Viavi 87339, Руководство по эксплуатации

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Principals of Operation
6 -7
Subframes 1 to 5 each provide a synchronization, hand-over word and a C/A code time
ambiguity removal. The remainder of the data is formatted as follows:
Subframe 1:
• Contains the time values of the transmitting satellite, including the parameters for
correcting signal transit delay and onboard clock time, as well as information on
satellite health and an estimate of the positional accuracy of the satellite.
Subframe 2 and 3:
• Ephemeris.
Subframe 4:
• Ionospheric model, UTC data, flags for each satellite indicating whether
anti-spoofing is on, almanac (approximate satellite ephemeris allowing the
receiver to select the best set of satellites or to determine which satellites are in
view) and health information for satellite number 25 and greater.
Subframe 5:
•
Almanac and health information for satellite number 1 to 24.
The reception and decoding of the data stream is performed automatically by a receiver
without any intervention by the operator. The information within this data is critical to
GPS operation. If a GPS receiver has never seen the GPS constellation before and does
not know its approximate location, the first action of the receiver is to acquire any SV in
view. Once a satellite has been acquired, the almanac is downloaded.
Refer to Fig. 6-8. Subframes 4 and 5 contain almanac information. Once the almanac is
acquired, the information it contains is used to determine which satellites are in view and
select the set of satellites with the best geometry. The almanac structure for one SV PRN
is shown in Fig. 6-8. Almanac data is typically updated every 24 hours. Data for a few
weeks is also provided in case of a delay in update. Once a receiver has an almanac
loaded in its memory, it can be used for a faster acquisition of satellites on the next
occasion of use. Once a satellite has been acquired, it’s ephemeris information can be
obtained.
Fig. 6-8 GPS Almanac
The almanac provides a basic description of each satellite orbit. Refer to Fig. 6-9. Two
parameters are commonly displayed on GPS receivers which describe the basic position
of a satellite relative to a GPS receiver at a specific location.
Elevation describes the angle of a satellite relative to the horizontal plane. If a satellite
is directly above the point of observation on the ground, then the elevation is 90°. If the
satellite is at the horizon, then the elevation is 0°.
Azimuth is the angle between a reference plane and a point. In the case of satellites the
reference plane is the plane of the horizon based on true North. The Azimuth is the angle
between the satellite and true North (North = 0°, East = 90°, South = 180°, West = 270°).
Of course many other parameters are used to define satellite orbit. Each satellite will
downlink a more precise description of its orbit, which is contained in sub-frames 2 and 3,
and is known as ephemeris. With this information the receiver can determine the
******** Week 572 almanac for PRN-03 ********
ID: 03
Health: 000
Eccentricity: 0.1350402832E-001
Time of Applicability(s): 405504.0000
Orbital Inclination(rad): 0.9279594421
Rate of Right Ascen(r/s): -0.8203642210E-008
SQRT(A) (m 1/2): 5153.669922
Right Ascen at Week(rad): 0.3102001
Argument of Perigee(rad): 1.020882249
Mean Anom(rad): -0.7818000
Af0(s): 0.5941390991E-003
Af1(s/s): 0.3637978807E-011
week: 572