle and SPAS select the same four SVs for solution were investigated. For the Space Station, DECAT was used to investigate GPS coverage