Global Positioning System Reference
In-Depth Information
12.5
12.0
1
TLM
HOW
Clock corrections and SV health/accuracy
2
TLM
HOW
Ephemeris parameters
1.0
3
TLM
HOW
Ephemeris parameters
1
0.5
TLM
HOW
Clock corrections and SV health/accuracy
25
1
4
0
TLM
HOW
Clock corrections and SV health/accuracy
TLM
HOW
Almanac, ionospheric model, dUTC
2
TLM
HOW
Ephemeris parameters
0
1
25
TLM
HOW
Clock corrections and SV health/accuracy
2
5
TLM
HOW
Ephemeris parameters
TLM
HOW
Almanac
3
6
TLM
HOW
Ephemeris parameters
2
TLM
HOW
Ephemeris parameters
3
3
TLM
HOW
Ephemeris parameters
4
12
TLM
HOW
Almanac, ionospheric model, dUTC
3
2
TLM
HOW
Ephemeris parameters
4
3
TLM
HOW
Almanac, ionospheric model, dUTC
5
18
TLM
HOW
Almanac
1
4
2
TLM
HOW
Almanac, ionospheric model, dUTC
5
TLM
HOW
Almanac
24
1
5
TLM
HOW
Almanac
30
FIGURE 2.9. GPS navigation data structure. Each subframe containing 300 bits lasts 6 s.
Subframes 1, 2, and 3 repeat every 30 s while subframes 4 and 5 have 25 versions before re-
peating. That is, the entire navigation message repeats after 12.5 minutes. Courtesy: Frank
van Diggelen.
navigation data. Figure 2.9 shows the overall structure of an entire navigation
message.
The basic format of the navigation data is a 1500-bit-long frame containing 5
subframes, each having length 300 bits. One subframe contains 10 words, each
word having length 30 bits. Subframes 1, 2, and 3 are repeated in each frame. The
last subframes, 4 and 5, have 25 versions (with the same structure, but different
data) referred to as page 1 to 25. With the bit rate of 50 bps, the transmission of
a subframe lasts 6 s, one frame lasts 30 s, and one entire navigation message lasts
12.5 minutes.
2.6.1 Telemetry and Handover Words
The subframes of 10 words always begin with two special words, the telemetry
(TLM) and handover word (HOW) pair.
TLM is the first word of each subframe and it is thus repeated every 6 s. It
contains an 8-bit preamble followed by 16 reserved bits and parity. The preamble
should be used for frame synchronization.
HOW contains a 17-bit truncated version of the time of week (TOW), followed
by two flags supplying information to the user of antispoofing, etc. The next three
bits indicate the subframe ID to show in which of the five subframes in the current
frame this HOW is located.
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