Earth‐Orbiting Satellites, Data Receiving and Handling Facilities
Authors: Force Charles T. , Horan Stephen 
Published: October 2003

Abstract
Ground support of spacecraft is often referred to as “TT&C,” for tracking, telemetry and command. Tracking generally refers to measuring the position of a spacecraft, telemetry to information carried on the radio‐frequency (RF) downlink signal, and command to information transmitted from the ground to a spacecraft. Increasingly, these terms are somewhat narrow to describe the action fully, as, for example, transmission of voice and video on the uplink. Though these functions were initially stand‐alone elements of space operations, technological advances and economic pressures have blurred distinctions, both among them and with related operational activities. Readers should be aware that the boundaries of data receiving and handling functions are dynamic.
The genesis of data receiving and handling facilities was primarily in the military launch ranges. Differences and limitations necessitated changes as space activities developed. Geographically, coverage expanded globally. Operationally, mission durations became more continuous and ground activities increasingly interactive.
Early space flights were brief. Operations were simple, few, and far between. With no existing infrastructure, supporting systems, procedures, and operators had to be put in place for each mission—usually unique for each flight. Much of this discussion focuses on NASA, it is in a larger sense the history of most space operations and thus has broader significance. NASA originally had three separate networks of ground stations for controlling and tracking spacecraft.
Telemetry and command systems were independent of each other, and tracking relied primarily on radio interferometery. The Deep Space Network (DSN) was used for interplanetary spacecraft; its notable characteristic was large antennae and very sensitive receivers. These stations were located around the world so that all directions in the celestial sphere were continuously in view of one of the stations. The Manned Space Flight Network (MSFN) was established just for “manned” missions.
Tracking stations were originally the focal points for executing space operations. Prepass planning was accomplished by the flight projects and the plan communicated—often via teletype messages on HF radio—to the stations. As ground communications became sufficiently reliable, control was centralized; the ground stations became remote communications and tracking terminals. Control eventually became real time and centralized at mission control centers as reliable communications became available.
The subsequent evolution of data receiving and handling methods was synergistically enabled by advances in technology, although arguably more influenced by competitive factors than by either technical performance or economics. Ground facilities were usually viewed as infrastructure and were funded separately from the spacecraft programs. Thus space–ground cost trades were seldom a major consideration. As a result, ground facilities must often accommodate a number of different approaches to command and telemetry. This has led to such situations as one NASA Center now using four different ways to transmit telemetric data to the ground: time division multiplex (TDM) and three different packet data formats—each requires singular ground data‐handling systems.
The economic benefits of standardization are persistently apparent, and progress is slowly being made.