sets of co-ordinates (one for each satellite) were com
puted every ten days. The deviations from the adopted
co-ordinates are given in table 9.
With respect to the results obtained with satellite 30190,
the improvement introduced by the NOVA satellite is
very significant and shows that a st.d. of 20 cm or less
is obtainable as soon as the reference orbit will be com
puted with an appropriate Earth gravity model.
A more spectacular way to note the positive contribution
of the NOVA satellite is given in fig. 4 and 5. For the
periods mentioned in table 6 they show the distribution
of the residuals (vertical axis) of each individual observa
tion; for both figures the horizontal axis has the same
scale and ranges from plus to minus 0,040 Hz. All resi
duals larger or less than 0,040 Hz - 0,040) are taken
equal to 0,040 Hz (- 0,040). Fig. 4 and 5 are related
respectively to satellite 30480 (NOVA) and 30190 and
both contain 8550 residuals. Here again, the advantage
of NOVA is demonstrated clearly.
To recognize the evolution of the quality of the measure
ments of the TRANSIT system it is interesting to com
pare the last two figures with fig. 6 which gives the
distribution of residuals of a TRANSIT satellite (30140)
observed in 1973. Compared to the residual distribution
of fig. 4, the improvement is only the consequence of
hardware up-dating.
1I1I4
Fig. 4.
Fig. 5.
UiUki
Fig. 6.
Distribution of residuals (in hertz) of 8550 equations of observation
(Vertical axis number of residuals).
In absolute positioning several experiments were con
ducted with GPS. The results seem to be of the same
quality (st.d. 50 cm) as obtained with TRANSIT, but
they are obtained faster. With a better ground support
for orbit determination we can believe that in the future
the absolute co-ordinates deduced from GPS can be
slightly improved.
In relative positioning knowledge of the reference orbit
is not so critical as for absolute positioning. Here orbital
parameters are improved together with the co-ordinates
of the ground network. For the TRANSIT system the
precision obtained, for relative positioning using broad
cast ephemeris, is of the same order as the absolute
positioning using precise ephemeris [C. Boucher et al,
1978]. The main advantage is the possibility to collect
observations on several satellites and thus be able to
operate faster. Baselines may range to 1 000 or 2 000 km
without a large decrease of the precision. During the last
few years the Global Positioning System (GPS) has
demonstrated its full capability for relative positioning
over short distances (refer to table 1). There is no doubt
that during the next decade the GPS system will remain
an ideal system for relative positioning over short dis
tances.
6. The European project POPSAT
In 1978 the SONG Workshop, organized by ESA, pro
posed a space programme in which precise positioning
from space was one of the main requests to be able to
use space vehicles for geophysical studies. The process
to reach such an objective is now developing; during the
last few years ESA initiated several studies to define an
appropriate satellite and a ground tracking system that
could meet the requirements of positioning and geo
physics. The project is called POPSAT (Precise Orbit
Positioning of Satellites). The orbit has been chosen to
be in minimum resonance with the Earth's gravity field
and the satellite will radiate continuously two coherent
frequencies (2 and 8 GHz). The measurement will be
both of range and range rate type. Observations will be
collected at the observing site or collected on-board of
the satellite.
The fundamental ground tracking station called MEX
(Mission execution) will provide data for a precise orbit
determination; the range and/or range rate measure
ments will be collected on-board and retransmitted from
time to time to a ground control center. In principle the
user stations will be operated independently of the MEX
stations, with the possibility to perform also range and
range-rate measurements; broadcast ephemeris will be
available in real time whereas the delivery of the precise
satellite positions will have a delay of a few days. A third
type of station, called GEO (Geophysical) will be de
signed to operate in station arrays, for example, for
control of seismic areas. The main orbital parameters of
POPSAT are expected to be:
inclination i 98,6 degrees
semi-major axis a 13370 km
eccentricity e 0,005
High satellite altitude, transmission at two high frequen
cies, all weather capability allowing permanent tracking
yields an exceptional system performance:
a. Absolute positioning after three days of observation:
- two-way range and range-rate 5 cm
- one way range rate 8 cm
b. Relative positioning over distances less than 1 000 km
and after three days of observations:
- two-way range and range-rate, horizontal and ver
tical accuracy respectively at 1,5 and 2,5 cm
- one way range-rate, horizontal and vertical accu
racy respectively at 5 cm
c. Earth rotation monitoring on a daily basis:
- pole position 4 cm
- length of day variation 0,15 ms
d. Determination of the orbit:
- definitive one day's orbit, error less than 30 cm
(RMS)
NGT GEODESIA 85
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