The Transit system, also known as NAVSAT or NNSS (for ''Navy Navigation Satellite System''), was the first
satellite navigation system
A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisoto ...
to be used operationally. The
radio navigation
Radio navigation or radionavigation is the application of radio frequencies to determine a position of an object on the Earth, either the vessel or an obstruction. Like radiolocation, it is a type of radiodetermination.
The basic principles ...
system was primarily used by the
U.S. Navy to provide accurate location information to its
Polaris
Polaris is a star in the northern circumpolar constellation of Ursa Minor. It is designated α Ursae Minoris ( Latinized to ''Alpha Ursae Minoris'') and is commonly called the North Star or Pole Star. With an apparent magnitude that ...
ballistic missile submarine
A ballistic missile submarine is a submarine capable of deploying submarine-launched ballistic missiles (SLBMs) with nuclear warheads. The United States Navy's hull classification symbols for ballistic missile submarines are SSB and SSBN ...
s, and it was also used as a navigation system by the Navy's
surface ships, as well as for
hydrographic survey
Hydrographic survey is the science of measurement and description of features which affect maritime navigation, marine construction, dredging, offshore oil exploration/ offshore oil drilling and related activities. Strong emphasis is placed ...
and
geodetic surveying. Transit provided continuous navigation satellite service from 1964, initially for Polaris submarines and later for civilian use as well. In the Project DAMP Program, the missile tracking ship
USAS American Mariner also used data from the satellite for precise ship's location information prior to positioning its tracking radars.
History
The Transit satellite system, sponsored by the Navy and developed jointly by
DARPA
The Defense Advanced Research Projects Agency (DARPA) is a research and development agency of the United States Department of Defense responsible for the development of emerging technologies for use by the military.
Originally known as the Ad ...
and the Johns Hopkins
Applied Physics Laboratory, under the leadership of Dr. Richard Kershner at Johns Hopkins, was the first satellite-based geopositioning system.
Just days after the
Soviet
The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen national ...
launch of
Sputnik 1, the first man-made earth-orbiting satellite on October 4, 1957, two physicists at APL, William Guier and George Weiffenbach, found themselves in discussion about the radio signals that would likely be emanating from the satellite. They were able to determine Sputnik's orbit by analyzing the
Doppler shift
The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who ...
of its radio signals during a single
pass.
Discussing the way forward for their research, their director Frank McClure, the chairman of APL's Research Center, suggested in March 1958 that if the satellite's position were known and predictable, the Doppler shift could be used to locate a receiver on Earth, and proposed a satellite system to implement this principle.
Development of the Transit system began in 1958, and a prototype satellite, Transit 1A, was launched in September 1959. That satellite failed to reach orbit. A second satellite, Transit 1B, was successfully launched April 13, 1960, by a
Thor-Ablestar rocket. The first successful tests of the system were made in 1960, and the system entered Naval service in 1964.
The Chance Vought/LTV
Scout
Scout may refer to:
Youth movement
* Scout (Scouting), a child, usually 10–18 years of age, participating in the worldwide Scouting movement
**Scouts (The Scout Association), section for 10-14 year olds in the United Kingdom
** Scouts BSA, sect ...
rocket was selected as the dedicated launch vehicle for the program because it delivered a payload into orbit for the lowest cost per pound. However, the Scout decision imposed two design constraints. First, the weight of the earlier satellites was about each, but the Scout launch capacity to the Transit orbit was about , but this was later increased significantly. A satellite mass reduction had to be achieved, despite a demand for more power than APL had previously designed into a satellite. The second problem concerned the increased vibration that affected the payload during launching because the Scout used solid rocket motors. Thus, electronic equipment that was smaller than before and rugged enough to withstand the increased vibration of launch had to be produced.
Meeting the new demands was more difficult than expected, but it was accomplished. The first prototype operational satellite (Transit 5A-1) was launched into a polar orbit by a Scout rocket on 18 December 1962.
The satellite verified a new technique for deploying the solar panels and for separating from the rocket, but otherwise it was not successful because of trouble with the power system. Transit 5A-2, launched on 5 April 1963, failed to achieve orbit. Transit 5A-3, with a redesigned power supply, was launched on 15 June 1963. A malfunction of the memory occurred during powered flight that kept it from accepting and storing the navigation message, and the oscillator stability was degraded during launch. Thus, 5A-3 could not be used for navigation. However, this satellite was the first to achieve
gravity-gradient stabilization, and its other subsystems performed well.
