The launching last spring of the Navy’s first experimental Transit satellites marked the beginning of a new era in the history of navigation.
As long as man has put to sea in ships, he has sought better ways to fix his position when beyond sight of land. For centuries, his inability to navigate on the high seas limited the “civilized” world to the European continent and the lands bordering the Mediterranean. Only the dawning of the science of celestial navigation led to man’s discovery of the riches, the wilderness, and the vast civilizations that lay beyond the horizons of Europe.
No one knows who first detected the relationship between observed positions of heavenly bodies and the observer’s position on the earth. Perhaps it was an early astronomer who noted that his observations of “fixed” stars varied when he travelled some distance from his primitive observatory. More likely it was a seaman who saw that the sky changed as he sailed up the coast of Asia Minor or down the Scandinavian Peninsula. In either case, celestial navigation probably was the discovery of a man who was looking for something else.
So was the idea of the Transit navigation system.
It was October 1957 when the U.S.S.R. startled the world by launching Sputnik I, a 184-pound artificial satellite that circled the earth every 96 minutes broadcasting continuous wave “beeps” that could be heard by anyone with a high frequency or very high frequency radio receiver. Two of the men who listened to Sputnik’s signals were Dr. William H. Guier and Dr. George Wieffenbach of the Johns Hopkins University’s Applied Physics Laboratory at Silver Spring, Maryland. Guier and Wieffenbach weren’t engaged in satellite research or anything remotely approaching it, but they were curious about the Russian achievement and set about monitoring its signals with a borrowed radio and Dr. Guier’s home tape recorder. They noted an apparent shift in the continuous wave signal’s frequency as the satellite approached, passed overhead, and disappeared over the horizon.
The two scientists correctly identified this shift as a manifestation of the Doppler effect, a scientific phenomenon discovered by an Austrian physicist in 1842. The principle is a simple one: A body emitting wave energy of a constant frequency—a continuous wave transmitter or a tuning fork, for example— produces pulsations that travel at a constant speed and at a given distance apart. The distance between pulsations is a function of their frequency and the speed at which they move through the air (or other medium) and is called wave-length. The relationship between frequency and wave-length is an inverse one. Frequency is what determines pitch.
If a tuning fork is struck and then moved rapidly toward the listener, each successive wave is emitted from a point slightly closer to the observer than was the last. Thus the distance between pulses—in other words, the wave-length—is shortened by whatever distance the tuning fork moved in the time interval between the two pulses. The pulses, now crowded closer together and still travelling at the same speed, arrive more frequently. Thus they appear to the listener to be of slightly higher pitch.
At the moment the fork passes the listener, this effect is cancelled. Momentarily he hears the fork at its true pitch. Then the pitch seems to drop off as the motion of the fork away from his ears spreads the waves further apart, lengthening them and in effect reducing their frequency at the point of observation. The classic illustration of the Doppler effect is the apparent change in pitch of a train whistle as it approaches, crosses, and then speeds away from a grade crossing.
The amount of the Doppler shift, that is the apparent change in pitch, is a direct function of the rate at which the distance between the radiating body and the observer is changing at any instant.
Knowing their exact geographic position and the frequency of the satellite’s radio signals, and measuring the Doppler shift as the artificial moon passed across the sky, Guier and Wieffenbach laboriously computed the parameters of Sputnik’s orbit. By the middle of the winter they had proved that the orbit of any radiating satellite could be computed accurately from Doppler shift data observed at any known point (or points) without other information. It remained for Dr. Frank T. McClure, head of the Research Center at the Applied Physics Laboratory, to suggest that the process might be reversed and the coordinates of a point on earth determined from the Doppler shift of the signal from a satellite in a known orbit. Given the known position and movements of the satellite and an accurately measured Doppler shift, it should be possible, he reasoned, to fix the point at which the shift was observed. If such
a system could be developed, it would be the only all-weather, all-points navigational aid in existence. Thus was Transit born from the curiosity of two scientists and the ingenious application of their “pure” research.
