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Chapter 3 / Sensors / NAVSTAR GPS
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10 Hz. The model is based on information from STANAG 4294 [N.8] and
papers on Navigation (GPS) published by the Institute Of Navigation [N.9] .
3.11.3 Satellite Ephemeris Data
Five monitoring stations have been set up in various parts of the world to
continuously track the orbit of each satellite. The orbital data collected is
passed to a master control station where precise values of each satellite's
orbital "constants" for the next two weeks are predicted using Kalman
Filtering techniques. The 4 ground transmission stations up-link a fresh set
of predicted orbital constants, atomic clock corrections and atmospheric
composition to each satellite as they pass overhead. These can be used with
confidence for approximately 1.5 hrs, Dierendonck (D.17) . The Ephemerides
contains corrections to these constants to account for solar radiation
pressure, polar wander, earth wobble, gravitational distortion due to the Sun
and Moon, etc.
The model is based on a 24 satellite symmetrical constellation that is never
found in practice but gives an even coverage over the earth surface without
the need to determine satellite position from their Ephemerides. In
Appendix G, whilst discussing Newton’s universal law of gravitation, it was
muted that position in space requires 6 constants of integration. In practice
each satellite’s position is defined in terms of 5 of the 6 classical Keplerian
orbital parameters transmitted in the Navigation message together with the
time of applicability (tREF) accurate to 1 s:
Semi-Major Axis ( √ Pr,s := √ 20183000 m 1/2 )
The semi-major axis is the orbital radius of the satellite at the reference time
measured from the earth’s geometric centre. This is transmitted in terms of
its square root to preserve accuracy. The navigation message also contains
correction coefficients and therefore the semi-major axis,
r,
s
2 ( P ) + C ⋅ sin ( 2⋅
ϕ ) + C ⋅ cos ( 2 ⋅ )
P : =
ϕ
r,
s
Where (ϕ) is the argument of latitude defined later.
Eccentricity ( eS )
rs
3.11-4
S
rc
S
Equation 3.11-1
The eccentricity of the satellite orbit. Although the orbits are supposed to
be circular (eS := 0), orbital perturbations lead to values of some 0.005.
Orbital Inclination ( ιO )
The orbital inclination of the orbital plane relative to the equatorial plane at
the reference time is nominally 55°. The navigation message contains the
inclination angular rate and correction coefficients,