Other Satellite Navigation Systems - Understanding GPS: Principles and Applications - page 608

frequency modification will have little effect on terrestrial users who cannot see
antipodal satellites simultaneously. Space-based receivers may require special dis-
criminating functions, such as Doppler checks, in order to track the proper satellite.
This ability to discriminate antipodal satellites is important, since the antenna beam-
widths on the GLONASS-M satellites are specifically designed to accommodate
space-based users [1, 25].
The values of K listed earlier are the proposed values for satellites operating
under normal conditions. Other values of K may be assigned for certain command
and control processing tasks or under “exceptional circumstances,” according to the
Russians [1].

11.1.9.2 Modulation
In a similar manner as GPS, each satellite modulates its L1 carrier frequency with two
PRN ranging sequences. (As shown in Figure 11.4, both sequences are modulo-2
added with navigation data before modulating the carrier.) One sequence, called the
P code, is reserved for military purposes. The other sequence, called the C/A code, is
for civil use and aids acquisition of the P code. Each satellite modulates its L2 carrier
frequency solely with the modulo-2 summation of P code and navigation data. The P
code and C/A code sequences are the same for all satellites [1, 22, 23].

11.1.9.3 Code Properties
Both GLONASS and GPS use pseudorandom codes that facilitate satellite-to-user
ranging and have inherent interference rejection. GLONASS C/A code and P code
sequences are described next [1, 22, 23].

GLONASS C/A Code
The GLONASS C/A code has the following characteristics:

• Code type: Maximal length 9-bit shift register;
•

Code rate: 0.511 Mchips/s;

Code length: 511 chips;

•

Repeat rate: 1 ms.

•

A maximal-length code sequence exhibits predictable and desirable auto-corre-
lation properties (see Section 4.3). The 511-bit C/A code is clocked at 0.511
Mchips/s; thus, the code repeats every millisecond. This use of a relatively short code
clocked at a high rate produces undesirable frequency components at 1-kHz inter-
vals that can result in cross-correlation between interference sources, reducing the
interference rejection benefit of the spread frequency spectrum. On the plus side, the
FDMA nature of the GLONASS signal significantly reduces any cross-correlation
between satellite signals due to the frequency separation. The reason for the short
code is to allow quick acquisition, requiring a receiver to search a maximum of 511
code phase shifts. The fast code rate is necessary for range discrimination, with each
code phase representing approximately 587m.