Other Satellite Navigation Systems - Understanding GPS: Principles and Applications - page 610

sages are 50-bps data streams. The primary purpose of these messages is to provide
information on satellite ephemeris and channel allocations. The ephemeris informa-
tion allows the GLONASS receiver to accurately compute where each GLONASS
satellite is located at any point in time. Although ephemeris is the predominant navi-
gation information, there is an assortment of other items provided, such as:

• Epoch timing;
•

Synchronization bits;

Error correction bits;

•

Satellite health;

•

Age of data;

•

Spare bits.

•

In addition, the Russians plan on providing data that will facilitate the combined
use of GPS and GLONASS, particularly differences between GLONASS system time
and GPS system time as well as differences between WGS-84 and PZ-90. An over-
view of the C/A code and P code navigation messages is provided next [22, 23].

C/A Navigation Message
Each GLONASS satellite broadcasts a C/A code navigation message that contains a
superframe consisting of five frames. Each frame contains 15 lines, with each line
containing 100 bits of information. Each frame takes 30 seconds to broadcast, so the
entire superframe is broadcast once every 2.5 minutes [22].
The first three lines of each frame contain the detailed ephemeris for the satellite
being tracked. Since each frame repeats every 30 seconds, a receiver will receive a
satellite's ephemeris within 30 seconds once data reception begins [22].
The other lines of each frame consist primarily of approximate ephemeris (i.e.,
almanac) information for all of the other satellites in the constellation. Each frame
can hold the ephemeris for five satellites. Since the constellation will have 24 satel-
lites, all five frames must be read in order to get the approximate ephemeris for all
the satellites. This takes approximately 2.5 minutes [1, 22].
The approximate ephemeris information is not as accurate as the detailed
ephemeris and is not used for the actual ranging measurement. Nonetheless, the
approximate ephemeris is sufficient to allow the receiver to quickly align its code
phase and acquire the desired satellite. Once acquired, the satellite's detailed
ephemeris is used for the ranging measurement. As with GPS, the ephemeris infor-
mation is often valid for hours. Therefore, a receiver does not need to continually
read the data message in order to compute accurate position.

P Code Navigation Message
The Russians have not publicly published any specifics on their P code. Nonetheless,
a number of independent organizations and individuals have investigated the P code
waveform and published their results [22]. The following information is extracted
from the published information. The important thing to remember is the Russians
publicly provided the detailed information on their C/A code data message and have
given certain guarantees regarding its continuity. No such information or guaran-