" . .. So if we only had a ring like that, not round the equator of the world- as Saturn's ring is around Saturn- but vertical to the plane of the equator, as the brass ring of an artificial globe goes, only far higher in proportion,- from that ring," said Q., pensively, "we could calculate the longitude."
From Edward Everett Hale's "The Brick Moon," 1869
From the earliest days of seafaring, the problems of accurate navigation have plagued the mariner. The great steps of navigation improvement have always led to great steps of improvement in seapower and national strength.
The earliest form of navigation was piloting. Piloting is navigation by observing visually the relative location of the navigator and known landmarks. Piloting techniques were improved by dead reckoning, a corrupted abbreviation of deduced reckoning. This can be described as an educated guess as to where you are or will be based upon an extrapolation of a previous position with estimates of course, speed, winds, currents, etc.
Thus, in the above navigation procedures, the ancient mariner faced the same basic problems we face today in modern navigation, though today's technology has severely increased the requirements for navigation. The advent of the nuclear submarine with long underwater and under-ice cruises, the fleet ballistic missile with its need for precise absolute reference between open seas and the heartland of the continents, and the dynamic swiftness and speed of modern warfare which requires precise navigation data without always the ability of selecting weather or convenient landmarks, all contribute to intensifying the navigator's task.
The same dynamic technology that brought us the nuclear submarine, the ballistic missile and the supersonic aircraft, however, also brings us an exciting new development which portends to solve two of the major problems of navigation science, perhaps even more.
The year 1960 saw the first successful steps in the practical application of space technology with the development of a meteorological satellite, Tiros, a series of communications satellites, Echo and Courier, an early-warning satellite, Midas, and the Navy's navigation satellite, Transit. The Transit navigation satellite has already demonstrated the feasibility of serving as a navigation tool beyond the greatest expectation of scientists and naval officers associated with the program.
It may be remembered that as recently as the summer of 1955, when the original fleet ballistic missile concept was being presented to the Navy and the office of the Secretary of Defense, the problems of navigation of the launching ship or submarine were considered too difficult to solve. Because of this fear, the Navy's FBM was nearly by-passed by the Secretary of Defense in favor of land-based systems, such as the IRBM's and the ICBM's, because convincing solutions to the navigation requirements were not at hand. With the approval of the FBM and the Polaris concept, however, the Navy began the most intensive program ever undertaken to advance the navigation art. In fact, so prominent was the problem of navigation in the minds of the FBM planners that the missile's name was selected as Polaris, the epitome of navigation aids.
Within days of the date of commencement of the FBM program in 1955, the Navy started development of another revolutionary project- the artificial earth satellite, Vanguard. The Vanguard, though dramatic in concept, was little appreciated at the time for the potential it and its successors would develop. Vanguard was a scientific experiment, part of the International Geophysical Year's series of experiments, intended to explore near space. It was not until almost two years later, when the first Soviet Sputnik was placed in orbit, that a serious appraisal of the potential of earth satellites was made by naval authorities. But with the practical demonstration of orbital flight, intense investigation of the employment of satellites for military purposes began.
The use of an artificial satellite for surface navigation was one of the first objectives undertaken by the Navy.
Navigation using celestial bodies, either natural or artificial, can be accomplished by any one of the three following methods:
(1) By angle-only measurements (altitude and azimuth)
(2) By range-only measurements
(3) By a combination of range and angle measurements
For observation of fixed stars, the relative motion of the star and the observer changes very slowly with time, hence good angle measurements can be made. For measurements of azimuth and altitude of an artificial satellite, the problem is more difficult because the satellite passes close to the Earth at a great speed. To simplify measurements relative to time and to introduce all-weather capabilities, a continuous radio signal needs to be transmitted from the satellite and an electronic sextant is needed by the observer to measure the satellite position.
Both stellar observation and angle-measuring of an artificial satellite require a stable vertical aboard ship for angle reference. Further, both optical and electronic signals are subject to refraction caused by the atmosphere or ionosphere, though solutions seem feasible here. There is also required an accurate clock and accurate knowledge of the continuous positions of the star or the satellite.
