GPS Receiver-Related Subjects - Fundamentals of Global Positioning System Receivers: A Software Approach - page 342

TABLE 12.7 Almanac Data from One Satellite

******** Week 240 almanac for PRN-01 ********
ID 01 (Satellite number)

Health 000 (Health condition)

Eccentricity 0.5343914032E − 002 ( e s : Equation(3.23))

Time of applicability (s) 503 808.0000 ( t yu : Time of YUMA data)

Orbital inclination (rad) 0.9784364629 ( i : Equation(4.45))

( ž

Rate of right ascen ( r/s ) − 0.8034620388E − 008 : Equation(4.14))

( √ a s : Equation(3.17)) SQRT(A) (m 1 / 2 ) 5153.525879

Right ascen at week
(rad)

− 0.2265141720E + 001 ( e : Equation (4.14))

−

Argument of perigee
(rad)

1.671651819 ( ω : Equation (4.12))

Mean anom (rad) − 0.1932000116E + 001 ( M 0 : Equation (3.27))

Af 0( s ) 0.3376007080E − 003 ( a f 0 : Satellite clock correction

Section 5.12)

Af 1 (s/s) 0.0000000000E + 000 ( a f 1 : Satellite clock correction

Section 5.12)

Week 240 (Week of YUMA data)

as a subset of ephemeris data. It does not contain most of the error correction
parameters. Therefore, the satellite positions calculated will not be as accurate,
but the information is adequate to simplify the acquisition program, as discussed
in Section 12.2.
The basic approach to calculating the satellite position is similar to that dis-
cussed in Section 4.9. There are two main differences. First, in Section 4.9, the
time is obtained from the ephemeris data, whereas in this approach of the time
is an input parameter. Second, the user location is given. In Section 4.9, the user
location was the final output of the receiver. In the present case, the user location
can be used to determine the satellites visible in the sky. The velocities of the
satellites can be found from the positions of the satellites as a function of time,
such as find the satellite position every second.
An almanac file contains all the satellites in the sky. The data for each satellite
are as listed in Table 12.7. The values in Table 12.7 were printed directly from
YUMA satellite data, and the information in parentheses relates the data to the
symbols used in this topic. The following steps are required to find the satellites:

Step1. Change fromlocal time toGPS time . Suppose that the input time is
the local time [given as year, month, date, hour, minute, second]. This time must
be changed to GPS time first. The year, month, and date are used to determine
the GPS week number (WN). The reference date of WN is 1999 August 21/22,
although the GPS WN started on midnight January 5/morning January 6 1980. Its
roll over happened in 1999. The number of dates passed since August 21, 1999,
divided by 7 gives the WN and the remainder is the date of the week (DOW).