Surveyors used Transit to locate remote
benchmarks by averaging dozens of Transit fixes, producing sub-meter accuracy. In fact, the elevation of
Mount Everest
Mount Everest (; Tibetan: ''Chomolungma'' ; ) is Earth's highest mountain above sea level, located in the Mahalangur Himal sub-range of the Himalayas. The China–Nepal border runs across its summit point. Its elevation (snow ...
was corrected in the late 1980s by using a Transit receiver to re-survey a nearby benchmark.
Thousands of warships, freighters and private watercraft used Transit from 1967 until 1991. In the 1970s, the
Soviet Union
The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, ...
started launching their own satellite navigation system
''Parus'' (military) /
Tsikada (civilian), which is still in use today besides the next generation
GLONASS
GLONASS (russian: ГЛОНАСС, label=none, ; rus, links=no, Глобальная навигационная спутниковая система, r=Global'naya Navigatsionnaya Sputnikovaya Sistema, t=Global Navigation Satellite System) is ...
. Some Soviet warships were equipped with
Motorola
Motorola, Inc. () was an American multinational telecommunications company based in Schaumburg, Illinois, United States. After having lost $4.3 billion from 2007 to 2009, the company split into two independent public companies, Motorola ...
NavSat receivers.
The Transit system was made obsolete by the
Global Positioning System (GPS), and ceased navigation service in 1996. Improvements in electronics allowed GPS receivers to effectively take several fixes at once, greatly reducing the complexity of deducing a position. GPS uses many more satellites than were used with Transit, allowing the system to be used continuously, while Transit provided a fix only every hour or more.
After 1996, the satellites were kept in use for the Navy Ionospheric Monitoring System (NIMS).
Description
Satellites
The satellites (known as ''OSCAR'' or ''NOVA'' satellites) used in the system were placed in
low
Low or LOW or lows, may refer to:
People
* Low (surname), listing people surnamed Low
Places
* Low, Quebec, Canada
* Low, Utah, United States
* Lo Wu station (MTR code LOW), Hong Kong; a rail station
* Salzburg Airport (ICAO airport code: LO ...
polar orbit
A polar orbit is one in which a satellite passes above or nearly above both poles of the body being orbited (usually a planet such as the Earth, but possibly another body such as the Moon or Sun) on each revolution. It has an inclination of abo ...
s, at an altitude of about , with an orbital period of about 106 minutes. A ''constellation'' of five satellites was required to provide reasonable global coverage. While the system was operational, at least ten satellites – one spare for each satellite in the basic constellation – were usually kept in orbit. Note that these ''OSCAR'' satellites were not the same as the
OSCAR series of satellites that were devoted to use by
amateur radio
Amateur radio, also known as ham radio, is the use of the radio frequency spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emergency communi ...
operators to use in
satellite communications
A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth. ...
.
The orbits of the Transit satellites were chosen to cover the entire Earth; they crossed over the poles and were spread out at the equator. Since only one satellite was usually visible at any given time, fixes could be made only when one of the satellites was above the horizon. At the equator this delay between fixes was several hours; at mid-latitudes the delay decreased to an hour or two. For its intended role as an updating system for SLBM launch, Transit sufficed, since submarines took periodic fixes to reset their
inertial guidance system
An inertial navigation system (INS) is a navigation device that uses motion sensors ( accelerometers), rotation sensors (gyroscopes) and a computer to continuously calculate by dead reckoning the position, the orientation, and the velocity (d ...
, but Transit lacked the ability to provide high-speed, real-time position measurements.
With later improvements, the system provided single-pass accuracy of roughly , and also provided
time synchronization
Synchronization is the coordination of events to operate a system in unison. For example, the conductor of an orchestra keeps the orchestra synchronized or ''in time''. Systems that operate with all parts in synchrony are said to be synchronou ...
to roughly 50 microseconds. Transit satellites also broadcast encrypted messages, although this was a secondary function.
The Transit satellites used arrays of
magnetic-core memory
Magnetic-core memory was the predominant form of random-access computer memory for 20 years between about 1955 and 1975.
Such memory is often just called core memory, or, informally, core.
Core memory uses toroids (rings) of a hard magneti ...
as mass data storage up to 32 kilobytes.
