Apollo Communications

DR. CRANFIELD:

In the summer you met and heard Captain James Lovell, the Astronaut, tell the dramatic story of the flight of Apollo XIII and his experiences in earlier space journeys. At that time I promised you that there would be a sequel in which a space scientist would tell you some little bit about how communications' contact and control of these fantastic flights is kept. The man who is to tell us about this has spent his adult life preparing for this career and Captain James Lovell may have been glad of it when he and his companions of Apollo XIII were returned safely. Our speaker was born in North Dakota April 5th, 1920. With the exception of that occasion when his mother insists he was four days late he has been on time ever since. Prophetically his birthplace was a town called "Banner." I am sure it is more than a flag stop on the railway, indeed it must be "Banner" as the dictionary puts it, "The Flag of a king." From his age at the time of "Pearl Harbor" I would judge he went directly into the U.S. Air Force where he spent four valorous years. He came out of it to attend the University of Minnesota and here he studied electrical engineering and from that excellent university he obtained his Bachelor of Science degree in 1949. Once qualified, he went almost directly into the Collins Radio Company with whom he has remained with distinction ever since. It must give astronauts great confidence to appreciate that the persons in charge of the design, installation and operation of the systems of navigation and communication upon which they depend, have explored scientifically the atmosphere and beyond for several decades. They have precise knowledge of the earth's intimate atmosphere the Troposphere which extends upwards around us for ten miles at the equator and half that at the poles and which is delineated by the graded decrease in temperature as one ascends. Beyond this is the ionosphere where the Aurora Borealis is familiar to you at its upper limits as are the falling stars that burn out at the nearest edge of this electrically charged atmosphere. Knowledge of the characteristics of such space had to be known before a manned moon shot could be successful. It is somewhat more complicated than sending smoke signals to keep contact with astronauts in flight, and on the moon, as you shall hear.

The concept of using satellites for communication depended upon work done under our speaker's direction in 1960 in his development of antenna through careful research. Following this, our speaker held progressively more advanced positions in the Engineering Division at Dallas to his present appointment which he first held in 1966. He is currently Vice President, Telecommunications Systems Engineering of the Collins Radio Company. For the benefit of any gentlemen farmers in the audience he has a reputation as a breeder of Registered Black Angus cattle.

I give you now Arlon J. Svien, Vice-President of the Collins Radio Company. His topic will be "Apollo Communications."

MR. SVIEN:

When the courageous Commander Armstrong made his "small step for man and giant leap for mankind" at 10:56 p.m. on July 25, 1969, it dramatically confirmed President John F. Kennedy's statement of May 1961 that the United States would place a man on the moon "before this decade is out." That commitment set in motion the greatest mobilization of men and resources ever undertaken for a peaceful project of science and exploration. The building of the hardware and associated systems required the concerted efforts of 20,000 industrial contractors, scores of university laboratories, and 400,000 people--all guided by or co-operating with the National Aeronautics and Space Administration (NASA).

Preparation of the Apollo communication system was one of those tasks.

Never before had man assembled such a vast team of personnel dedicated to a single communications requirement. The problem was indeed formidable. Not only did the communications system have to provide a means for transmitting the voice and television signals, but it also had to transmit to earth the physical status of the spacecraft and the astronauts. Through this communications link, the astronauts had at their disposal the vast earth support team and all of their analysis equipment.

According to John Noble Wilford of the New York Times, "Of all the vehicles and instruments developed for the moon mission, nothing quite compared in breadth and sophistication with the worldwide network of tracking antennas, (and) communications relay stations . . . that supported each Apollo flight."

The "voice-link" made it possible for the astronauts to describe the view from space, tell of their head colds and talk with their wives back on earth. It also allowed ground controllers to give verbal instructions to the crew, read out new data for the spacecraft's computer and relieve the monotony of space travel by relaying baseball scores and the morning headlines. In addition, there was coded data from the spacecraft--called telemetry. Telemetry involved transmissions by radio signals of information from onboard sensors to ground stations. This function provided Mission Control with essential data about flight conditions.

To all these communications demands was added the requirement that all transmission be carried by a single radio frequency; hence the term "Unified S-Band System." SBand is a term used to identify the general band of radio frequencies used. Previous Mercury and Gemini earth-orbiting spacecraft used four different sets of radio and radar equipment operating on different frequencies to satisfy the above requirements.

The spacecraft and ground stations represented two widely different problems involving equipment of similar capability. Because of limited power and space aboard the lunar module, the ground station had to provide the greater part of total system sensitivity and transmission efficiency. This was accomplished with large dish-type antennas and high powered transmitters.

The radio frequencies used provided only line-of-sight communication. The spacecraft orbits the earth at a relatively low altitude, so at any instant, its horizon is limited. For example, if we scale the earth down to a ball, seven feet in diameter, the spacecraft would be orbiting only one inch above the surface.

Therefore, to keep the spacecraft in sight, the earth orbital tracking network consisted of 12 strategically located ground stations. These stations used antennas of 30 feet in diameter. Four instrumented ships and half a dozen or more instrumented aircraft were used for backup.

Because of its ideally located land masses and islands, the British Commonwealth was called upon to provide many of the sites for the tracking stations. These included sites in Grand Bahama Island, Antigua, the Ascension Island, Bermuda, and Australia.

Three stations with 85-foot diameter antennas, designed for the greater distances involved in the lunar phase of the flight, were installed at Madrid, Spain; Goldstone, California; and Canberra, Australia. These three deep-space antenna stations were situated so that, as the earth rotated, at least one always faced the moon so as to provide constant visibility to the spacecraft during the lunar phase. In addition, there were two 210-foot antennas, one at Goldstone and one in Australia, that were pressed into use. These antennas were originally installed for interplanetary missions but were modified for Apollo support. They provided the extra power needed for color television.

