Technical data
Tables
To measure the range from the satellite to the receiver, two criteria are
required: signal transmission time, and signal reception time. All GPS
satellites have several atomic clocks which keep precise time and these
are used to time-tag the message (i.e. code the transmission time onto
the signal) and to control the transmission sequence of the coded
signal. The receiver has an internal clock to precisely identify the arrival
time of the signal. Transit speed of the signal is a known constant (the
speed of light), therefore: time x speed of light = distance.
Once the receiver calculates the range to a satellite, it knows that it lies
somewhere on an imaginary sphere whose radius is equal to this range.
If a second satellite is then found, a second sphere can again be
calculated from this range information. The receiver will now know that
it lies somewhere on the circle of points produced where these two
spheres intersect.
When a third satellite is detected and a range determined, a third
sphere would intersect the area formed by the other two. This
intersection occurs at just two points. The correct point is apparent to
the user, who will at least have a very rough idea of position. A fourth
satellite is then used to synchronise the receiver clock to the satellite
clocks.
When the Symmetricom™ GPS Receiver has calculated a position
solution, the timing element (of this solution) is used to control the
ovenized crystal oscillator. The oscillator’s frequency is automatically
controlled to ensure synchronisation with GPS time. When this
synchronisation has been achieved, time and frequency outputs from
GPS Receiver are enabled.
Utilizing its considerable processing power, the Symmetricom™ GPS
Receiver rapidly updates these calculations from satellite data to
provide Timing and Frequency outputs.
Standard Positioning Service (SPS)
Civil users world-wide are able to use the SPS without restriction or
charge. Accuracy of the system is intentionally degraded by the DoD
through the application of Selective Availability (SA). This degradation
is achieved by the system deliberately broadcasting extra errors into the
satellite orbit information, and by ‘dithering’ the satellite clocks.
A predicted accuracy for the SPS has been published in the 1994
Federal Radionavigation Plan as:-
100 meter horizontal accuracy
156 meter vertical accuracy
340 nanosecond time accuracy
GPS Timing Receiver Installation and Operation Manual
D-3