Ch8-1

Computers in Spaceflight: The NASA Experience

- Chapter Eight -

Computers in mission control

[242] Mission control begins when launch processing ends. At the point a missile is committed to flight-as when the Shuttle solid rockets are fired or a liquid-fueled booster rises an inch off the pad-responsibility for monitoring and control of the spacecraft shifts from the launch director and his crew to the flight director's team. Three major tasks occupy the flight controllers: sampling the telemetry stream to make certain everything is going well and to collect science data, doing navigation calculations, and sending commands. Manned and unmanned spacecraft require this support, with manned spacecraft having the advantage of carrying observers and decision makers to supplement what can be done from the ground. To successfully support both types of missions, digital computers must operate on massive amounts of data in real time. Mission control tasks are beyond the abilities of humans alone.

Mission control centers and their equipment are located far from the launch site. NASA's manned mission control began in 1961 with Project Mercury at the Cape Canaveral launch area, but its computers were at Goddard Space Flight Center near Washington, D.C. Since 1964, early in the Gemini program, both computers and controllers have been housed in Building 30 at the Johnson Space Center in Houston. NASA's unmanned near-earth missions are controlled mostly from Goddard, with most deep space missions handled through the Jet Propulsion Laboratory's (JPL) Spaceflight Operations Facility in Pasadena, California.

In addition to control centers, mission support requires numerous tracking stations to collect and format telemetry and radar data to help in monitoring and navigation and to transmit commands. These widely scattered stations and the control centers are linked together by the NASA Communications Network (NASCOM), headquartered at Goddard. The Space Tracking and Data Acquisition Network (STADAN), used to specialize in unmanned spacecraft but, having combined with the Manned Spaceflight Network (MSFN) in 1972, has become the general network. When all the specified Tracking and Data Relay Satellites are in place, they will take over much of the manned flight communications, yet tracking is still a STADAN responsibility. Lunar and planetary probes are the venue of the Deep Space Network, which operates three main stations at Goldstone, California, Madrid, Spain, and Canberra, Australia, each with a variety of antennas ranging up to 64 meters in diameter. The Deep Space Network helped with manned lunar missions when the Apollo spacecraft [243] passed a distance of 10,000 miles from earth^*.

In contrast with on-board computers, computer systems used in control centers and tracking stations have primarily consisted of off-the-shelf equipment. NASA could take this approach to procurement because, so far, adequate processing power to achieve mission objectives has been available in commercial systems. When mission control began in the late 1950s and early 1960s, software technology had not reached the necessary level of sophistication. The prime contractor had to develop completely new operating system software for the Vanguard, Mercury, and Gemini programs, but was able to incorporate large chunks of existing operating systems into those used for Apollo and Shuttle, as well as some later deep space missions. This was possible in part because experience and techniques learned from designing the original operating systems were used in new commercial products.
^* For the story of the tracking and communication networks, see William R. Corliss, Histories of the Space Tracking and Data Acquisition Network (STADAN), the Manned Space Flight Network (MSFN). and the NASA Communications Network (NASCOM), NASA CR-140390, June, 1974, and N.A. Renzetti, ed., A History of the Deep Space Network From Inception to January 1, 1969. Jet Propulsion Laboratory TR 32-1533, September 1, 1971. Each has considerable detail about the technical developments involved, including the decision to use computers at stations.