Transit: The GPS Forefather
Transit: The GPS Forefather
Before there was GPS, there was the Navy navigation satellite system called Transit. Development began in 1958 at the Johns Hopkins University Applied Physics Laboratory; it was declared operational in 1964 and continued until 1996. The satellites were tracked by a series of ground stations and a command center that operated the satellites and generated their navigation messages.
Transit operated on a Doppler ranging principle. Motion of the satellite relative to the user produced a Doppler frequency shift in the satellite signal received. The user’s receiver generated a reference signal that was compared with the received signal by generating the difference frequency between the two. The receiver counted the number of cycles of the difference frequency over an interval (often about 23 seconds) to form the “Doppler count.” Essentially, the Doppler count was a measure of the change in the slant range (distance) of the satellite and the user between the start and end of the interval. In practice, a position fix would use several successive Doppler counts to compute the user position.
One of the strengths of Transit was that it required a nominal constellation of only four satellites, because a position fix required only one satellite at a time; in practice, the constellation generally had between four and six satellites. These were in circular, polar, low Earth orbits (about 1075-kilometer altitude), which ensured good Doppler shifts and reduced the required broadcast power and the size of the required launch vehicle (Scout). It was a system with an unlimited number of passive users anywhere in the world, and it could operate in essentially all weather conditions. But the Doppler principle also meant that a position fix could take 30 minutes to compute, and any motion of the receiver (especially for airborne users) complicated the position calculation. It was generally considered only a 2-D system (latitude and longitude), and it was noncontinuous in many areas (since 30 minutes might elapse before the next satellite came into view).
In contrast, the GPS system is based on determining the range between the user and a GPS satellite; the user essentially computes the time required for the satellite signal to reach the receiver. Range measurements to four GPS satellites allow the user’s receiver to compute its 3-D position and correct for errors in its internal clock. GPS leveraged the lessons of the Navy’s Transit and Timation (time navigation—passive ranging by measuring the time difference between electronic clocks located within the satellite and in the navigator’s receiver). An atomic frequency standard provides a stable, precise signal, whose timing is synchronized across the constellation. The code-division, multiple-access technique allows all satellites to transmit on the same center frequency. An additional navigation signal on a second frequency allows the user to correct for ranging errors introduced as the signals pass through the ionosphere.
This precise ranging to four GPS satellites enables rapid and accurate real-time positioning for dynamic users, which addresses a shortcoming of the earlier systems. But it comes at a price: If you want continuous, real-time positioning anywhere at any time, four satellites need to be in continuous view. Moreover, they need to have “good geometry” to ensure an accurate solution and be deployed in stable, predictable orbits. These conditions led GPS to its constellation of 24 to 30 satellites. The Russian GLONASS system and the European Union’s Galileo system each define their own constellation, but are about the same size range when fully populated.
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