Global Positioning System Reference
In-Depth Information
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incoming signal by a locally generated replica of the code. This correlation process
results in a signal that is well above the noise level.
3.2.2.2 Navigation Message The Master Control Station, located near Col-
orado Springs, uses data from a network of monitoring stations around the world to
monitor the satellite transmissions continuously, compute the broadcast ephemerides,
calibrate the satellite clocks, and periodically update the navigation message. This
“control segment” ensures that the SPS and PPS are available as specified in SPS
(2001).
The satellites transmit a navigation message that contains, among other things,
orbital data for computing the positions of all satellites. A complete message consists
of 25 frames, each containing 1500 bits. Each frame is subdivided into 5 300-bit
subframes, and each subframe consists of 10 words of 30 bits each. At the 50 bps rate
it takes 6 seconds to transmit a subframe, 30 seconds to complete a frame, and 12.5
minutes for one complete transmission of the navigation message. The subframes 1,
2, and 3 are transmitted with each frame. Subframes 4 and 5 are each subcommutated
25 times. The 25 versions of subframes 4 and 5 are referred to as pages 1 through 25.
Thus, each of these pages repeats every 12.5 minutes.
Each subframe begins with the telemetry word (TLM) and the handover word
(HOW). The TLM begins with a preamble and otherwise contains only information
that is needed by the authorized user. The HOW is a truncation of the GPS time of
week (TOW). HOW, when multiplied by 4, gives the X 1 count at the start of the
following subframe. As soon as a receiver has locked to the C/A-code, the HOW
word is extracted and is used to identify the X 1 count at the start of the following
subframe. In this way, the receiver knows exactly which part of the long P(Y)-code is
being transmitted. P(Y)-code tracking can then readily begin, thus the term handover
word . To lock rapidly to the P(Y)-code, the HOW is included on each subframe (see
Figure 3.11).
GPS time is directly related to the X 1 counts of the P(Y)-code. The Z count is a
twenty-nine-bit number that contains several pieces of timing information. It can be
used to extract the HOW, which relates to the X 1 count as discussed above, and the
TOW, which represents the number of seconds since the beginning of the GPS week.
A full week has 403,199 X 1 counts. The Z count gives the current GPS week number
(modulo-1024). The beginning of the GPS week is offset from midnight UTC by the
accumulated number of leap seconds since January 5-6, 1980, the beginning of GPS
time.
Subframe 1 contains the GPS week number, space vehicle accuracy and health
status, satellite clock correction terms a f 0 , a f 1 , a f 2 and the clock reference time t oc
(Section 5.3.1), the differential group delay, T GD (Section 5.4), and the issue of date
clock (IODC) term. The latter term is the issue number of the clock data set and can
be conveniently used to detect any change in the correction parameters. The messages
are updated usually every 4 hours.
Subframes 2 and 3 contain the ephemeris parameters for the transmitting satellite.
The various elements are listed in Table 3.5. These elements are a result of least-
squares fitting of the predicted ephemeris over a well-specified interval of time. The
[81
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