Global Positioning System Reference
In-Depth Information
GLONASS P Code
The Russians have emphatically stated numerous times that the P code is strictly a
military signal. As such, there is very little Russian information available on the
GLONASS P code. Most P code information is derived from analysis of the code
performed by various independent individuals or organizations such as that pro-
vided in [23]. Based on [23], the P code characteristics are:
Code type: Maximal length 25-bit shift register;
Code rate: 5.11 Mchips/s;
Code length: 33,554,432 chips;
Repeat rate: 1 second (repeat rate is actually at 6.57-second intervals, but
chipped sequence is truncated such that it repeats every 1 second).
As with the C/A code, the maximal length code has exceptional, predictable
auto-correlation properties. The significant difference between the P code and the
C/A code is that the P code is much longer compared to its clock rate, thus repeating
only once every second. Although this produces undesirable frequency components
at 1-Hz intervals, the cross-correlation problem is not as severe as with the C/A
code. As with the C/A code, FDMA virtually eliminates any problems involving
cross-correlation between GLONASS satellite signals. While the P code gains in
terms of correlation properties, it sacrifices in terms of acquisition. The P code con-
tains 511 million code phase shift possibilities. Thus, a receiver typically acquires
C/A code first and then uses the C/A code to help narrow the number of P code
phase shifts to search. Each P code phase, clocked at 10 times the C/A code, repre-
sents 58.7m in range. A HOW like the one used in GPS to facilitate handover to
P(Y) code is not necessary. The GLONASS P code repeats once every second, mak-
ing it possible to use the timing of the C/A code sequence to assist in the handover
process. This is an example of one more design tradeoff between the desired security
and correlation properties of a long sequence and the desire for a faster acquisition
scheme. GPS employs the former implementation, while GLONASS employs the
latter [23].
Comparison of GLONASS Codes to GPS Codes
Because of the CDMA nature of GPS, the GPS design could not ignore the effect of
cross-correlation between satellite signals. The Gold codes used by GPS were specif-
ically chosen because of the ability to mathematically bound the auto-correlation
and cross-correlation properties of the C/A codes. Nonetheless, in most respects, the
GLONASS and GPS C/A codes are comparable in terms of correlation properties.
By contrast, the longer GPS P code means GPS has better correlation properties than
GLONASS P code. However, under certain configurations, the shorter GLONASS
P code may make it easier to directly acquire than the GPS P(Y) code.
11.1.9.4 Navigation Message
Unlike GPS, GLONASS has two types of navigation messages. The C/A code navi-
gation message is modulo-2 added to the C/A code at the satellite, whereas a P code
unique navigation message is modulo-2 added to the P code. Both navigation mes-
Search WWH ::




Custom Search