N ZM, p 4810^)
it is still designed for navigation but in a general sense:
for space, ground or maritime navigation. Its first goal is
to provide a quasi-instantaneous positioning with a pre
cision of 10 meters. For geodetic purposes, experiments
with GPS have demonstrated that the absolute posi
tioning is of about the same quality as the TRANSIT
system but it is two or three times faster; however, till
now, the main success of GPS is due to its remarkable
performance in relative positioning over short distances.
Over a few tens of kilometers GPS agrees, at the centi
meter level, with the classical terrestrial methods
(table 1).
Date
Stations
Length
Differences (cm)
from - to
km
classical-GPS
1-19-83
7 6
12,8
-0,8
7 5
18,5
-0,1
5 6
8,7
0,6
1-20-83
7 8
42,1
2,1
7 5
18,5
0,0
5 8
34,6
-1,7
1-21-83
7 8
42,1
2,7
7 5
18,5
-0,4
5 8
34,6
1,0
Table 1. Tests performed by the US Federal Geodetic Control Com
mittee IFGCC). Comparisons of baseline measured by classical
technique and GPS.
More recently the European Space Agency (ESA) ini
tiated studies for an advanced satellite whose main ob
jectives are: Earth rotation monitoring and absolute and
relative ground positioning (respectively with an accu
racy of 10 cm or better and 5 cm over distances of a few
hundreds kilometers). This project is called POPS AT
(Precise Orbit Positioning Satellite) and is expected to be
launched early in the nineties (1992?).
For positioning, the development of geodesy via space
radioelectric methods has been very fast and is still in full
evolution. It still opens new approaches in geodesy and
new orientations in geophysics. Let us note as ex
amples:
- the development of a Space-Time terrestrial refe
rence system;
- the monitoring of the Earth rotation parameters, the
ocean and ice dynamics;
- the positioning of any point at the surface of the
Earth in a geocentric reference frame with subdeci
meter accuracy;
- the control of plate deformation in seismic areas.
In the following sections we shall review briefly some of
the mentioned perturbations as well as the TRANSIT
and POPSAT systems. GPS is covered during this meet
ing by dr. Seeber.
2. Observed quantities and atmospheric pertur
bations
In radioelectric satellite tracking the observed quantities
are generally „distance and/or range rate" which de
notes respectively as the time interval required by a radio
wave to travel from station to satellite and back, or the
variation of a fixed frequency transmitted by the satellite
and received at the station. The latter is the well-known
Doppler effect, extensively used in satellite geodesy
since 20 years.
With distance measurements the relative position of the
station with respect to the satellite is located at the point
NGT GEODESIA 85
of the intersection of spheres centered at successive
satellite positions. By using several satellites, a quasi-
instantaneous positioning is possible.
With range rate measurements the station position is de
termined by the classical method of hyperbolic navi
gation. The precision with which the orbit is known is
directly reflected on the quality of the station posi
tioning.
For distance and range rate, the signal, transmitted by
the satellite and received at a ground station, is passing
through the atmosphere and the resulting perturbation is
divided into two components. To show the present
situation, both will be briefly described and possible
actions to remove their effects will be indicated.
2.1. The tropospheric perturbation
The tropospheric refraction concerns the lower part of
the atmosphere and is independent of the transmitted
frequency as far as it remains below a few tens of GHz.
The delay in the propagation of the electromagnetic wa
ve is equivalent to a length error, represented by
zlr p nds - /G ds
where n is the index of refraction, which varies along the
signal path; P represents the actual path and G the geo
metrical one.
In 1963, H. S. Hopfield demonstrated that if the first in
tegral is estimated along the geometrical path the result
ing error is of the second order for elevations above 3 or
4 degrees. Excluding observations in the vicinity of the
horizon we write
zlr /G (n - 1) ds /G Nds (1)
where N is the index of refractivity given by
Tk Tk
where
K the temperature in degrees Kelvin;
P the atmospheric pressure in mbars;
e the partial water vapour pressure in mbars.
From knowledge of these three parameters along the
signal path, a numerical computation of (1) is possible
and several models have been proposed to deduce their
value from ground measurements [Hopfield, 1971; Saas-
tamoinen, 1973]. Moreover, as the influence of the
water vapour is negligible above 10 km while the air
pressure is still important till about 40 km, the correction
is divided in two parts:
- a dry component Nd 77,6 P/TK
- a wet component Nw 77,6 (4810 e/T2K)
In table 2 an estimate is given of the order of magnitude
of the dry and wet components. Between the zenith and
an elevation of 5 degrees, the wet component is about
10% of the total tropospheric effect. Below 5 degrees it
can reach 20 or 25%. From table 2 it is also interesting
to note that the models are excellent for the dry compo
nent, the errors remain indeed below 0,2%; unfortunate-
Elevation
Total effect
Contribution (m)
Error (cm)
in m
dry
wet
dry wet
5
24,4
22,0
2,4
5, 49,
10
13,3
12,0
1,3
3, 27,
20
6,9
6,2
,7
1, 14,
30
4,7
4,2
,5
1, 9,
90
2,4
2,2
,2
,5 5,
Table 2. Tropospheric refraction (Hopfield, 1971).
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