Editing Transit (satellite) (section)

===Determining ground location===
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Determining a location, also known as "taking a fix", normally requires two or more measurements to be taken to produce a 2D location. In the case of the modern GPS system, dozens of such measurements may be taken depending on which satellites are visible at that time, each one helping improve accuracy. In the case of Transit, only a small number of satellites were in orbit and were spread out. This generally meant there was only one satellite visible at any time. Some other method of determining a second measurement was needed.

Transit did this by measuring the signal's Doppler shift. The spacecraft traveled at about {{convert|17000|mph|km/h|abbr=on}}, which could increase or decrease the frequency of the received carrier signal by as much as 10&nbsp;kHz as measured on the ground. While the satellite is approaching the ground station its signals will be shifted up in frequency, and as it recedes they will shift down again. The precise moment when the frequency is exactly equal to the broadcast frequency is when the satellite's [[ground track]] passes the ground location's location (with some corrections). This provides one of the two measurements needed.

For the second measure, one has to consider the pattern of the Doppler shift. If the satellite passes directly overhead, its angular velocity as it passes will be more than if it passes to one side. In the extreme case, with a satellite near the horizon, the relative velocity change is minimized. Thus the rapidity of the change in frequency is an indication of the relative longitude between the station and the satellite. Additionally, the rotation of the Earth provided another Doppler correction which could be used to determine whether the satellite was to the east or west of the ground station.

These measurements produce a relative location compared to the satellite. To determine the actual location, that relative measure is applied to the location of the satellite. This is provided by periodically sending out precise time hacks (every two minutes), plus the satellite's six [[orbital elements]] and orbit [[Perturbation (astronomy)|perturbation]] variables. The ground receiver downloaded these signals and calculated the location of the satellite while it was measuring the shifts. The orbit [[ephemeris]] and clock corrections were uploaded twice each day to each satellite from one of the four Navy tracking and injection stations.

The Transit satellite broadcast on 150 and 400&nbsp;MHz. The two frequencies were used to allow the [[refraction]] of the satellite radio signals by the ionosphere to be canceled out, thereby improving location accuracy. The Transit system also provided the first worldwide timekeeping service, allowing clocks everywhere to be synchronised with 50 microsecond accuracy.

Calculating the most likely receiver location was not a trivial exercise.  The navigation software used the satellite's motion to compute a 'trial' Doppler curve, based on an initial 'trial' location for the receiver.  The software would then perform a [[least squares]] curve fit for each two-minute section of the Doppler curve, recursively moving the trial position until the trial Doppler curve 'most closely' matched the actual Doppler received from the satellite for all two-minute curve segments.

If the receiver was also moving relative to the earth, such as aboard a ship or airplane, this would cause mismatches with the idealized Doppler curves, and degrade position accuracy.  However, positional accuracy could usually be computed to within 100&nbsp;meters for a slow-moving ship, even with reception of just one two-minute Doppler curve.  This was the navigation criterion demanded by the U.S. Navy, since American submarines would normally expose their UHF antenna for only 2&nbsp;minutes to obtain a usable Transit fix.  The U.S. submarine version of the Transit system also included a special encrypted, more accurate version of the downloaded satellite's orbital data.{{Citation needed|date=August 2017}} This enhanced data allowed for considerably enhanced system accuracy [not unlike [[Selective Availability]] (SA) under GPS]. Using this enhanced mode, accuracy was typically less than 20 meters, (i.e. the accuracy was between that of [[LORAN C]] and GPS) For a typical 12 - 15 minute high satellite altitude pass accuracy was under ten meters. Certainly, Transit was the most accurate navigation system of its day.

The basic operating principle of Transit is similar to the system used by [[emergency locator transmitter]]s (ELTs), except that in the latter case the transmitter is on the ground and the receiver is in orbit. ELTs measure the Doppler shift of the transmitter on the boat or aircraft as it passes overhead and forwards that data to the ground where the location of the craft can be determined.