International Geophysical Year, 1957-1958
It was 12 years after WWII ended. Dwight D. Eisenhower was president of the United States. Radar technology had grown since the war to provide proximity fuses, guided missiles, and land-based navigation systems like the long range navigation system, (LORAN).
The International Council of Scientific Unions (ICSU)—now known as the International Council for Science (ICS)—a non-governmental organization of scientists, had declared 1957-1958 to be the International Geophysical Year (IGY). In the planning since 1955, the 18-month long IGY, (July 1957 to December 1958) was scheduled to coincide with the Solar Cycle 19, a time when maximum sunspot activity was expected. The IGY focused international efforts on exploring Earth. Both the United States of America (US) and the United Soviet States of Russia (USSR) had announced satellite launchings, the USSR aiming for late 1957 and the US for the spring of 1958.
On September 30, 1957, the National Academy of Sciences in Washington DC hosted a week long International Conference on Rockets and Satellites. American and Soviet scientists presented their latest findings and gave details on the progress of their respective satellite programs. It was confirmed that the satellites would carry radios to transmit signals back to Earth as a way to track the orbits and to confirm the launch. External lighting was also discussed as well as weight, dimensions, and other information.
The New York Times carried daily updates from the conference by Walter Sullivan. His column from Tuesday, October 1st mentioned that, when tracking the satellite by the emitted radio signals, the frequency would drop slightly as the satellite passed the receiving station, comparing it to the sound of a train whistle as it passed the train station. The Doppler Effect was not mentioned by name; what was mentioned was the time of the drop in frequency would indicate the instant the satellite passed a receiving station and the degree of drop would indicate the distance of the satellite's path from the station.
Sullivan's subsequent columns reported that the Soviets, when pressed for the possible timing of their launch, deflected the questions but the hinted at possibly launching soon. At the final meeting of the conference, a Soviet scientist confided that he felt a launch had been imminent when he left for the conference. The scientists were at a dinner party that night when the announcement came. Sputnik was launched October 4, 1957, at approximately 5pm Washington, DC time.
Sputnik announced. Image courtesy of Coalwood, West Virginia
The Space Age
Sputnik I's launch took the world by storm. Newspapers headlined the story from October 5th through the following week. The official release from the Soviet news agency TASS revealed a steel radio transmitter on board, transmitting on two frequencies: 20.005 MHz and 40.002 MHz.
The schematic for Sputnik's transmitter. Image courtesy of Stefan Heesch, adapted from Radio.ru (PDF)
The frequencies corresponded to the 15-meter and 7.5-meter bands allocated to Amateur Radio. The US standard for satellite transmission was 108 MHz. This means that, in order to track Sputnik, US satellite receiving stations had to be recalibrated. Amateur radio operators (Hams), like those in the St. Joseph High School Radio Club, however, were ready to track Sputnik.
The American Relay Radio League (ARRL) suggested that US Hams attempting to tune in the signal first tune to WWV, to the Bureau of Standards timing station operating on 20MHz and then moving up to the correct frequency. The actual signal could not be 'heard', beat oscillators in the receiving circuitry made an audible 'chirp' which could be recorded.
A series of "chirps" from Sputnik. Audio courtesy of NASA.
The USSR requested all Russian Hams and commercial radio stations tape the recordings and send them to Moscow for post-processing. They had made schematics for the receiving circuits available. The USSR hoped the rest of the world would also send recordings. The Naval Research Laboratory (NRL) acted as the collection point for US recordings of transmissions and sightings. Commercial, military and amateur radio stations were encouraged to forward their data to the NRL.
The October 7th edition of the New York Times reported Moscow was tracking Sputnik using the Doppler Effect as shown below:
The Doppler Effect
The Doppler Effect, named for Christian Doppler, who proposed it in 1842, refers to the physical phenomenon experienced with waves and motion. The Doppler Effect or Doppler Shift, is perceived changes in wave energy between two points due to relative motion between the two points. The motion can be due to either the velocity of the source (emitter) or the receiver or both. It is in evidence with audio waves, radio waves, electromagnetic waves, light waves. How the difference is perceived depends on the wavelength. With audio signals, we hear a change in pitch (as when an ambulance with its siren blasting rushes past us). With visible light, we see changes in color (as with the redshift in astronomy). It holds in classical physics where velocities are much less than the speed of light and at relativistic speeds. There is no change in the emitted frequency, it is only in perception due to the Doppler Effect that the change is apparent.
Doppler data is the difference between the observed and emitted frequency as a function of time. With a satellite, the orbit is tracked by recording and measuring the actual frequencies received as the satellite approaches a receiver. The observed frequency of approach, passing and receding are plotted against time. The Doppler indicates the relative position and motion of the observer to the emitter.
