Transit-1B, the first experimental navigational satellite has been launched on 13 April, 1960, laying the foundations of - the now ubiquitous - space-based navigation.

Events have started - as in other American space programmes - with Sputnik. The first satellite of the world started to beep on a Friday and only on Monday did the idea of measuring its orbit come to western researchers. Scientists at John Hopkins University tested their new receiver with success, measuring the Doppler-shift of the radio signals and from them proving mathematically that the broadcast indeed came from an object in low Earth orbit. That evening they made estimations for the orbital elements. The next few days were in fever of calculating the exact orbit. George Weiffenbach and William Guier developed an algorithm for the orbital elements using the Doppler-shift and the known position of the radio receiver. It did not take long to create the inverse theorem: knowing the satellite's orbit and measuring its radio signals we can determine the location of a receiver on Earth. The use of such a method? The most obvious use was for mobile receivers, of which the USA had several: ships, submarines, airplanes (as the military is usually the first to fund new ideas..). The Navy soon adopted the idea.

Development has started in 1958. A total of 5 satellites were determined to be enough to cover the globe and the receiver needed only to "see" one satellite for localization (with a 200m accuracy). A fleet of 10 satellites were finally commissioned, each of the probes having a backup.

The satellite system was christened Transit by NASA, though the Navy used the name NavSat (Navy Navigation Satellite System). Transit-1 has started the series with a failure in 1959 (due to the failing of the Thor-Able rocket). Its backup, Transit-1B has been laucnhed on 13 April 1960. It was working on a polar orbit at altitude of 1100 km, taking 106 minutes per orbit. Two UHF transmitters (on frequencies 150MHz and 400MHz) broadcast signals containing time and orbital element data every two minutes. The orbit ephemeris and clock correction data were uploaded twice daily to the satellites from the Navy tracking stations. The use of two frequencies allowed to reduce the effects of the ionosphere (note the the civil version of today's GPS only accesses one frequency, achieving worse accuracy than the two-frequency military receivers).

Prototype of Transit-1. (Source: USAF)

The other satellites followed quickly, the system being operational by 1964. By today's standards, it was quite limited: the ships updated their data every few hours and achieved an accuracy of 100-200 meters.

Transit had other services as well. It was the first worldwide time signal system, having an accuracy of 50 microseconds. Moreover, it allowed the Navy to send encrypted messages.

The system remained a military secret until 1967, when the encryption was removed and thus commercial or civilian ships were able to use it. Some data point to the Soviet navy using Motorola NavSat receivers. Later surveyors found a way to use the system, using averaging to achieve accuracy under a meter. This was used to re-measure the height of Mount Everest to 8850m from 8848m in the end of the 1980s.

Doppler measurements were used for geodetic purposes in Hungary as well, in the Cosmic Geodetic Observatory of Penc. The upper photo shows a JMR-1 receiver - compare to a modern GPS receiver...

The bottom photo show the antennas used for calibration in 1983. (Photos: FÖMI KGO)

Accuracy had been increased in tine. New data analysing methods, for military use only, have improved one-time measurement accurarcy to 20 meters. Transit was used for navigation until 1991, being replaced by GPS. The system was deactivated in 1996, though the remaining satellites are still used by the Navy as part of the ionosphere research system.

Credit: Béla Dancsó - Űrvilág

Translation by Márk Horváth