GPS C/A Code Signal Structure - Fundamentals of Global Positioning System Receivers: A Software Approach - page 80

consecutive 10 ms of data. Between two consecutive sets of 10 ms of data there
is at most one navigation data bit phase transition. Therefore, one set of these data
will have no data bit phase transition and coherent acquisition should produce
the desired result. Thirty data bits make a navigation word and 10 words make
a subframe. Figure 5.3 shows these relations.
The GPS time is given by the number of seconds in one week and this value
is reset every week at the end/start of a week. At end/start of a week the cyclic
paging to subframes 1 through 5 will restart with subframe 1 regardless of which
subframe was last transmitted prior to end/start of week. The cycling of the 25
pages will restart with page 1 of each of the subframes, regardless of which
page was the last to be transmitted prior to the end/start of week. All upload and
page cutovers will occur on frame boundaries (i.e., modulo 30 seconds relative
to end/start of week). Accordingly, new data in subframes 4 and 5 may start to
be transmitted with any of the 25 pages of these subframes.
In the following sections the navigation data will be discussed. Only the lim-
ited information used to determine the user position will be included. Detailed
information can be found in references 3 and 7.
5.9 TELEMETRY (TLM) AND HAND OVER WORD (HOW) ( 2,3,7 )
As previously mentioned, five subframes make a page. The first two words of all
the subframes are the telemetry (TLM) and hand over word (HOW). Each word
contains 30 bits and the message is transmitted from bit 1 to bit 30. These two
words are shown in Figure 5.7. The TLM word begins with an 8-bit preamble,
followed by 16 reserved bits and 6 parity bits. The bit pattern of the preamble is
shown in this figure. The bit pattern of the preamble will be used to match the
navigation data to detect the beginning of a subframe.
The HOW word can be divided into four parts.
1. The first 17 bits (1 - 17) are the truncated time of week (TOW) count that
provides the time of the week in units of 6 seconds. The TOW is the
truncated LSB of the Z count, which will be discussed in the next section.
2. The next two bits (18, 19) are flag bits. For satellite configuration 001
(block II satellite) bit 18 is an alert bit and bit 19 is antispoof. Satellites are
procured in blocks. Most block I satellites are experimental ones and all
the satellites in orbit are from block II. When bit 18 = 1, it indicates that
the satellite user range accuracy may be worse than indicated in subframe
1 and the user uses the satellite at the user's own risk. Bit 19 = 1 indicates
the antispoof mode is on.
3. The following three bits (20 - 22) are the subframe ID and their values are
1, 2, 3, 4, and 5 or (001, 010, 011, 100, and 101) to identify one of the
five subframes. These data will be used for subframe matching.
4. The last 8 bits (23 - 30) are used for parity bits.