The BPSK modulation produces the classical sinc

2

x (sinc x = sin x over x) spectrum

centred on the carrier frequency with nulls at  multiples of the bit rate.  The signal

appears to be filtered before transmission to limit the transmission bandwidth to only

the first two nulls.  A typical spectrum is illustrated in Fig A 2.  Some GLONASS

satellites, but not all have been observed, Ref  2, to contain additional spectral lines

precisely at the spectral nulls of +/  the code rate.

Two theories have been put forward to explain the appearance of spectral lines at code

nulls. The first postulates that the lines are present by accident as a result of poor

implementation of the carrier modulation circuitry.  An alternative theory that the lines

were deliberate in order to provide instantaneous satellite velocity information has not

been substantiated.  The occurrence of the phenomena has been intermittent,

GLONASS 19 and 20 had very pronounced lines.  No lines have been observed on any

 of the satellites launched during 1986 1988 but they  have reappeared on satellites

launched in 1989 (GLONASS 42).

A.12  RANGING CODES

GLONASS employs ranging codes similar to GPS, but with the same code transmitted

by all satellites at different frequencies.  The equivalent of the GPS C/A code uses a

maximum length sequence with a period of 1 ms and a bit rate of 0.511 Mchips/s, a

length of 511 bits as compared to the GPS C/A code of 1023 bits, Ref 5.  The code is

generated by a feedback 9 element shift register, (2

9

  1 = 511bits) with feedback from

the 5th and 9th taps.  The output is taken from the 7th tap. 

The P code appears to be a truncated maximum length sequence generated by a 25  bit

shift register with feedback from the 3rd and 25 taps.  A maximum length code from

such a generator has a length of 2

25 

 1 = 33554431 bits.  At a clock rate of 5.11 MHz

the code would be 6.5664 seconds, but the sequence is short cycled at the second

boundary and the register reset to all ones.

A.13  NAVIGATION DATA

Navigation data is transmitted at 50 baud, Ref 6.  In common with GPS, the data is

formatted into frames, sub frames and words.  A frame has a duration of 150 seconds

and is sub divided into five sub frames.  Each sub frame is divided into 15 lines, of 2

seconds duration.  The first part of each line 1.7 seconds duration contains a preamble

(always 0), line number (4 bits), data parameters (72), parity bits (8).  The C/A code

navigation data includes a  meanda  code at double chip frequency 100 Hz, 0101010....

 is modulo 2 added to the data, resulting in a  Manchester  modulation.  The remaining

0.3 seconds is composed of a time mark at 100 Hz,

111110001101110101000010010110, the last bit is aligned with even integer seconds

from the beginning of the day Moscow time UT(SU).  P code navigation data is a non

return to zero 50 baud data message with several differences from the C/A code Ref 7.

A 12