The Doppler will show a steep drop in frequency when a satellite is overhead (90 degrees to the horizon) and less of a drop when the satellite is at a lesser angle to the receiver. Comparing the following two figures, the Doppler at 90 degrees shows a dramatic change in frequency as the satellite approaches then recedes. At 20 degrees, the maximum frequency shift is lower, and the slope not as steep. As the satellite approaches the receiver, the frequency differences can be hundreds of Hz.
Tracking, Predicting, Locating
Among those tracking Sputnik were Drs. William Guier and George Weiffenbach, researchers at the Johns Hopkins University Applied Physics Laboratory (APL). With backgrounds in mathematics, physics, and microwaves, they not only tracked the satellite but were able to predict the position of Sputnik using doppler data from a single pass of the satellite.
Using vector analysis to solve eight equations with eight unknowns, they were able to solve for six orbital elements: the period, eccentricity, inclination, argument of perigee, latitude of perigee, and longitude of ascending node. The other variables were to quantify the effects of ionospheric refraction and errors. They used APL's UNIVAC computer for solving the mathematical equations. When Sputnik's batteries gave out, their work continued with the launch of Sputnik II in November 1957.
With success in predicting orbits, the APL's deputy director, Dr. Frank McClure, posed a question: If they could predict the orbit of a moving satellite from knowing the doppler effect on a stationary receiver, could they reverse the equation? Could knowing the orbit and position of space-based satellites allow them to locate an object on Earth? The two scenarios are shown in the figure below. For a navigation system, only the longitude and latitude would be needed.
Locating objects on Earth via satellite.
It turned out, they could!
Why the Question?
The Navy was deploying the Polaris Missile, a missile launched from a submarine. The missile guidance needed accurate launch coordinates for targeting. Existing navigation systems were not accurate enough for submarine launching. Could a Doppler based system work?
The April 1960 edition of the Proceedings of the Institute of Radio Engineers (IRE) contained two articles: 'A Satellite Doppler Navigation System' by W. H. Guier and G. C. Weiffenbach and 'Measurement of the Doppler Shift of Radio Transmissions from Satellites' by George C. Weiffenbach. The elements needed for a Doppler navigation system were listed:
- A network of satellites at optimum altitudes for tracking and navigation
- A network of stations to supply tracking data to a computing complex to maintain the satellite data
- A way to get each satellite’s data to the navigator
- Independent gear to process the satellite’s data and provide a navigation solution
This system became the basis of the Transit, the first Navy navigational satellite system. Transit became operational in 1964 providing navigation for US Navy submarines and ships. It consisted of 6 satellites, providing bearings every 90-110 minutes.
Transit -1's satellite prototype
While Transit was being deployed, other systems were being developed simultaneously. After all, being able to get a true position every hour and a half is not optimal when you may need to fire missiles in self-defense.
Both the Air Force and the NRL proposed systems. The Air Force's 621B used CDMA modulation for navigation signals; the NRL's Timation (timed navigation) used a new methodology which depended on precise timing. These efforts were combined in 1973 and the Timation III became the first satellite in the Navigation System using Timing and Ranging (NAVSTAR) GPS. NAVSTAR satellites were launched between 1978 and 1989; the system became fully operational between 1995-1996.
Although developed for the military, GPS signals were made available to anyone, free of charge. There have been upgrades to the satellites in the years since. Transit stopped providing navigational data when NAVSTAR went live.
Today’s GPS, maintained by the US Department of Defense, provides navigation coordinates and time of day to any GPS receiver, 24/7. There are three components, which are not so different from the initial requirements given by Guier and Weiffenbach:
1) The space segment: a constellation of a minimum of 24 satellites in semi-synchronous orbits, circling the Earth twice a day, at least 4 satellites are available from any place on Earth at any time.
2) The control segment: ground control stations, as well as monitoring and antenna control stations.
3) The user segment: the GPS receiver. Depending on the type of receiver in use, this also includes databases, maps and displays to provide visual information.
GPS has been useful in all aspects of military guidance, with the civilian use continuing to grow. From surveying to fleet monitoring, crop dusting to harbor navigation, GPS receivers are available as stand alone units or incorporated into all types of vehicles; GPS is also incorporated into mobiles and wearables.
Other countries have fielded their own navigation systems. Systems currently in place or planned are:
- Russia: Global Navigation Satellite System (GLONASS)
- China: BeiDou Satellite Navigation System (BDS) - regional; Compass (2020, global)
- European Union: Galileo
- India: Indian Regional Navigation Satellite System (IRNSS)
- Japan: Quasi-Zenith Satellite System (QZSS)
As receivers develop to use multiple system data, positioning will be more precise.
As we celebrate the launching of the space age and the Voyager missions, we also have to point to that remarkable achievement as the beginning of GPS, both events which have profoundly changed our technological world.