Days of the year
Satellite
023 066
067 122
123 - 159
30190 30480
X X
x
X X
Table 6. Availability of the precise ephemeris for satellites 30190
and 30480, for the first part of 1984 (days 023 - 159).
Using the software developed in Brussels the along track
and range displacements were computed for all passes
observed during the periods given in table 6 and their
distribution is given in fig. 2 (along track) and fig. 3
(range); the mean and standard deviation are given in
table 7.
Satellite
Along track
Range
Number
Mean
Std
Mean
Std
passes
30480
,36
1,36
,40
1,56
341
30190
-,12
2,30
,38
2,38
494
Table 7. Along track and range displacements (m) of satellite
position determined in the Guier plane.
Fig. 2. Distribution of along track residuals of satellite positioning
(in meters). Number of passes 341.
Fig. 3. Distribution of range residuals of satellite positioning (in
meters). Number of passes 341.
The mean represents the systematic deviation but it can
not be interpreted because it depends of the adopted
station co-ordinates. The standard deviation however, is
independent of this choice and as the ground station
was fixed during the complete period of observation, it
represents a good estimate of the precision of the orbit
determination. With respect to the classical TRANSIT
satellites, the NOVA satellite reduces the orbital uncer
tainties by about 60%. This is in fact, a first approxima
tion because the Earth's gravity field used to process the
NOVA data is still the one determined with the non-drag
free satellites. Thus a significant improvement can be
expected, as soon as a new Earth model will be deduced
from the observations of the NOVA satellites.
5. Precision in point positioning
In point positioning it must be specified whether the co
ordinates refer to an Earth geocentric reference system
(absolute positioning) or are determined with respect to
one or several ground reference points (relative posi
tioning).
In absolute positioning the quality of the orbit of refer
ence is directly reflected on the station co-ordinates. It
is well-known that, with the TRANSIT satellites and
using the broadcast ephemeris, the co-ordinates are de
termined with a standard deviation (st.d.) of 4 or 5
meters whereas the precise ephemeris reduces the st.d.
by a factor 10. This is confirmed by results obtained from
different softwares applied on the same data set; ex
amples were given with the EDOC-2 data processed with
three different softwares (IGN - Paris; GEODOP-IFAG,
Frankfurt; ORB - Brussels). The same order of magni
tude of the co-ordinate uncertainties is observed be
tween the DMA and ORB softwares, both applied on 98
stations of the African Doppler campaign (ADOS).
Table 8 summarizes these comparisons.
Campaign EDOC-2
(36 stations)
Software
At
zIY
ZlZ
GE0D0P-0RB
IGN GE0D0P
0RB-IGN
- 0,24 0,54
0,42 0,63
- 0,18 0,15
0,34 0,57
- 0,24 0,57
- 0,10 0,31
- 0,44 0,65
0,28 1,06
0,16 0,73
Campaign ADOS
(98 stations)
ORB-DMA
- 0,10 0,57
- 0,06 0,54
0,10 0,50
Table 8. Comparison of results obtained with different softwares
(in meters).
From the various analyses we should note that the differ
ence between two softwares can of course be the result
of the difference of models; however, even if the same
model is used, the differences at sub-meter level are
mainly generated by the statistical rules applied to re
move suspected observations.
More recently, since day 023 of 1984, the precise
ephemeris has been made available not only for the
TRANSIT satellite 30190 but also for the NOVA 30480;
it is interesting to compare the set of co-ordinates de
rived from the observations of each of the two satellites.
During the common period of observation (table 7) two
Solution with satellite 30430
Period
X
Y
Z
EX
EY
EZ
in days
23 29
,08
- ,41
,60
,08
,13
,08
30 - 39
,83
,19
,40
,06
,09
,05
40 - 49
,09
,10
,45
,06
,10
,06
50 - 59
,05
- ,70
,39 v.
,08
,13
,07
60 - 69
,06
- ,67
,39
,09
,13
,08
123 - 129
,20
- ,25
,82
,06
,10
,06
130 - 139
,38
- ,26
1,09
,05
,09
,05
140 - 149
,32
- ,18
,86
,06"
,09
,05
150 - 159
,58
,06
,70
,06
,10
,05
MEAN
,30
- ,23
,63
ST. D.
,27
,32
,25
Solution with satellite 30190
20 - 29
,11
- ,84
1,29
,28
,36
,25
30 - 39
1,65
-1,15
,68
,37
,26
40 49
1,44
,36
,54
,35
,23
50 - 59
1,16
,95
-,71
71
,32
,19
60 69
1,13
- ,06
-,23
,21
,13
123 129
,75
,26
,10
,12
,17
,10
130 - 139
,47
- ,65
,03
,11
,15
,10
140 149
,24
,20
,32
,11
,16
,10
150 - 159
,49
- ,19
-,75
,11
,18
,10
MEAN
'82
- ,12
,14
ST. D.
,54
,66
,65
Table 9. Comparison of co-ordinates derived from observations of
satellites 30480 and 30190 (in meters).
78
NGT GEODESIA 85
