GPS Satellite Signal Characteristics - Understanding GPS: Principles and Applications - page 143

The GPS navigation message is transmitted in five 300-bit subframes, as shown
in Figure 4.19. Each subframe is itself composed of ten 30-bit words. The last 6 bits
in each word of the navigation message are used for parity checking to provide the
user equipment with a capability to detect bit errors during demodulation. A (32,
26) Hamming code is employed. The five subframes are transmitted in order begin-
ning with subframe 1. Subframes 4 and 5 consist of 25 pages each, so that the first
time through the five subframes, page 1 of subframes 4 and 5 are broadcast. In the
next cycle through the five subframes, page 2 of subframes 4 and 5 are broadcast
and so on.
Although there are provisions for a loss of ground contact, normally the control
segment uploads critical navigation data elements once or twice per day per satel-
lite. In this nominal mode of operation, the same critical navigation data elements
(e.g., satellite ephemeris and clock correction data) are broadcast repeatedly over
2-hour time spans (except if an upload occurs during this interval). On 2-hour
boundaries, each satellite switches to broadcasting a different set of these critical
elements, which are stored in tables in the satellite's RAM. The control segment
generates these message elements based upon its current estimates of each satellite's
position and clock error and prediction algorithms on how these parameters will
change over time.
The first two words of each subframe (bits 1-60) contain telemetry (TLM) data
and a handover word (HOW). The TLM word is the first of the 10 words in each
subframe and includes a fixed preamble, a fixed 8-bit pattern 10001011 that never
changes. This pattern is included to assist the user equipment in locating the begin-
ning of each subframe. Each TLM word also includes 14 bits of data that are only
meaningful to authorized users. The HOW, so-named because it allows the user

bit 1 60 300

GPS week number, SV accuracy and health, clock
correction terms

Subframe 1

TLM HOW

Subframe 2

TLM HOW Ephemeris parameters

Subframe 3

TLM HOW Ephemeris parameters

Subframe 4
(pages 1-25)

Almanac and health data for SVs 25-32, special messages,
satellite configuration flags, ionospheric and UTC data

TLM HOW

Subframe 5
(pages 1-25)

Almanac and health data for SVs 1-24, almanac reference
time and week number

TLM HOW

300 bits (6 s at 50 bps)

Figure 4.19 Navigation message format.