0
30
60
300
BIT NO.
SUBFRAME
TELEMETRY
HANDOVER
CLOCK CORRECTION
6 SEC
1
WORD
WORD
300
330
360
600
TELEMETRY
SUBFRAME
HANDOVER
EPHEMERIS
2
WORD
WORD
12 SEC
600
630
660
900
SUBFRAME
TELEMETRY
HANDOVER
3
WORD
WORD
EPHEMERIS
18 SEC
900
930
960
1200
SUBFRAME
TELEMETRY
HANDOVER
24 SEC
WORD
(MULTIPLEX)
4
WORD
MESSAGE (CHANGES THROUGH 25 FRAMES)
1200
1230
1260
1500
TELEMETRY
SUBFRAME
HANDOVER
ALMANAC/HEALTH STATUS
5
WORD
WORD
(MULTIPLEX)
(CHANGES THROUGH 25 FRAMES)
30 SEC
*12.5 MINUTES BEFORE THE ENTIRE MESSAGE REPEATS
Figure 1 5. The Navigation Message
Subframes 4 and 5 have data which cycle through the 25 data frames. They contain data which is
common to all satellites and less critical for a receiver to acquire quickly. Subframes 4 and 5
contain almanac (coarse orbital) data and low precision clock corrections, simplified health and
configuration status for every satellite, user text messages, and the coefficients for the ionospheric
model and UTC calculation. A comprehensive description of the navigation message is provided
in "Technical Characteristics of the Navstar GPS", together with the standard algorithms needed
to use the data correctly.
1.4.1.4 Satellite Signal Modulation
The L1 carrier is BPSK modulated by both the C/A and P(Y) codes plus the navigation message
superimposed on both codes. The L2 carrier is BPSK modulated by the P(Y) code superimposed
with the navigation message. The BPSK technique reverses the carrier phase when the
modulating code changes from logic 0 to 1 or 1 to 0. On L1, the C/A code is 90 degrees out of
phase with the P(Y) code. Figure 1 6 shows this modulation scheme in schematic form.
1 8
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