Transit was proposed to the Bureau of Ordnance, which forwarded the project to the Advanced Research Projects Agency, recommending approval. ARPA officially sponsored the project, and in December 1958 assigned technical responsibility to the Navy Department. The project was assigned to the Bureau of Ordnance which logically named the Applied Physics Laboratory as prime contractor for the payload. Responsibility for placing the payload in orbit for the Navy was assigned to the Ballistics Missile Division of the Air Force.
The first satellite was ready for launching in less than a year.
Transit I-A was a 265-pound, 36-inch sphere containing two ultra-stable oscillators each of which broadcast two frequencies, two receivers, telemeter transmitting equipment, an infra-red scanner to measure satellite rotation, and batteries and solenoids to power the sophisticated device. A silver spiral antenna was painted on the outside of the sphere, giving the package a bizarre, almost carnival appearance. It was launched from the Atlantic Missile Range at Cape Canaveral in September 1959, by a three-stage Thor-Able rocket. The launching was a failure, in that an orbit was not achieved, but the payload, during its short life, was an unqualified success.
Some time before the huge Thor blasted off its launching pad, the tiny radios in the Transit package began transmitting the signals they were to send back from space. The signals were clear and right on pitch. Exterior power was cut off and the signals continued, powered by chemical batteries and the two belts of solar cells immediately above and below the sphere’s equator. Then the rocket ignited and the Thor, with its candy-striped payload in its nose, rose from the launching pad.
The flight lasted only 25 minutes. The first and second stages burned properly and boosted the would-be satellite 400 miles into the sky and an eighth of the way around the world. Then the third stage failed to fire and the payload, with the empty second stage and the unburned third stage, re-entered the atmosphere. It burned in the sky somewhere west of Ireland. But from the moment of launching until the satellite’s death, tracking stations recorded its signals, and special equipment at the Naval Air Station in Argentia, Newfoundland, recorded the same information that would have been received if the satellite had achieved orbit.
The Transit satellite was programmed to transmit on four frequencies. Radio signals entering the earth’s atmosphere are refracted by the ionosphere much as sunlight is refracted by the surface of a swimming pool. The amount of refraction varies with the frequency of the wave energy. Any bending or other distortion of the satellite’s signals would, of course, lessen the accuracy of calculations based on the Doppler shift. Transmitting on two or more frequencies, however, would permit observers to compare signal variations and to calculate the amount of refraction of each due to the ionosphere. Thus it was possible to distinguish such “false” variations from the Doppler shift that the system was seeking to measure. The single pass by Transit I-A over the tracking station at Argentia was enough to make preliminary calculations of the effect of ionospheric refraction. These one-time calculations, of course, are being refined by observations of signals from the satellites now in orbit, but they were sufficiently accurate to be scientifically useful. Thus the Transit I-A payload, while it failed to orbit, was itself a success.
Transit I-B, as the vehicle launched in April is called, is essentially identical to the first payload. The sphere is Fiberglass coated internally with gold to reflect the radiant heat of the sun. Its diameter is 36 inches and it weighs 265 pounds. It was launched from Cape Canaveral by a two-stage Thor-Able-Star at 0703 13 April. A few minutes later its signals were coming in at Howard County, Maryland, then at Argentia, then at London. An Air Force station in Germany reported the frequency jump that signified separation of the payload from the second stage. At 0836 its signals were heard in New Mexico and a few minutes later Dr. Richard B. Kershner of the Applied Physics Laboratory, director of the Transit project, announced triumphantly that it was in orbit.
The launching vehicle was unique in having a second stage which automatically stopped burning at a given height and then restarted to place the payload into orbit. The orbit obtained—apogee of 463 miles and perigee of 233 miles—was some distance from the 500-mile-high circular path intended but this has not seriously affected the accuracy of observations.
