Figure A 2.  GLONASS L1 C/A and P(Y) Code Spectrum

The data in each subframe is divided into two sections; the first containing the

coordinates and clock parameters of the transmitting satellite and the second almanac

parameters for all other satellites currently in the system.  Various flags occur in the

message relating to validity of specific data, status and health of particular satellites. 

Several data message have not been published or are only partially understood, for

example, the luni solar  correction term in the almanacs.

GLONASS ephemerides are similar to the format used by Cicada satellites.  Both systems

encode the satellite s instantaneous position and velocity at fixed time intervals in an earth 

centred earth fixed (ECEF) rectangular coordinate system.  Positions and velocities at

intermediate times are calculated by the user using interpolation.  In addition to positional

data at reference times, GLONASS also transmits in the ephemeris two parameters relating

to the on board clocks.  The first is a time correction for the instantaneous time difference

between space vehicle time and GLONASS  system time.  The second parameter a

frequency correction gives the rate of change of space vehicle time offset.  An age of 

ephemeris data (AODE) parameter is included to allow the  user to calculate the satellites

time and frequency offset at the time the transmission occurred.

There is greater similarity between Navstar and GLONASS in the transmission of

almanacs. Both systems transmit the basic elements of a osculating Kepler ellipse,  as

illustrated in Table A 3.  In terms of using almanacs to predict satellite position from the

reduced Kepler orbit, the two  sets  of  data  are  similar.  Where  differences  occur 

(parameter 8), the terms are seen to be

A 13