-
-
-
TRANSIT satellites, showed clearly that the second
order effect of the perturbation caused by the iono
sphere is not negligible, at least not with the pair (150,
400) MHz. From this analysis an apparent variation of
the height of the station appeared with two characte
ristic periods; the first corresponds to the solar cycle
activity (11 yrs) and the second one is an annual compo
nent [Dehant et al, 1983]. The corresponding amplitudes
are both approximately 50 cm (fig. 1).
The use of higher frequencies will significatively reduce
the ionospheric error. The TRANSIT system uses 150
and 400 MHz, because at the time of system concept
(1962) the technology did not allow to design any hard
ware using higher frequencies. Now the GPS system is
using the pair (1227, 1575) MHz whereas the European
project POPSAT will use the pair (2 000, 8 000) MHz.
o> 1.0
■C
T3
41
Q.
O 0.0
E
0
2
2
s
1
Nevertheless, to improve the determination of all com
ponents, a strong effort is expected during the next ten
years; table 5 has been taken from a report prepared by
the Solid Earth Working Group (SEWG) of ESA and
contains the past and future missions that were or could
be conducted by NASA and ESA to this critical problem.
Any spacecraft
Dedicated
gravsat
Terrestrial
methods
Wavelength
(km)
Orbit
perturbation
analysis
Altimetry
(over
oceans
only)
SST
high - low
Gradiometry
SST
low - low
2000 - 10000
XXX
X
X X
X
700 2000
X
X
X X
X X
X
100 700
XXX
XXX
X X
100
X X
X
XXX
(over acc-
sible land
areas)
Past missions
Many
GE0S3
SEASAT
ATS, GE0S-3
APOLLO
S0YUZ
Future
concepts
ERS-1
TOPEX
POSEIDON
P0PSAT/ERS
GRAVSAT
SLALOM
GRADI0
xxx - Best technique
xx - Good candidate
x - Useful, but of reduced value
Raw data
Model BT6
Fig. 1. Apparent height variations of the Doppler station of Brussels
(1972- 19811.
Table 5. Past and expected future missions for the determination
of the Earth's gravity field.
3. Earth's gravity field
The usual approach to determine the lengths of the
Earth's gravity field is the analysis of near Earth satellite
orbit perturbations; to separate the different compo
nents of the spherical harmonics development, many
satellite orbits with different inclinations, excentricities
and semi-major axis have to be used. The short wave
length components were determined over the continents
by terrestrial gravimetry whereas more recently the
oceanic contribution was deduced from satellite alti-
metry.
Prior to any estimation of the gravity field, the effects on
the orbits of the non-gravitational forces must be remo
ved; it means that atmospheric drag and solar pressure
have be modelled carefully. In practice this is not yet the
case and most of the published Earth's gravity models
are still affected by some un-modelled contributions of
non-gravitational forces. To limit this effect, the new
geodetic satellites are or will be at higher altitude
(LAGEOS 5 000 km, POPSAT 6 500 km) and the external
shape of the satellite will be about a sphere so that the
modelisation of the drag effect will be simpler.
Another solution is given by NOVA satellite, a drag free
satellite of the TRANSIT series, for which the non-
gravitational forces are eliminated by keeping continu
ously their resultant almost equal to zero by use of reac
tion forces. As we shall see later, using the Doppler
technique, the NOVA improves the positioning by a
factor 2.
In conclusion, at present an Earth's gravity model suited
for precise orbit computations does not exist for low
satellite orbits (1 000 km); thus the choice of an higher
altitude has the advantage of not only reducing the non-
gravitational forces but also reducing the effects of the
short wave-length components of the gravity field.
NGT GEODESIA 85
4. Estimation of the error in orbit determination of
TRANSIT satellites
Having no access to the direct determination of the orbit
of the TRANSIT satellites, an external criterion must be
defined and a very good approach is to determine, in the
Guier plane, the satellite position rather than the ground
station. Let us remind that the Guier plane is defined by
the first approximation of the station position, and the
along track component of the satellite motion, at the ti
me of the closest approach (TCA). To define the station
displacement with respect to the satellite orbit Guier
uses a reference system with its origin at the satellite
position at TCA whith the axes:
R the range axis from station to satellite;
L the along track component positively orientated in
the sense of the satellite motion;
Z the third axis perpendicular to the plane (R, L).
The TCA, the along track and range axis are derived
from the adopted station position and the precise ephe-
meris delivered by the Defence Mapping Agency Hydro-
graphic and Topographic Center (DMAHTC).
The Guier elements (R, L) are often used to filter the
observations and also to determine the station position
[Usandivaras et al, 1976], Inversely, it can be used to
estimate the deviation of the true satellite position from
the one given by the precise ephemeris. This principle
has been applied to the observations of two satellites
observed during the first part of 1984 by the permanent
TRANET-2 station operated at the Observatoire Royal
de Belgique at Brussels. One of the two satellites is a
classical TRANSIT (30190) in operation since 1970. The
other is the first NOVA (30480), in operation since 1981,
for which for the first time the precise ephemeris has
been made partially available. Table 6 gives the period
during which the precise ephemeris has been received
during the first part of the year.
77
