ofd
Past and future of space radioelectric
measurement*)
nederlands geodetisch tijdschrift
by dr. P. Paquet, working at the Observatoire Royal de Belgique at Brussels.
1. Introduction
Navigation and positioning by satellites are in common
use since about 15 years; the accuracy ranges from 100
meters to a few decimeters according to the require
ments and available technical facilities. Within the next
ten years, absolute positioning at the subdecimeter
accuracy will be available while over short distances
(50 km) relative positioning with an accuracy of one or
two centimeters is already available. Such a success is
not only the consequence of technical research and
development but results also from the contribution of
fundamental astronomy, geophysics, geodesy and geo
metrical and dynamical knowledge of the Earth and its
environment.
From the point of view of geodesy, the evolution can be
summarized in six phases:
1. 1957 - 1964. The inconsistency of the various classi
cal terrestrial systems in use all over the world and
the complexity of the Earth's gravity field are recog
nized. Actions are taken to perform relative positio
ning at continental and intercontinental scales.
2. 1964- 1969. Many intensive geodetic campaigns are
organized for observing of dedicated satellites like
ECHO and PAGEOS; using a technique of geometri
cal analysis, relative site positions are derived with a
precision of 2 or 3 meters.
3. 1966 - 1970. Publication of the first models describing
the gravity field of the Earth. The station co-ordinates
are determined in a geocentric reference system,
simultaneously with the orbit computation. The pre
cision for world networks is typically around 5 or 10
meters.
4. 1969-1979. Space geodesy detects polar motion
with the same precision as the astronomical network
(40 cm). The TRANSIT system is released for public
use.
5. 1979 Space methods determine the fluctuations
of the Earth's rotation with the same quality as astro
nomy.
6. 1983Satellites with new technology (NOVA,
GPS) are launched and of current use. They allow
Inleiding gehouden op 18 september 1984 te Wageningen
tijdens de studiedag „Radiosatellieten plaatsbepaling; een me
thode in de dagelijkse praktijk van de geodeet van morgen?"
74
partial removal of some perturbations which can
hardly be modelled (gravity field, drag). Such pro
gresses are possible by working simultaneously on
different inputs leading to the determination of pre
cise orbits which is the prerequisite for accurate ab
solute positioning.
Let us remind that for determination of the orbit of a
satellite the usual procedure consists of:
a. fixing the co-ordinates of a ground tracking network;
b. adopting a model for the gravity field and for the at
mosphere;
c. fitting the observed quantities with respect to
- six orbital constants;
- an atmospheric density scaling factor;
- a radiation pressure scaling factor;
- some complementary unknowns depending on
the station characteristics;
while the main sources of errors are:
a. the Earth's gravity field;
b. the atmospheric perturbations acting on signal pro
pagation;
c. inhomogeneity and poor distribution of the ground
tracking network;
d. the atmospheric drag, direct and indirect solar
pressure.
In spite of the progress realized during the last few years,
the non-newtonian part of the gravity field, the drag and
the atmospheric refraction are still identified to be the
main perturbations. To overcome part of the difficulties
the new generation of radioelectric satellites have or will
have a higher altitude to reduce the effects of the in
homogeneity of the gravity field and of the atmospheric
drag; moreover, to reduce the ionospheric perturba
tions, they will transmit at higher frequencies. Some
other types of satellites, like the NOVA series, are
equipped with a drag compensating device.
For positioning using radioelectric signals two systems
are at present in use: TRANSIT and GPS. The first one
has been designed by the US-NAVY for navigation pur
poses; it has been a pioneer in modern geodetic posi
tioning and one of the main contributors to the develop
ment of space geodesy.
The GPS system, initiated in 1973, is at present in pro
gress to become fully operational in 1987. As TRANSIT
NGT GEODESIA 85
