Satellite ephemeris is generated by observation of the ranging signals by the NFCE. 

However, GLONASS satellite also have a laser reflector that enables an independent

measurement of range and orbit parameters. 

A.5   SPACE SEGMENT

Satellite orbital position and the overall numbers in orbit are driven by the requirements

for accuracy and availability.  The constellation is made up of 24 satellites located in

three orbit planes of eight satellites.  The planes are spaced at 120 degree intervals

around the equator and inclined at 64.8 degrees.  (The longitude of the point where the

satellite crosses the equation in a northerly directions is known as the Right Ascension

of the Ascending Node, RAAN).  The satellites are equally spaced in the orbit plane in

circular orbits, maximum eccentricity 0.01, with a period around 11 hrs 15 mins at a

height of 19100 kms.  (These parameters are very near that planed originally for GPS

before the 6 plane constellation was introduced). 

It is reported Ref 1, that the constellation will be populated by filling pre defined orbital

"slots". Thus while the GLONASS system is being completed and when older satellites

reach the end of their life, gaps will appear in the constellation.  Based on the

performance of all 24 satellite GLONASS controllers will determine the  best  21 to

activate.  The remaining three will be held back in reserve.  Periodically the mix will be

evaluated and if necessary a new best set of 21 defined.  When necessary to maintain

system accuracy three new satellite will be launched. 

Taking plane 1 with an RAAN of 128 degrees on day 240, 1995 as reference, planes 2

and 3 have RAAN s of +120 and +240 degrees respectively as illustrated in Figure A 1.

 Separation  in the argument of latitude or orbital phase in the plane is a multiple of 45

degrees.  There is a displacement of +30 degrees and  30 degrees for satellites in

planes 2 and 3 respectively with reference to plane 1.  Phase angle is measured

clockwise with the satellite direction of travel being anti clockwise.  Relative positions of

satellites appear to remain stable over long periods. The rate of change of RAAN is

approximately  0.03 degrees/day, resulting in the RAAN of plane 1 changing from 167

degrees  in 1989 to 128 degrees in 1995. 

All satellites have the same nominal orbital period of 11 hrs 25 mins.  The orbit period

is equivalent to a movement of 675.73 degrees of longitude, a change of 169.41

degrees W each orbit.  The orbit produces a ground track repeat every 17 orbits that is

8 days less 32.56 minutes.  This diurnal offset of DT = 4.07 minutes from a full 24 hour

day coincides with that of Navstar GPS and is very nearly the difference between a

solar and  sidereal day (3.93 minutes).  This implies that each complete day  less DT

minutes a satellite performs 17/8 orbits or 2 whole revolutions plus an additional 1/8

revolution, equivalent to 45 degrees.  Therefore two satellites in the same plane but

separated by 45 degrees in orbital phase, appear at precisely the same position on

successive days less DT minutes.  Over a ground track repeat interval of 8 days, all

satellites in the same plane with separation of 45 degrees appear in turn at the same

position at intervals of 1 day less DT minutes.  After 8 days, the whole cycle naturally

repeats.

A 3