Such an artificial satellite used for angle measuring navigation is considered feasible and is under study by the Navy Department. The name of the project is Pathfinder. Active development is not underway at this time because of the promise of a range-only navigation system, which will offer reduced complexity aboard ship by elimination of the radio sextant antenna and the stable vertical. This system for surface navigation from an artificial earth satellite based upon range-only measurements is known as Project Transit.
This system is currently under development within the Navy at top priority and, based upon present schedules, is expected to be operational in late 1962. The Transit system is especially attractive for shipboard use, because it is dependent upon the measurement of only two quantities aboard ship- time and frequency, both of which are independent of ship motion and do not require complex installations aboard ship.
Project Transit is the satellite Doppler navigation system now being developed by the Navy Bureau of Weapons under contract with the Applied Physics Laboratory, Johns Hopkins University. This system will provide extremely accurate, world-wide, all-weather position information for surface and subsurface vessels and for aircraft. Navigational fixes accurate to 0.1 mile or better can be expected.
Transit will also be used in determining the Earth's geoid, or shape, to a higher degree than now possible, and for the refinement of the mapping of the Earth.
The Transit navigation system is based on the Doppler shift phenomenon, that is, the apparent change in frequency of the radio waves observed when the distance between the source of radiation (in this case, the satellite) and the observer or receiving station is increasing or decreasing because of the motion of either or both. The amount of shift in either case is proportional to the velocity of approach or recession. The frequency is shifted upward as the satellite approaches the receiving station and shifted downward as the satellite passes and recedes. The exact amount of this shift depends on the exact location of the receiving station with respect to the path of the satellite. Accordingly, by a very exact measure of the Doppler shift in frequency, and with knowledge of the path of the satellite, it is possible to calculate the location of the station on Earth.
Accuracies of 0.5 mile obtained by using this Doppler shift technique are possible because of two facts. First, the quantities we measure, frequency and time, are quantities that can be measured readily to an accuracy of one part in a billion. Second, the satellite is very precisely restricted by Newton's laws of motion. A satellite is a small planet moving inexorably where it must under the gravitational pull of the earth. Of all the possible satellite paths permitted by Newton's laws, there is only one which would result in a particular curve of Doppler shift. So, from this curve the position of a satellite with respect to the receiving station at any one time can be determined.
Two basic problems are inherent in this seemingly simple navigation system. One is that the ionosphere bends radio waves and thus gives a false position of the satellite. To overcome this problem, the satellites will transmit on two or more frequencies. Each of these frequencies will be bent a different amount. By comparing the Doppler signal received on these frequencies we can determine with great accuracy the refraction effect of the ionosphere.
The other problem inherent in this system is the shape of the earth. If the Earth were a perfect sphere, the path of the satellite outside the atmosphere would be easy to calculate. Once established in orbit, the satellite would describe a perfect ellipse with only minor variations introduced by a gravity of the Sun and Moon. But the Earth is not a perfect sphere; it has a considerable bulge at the equator causing the satellite's path to deviate. After the satellite crosses the equator, this same attraction will give a deviation in the opposite direction. Hence the oblateness of the Earth produces wriggles in the orbit, which must be compensated for in the tracking. Conversely, an accurate measurement of these wriggles provides a means for measuring the size of the Earth's equatorial bulge.
The Transit operational system will consist of five groups of equipment; several satellites, a network of tracking or receiving stations, a computing center, an injection station, and navigational receiving equipment (see schematic drawing, page 81).
In this system: (a) The satellites orbit at altitudes optimum for accurate tracking. (b) The tracking stations receive and record Doppler shifts radiated by each satellite on any pass within receiving range. This information is corrected for refraction and the resulting vacuum Doppler data transmitted to the computing center in digital form. (c) The computing center uses the Doppler data to calculate orbital parameters of the satellites for a minimum of one day in advance and transmits the information to the injection station. (d) The injection station transmits the orbital data to the satellite twice a day as each satellite passes within range. These signals first erase the satellite memory unit and then read in new orbital parameters and the time correction. The satellite retransmits data received from the injection station once each minute either as a modulation of one of the two stable frequencies or on a separate frequency. (e) The navigational equipment receives and records the Doppler shift of each of the satellite frequencies. It also records orbital information transmitted by the satellite. From these data, together with the accurate time signals available from the satellite, the navigator's latitude and longitude are computed.