The satellite contains two ultra-stable oscillators identical to those of its predecessor. They are housed in thermally isolated Dewar flasks, which minimize frequency shifting due to temperature changes. They transmit beeps at one-minute intervals, one on 54 and 216 megacycles and one on 162 and 324 megacycles. One of the frequencies is pulse-modulated; i.e., information about conditions within the tiny moon, such as temperatures of its various components and the spin rate, is telemetered back to earth by lengthening or shortening the duration of its beeps. The frequencies are separated widely enough to permit further measurement of ionospheric refraction as tracking stations record Doppler shift data.
One of the oscillators is powered by nickel-cadmium batteries of relatively short life. Its two frequencies will go off the air when these batteries are consumed. The solar batteries will keep the other oscillator transmitting for perhaps a year. With the orbit achieved, it is expected that the satellite itself will last a minimum of 16 months.
Data from Transit I-B is being recorded at seven stations: the Applied Physics Laboratory’s Field Station in Howard County, Maryland; the Defense Research Laboratory of the University of Texas at Austin; the University of Washington, Seattle; the University of New Mexico at Las Cruces; the Naval Air Station at Argentia, Newfoundland; the Royal Aircraft Establishment, Lasham, Hants, England; and San Jose Dos Campos, Brazil. Doppler shift information is transmitted by teletype to the Applied Physics Laboratory by the other five stations. There it is fed into a Univac 1103-A high speed digital computer which calculates the satellite’s positions.
From these data the precise orbit of the Transit satellite is calculated and extrapolated for several days in advance. This information, coupled with the measured Doppler shift at any point, is sufficient to fix the location of the point at which the shift is recorded.
An idea of the complexity of satellite engineering may be obtained from the de-spin systems installed in Transit. It is necessary to impart an axial spin to the satellite during launching. This rotation later causes an undesirable Doppler shift and should be eliminated in the interest of more accurate recording of the Doppler shift due solely to the orbital movement of the satellite. The mechanical de-spin device consists of two weights attached to cables wrapped around the sphere’s equator. After seven days in orbit, a timer frees the weights which fly out, due to centrifugal force, and separate from the satellite. The act of hurling the weights outward absorbs much of the ball’s rotational energy and thus stops most of the spin.
Also in the sphere is a magnetic de-spin device consisting of metal rods inside a short-circuited solenoid lying in the plane of the satellite’s equator. Motion of the satellite through the earth’s magnetic field magnetizes the rods. As the satellite rotates about its axis, these “compass needles” are turned against the force of the earth’s magnetic field, neutralized, and re-magnetized with reversed polarity. To move a magnet against a field, de-magnetize it, and magnetize it again with opposite polarity requires energy. Again, this energy is provided by the satellite’s spin. The magnetic de-spin device absorbs a little of the residual spin energy in every axial rotation of the sphere until its spin rate gradually approaches zero.
Telemetered data indicates that the spin of Transit I-A was reduced from 2.815 revolutions per second to 0.004 in its first two months of operation. The major reduction came from the mechanical device while the magnetic rods continue to act as a gentle brake.
The second Transit satellite put into orbit, called Transit II-A, was launched 21 June. Again, the job was done at Cape Canaveral by an Air Force Thor-Able-Star rocket with its stop-start second stage. Orbit was achieved at a 66.7 degree inclination to the earth’s equator with apogee of 665 statute miles and perigee of 385 miles. The satellite’s orbital period is 101.5 minutes.
Transit II-A weighs only 223 pounds. Weight reduction was made possible by powering both oscillators with solar cells and eliminating the heavier chemical batteries which powered one oscillator in each of the two earlier models. A small nickel-cadmium battery stores power for use when the satellite is in the earth’s shadow. Another important refinement added to II-A was an extremely accurate clock. Stations listening to Transit I-A must also monitor WWV for time signals to complete data needed for navigation. II-A and subsequent Transit satellites will broadcast their own time data, simplifying reception and computation.
A hospitable satellite, Transit II-A carried two hitch-hikers, a Canadian package designed to measure cosmic noise above the ionosphere, and a pickaback satellite built by the Naval Research Laboratory, Washington, to measure solar radiation.
