SPACE Shuttle Atlantis is ready to head for orbit to service one of the most important facilities the US National Aeronautics and Space Administration possesses.It's the Tracking and Data Relay Satellite (TDRS) system. Unmanned scientific satellites such as the Hubble space telescope and orbiting astronauts alike depend on this planet-girdling network to return their data and keep virtually constant communication with Earth. In fact, the system is so critical that TDRS network official Philip Liebrecht at the Goddard Space Flight Center in Greenbelt, Md., says, "It's not possible to operate the NASA space network without [it]." At this writing, the Atlantis countdown at the Kennedy Space Center in Florida was proceeding toward a launch July 24 sometime between 10:55 a.m. and 3:12 p.m. Eastern daylight time. NASA wants to make sure the vital TDRS communications system, which has replaced the old ground-based tracking network, has ample capacity to continue to function reliably. So Atlantis is to carry up a $120 million TDRS unit to add to the present three-satellite network. Deploying this satellite about six hours after launch is the single most important job mission commander John E. Blaha and his four-man crew have on their mission of roughly nine days. The bulk of their time will be devoted to a variety of tasks and experiments. Among other activities, they plan to make precise measurements of the stratospheric ozone layer. These data will be used to check the accuracy of regular ozone monitoring instruments on several NASA satellites. To some extent, the development of the TDRS system has paralleled that of the shuttle itself. The old way of tracking earth-orbiting satellites from a network of ground stations was inefficient and becoming too costly by the late 1970s. Especially with the advent of the space shuttle, there was a need for continuous communication with spacecraft in low-Earth orbit. The solution was a system of communications relay satellites in geosynchronous orbit. In this orbit at 22,300 miles height, a satellite moves at the same rate Earth turns and remains roughly over a given spot on the ground. A network of two to three satellites could relay communications with spacecraft in lower orbits using a single ground station - in this case, at White Sands, N.M. Like the space shuttle, the TDRS system has provided its share of drama. Indeed, its history is intimately linked with that of the lost shuttle Challenger. The first TDRS was deployed April 4, 1983, during Challenger's maiden flight. It immediately suffered a spectacular failure. The booster rocket that was to lift it to geosynchronous orbit malfunctioned. The satellite was stranded. This started one of the most demanding spacecraft rescue efforts NASA had ever undertaken. Controllers used the craft's small maneuvering rockets to nudge it to its proper orbit over a 57-day period. Afterward, project manager Ronald K. Browning at Goddard remarked that "we've saved spacecraft before but never anything of this magnitude." Then, in January 1986, came the heartbreak of the Challenger disaster. The second TDRS that was to give NASA nearly complete communications coverage was lost along with the shuttle. With the return of the shuttle system to service, two more TDRS's have been orbited, in September 1988 and March 1989. Now a fifth unit is to be deployed on the 42nd shuttle mission - the ninth flight for Atlantis.