For example, it will be necessary to transmit coded commands to a remotely controlled lunar vehicle to start/stop TV cameras, or to transmit commands to an interplanetary spacecraft near Venus for refinement of trajectory or attitude.

(d) Communications (ground-to-spacecraft-to-ground): The vital process of exchanging information between astronauts and Earthbased personnel. High-quality voice communications at predetermined times is required to insure the safety of the astronaut.

(e) Communications (ground-to-ground): The very necessary transmission of scientific data and operational information between ground stations and a central control point. These circuits must be very nearly 100-percent reliable during conduct of operations to enable ground station personnel to transmit to each other and to central control.

(f) Tracking and data acquisition computations: The injection conditions; i.e., the direction, velocity, and position of the spacecraft at time of injection into orbit or transfer ellipse and the orbital parameters; i.e., the parameters that completely define an orbit, are calculated by a complex of computers which process telemetry data, determine a responsive course of action, and generate appropriate commands. Success of a mission often depends upon decisions made as a result of monitoring computer outputs during flight.

The functions just described are accomplished by several types of networks of ground stations. Three types of networks have been developed with capabilities which are especially suited to match the differing needs of the space flight missions: The satellite network, the deep space network, and the manned space flight network.

Figure 224 indicates the types of missions, the general requirements of these missions which affect and largely determine the differing characteristics of each network, and the network characteristics which match the differing mission requirements.

Scientific and applications satellites may be launched into orbit in any plane from equatorial to polar because of the need for scientific information, weather data, or space communications over all portions of the Earth. To most effectively and economically meet these needs of the scientific and applications satellites, ground stations are at 14 locations throughout the world. Mobile stations are used to meet special requirements, such as monitoring the insertion of a spacecraft into orbit or the transmission of a critical command, when these events occur outside the visibility of the fixed stations.

Diverse requirements are characteristic of the scientific and applications satellites. The missions require a wide variety of orbits from polar to equatorial, wide variations in instrumentation accuracy and great variations in the amount of data that must be acquired from individual satellites. To match these diverse needs, the satellite network employs an appropriate variety of antennas ranging in size from a simple type similar to a television antenna to the 85-foot diameter parabolic reflector. They also utilize two basic types of tracking systems, and where speed is paramount or where great volumes of data must be handled, automatic data handling equipment is used.

Lunar and planetary missions require tracking and data acquisition support for spacecraft at extreme range or distance from the Earth. To receive the exceedingly weak signals arriving on Earth from distances of millions of miles calls for the use of large antennas and for the best state-of-the-art in receiver technology. Likewise, powerful transmitters are required to transmit readable signals from the Earth to the spacecraft at these distances. These missions also require a capability of continuous two-way contact which is met with stations spaced approximately 120° in longitude around the Earth.

Of prime consideration for the manned space flight missions is the astronauts' safety, a requirement which demands high reliability. The manned space flight network must meet this demand by employing extremely reliable equipment, backup facilities, and computing techniques for rapidly processing vital data. If a mission is to succeed and the safety of astronauts to be assured, vital decisions must be made in near real time. The network employs a number of ground stations, instrumentation ships, and, in the case of the APOLLO lunar flight, instrumented aircraft.

LAUNCH PHASE AND MISSION PHASE

Before proceeding to a description of each of these three networks, it should be pointed out that they are designed primarily to meet the specialized requirements for tracking, data acquisition, and command support once the spacecraft has been launched into its mission Earth orbit or Earth escape trajectory toward the Moon or planets.

NASA spacecraft are launched from either the Atlantic Missile Range, the NASA Wallops Station, or the Pacific Missile Range. These ranges supply most of the specialized launch vehicle tracking and data acquisition facilities to assure proper launch vehicle performance. NASA provides the specialized spacecraft tracking and data acquisition equipment at the launch sites and down range stations which are required to determine that the spacecraft is ready for launch and to monitor spacecraft conditions during the launch phase. Launch vehicle tracking and data acquisition facilities were in general designed to work with the specialized tracking, telemetry, and command transponders that were developed for the testing of ballistics missile rocket systems. As these rockets were adapted for use as launch vehicles for spacecraft the ground support systems and corresponding transponders aboard the launch vehicles were modified only slightly to carry out these new missions of the launch vehicles.

