Other Satellite Navigation Systems - Understanding GPS: Principles and Applications - page 611

tees exist regarding the P code data. Thus, the P code data structure described here
may change at any time without notice.
Each GLONASS satellite broadcasts a P code navigation message that contains
a superframe consisting of 72 frames. Each frame has five lines, with each line con-
taining 100 bits of information. Each frame takes 10 seconds to broadcast, so the
entire superframe is broadcast once every 12 minutes [22].
The first three lines of each frame contain the detailed ephemeris for the satellite
being tracked. Since each frame repeats every 10 seconds, a receiver will receive a
satellite's ephemeris within 10 seconds once data reception occurs. The other lines
of each frame consist primarily of approximate ephemeris information for the other
satellites in the constellation. All 72 frames must be read to get all the ephemeris,
which takes 12 minutes [22].
The two most distinguishing differences between the data messages deal with
the length of time required to obtain ephemeris information. The time to obtain
detailed ephemeris is:

• P code: 10 seconds;
•

C/A code: 30 seconds.

The time to obtain almanac (approximate ephemeris) for all satellites is:

P code: 12 minutes;

•

C/A code: 2.5 minutes.

•

11.1.10 System Accuracy
GLONASS provides two levels of accuracy similar to GPS. The high-accuracy ser-
vice is exclusively for Russian military use, while the lower accuracy service is for
civil use. The high-accuracy service has an antispoofing capability that is under the
control of the Russian Ministry of Defense. Based on Western observations, this fea-
ture is typically not activated, and the service is available for navigation. Observa-
tions by the University of Leeds in the United Kingdom and 3S Corporation during
the mid-1990s, when the constellation was almost fully populated, indicate that the
accuracy provided by the military service is similar to the specification for the GPS
PPS—approximately 20m (2 drms, 95% probability) in the horizontal plane and
34m (2 sigma), in the vertical dimension. However, according to the Russians, the
military accuracies for GLONASS remain classified [1, 22].
The specification for GLONASS civil accuracy is 100m (2 drms, 95% probabil-
ity) in the horizontal, 150m (2 sigma) in the vertical, and 15 cm/sec (2 sigma) in
velocity. A full GLONASS constellation (21 satellites plus 3 active spares) is
designed to have a 94.7% probability of providing civilian navigational informa-
tion to the previously mentioned design accuracy. The specification for derived
time is within 1m of GLONASS system time and within 5 ms of UTC (SU) [1].
In practice during the mid-1990s when the constellation was almost fully popu-
lated, GLONASS accuracy was much better than the specified values cited here.
Tests of the operational system have demonstrated civil accuracies of 26m (2 drms,