The Transit system insures reliable, completely passive, all-weather navigation. To insure against jamming of the system, the satellites themselves contain means for supplying their data to the navigators. Thus the navigation station need neither interrogate the satellite nor receive orbital information by other communication links.
The system is inherently all-weather, since line-of-sight radio frequencies are used for both tracking and navigation. The system is reliable, because the number of statistically independent data points is so great that temporary interruption of reception does not preclude the use of the data that are received to obtain an accurate navigational fix.
The operational satellite will weigh about 100 pounds and will have an orbital life of nearly five years. It will contain an ultra stable oscillator for generating the time standard markers and the two very stable radio frequencies. It will also contain a miniaturized digital memory for storing orbital information received from the injection station, plus a modulator for retransmittal of this information on a radio frequency to the navigating stations. The memory and injection system provides a communications backup capacity for transmission of operational messages to ships or submarines. The satellites will be completely transistorized and will use solar power.
With only four satellites in orbit at any one time, it appears possible to guarantee navigational fixes on nearly any part of the globe at least once every hour and a half, and in some areas two to four times as often. Occasionally it will prove necessary to launch another satellite, either to replace a dying satellite or to fill a developing gap in the satellite grid owing to the different precessional rates of the satellites when in slightly different orbits. This satellite replenishment is expected to require one launching per year.
During Transit's research and development stage, five experimental satellites have been designed: they are designated the 1A, 1B, 2A, 2B, and 3A respectively. All have been launched. Seven ground receiving stations are operable; others are being designed and constructed by the Naval Ordnance Test Station, China Lake, California. An experimental computing center is in operation at the Applied Physics Laboratory, Johns Hopkins University, Silver Spring, Maryland. The requirements of the injection station have been determined, and the design of the station is underway. Mathematical analyses of computation procedural development, ionospheric refraction, geodesy, and navigation are being made. These analyses will be continued for some time.
The Transit 1A satellite was launched on 17 September 1959 by a Thor Able vehicle at Cape Canaveral. Although the launching vehicle failed to place the satellite into orbit, sufficient data were obtained to verify the feasibility of satellite tracking and of navigation by Doppler analysis. Many of the objectives of the launching, however, were not achieved because the satellite failed to orbit. These objectives, which concerned the verification of the satellite design, were achieved by the 1B launching.
Transit 1B was successfully launched on 13 April 1960 by the two-stage Thor-Able-Star vehicle at Cape Canaveral. The 1B satellite is similar to the 1A in appearance and function. It is a 36-inch diameter sphere and weighs approximately 265 pounds. Structurally, it consists of a shell divided into two hemispheres, a central support tube and an instrument tray. The shell is a lamination consisting of two pieces of fiberglass with a honeycomb plastic filler. This makes a highly insulated, strong, nonmetallic structure. The tube, made of pressure-laminated fiberglass, connects the hemispheres and supports the instrument tray. The tray, of aluminum sheet metal, is tied to the shell by nylon lacing.
The components on the tray consist of two transmitting systems, each transmitting on two frequencies, 54 mcs. and 324 mcs., and 162 mcs. and 216 mcs.; silver-zinc and nickel-cadmium batteries; two command receivers; a de-spin system; and two radiation and insulation shields, one over the tray and one under the tray. Four metal rods of high magnetic permeability, part of the de-spin system, are mounted on each shield. Two banks of solar cells for charging the nickelcadmium batteries and two de-spin weights are mounted on the outside of the shell around its equator. Broadband antennas are painted on the shell in a spiral pattern.
An infrared scanner, which is a self-contained unit designed for special studies by the Naval Ordnance Test Station, is also on the tray. A hole for this scanner is in the upper hemisphere.
