Galileo OS SIS ICD.indd - GSA - Europa

5.1.10. Almanac
The CRC is composed of a sequence of 24 parity bits p i; for any i from 1 to 24, p i is the
coeffi cient of X 24-i in R(X) where:
● R(X) is the remainder of the binary polynomial algebra division of the polynomial
m(X) X 24 by G(X) and
● m(X)=m 1X k-1 +…+ m k-2X 2 + m k-1X + m k with m 1, m 2,..., m k the sequence of k-bits information
to be protected by the CRC, and m 1 as the MSB.
The almanac data is a reduced-precision subset of the clock and ephemeris parameters
of the active satellites in orbit. The user receiver utilises the algorithm described in
paragraph 5.1.1 to compute the positions of the Galileo satellites. All other parameters
appearing in the equations of Table 62, but not included in the content of the almanac,
are set to zero for satellite position determination.
The Galileo almanac orbital parameters consist of
● Mean of semi-major axis
● Eccentricity
● Inclination
● Right ascension of the ascending node
● Argument of perigee
● Mean anomaly
A reduced set of clock correction parameters is provided as well in the almanac for
each satellite including the time of applicability t oa of the almanac data. This almanac
reference time t 0a is referenced to the almanac reference week (WN a). The WN a term
consists of two bits which is a Modulo 4 binary representation of the GST week number.
Additionally, a predicted satellite health status is provided for each of these satellites,
giving indications on the satellite’s signal components health and navigation data
health. Finally, the IOD a allows identifying without ambiguity an almanac batch. The
almanac parameters are transmitted according to the characteristics stated in Table 79.
Parameter Defi nition Bits
SVID Satellite ID (1 constellation of 36
satellites)
Δ(a 1/2) Difference with respect to the square
root of the nominal semi-major axis
(see Table 61)
54 © European Union 2010
Document subject to terms of use and disclaimers p. ii-iii
OD SIS ICD, Issue 1, February 2010
Scale
factor
Unit
6 1 dimensionless
13* 2 -9 meters1/2
e Eccentricity 11 2 -16 dimensionless
δ i
Inclination at reference time relative to
i 0 = 56°
11* 2 -14 semi-circles***
Ω 0 Right ascension 16* 2 -15 semi-circles***
●
Ω Rate of right ascension 11* 2 -33 semi-circles/s***
ω Argument of perigee 16* 2 -15 semi-circles***
M 0
Satellite mean anomaly at reference
time
16* 2 -15 semi-circles***