Transit (satellite)
The TRANSIT system, also known as NAVSAT (Navy Navigation Satellite System), was the first satellite navigation system to work. In its first stage, it was used by the US Navy to obtain precise information for the launch of submarine missiles and for the navigation of ships and submarines, it was also used for topographic, geotechnical and hydrographic studies.
Days after the launch of Sputnik (USSR – October 4, 1965), scientists George Weiffenbach and William Guier wanted to determine the satellite's orbit by analyzing the Doppler shift in its radio signals. It is then that it was suggested that if the position of the satellite could be predicted, the Doppler effect could be used for locations on earth.
From the satellites of the TRANSIT system, two carrier signals (UHF) are transmitted periodically (every two minutes). Orbit incidents and clock corrections are updated twice a day. With this information, the position of the satellite is calculated over time. By using two signals, the number of errors is reduced. This system made it possible to synchronize clocks around the world with a precision of 50 microseconds.
The procedure that allows the receiver to obtain information on the location of an object is through the Doppler effect (apparent change in frequency of a wave produced by the movement of the source with respect to its observer). The transmitter of the signal travels in the satellite at about 27,000 km/h, which increases the frequency of the signal by at most 10 kHz. This effect is unique for each location within the line of sight of the satellite.
Calculating the optimal location of the receiver is a fairly complex process, with successive adjustments and updates of the object to be located. If the receiver is moving, mismatches and loss of precision in the effectiveness of the location will also occur. The accuracy of the measurement is also influenced by the accuracy of the watch. Transmission is carried out at 150 and 400 MHz. These two frequencies are used to minimize the effect of the ionosphere on the signals and thus achieve a more precise location.
At first, the locations could only be made while the satellite was on the visible horizon with the terrestrial portion to be analyzed. This meant that there were areas that took easily over two hours to receive new data.
This system pioneered many of its features, such as correction for the effect of the ionosphere, or solar altitude detectors, which meant great advances in many areas of science.
The launch of this satellite also implied the improvement of other systems such as microprocessors, achieving improvements in their size, computing power and other characteristics (for example, the AN/UYK-1, installed in the submarines of the American Navy)..
Still today, the legacy of Navsat can be seen in the application to many innovations and scientific discoveries, such as stabilization systems, knowledge of the Earth's gravitational field or biomedical applications (rechargeable pacemakers or defibrillators…)
This system became obsolete after the appearance of the GPS (Global Positioning System), since electronic improvements allowed the GPS system to perform measurements and calculations much more efficiently, so the NAVSAT system stopped working in the year nineteen ninety six.