The support tube and nylon lacing used in securing the tray and other components absorb the shocks produced in launching.
The insulation shields, made of layers of aluminum foil and fiberglass, minimize the temperature changes encountered as the satellite travels through solar radiation and through the Earth's shadows. The support tube and nylon lacings also help maintain a more constant temperature by reducing heat transfer by conduction from the outer shell to the instrument tray. White paint on the shell also helps in controlling the temperature.
Each transmitting system can be powered by either the silver-zinc batteries or the nickel-cadmium batteries (solar power). On signals from the ground station, the command receivers can switch the source of power for the two pairs of transmitted frequencies. All signals from the satellite can be cut off at any time after the silver-zinc batteries have run down by switching both transmitting systems over to these batteries.
A spin rate of approximately 180 r .p.m. is imparted to the satellite by the launching vehicle before release from the second stage. After a pre-set time the de-spin system releases two weights. They unwind from their cables that encircle the shell, owing to centrifugal force. These counteracting forces, together with those engendered by the metal rods and the earth's magnetic field, stop the spinning of the satellite.
At the designated time of injection into orbit, the Transit 1B satellite separated satisfactorily from the second stage of the ThorAble- Star launching vehicle. It was transmitting on all four frequencies, and within a few hours, its orbit was determined. Later, after the tracking stations received additional and more precise data, minor corrections in the orbital determination were made.
Tracking operations and analysis of telemetered data indicate that Transit components operated satisfactorily. From the tetemetered data, we know that the temperatures of the outer skin and of the internal components are within a degree or two of the temperatures estimated and that the solar cell and nickel-cadmium battery power supply is functioning satisfactorily. We also know that de-spin of the satellite from its initially rapid rotational rate was accomplished at the set time.
The orbit of the Transit 1B satellite has an apogee of 475 statute miles, a perigee of 235 statute miles, and an inclination of 51 degrees to the plane of the equator. The satellite period is 95.5 minutes, that is, Transit makes one complete revolution in its orbit every hour and a half.
Several navigation experiments have been made with the data received from the satellite. The results are indeed gratifying. In one of these experiments, a navigation fix was made on Austin, Texas, and this fix agreed with the first order survey position within one-quarter of a mile. This demonstrates the ability to perform navigation with the precision that may be required for almost any military application. Navigation experiments, however, will continue to be made.
Also, in spite of the undesirably low perigee of the orbit, active investigation of the earth's gravitational field and of the geodetic surface of the earth has been started and effective results already obtained.
On 22 June 1960, the Navy launched the second experimental Transit satellite, the Transit 2A. It differs from the 1A and 1B in that it includes more solar cells, an improved telemetering system, and an electronic clock. The clock, running from the satellite stable oscillator, will provide an accurate correlation of time between the ground receiving stations, which is necessary for the accurate determination of satellite orbits. Transit 2A was also launched at a higher inclination to the equator, 67½ degrees, in order to obtain additional data on the earth's geoid.
On 21 February 1961, the Transit 3B was launched. This satellite incorporates an additional electronic unit in the form of a complete data storage system capable of storing a small number of bits of digital information. A storage loading technique and readout system it used to insure correct data insertion. This unit provided the first flight test of the data system working in conjunction with an injection station. Twice a day, as each satellite passes within range, the injection station transmits a mathematical description of the satellite orbit to the satellite, which then retransmits the signals to the navigator. The satellite will transmit the same signals, approximately, until new information is received. As new bits of data from the injection station are read in, the old bits are erased.
Injection at 12-hour intervals provides maximum accuracy for orbital parameters and time data. Failure, however, of the station to inject at one or two intervals will not incapacitate the system, since the satellite memory is capable of retaining orbital parameters for several days in advance of each injection.
Such a memory and injection system provides a communication backup capacity for transmission of operational messages to ships or submarines. Since the satellite retransmits the signals on a UHF line-of-sight radio link, the navigation stations can receive the data quickly, without fear of jamming, and in all weather conditions.
