GPS Satellite Signal Characteristics - Understanding GPS: Principles and Applications - page 144

equipment to “handover” from C/A code tracking to P(Y) code tracking, provides
the GPS time-of-week (TOW) modulo 6 seconds corresponding to the leading edge
of the following subframe. The HOW also provides two flag bits, one that indicates
whether antispoofing is activated (see Section 4.3.1), and one that serves as an alert
indicator. If the alert flag is set, it indicates that the signal accuracy may be poor and
should be processed at the user's own risk. Finally, the HOW provides the subframe
number (1-5).
Subframe 1 provides the GPS transmission week number, which is the number
of weeks modulo 1,024 that have elapsed since January 5, 1980. The first rollover of
the GPS week number occurred on August 22, 1999. The next rollover will occur in
April 2019. It is prudent that the GPS receiver designer keep track of these rare but
inevitable rollover epochs in nonvolatile memory. Subframe 1 also provides the fol-
lowing satellite clock correction terms: a f 0 , a f 1 , a f 2 , and time of clock, t oc . These terms
are extremely important for precise ranging, since they account for the lack of per-
fect synchronization between the timing of the SV broadcast signals and GPS system
time (see Section 7.2.1). A 10-bit number referred to as issue of data, clock (IODC)
is included in subframe 1 to uniquely identify the current set of navigation data. User
equipment can monitor the IODC field to detect changes to the navigation data. The
current IODC is different from IODCs used over the past seven days. Subframe 1
also includes a group delay correction, T gd , a user range accuracy (URA) indicator, a
SV health indicator, an L2 code indicator, and an L2 P data flag. T gd is needed by sin-
gle-frequency (L1- or L2-only) users since the clock correction parameters refer to
the timing of the P(Y) code on L1 and L2, as apparent to a user that is using a linear
combination of dual-frequency L1/L2 P(Y) code measurements to mitigate iono-
spheric errors (see Section 7.2.4.1). The URA indicator provides the user with an
estimate of the 1-sigma range errors to the satellite due to satellite and control seg-
ment errors (and is fully applicable only for L1/L2 P-code users). The SV health indi-
cator is a 6-bit field that indicates whether the satellite is operating normally or
whether components of the signal or navigation data are suspected to be erroneous.
The L2 code indicator field indicates whether the P(Y) code or C/A code is active on
L2. Finally, the L2 P data flag indicates whether navigation data is being modulated
onto the L2 P(Y) code.
Subframes 2 and 3 include the osculating Keplerian orbital elements described
in Section 2.3 that allow the user equipment to precisely determine the location of
the satellite. Subframe 2 also includes a fit interval flag and an age of data offset
(AODO) term. The fit interval flag indicates whether the orbital elements are based
upon a nominal 4-hour curve fit (that corresponds to the 2-hour nominal data trans-
mission interval described earlier) or a longer interval. The AODO term provides an
indication of the age of the elements based on a navigation message correction table
(NMCT) that has been included in the GPS navigation data since 1995 [15]. Both
subframes 2 and 3 also include an issue of data ephemeris (IODE) field. IODE con-
sists of the 8 least significant bits (LSBs) of IODC and may be used by the user equip-
ment to detect changes in the broadcast orbital elements.
Pages 2-5 and 7-10 of subframe 4 and pages 1-24 of subframe 5 contain alma-
nac data (coarse orbital elements that allow the user equipment to determine
approximate positions of other satellites to assist acquisition) for SVs 1-32 (see Sec-
tion 2.3). Page 13 of subframe 4 includes the NMCT range corrections. Page 18 of