Table A 3.  Almanacs

Parameter

 NAVSTAR

   GLONASS

1

  week of validity  

 day of validity 

2

  identifier  

 channel number

3

  eccentricity

4

  inclination

5

  time of almanac

 equator crossing time

6

  health 

 validity of almanac 

7

  right ascension ascending node (RAAN)

 equator crossing longitude

8

  rate of change of RAAN

9

  root of semi major axis

 orbital period  

10

  argument of perigee 

 argument of perigee

11

  mean anomaly

12

 luni solar term

13

  time offset 

 time offset

14

  frequency offset

equivalent.  The primary purpose of almanac data is to allow the user to predict in

approximate terms the visible satellites and their geometry. 

Almanac data provides a position of each satellite to within 100 200 m similar to GPS

almanacs. However, the inclusion in the GLONASS almanac of a luni solar  correction

term implies a position error perhaps an order of magnitude better than a Navstar

almanac over an extended time period.  The luni solar term remains substantially

constant for satellites with the same Right Ascension.  Although the almanac is valid for

 several days they are usually but not always  changed every day in GLONASS at local

 midnight.

It is interesting to observe that the GLONASS almanacs differ from the earlier Cicada

almanacs in one major respect.  The earlier almanacs were based on an equitial tial

Kepler  set where eccentricity and argument of the perigee  are transmitted as h = e x

sinw and 1 = e x cosw.  The equinoctial set of elements is suitable for orbits with small

eccentricity since it leads to equations with no singularities when e tends to zero.

A.14   NAVIGATION REFERENCE FRAME

GLONASS employs a geocentric cartesian system designated SGS85.  The difference

from the GPS WGS84 frame is not large.  Misra, Ref 8 reported differences of less than

 20 m RMS.  The two coordinate frames may be brought together by a small rotation

0.6" (3.10

 6

 rad) of the z axis and a 4 m displacement of the origin along the z axis.

Recently there has been a suggestion that the GLONASS coordinate frame has been

updated to a SGS90 designation.

A 14