Determining ground location
Determining a location, also known as "taking a fix", normally requires two or more measurements to be taken to produce a 2D location. In the case of the modern GPS system, dozens of such measurements may be taken depending on which satellites are visible at that time, each one helping improve accuracy. In the case of Transit, only a small number of satellites were in orbit and were spread out. This generally meant there was only one satellite visible at any time. Some other method of determining a second measurement was needed.
Transit did this by measuring the signal's Doppler shift. The spacecraft traveled at about , which could increase or decrease the frequency of the received carrier signal by as much as 10 kHz as measured on the ground. While the satellite is approaching the ground station its signals will be shifted up in frequency, and as it recedes they will shift down again. The precise moment when the frequency is exactly equal to the broadcast frequency is when the satellite's
ground track
A ground track or ground trace is the path on the surface of a planet directly below an aircraft's or satellite's trajectory. In the case of satellites, it is also known as a suborbital track, and is the vertical projection of the satellite' ...
passes the ground location's location (with some corrections). This provides one of the two measurements needed.
For the second measure, one has to consider the pattern of the Doppler shift. If the satellite passes directly overhead, its angular velocity as it passes will be more than if it passes to one side. In the extreme case, with a satellite near the horizon, the relative velocity change is minimized. Thus the rapidity of the change in frequency is an indication of the relative longitude between the station and the satellite. Additionally, the rotation of the Earth provided another Doppler correction which could be used to determine whether the satellite was to the east or west of the ground station.
These measurements produce a relative location compared to the satellite. To determine the actual location, that relative measure is applied to the location of the satellite. This is provided by periodically sending out precise time hacks (every two minutes), plus the satellite's six
orbital elements
Orbital elements are the parameters required to uniquely identify a specific orbit. In celestial mechanics these elements are considered in two-body systems using a Kepler orbit. There are many different ways to mathematically describe the same ...
and orbit
perturbation variables. The ground receiver downloaded these signals and calculated the location of the satellite while it was measuring the shifts. The orbit
ephemeris
In astronomy and celestial navigation, an ephemeris (pl. ephemerides; ) is a book with tables that gives the trajectory of naturally occurring astronomical objects as well as artificial satellites in the sky, i.e., the position (and possibly ...
and clock corrections were uploaded twice each day to each satellite from one of the four Navy tracking and injection stations.
The Transit satellite broadcast on 150 and 400 MHz. The two frequencies were used to allow the
refraction
In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomen ...
of the satellite radio signals by the ionosphere to be canceled out, thereby improving location accuracy. The Transit system also provided the first worldwide timekeeping service, allowing clocks everywhere to be synchronised with 50 microsecond accuracy.
Calculating the most likely receiver location was not a trivial exercise. The navigation software used the satellite's motion to compute a 'trial' Doppler curve, based on an initial 'trial' location for the receiver. The software would then perform a
least squares
The method of least squares is a standard approach in regression analysis to approximate the solution of overdetermined systems (sets of equations in which there are more equations than unknowns) by minimizing the sum of the squares of the r ...
curve fit for each two-minute section of the Doppler curve, recursively moving the trial position until the trial Doppler curve 'most closely' matched the actual Doppler received from the satellite for all two-minute curve segments.
If the receiver was also moving relative to the earth, such as aboard a ship or airplane, this would cause mismatches with the idealized Doppler curves, and degrade position accuracy. However, positional accuracy could usually be computed to within 100 meters for a slow-moving ship, even with reception of just one two-minute Doppler curve. This was the navigation criterion demanded by the U.S. Navy, since American submarines would normally expose their UHF antenna for only 2 minutes to obtain a usable Transit fix. The U.S. submarine version of the Transit system also included a special encrypted, more accurate version of the downloaded satellite's orbital data This enhanced data allowed for considerably enhanced system accuracy
ot unlike Selective Availability (SA) under GPS">Selective_Availability.html" ;"title="ot unlike Selective Availability">ot unlike Selective Availability (SA) under GPS Using this enhanced mode, accuracy was typically less than 20 meters, i.e. the accuracy was between that of LORAN C and GPS. Certainly, Transit was the most accurate navigation system of its day.
The basic operating principle of Transit is similar to the system used by emergency locator transmitters (ELTs), except that in the latter case the transmitter is on the ground and the receiver is in orbit. ELTs measure the Doppler shift of the transmitter on the boat or aircraft as it passes overhead and forwards that data to the ground where the location of the craft can be determined.