The satellite tracking stations are in an advanced state of development. Each of the satellite frequencies is monitored on special receivers, and continuous recording of the received signal is made. An ultra stable oscillator, similar to the one in the satellite, serves as a frequency standard at the ground station. The amount of Doppler shift is measured by comparing the signal from the reference oscillator at the ground station with the signal being transmitted from the satellite. The Doppler signal is received in digital form to provide an automatic printed record. The output is also punched on standard paper tape, which is fed directly into the computer for Doppler shift analysis and determination of the satellite orbit. Final results are printed by a high-speed printer.
The computing and analysis process must continue in order that the ultimate data handling process can be provided consistent with the high-accuracy navigation objectives of the Transit system. The computing programs are concerned primarily with the routines devoted to the Doppler tracking of satellites on a day-to-day basis and with studies of ionospheric refraction, geodesy, and tracking accuracy.
Computation and analysis require (1) receivers for the stable frequencies transmitted by the satellite, (2) a refraction correction device for generation of the vacuum Doppler shift, (3) digitalization equipment for presenting the vacuum Doppler signal in a form suitable for computation, (4) receiving and decoding equipment for the reception and display of the orbital information transmitted by the satellite, and (5) a computing program for a digital computer to determine latitude and longitude.
The shipboard navigating receivers are considerably simplified relative to the tracking stations, as they need only determine latitude and longitude, not complete orbits.
The Transit navigation system will be in operation next year. Then ships anywhere and everywhere can get not only navigational information, but also the exact time and the predicted future orbits of the satellites. This system insures reliable, completely passive, all-weather navigation. Ocean voyages for all ships of all nations can thus become safe and more efficient.
The Transit navigation system has been under development since early 1958. Originally, the project was under the Director of the Advanced Research Projects Agency, with management and technical direction under the Navy. This phase of the program was concerned primarily with the proving of feasibility. That goal was accomplished with the successful Transit orbit achieved on 13 April 1960.
The credit for this achievement belongs to a great many individuals. Perhaps it would not be out of place here to mention the names of a few dedicated men who have made Transit succeed. Dr. F. T. McClure had the original idea which led to Transit; Drs. W. H. Guier and George Weiffenbach first demonstrated the feasibility of using Doppler techniques to track satellites; Dr. R . B. Kershner was program director for the Johns Hopkins Applied Physics Laboratory which was the system developer; John Nicolaides of the Naval Weapons Laboratory did much of the computational work; and Thomas F. Griffin, of the Bureau of Naval Weapons, was the project engineer.
In early May 1960, the management of the Transit program was formally transferred from ARPA to the Navy, and the second phase of the program got underway. This phase is the development of an engineering prototype of the operational system, including the development of shipboard navigation gear. During this phase, intense effort will continue on:
(1) development of increased system reliability
(2) accuracy improvement including research and experiments in the refraction and geodetic areas
(3) system simplification and shipboard operation
(4) advanced development of aircraft navigation
Transit has, so far, been a completely successful experiment. The operation of all subsystems is normal. All four stable frequencies transmit normally at an extremely low noise level. Excellent refraction data are available as well as limited geo-id data. Position determination experiments have been made with station fixes being well within one-quarter of a mile of various geodetic bench marks. All electrical and mechanical aspects of the satellite are proven, including solar cell operation, command off-on operation, de-spin devices, temperature stability, and the like. In addition, ground tracking and computation has proven completely successful. As a sidelight, the precision tracking technique developed under Transit is being utilized on other satellites, notably the U. S. Air Force Discoverer satellites launched from the Pacific Missile Range, Point Mugu, California.
Project Transit thus represents a fundamental new attack on the age-old problem of safe and precise navigation at sea. But more than the solution of a specific problem, it represents, perhaps, the first real step towards using space and astronautics for the benefit of all mankind here on Earth. For, not only will Transit be a tool to improve the Navy's weapons; it is expected to be made available to mariners of all countries. This is more than a navigation advance, it is the first tool to make the space age a practical reality.