The Canadian device was a receiver and antenna designed by the Defense Research Telecommunications Establishment in Ottawa. It detected cosmic noise at a frequency of 3.8 megacylces, telemetering coded information to Transit tracking stations on the ground. Its antenna was connected to the satellite’s de-spin weights. When the weights were thrown off at the end of a week’s operation, the Canadian device, which had done its job, went off the air. Its findings will be applied to the design of a “top-side ionospheric sounding satellite” being developed by the Canadian agency.
Probably the most spectacular aspect of Transit II-A was the launching of the Naval Research Laboratory’s pickaback satellite. It is a 20-inch, 40-pound sphere that was clamped to Transit as they rode into orbit together in the rocket’s nose cone. Once in orbit, the two were separated by a spring. They are now in approximately the same orbit, still separating at the rate of a foot and a half each second. The separation device had been tested on Transit I-B, which carried aloft a metal disc and ejected it when orbit was achieved.
The Naval Research Laboratory satellite is designed to measure solar radiation above the ionosphere. It has six round “solar patches,” each nine inches in diameter, on its outer surface. They detect solar radiation and send telemetered information by radio to the earth. The system is powered by a 12-volt storage battery charged by solar cells.
Measurement of the sun’s radiation above the ionosphere is expected to reveal many of nature’s secrets, including a good deal about the ionosphere itself. Since the ionosphere, which extends from 35 miles to several hundred miles above the earth’s surface, is produced by solar radiation striking the beginnings of the earth’s atmosphere, a knowledge of solar radiation should shed much light on the nature of the ionosphere. This, in turn, will benefit Transit, for it is the refracting effect of the ionosphere on radio transmission which requires that Transit satellites transmit on multiple frequencies. Thus the Naval Research Laboratory, which will gain much in the area of pure research from the satellite, will pay for its ride to outer space with data of direct use to Transit.
The operational Transit navigation system will consist of four satellites. They will orbit, preferably in nearly circular paths, at altitudes of about 500 miles. Polar orbits will not be required, but the goal is to orbit the four satellites, properly spaced, two at angles of inclination in the neighborhood of 65 to 70 degrees to the equator, and two at angles of 20 to 30 degrees.
Operational satellites probably will weigh between 50 and 100 pounds. Each will contain only one oscillator, transmitting on only two frequencies. Their electronic systems will be completely transistorized. In addition to transmitters and receivers, they will carry tiny electronic memories. When a satellite’s transmissions have been analyzed and its orbit calculated for the next day or two, this information will be transmitted by a ground station back to the satellite’s memory. The satellite will then include this information on its own orbit in its transmissions until new orbital information is provided it by the so-called injection station. The operational satellites will carry clocks similar to that in Transit II-A and will broadcast on precise time schedules. Thus each satellite will transmit all the information a ship or aircraft needs to calculate its position from a single pass by the satellite. Listening will be wholly passive. It will not be necessary to interrogate the satellite.
A receiver capable of understanding the coded orbital data and measuring the Doppler shift as the satellite crosses the sky will give the navigator a position more precise than any celestial or electronic device now in use. Computing the satellite’s orbit from Doppler data and the known geodetic positions of two tracking stations, scientists have been able to compute the location of a third station to within a few hundred feet of its known, surveyed position using the experimental satellites now in orbit. With four operational satellites, it should be possible to obtain a fix, accurate to better than a quarter of a mile, at any point on the earth about once every 90 minutes. A less precise receiver could be made sufficiently compact for use in aircraft.
With early developmental equipment, the navigator probably will have to do some computing, but the advanced operational models will compute the ship’s position directly and print co-ordinates on tape, plot them on a chart, and even feed periodic corrections into an automatic system such as a dead reckoning tracer or ship’s inertial navigation system.