Determining the satellite orbits
A network of ground stations, whose locations were accurately known, continually tracked the Transit satellites. They measured the Doppler shift and transferred the data to 5-hole paper tape. This data was sent to the Satellite Control Center at Applied Physics Laboratory in Laurel, Maryland using commercial and military teleprinter networks. The data from the fixed ground stations provided the location information on the Transit satellite orbit. Locating a Transit satellite in earth orbit from a known ground station using the Doppler shift is simply the reverse of using the known location of the satellite in orbit to locate an unknown location on the earth, again using the Doppler shift .
A typical ground station occupied a small
Quonset hut
A Quonset hut is a lightweight prefabricated structure of corrugated galvanized steel having a semi cylindrical cross-section. The design was developed in the United States, based on the Nissen hut introduced by the British during World War ...
. The accuracy of the ground station measurements was a function of the ground station master clock accuracy. Initially a quartz oscillator in a temperature controlled
oven was used as the master clock. The master clock was checked daily for drift using a VLF receiver tuned to a US Navy VLF station. The VLF signal had the property that the phase of the VLF signal did not change from day to day at noon along the path between the transmitter and the receiver and thus could be used to measure oscillator drift. Later
rubidium
Rubidium is the chemical element with the symbol Rb and atomic number 37. It is a very soft, whitish-grey solid in the alkali metal group, similar to potassium and caesium. Rubidium is the first alkali metal in the group to have a density hig ...
and
cesium beam clocks were used. Ground stations had number names; for example, Station 019 was McMurdo Station, Antarctica. For many years during the 1970s this station was staffed by a graduate student and an undergraduate student, typically in electrical engineering, from the University of Texas at Austin. Other stations were located at New Mexico State University, the University of Texas at Austin, Sicily, Japan, Seychelles Island, Thule Greenland and a number of other locations. The Greenland and Antarctica stations saw every pass of every Transit satellite because of their near pole location for these polar orbiting satellites.
Portable Geoceiver
A portable version of the ground station was called a Geoceiver and was used to make field measurements. This receiver, power supply, punched tape unit, and antennas could fit in a number of padded aluminum cases and could be shipped as extra cargo on an airline. Data was taken over a period of time, typically a week, and sent back to the Satellite Control Center for processing. Therefore, unlike GPS, there was not an immediate accurate location of the Geoceiver location. A Geoceiver was permanently located at the South Pole Station and operated by United States Geological Survey personnel. Since it was located on the surface of a moving ice sheet, its data was used to measure the ice sheet movement. Other Geoceivers were taken out in the field in Antarctica during the summer and were used to measure locations, for example the movement of the
Ross Ice Shelf
The Ross Ice Shelf is the largest ice shelf of Antarctica (, an area of roughly and about across: about the size of France). It is several hundred metres thick. The nearly vertical ice front to the open sea is more than long, and between h ...
.
The AN/UYK-1 (TRW-130) Computer
Since no computer small enough to fit through a submarine's hatch existed (in 1958), a new computer was designed, named the AN/UYK-1 (TRW-130). It was built with rounded corners to fit through the hatch and was about five feet tall and sealed to be waterproof. The principal design engineer was then-UCLA-faculty-member Lowell Amdahl, brother of
Gene Amdahl. The AN/UYK-1 was built by the
Ramo-Wooldridge Corporation
TRW Inc., was an American corporation involved in a variety of businesses, mainly aerospace, electronics, automotive, and credit reporting.http://www.fundinguniverse.com/company-histories/TRW-Inc-Company-History.html TRW Inc. It was a pioneer ...
(later TRW) for the
''Lafayette'' class SSBNs. It was equipped with 8,192 words of 15-bit
core memory
Core or cores may refer to:
Science and technology
* Core (anatomy), everything except the appendages
* Core (manufacturing), used in casting and molding
* Core (optical fiber), the signal-carrying portion of an optical fiber
* Core, the centr ...
plus
parity bit
A parity bit, or check bit, is a bit added to a string of binary code. Parity bits are a simple form of error detecting code. Parity bits are generally applied to the smallest units of a communication protocol, typically 8-bit octets (bytes) ...
, threaded by hand at their Canoga Park factory. Cycle time was about one
microsecond
A microsecond is a unit of time in the International System of Units (SI) equal to one millionth (0.000001 or 10−6 or ) of a second. Its symbol is μs, sometimes simplified to us when Unicode is not available.
A microsecond is equal to 100 ...
