Editing Transit (satellite) (section)

===Determining the satellite orbits===
[[File:Doppler 019 outside, McMurdo Station, 1974.png|thumb|Vestibule and Quonset hut housing Transit satellite tracking station 019. 1. Triad satellite magnetometer down load antenna. 2. flag pole, 3. Utility pole in background, 4 Revolving light temperature alarm, 5 VLF antenna, 6-9 Doppler satellite tracking antennas, 10. stove pipe for heater, 11 Flood light for low visibility conditions, 12 fuel tank.]]
[[File:Doppler 019 inside, McMurdo Station, 1974.png|thumb|Some of the equipment inside Transit satellite tracking station 019. 1. Automatic Control Unit, 2. timer-counter, 3. Time burst detector, 4. time conversion chart, 5. satellite ephemeris, 6. tracking receiver, 7. time display, 8 Header-Tailer programmer, 9. Digitizer and main clock, 10. master oscillator, 11. strip chart recorder, 12. paper tape punch, 13. short wave receiver. Out of site: VLF receiver, refraction correction unit, backup battery system, power supplies, AC voltage regulators.]]
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]].  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 [[Crystal oven|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 standard|rubidium]] and [[Caesium standard|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.