With the earth stations distributed throughout the world in this manner, the problem of providing communication links from the stations to the Houston Mission Control Center required one of the world's most complex communications systems. A total of nearly one-half million miles of circuit paths were necessary. Virtually every type of transmission system was employed: wire lines, oceanic cable, microwave, short wave radio, aircraft, and satellite relay. Arranging this vast network required the co-operation of many nations. Typical was the network that provided the link to Canberra, which included land lines over two-thirds of Australia, transoceanic cable to Canada via Hawaii, cable and microwave to NASA Headquarters and microwave to the Houston Mission Control Center. You may be interested to know that a cable across Canada and another cable to London were included in the complex Apollo communications network.

The equipment--from the huge deep-space antennas to the smallest microcircuit--was designed to the most exacting performance standards.

For example, the weight of the 30-foot antenna dish installed at Grand Bahama Island, and other sites, is approximately 80 tons. Yet, despite this enormous weight, these antennas can be rotated at 3 to 5 degrees per second. They also can be moved as slowly as two one-thousandths of a degree per second--a pace slower than that of an hour-hand on a clock.

The S-Band antennas can withstand winds greater than 200 miles per hour.

These antennas can pick up from space a signal as weak as minus 170 dbm--which, expressed as a fraction of a watt, would be written as a numerator of one and a denominator of one followed by 20 zeros.

The antennas had to be built and installed with the extraordinary precision required to track an object over distances and at speeds such as those involved in the lunar flight programme. The surface accuracy of the huge parabola-shaped dish, for example, cannot vary more than the approximate thickness of a paper plate.

Although each tooth in the huge bull gears, or main drive gears, transmits a force of more than 41,000 pounds, tolerances on the gears are three to five one-thousandths of an inch, or about the thickness of a postage stamp.

In designing the system that would support Apollo earth-orbit and lunar missions, it was obvious that the quality of the network would have to be advanced far beyond the capabilities of previous space communications systems.

The progress made between the early Mercury days and the Apollo flights was amazing. For example, during the seven minutes a Mercury capsule was in range of the station at Carnarvon, Australia, the system was able to transmit data summarizing about 30 or 40 onboard functions. With Apollo, the data on 500 functions can be communicated to Houston Control Center within two seconds after the information has been generated in the spacecraft.

The challenge of designing a system to meet all of these requirements seemed overwhelming, and certainly, before the advent of solid state technology, would have been impossible. Indeed, if for no other reason than the communications and electronic requirements, the mission would have been impossible a decade earlier. Only the reduced weight and power requirements of solid state (transistor) devices permitted an adequate computational and communication system aboard the spacecraft. Certainly the computer system that guided the Lunar Expeditionary Module to a safe descent was one of the more important electronic elements of the system.

How well the electronic design challenge was met can be determined by examining the actual performance of the system during the several missions and its contribution to the total success of the program.

The whole world became aware of the success of the voice and television communications from the spacecraft and the lunar surface. However, a hidden bit of information that rode silently in the same carrier frequency that conveyed the pictures and the speech was, perhaps, more important to the success of the mission. This was the information which provided a continuous and accurate position and trajectory report of the spacecraft. First, by sensing the direction of the arrival of the signal, the large 85-foot diameter antennas provided a constant angular position report of the spacecraft with respect to a point on the earth. This was achieved with an accuracy of 0.015 milliradian, or a maximum error of 30 miles at lunar distances. More remarkable, perhaps, was the capability of the system in measuring a second dimension--distance from the receiving station. By measuring the round-trip time of a signal from the earth station to the spacecraft and back, it was possible to determine range within approximately 15 meters, or less than 50 feet. As the spacecraft neared the moon at a distance of 238,000 miles, the round trip time for a radio signal travelling at the speed of light was approximately 2 1/2 seconds. This distance was measured with maximum error of less than the length of the average house.

Thus, two of the three parameters necessary to the navigation of the spacecraft were measured. The third parameter, velocity, was measured by observing the change in frequency of the received signal as a result of doppler shift, the same effect that causes a locomotive whistle to change pitch as it approaches or retreats from the observer. Here the communication system performed with extreme accuracy. Measurements of velocity were made within 5 millimeters per second, or one-thousandth of a mile per hour. This information, derived from the communication signal, was constantly relayed to NASA facilities, where electronic computers continually checked the trajectory of the Apollo vehicle. When necessary, course corrections were transmitted to the astronauts. The speed and accuracy with which these calculations can be made was demonstrated during the Apollo 12 mission. As the spacecraft appeared from the far side of the moon on its lunar orbit, the earth station, through its tracking and ranging system, updated the course position at the earth station. The resulting calculations were transmitted to the spacecraft and a 4,200-foot landing position correction was entered into LEM descent computer. This resulted in an extremely precise landing, as witnessed by hundreds of millions of people around the world.

Man's greatest undertaking of the twentieth century is now history. The total effort that went into putting men on the moon is beyond measurement. From the dramatic liftoff and earth orbit. . . through translunar injection . . . the walk on the moon . . . and the triumphant return to earth, the feat was a remarkable example of the peacetime efforts of men working toward a common goal.

Certainly the communication system helped demonstrate the achievement to the entire world through the medium of television and voice transmission.

As a communicator, I can only hope that, properly directed, this worldwide communication network could foster among all peoples of the world a common bond of understanding much like the kinship that all of us felt for the astronauts during the historic voyage of Apollo 11.

Thanks of the meeting were expressed by Major Gen. George Kitching, C.B.E., D.S.O., O.ST.J., C.D.