Viavi 87339, Руководство по эксплуатации

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Principals of Operation
6 -19
The GPS-UTC offset field can accommodate 255 leap seconds (eight bits) which, given
the current rate of change of the Earth's rotation (with one leap second introduced
approximately every 18 months), should be sufficient to last until approximately the year
2300. As opposed to the year, month, and day format of the Gregorian calendar, the GPS
date is expressed as a week number and a seconds-into-week number.
Fig. 6-25 GPS v. UTC Time Format
The week number is transmitted as a ten-bit field in the C/A and P(Y) navigation
messages, and so it becomes zero again every 1,024 weeks (19.6 years). GPS week zero
started at 00:00:00 UTC (00:00:19 TAI) on January 6, 1980, and the week number
became zero again for the first time at 23:59:47 TC on August 21, 1999 (00:00:19 TAI on
August 22, 1999).
To determine the current Gregorian date, a GPS receiver must be provided with the
approximate date (to within 3,584 days) to correctly translate the GPS date signal. To
address this concern the modernized GPS navigation message uses a 13-bit field, which
only repeats every 8,192 weeks (157 years), thus lasting until the year 2137 (157 years
after GPS week zero).
6.1.6 GNSS Accuracy
A GNSS receiver determines its Position (horizontal and vertical), its Velocity and the
Time from the signals of at least four satellites by means of triangulation. The precision
of the computations by triangulation depends on the constellation of all satellites of which
the signals are taken into account (four or more). As the number and position of satellites
will seldom be ideal, the maximum obtainable precision will be diluted in practice. Here
we present the different terms of dilution of precision.
Dilution of precision (DOP) is a measure of the quality of the GPS data being received
from the satellites. DOP is a mathematical representation for the quality of the GPS
position solution. The main factors affecting DOP are the number of satellites being
tracked and where these satellites are positioned in the sky. The effect of DOP can be
resolved into HDOP, VDOP, PDOP and TDOP.
HDOP (Horizontal Dilution Of Precision) is a measure of how well the positions of the
satellites, used to generate the Latitude and Longitude solutions, are arranged. PDOP
less than 4 gives the best accuracy, between 4 and 8 gives acceptable accuracy and
greater than 8 gives unacceptable poor accuracy. Higher HDOP values can be caused if
the satellites are at high elevations.
VDOP (Vertical Dilution Of Precision) is a measure of how well the positions of the
satellites, used to generate the vertical component of a solution, are arranged. Higher
VDOP values mean less certainty in the solutions and can be caused if the satellites are
at low elevations.
TDOP (Time Dilution Of Precision) is a measure of how the satellite geometry is affecting
the ability of the GPS receiver to determine time.