A submarine can keep calculating its position while it travels underwater. The harder question is how to check that calculation against the outside world. Before GPS, the U.S. Navy used Transit: satellites whose changing radio frequency helped a receiver work out where it was.
The useful distinction is between a position fix and the navigation that continues afterward. Transit supplied an outside check during a satellite pass. Inertial instruments carried the submarine's position estimate forward between those checks.
How did listening to Sputnik become a way to navigate?#
The starting point was a known receiver listening to an unknown orbit. After Sputnik's launch in 1957, William Guier and George Weiffenbach at the Johns Hopkins Applied Physics Laboratory analyzed changes in its received radio frequency. Those changes contained information about the satellite's motion relative to their antenna.
In their later account, the researchers describe progressing from rough estimates to a computer demonstration that an orbit could be inferred from a single pass, with corrections for the atmosphere and transmitter behavior. Frank McClure then proposed reversing the problem: supply the orbit, and solve for the receiver's position. Their account dates that proposal to March 1958. The insight followed months of analysis rather than an instantaneous navigation system emerging from the first signal.
APL developed Transit for the Navy's ballistic-missile submarines. The system was operational in 1964. Its receivers helped determine their own location; this was a navigation service for the user, rather than a means of detecting an opposing submarine.
What did the changing frequency reveal?#
A radio transmitter emits cycles at a particular rate: its frequency. When the distance between transmitter and receiver shrinks, the received cycles arrive faster; when that distance grows, they arrive slower. This is the Doppler effect. The relevant motion is toward or away from the receiver, not simply the satellite's total speed.
One frequency reading cannot uniquely identify a place. Transit accumulated measurements as a satellite moved through successive known positions. Its receiver counted the difference between the incoming signal and a stable local reference over timed intervals. Those counts constrained changes in distance to the satellite. Combining intervals with the known orbit allowed a position solution at Earth's surface, including a correction for frequency offset.
The illustration simplifies this into a frequency curve. The changing shape across the pass carries information that a single high or low reading cannot provide. It is a conceptual model, with no measured frequencies or elapsed times plotted.
How could the receiver trust the satellite's position?#
The orbit had to be determined from observations and sent to the user. Transit therefore needed ground infrastructure as well as spacecraft. Tracking stations measured satellite signals; orbit predictions were uploaded to the satellites, which broadcast the information along with their navigation signals. An orbit description of this kind is called an ephemeris.
There were errors to control on both sides of the radio link. The ionosphere, a region of electrically charged gas above Earth, alters radio propagation. Transit transmitted near 150 and 400 megahertz—150 million and 400 million cycles per second. Comparing the two frequencies let receivers correct the principal ionospheric effect because that disturbance changes differently with frequency from the Doppler effect.
The user's own motion also mattered. A moving receiver contributes to the measured change in separation. Kershner's 1965 technical description explicitly distinguishes motion over Earth from speed through water: an unknown current can make those different. Satellite navigation still needed a sound account of the vessel's movement during the observation.
What happened when no usable satellite pass was available?#
The submarine continued navigating with its inertial system. It measured acceleration and direction to carry a position estimate forward from an established starting point. Small instrument errors accumulated, so the estimate needed periodic outside corrections.
A 1978 U.S. Government Accountability Office report describes strategic submarines using inertial systems as their primary navigation equipment, with Transit and other aids correcting cumulative errors. Transit supplied intermittent fixes, while the inertial calculation continued between them. Satellite visibility and the crew's access to a usable signal were separate constraints; a pass overhead did not guarantee a fix aboard a deeply submerged boat.
The same report states that Transit signals could not penetrate seawater to considerable depths. A drawing of a satellite beaming straight through the ocean to a deep submarine would therefore misrepresent the system. The diagram below shows the flow of information, with no claim about antenna arrangements, operating depths or procedures.
Image unavailable.
Continuous inertial position estimate receives a periodic correction from a Transit satellite fix.
Sources: [4]
How was Transit different from GPS?#
Transit built a fix from the change in radio reception over a satellite pass. GPS uses a different basic measurement: the travel time of coded signals from several satellites. Multiplying travel time by the speed of light gives an apparent distance. A GPS receiver normally needs a fourth satellite to solve for its clock error as well as three position coordinates.
That makes GPS well suited to frequent position solutions without waiting to observe the changing geometry of one satellite throughout a pass. Both systems require known satellite positions, but Transit was not an early version of the same receiver technology.
Transit also reached beyond its original naval users. APL's development history describes oceanographers using it aboard research ships, followed by a public and commercial release in 1967 and wider maritime and surveying applications. A method developed to check a submarine's location became useful wherever a reliable position mattered at sea.
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Sources & further reading
Follow the original research, records, and explanations behind this story.
- Genesis of Satellite Navigation Johns Hopkins APL Technical Digest; William H. Guier and George C. Weiffenbach
First-person retrospective; Sputnik analysis and March 1958 inversion.
- An Overview of Transit Development Johns Hopkins APL Technical Digest; Robert J. Danchik
Naval purpose, system components, early operation and civilian adoption.
- Present State of Navigation by Doppler Measurement from Near Earth Satellites APL Technical Digest; R. B. Kershner
1965 technical description: integrated Doppler, ephemeris, ionospheric correction and user motion.
- Navigation Planning—Need for a New Direction U.S. Government Accountability Office
1978 historical account of inertial navigation, outside corrections and reception limits.
- Innovations and Breakthroughs Johns Hopkins University Applied Physics Laboratory
Entry “Transit Becomes Fully Operational 1964.”
- Feasibility of Satellite Interferometry for Surveillance, Navigation, and Traffic Control Battelle Columbus Laboratories; NASA-sponsored report BCL-OA-TFR-76-2
February 1976 report, section 5.1.2.1, pp. 5-6–5-7: Transit orbit broadcasts, nominal frequencies and user-motion sensitivity.
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