The experimental Transit satellites now in orbit are accomplishing four specific tasks. They are making a practical test of the Doppler shift navigation system, which previously has been proved only in theory. They are adding to scientists’ knowledge of the effects of the ionosphere on radio wave transmission. They have provided their masters an opportunity to test and improve orbital tracking techniques. And, like Vanguard which first showed the earth to be pear-shaped and not the oblate spheroid it was thought to be, Transit is providing a basis for increased accuracy of geodetic measurements.
Two days before Transit II-A was launched, the Navy announced that Transit I-B had confirmed Vanguard’s finding that the gravitational field in the earth’s northern hemisphere is different from that in the south. Analysis indicates that the North Pole is about 50 feet farther from the equator than if the earth were a perfectly symmetrical body, and the South Pole is about 50 feet nearer the equator.
Accurate knowledge of the earth’s shape is important in fixing positions on the earth. This, in turn, is essential to accurate long range missilery as well as to eventual peaceful use of rockets for long haul transportation.
It will be necessary to launch at least two more experimental satellites before putting aloft an operational Transit system. Transit III-A is tentatively scheduled for launching in December. It will carry a prototype of the memory device for receiving, storing, and transmitting the satellite’s own orbital parameters, as well as a clock like the one in II-A. Transit III-A also will carry an experimental package for the Army and another pickaback satellite developed by the Naval Research Laboratory, this one a very low frequency experiment.
Once the Transit navigation system is in operation, it will cost the Navy about $3 million a year to maintain it, plus the cost of replacing the satellites. It is expected that each satellite will be useful for about five years. While three of the seven tracking stations now in use are located outside of the United States, Dr. Kershner has stated that the operational system could maintain its accuracy without the establishment of tracking installations on foreign soil.
Management of Transit was transferred from the Advanced Research Projects Agency of the Department of Defense to the Navy on 9 May, marking another step in its progress toward operational status.
The usefulness of Dutton, Bowditch, and H.O. 214 is far from past, but the remarkable success of Transit to date has brought closer the day—perhaps as soon as 1962—when ships, including ballistic missile ships, can fix their positions accurately in any weather, in any waters, and at virtually any hour of the day or night, using man-made stars.
Lieutenant Commander Castillo graduated from Northwestern University in 1945 and was commissioned from the NROTC program. He obtained a master’s degree in Public Relations from Boston University in 1954. He is the Head of the News Branch in the Office of Information, and has had a previous article published in the Proceedings.
★
ANTARCTIC HUMOR
Contributed by Commander V. Pendergraft, USN
Sailing south for Antarctica from New Zealand, we waited anxiously for our first ice. Camera fans amongst the crew were the most anxious of all and almost hourly they checked their cameras and light meters. Then late one evening came the electrifying announcement, “Ice, ice—ice on the bow!”
Up rushed the shutter-bugs only to find that the admiral had sent a steward on deck with a glass of ice cubes.
★ ★
NOT ENOUGH TO GO AROUND
Contributed by Lieutenant Commander H. S. Frey, USN
We were anchored in the harbor and one of our launches came alongside to return men who had been on liberty. Our Marine Captain was the officer-of-the-deck and, as the men came on board, he methodically checked packages to see that the regulations were not broken.
The last man, an old salt who had over-indulged ashore, stepped up on the quarterdeck with great effort carrying a package.
“What have you got in that package?” asked the Marine officer.
“Nothing sir,” mumbled the thick-tongued sailor.
“Let me see for myself,” said the Marine, and a moment later he ran around the deck roaring “Great thunder! Two bottles of whiskey! Quartermaster! Junior Officer of the deck! Get the Corporal of the guard! Get the Master-at-Arms! Get the Sergeant of the guard! Put this man on the report! Give me that whiskey for evidence!”
The old-timer stood there watching all the confusion and the rattled Marine Captain for a couple of minutes, and then he blurted out, “Hell! Don’t call everybody, we’ve got over 900 men aboard and I ain’t got but two bottles—just one for you, and one for me, sir!”
(The Naval Institute will pay $5.00 for each anecdote accepted for publication in the Proceedings.)