These launch vehicle tracking and data acquisition systems, while most suitable for monitoring and control of the launch vehicles, are not suitable for support of the spacecraft missions once they have been inserted into their mission orbit. The launch vehicle transponders were designed to perform only during the launch phase whereas the space vehicle transponders must perform for weeks, months, or years. The launch vehicle transponders can and do have much higher weights than can be accommodated aboard the orbiting spacecraft. The design of the ground systems to meet these varying requirements also must be tailored to the specific spacecraft mission.

An example should make this more clear. The THOR-DELTA rocket has had 22 successful launches from AMR.* Each launch vehicle has been supported by the tracking and data acquisition facilities of the Atlantic Missile Range. Of these 22 launches, spacecraft mission support has ranged from TIROS, RELAY, TELSTAR, SYNCOM, various EXPLORER scientific satellites, and the Interplanetary Monitoring Platform. Each of these satellites was launched using the same launch vehicle ground support facilities but once in orbit received support from facilities more suitable to meet the mission requirements. In general, the orbital facilities were used to provide crosstracking and data acquisition coverage except for the very specialized requirements of specific projects.

Examples could also be given for SCOUT, THOR-AGENA, ATLAS-AGENA, and the other launch vehicles in the national inventory in which both DOD and NASA make use of the specialized launch vehicle tracking and data acquisition support of the national ranges during launch and then assume the specialized tracking and data acquisition control once the spacecraft is in its mission trajectory. To summarize, although the following discussion covers only the main mission support phase of scientific and application satellites, lunar and planetary probes and manned space vehicles, all these missions require support during the launch vehicle phase by the tracking and data acquisition facilities of the national ranges.

NETWORK DESCRIPTION

Each of the three NASA networks will be described in some detail below. As mentioned before, the networks are: Satellite Network, Deep Space Network, and Manned Space Flight Network.

*Including the Beacon Explorer launch of March 19, 1964, 24 launches have been attempted and all but the first and last were successful.

Satellite network

Figure 225 shows the planned satellite network configuration of 14 stations.

The triangles show the stations which have been the basic stations of the satellite network since shortly after NASA was formed. Many of these stations were part of the original NRL vanguard network which was established during the IGY and then became part of NASA in late 1958. In 1960, NASA added the more northerly stations at Fairbanks, Alaska; East Grand Forks, Minn.; St. Johns, Canada; and Winkfield, England, to provide coverage of highly inclined and polar orbiting satellites.

These stations provide angular tracking information and data acquisition for the small scientific Earth satellites launched by SCOUT and DELTA class launch vehicles.

There has been a continued improvement in satellite technology which has meant a continued growth in satellite data gathering capability, an increasing complexity of satellite systems in orbit plus an increasingly specialized support required for specific satellites such as the SYNCOM synchronous communications satellites.

Figure 226 depicts how the satellite network requirements for data handling have been growing since 1962 and are expected to continue to grow. Despite the fact that the number of NASA-launched scientific and applications satellites decreased from 11 to 8 from 1962 to 1963, the data points collected per day increased from 600,000 to 7 million, or more than a tenfold increase in the same period. This increase in data has occurred for two primary reasons: (a) an increase

in satellite complexity and data-gathering ability; and (b) an increase in the usable life of satellites. The satellites scheduled for launch from 1964 through 1967 indicate a continuation of these trends.

To keep abreast of these growing requirements, a network improvement program is currently in progress which includes the installation of (a) large parabolic antennas, (b) telemetry and command systems that employ digital coding (digital coding is the basis for a communications language that lends itself to automation particularly in highspeed data processing), and (c) automated systems for data processing. The circles shown in figure 225 above are those stations that have parabolic antennas which are required to provide data acquisition support for the observatory satellites such as the Orbiting Astronomical Observatory (OAO), the Polar Orbiting Geophysical Observatory (POGO), and the Eccentric Geophysical Observatory (EGO). Three of the stations have 85-foot parabolic antennas. They are at Fairbanks, Alaska; Rosman, N.C.; and Canberra, Australia. The other three, Quito, Ecuador; Santiago, Chile; and Johannesburg, South Africa, have 40-foot parabolic antennas.

The 85-foot antenna facilities provide the necessary antenna gain to acquire data from highly eccentric satellites such as EGO at its approximately 60,000 mile apogee and to acquire very wide bandwidth transmissions from satellites such as the NIMBUS R. & D. weather satellite. They also provide geographical coverage for the observatory satellites such as POGO and ŎAO. The 40-foot facilities provide the additional geographical coverage needed to support the OAO and POGO satellites plus the perigee coverage of EGO which occurs in the Southern Hemisphere.