. The AN/UYK-1 weighed about .
The AN/UYK-1 was a
microprogrammed machine with a 15-bit word length that lacked hardware commands to subtract, multiply or divide, but could add, shift, form
ones' complement, and test the carry bit. Instructions to perform standard fixed and floating point operations were software subroutines and programs were lists of links and operators to those subroutines. For example, the "subtract" subroutine had to form the ones' complement of the subtrahend and add it. Multiplication required successive shifting and conditional adding.
In the AN/UYK-1 instruction set, the machine-language instructions had two operators that could simultaneously manipulate the arithmetic registers—for example, complementing the contents of one register while loading or storing another. It may have been the first computer that implemented a single-cycle indirect addressing ability.
During a satellite pass, a GE receiver would receive the orbital parameters and encrypted messages from the satellite, as well as measure the Doppler-shifted frequency at intervals and provide this data to the AN/UYK-1 computer. The computer would also receive from the ship's inertial navigation system (SINS) a reading of latitude and longitude. Using this information the AN/UYK-1 ran a
least squares
The method of least squares is a standard approach in regression analysis to approximate the solution of overdetermined systems (sets of equations in which there are more equations than unknowns) by minimizing the sum of the squares of the r ...
algorithm and provided a location reading in about fifteen minutes.
Other satellites
There were 41 satellites in the Transit series that were assigned the Transit name by NASA.
Transit 3B demonstrated uploading programs into the onboard computer's memory whilst in orbit.
Transit 4A, launched June 29, 1961, was the first satellite to use a
radioactive power source (RTG) (a
SNAP-3
The Systems Nuclear Auxiliary POWER (SNAP) program was a program of experimental radioisotope thermoelectric generators (RTGs) and space nuclear reactors flown during the 1960s by NASA.
Odd-numbered SNAPs: radioisotope thermoelectric generators ...
). Transit 4B (1961) also had a SNAP-3 RTG. Transit 4B was among several satellites which were inadvertently damaged or destroyed in a nuclear explosion, specifically the United States
Starfish Prime high-altitude nuclear test on July 9, 1962 and subsequent
radiation belt.
Transit 5A3 and Transit 5B-1 (1963) each had a
SNAP-3
The Systems Nuclear Auxiliary POWER (SNAP) program was a program of experimental radioisotope thermoelectric generators (RTGs) and space nuclear reactors flown during the 1960s by NASA.
Odd-numbered SNAPs: radioisotope thermoelectric generators ...
RTG.
Transit 5B-2 (1963) had a
SNAP-9A RTG.
In 1964, a rocket failed to boost Transit 5BN-3 with a SNAP-9A RTG into orbit. It "burned up during re-entry and ablated into small particles" together with its approximately 1 kilogram of Plutonium-238.
Transit 5B-5 resumed communicating again after an extended period of inactivity (a
zombie satellite).
Transit-9 and 5B4 (1964) and Transit-5B7 and 5B6 (1965) each had "a nuclear power source".
The
US Air Force
The United States Air Force (USAF) is the Aerial warfare, air military branch, service branch of the United States Armed Forces, and is one of the eight uniformed services of the United States. Originally created on 1 August 1907, as a part ...
also periodically launched short lived satellites equipped with radio beacons of 162 MHz and 324 MHz at much lower orbits to study
orbital drag. The Transit ground tracking stations tracked these satellites as well, locating the satellites within their orbits using the same principles. The satellite location data was used to collect orbital drag data, including variations in the upper atmosphere and the Earth's gravitational field.
Beacon Explorer-A
Explorer S-66 (also called BE-A, acronym of Beacon Explorer-A), was a NASA satellite launched on 19 March 1964 by means of a Thor-Delta B launch vehicle, but it could not reach orbit due to a vehicle launcher failure.
Spacecraft
Beacon Expl ...
and
Beacon Explorer-B
Explorer 27 (or BE-C or Beacon Explorer-C, Beacon-C or S-66C) was a small NASA satellite, launched in 1965, designed to conduct scientific research in the ionosphere. It was powered by 4 solar panels. One goal of the mission was to study in de ...
also carried Transit-compatible transmitters.
References
External links
Encyclopedia Astronautica
{{DEFAULTSORT:Transit (Satellite)
1960 in spaceflight
1961 in spaceflight
1962 in spaceflight
1963 in spaceflight
Military space program of the United States
Navigation satellite constellations
Satellites in low Earth orbit