400 MHz DORIS signal fades :
correlations with ionospheric scintillations
Objectives
Objectives
DORIS is a dual frequency radioelectric system. A 2 GHz channel is used for the main Doppler observation while a 401
MHz channel allows a very accurate correction of the ionospheric perturbation (1/f2 in distance).
The scintillations are due to small-scale variations of the electron density in the ionosphere. They can perturbate the
amplitude and phase of radio-signals even causing total fades. An analysis of the losses observed on the 401 MHz DORIS
signal has been done. We try to show correlations with the ionospheric scintillations.
scintillations. Nevertheless, remember that the
DORIS system is not severely affected by signal fades of any source (a few percents of the whole data).
Data
Data
SPOT2, SPOT4 and TOPEX/POSEIDON satellites
The whole DORIS stations network
J.J. Valette, B. Frayssinet (CLS)
B. Bonhoure (CNES)
Solar Flux F10.7
Geomagnetic Kp index
350
10
9
SPOT2
SPOT2results
results
Method
Method
sfu
300
The algorithm to isolate lost data in the 400 MHz telemetry is simple. For each observation, the applied criteria is a coherent 2 GHz quality bit
while the 400 MHz quality bit is not or the station identification number is not. This number is included in the beacon message such as the
meteorological parameters. The beacon message is transmitted with a modulation of the 401 MHz frequency.
8
250
7
200
6
5
150
4
3
100
2
50
Additional tests are applied to extract losses issued from the beacon failures. The ionospheric scintillations origin signal fades have not been separated from external jamming or internal jamming (iso
-Doppler).
(iso-Doppler).
This last source of perturbation could have been calculated and a-priori deduced. However, in this case, the geographical distribution signature is clearly visible with aligning points along the satellite ground track
located in areas of 2 stations with co-visibility of the satellite.
1
0
1997
0
1998
1999
1997
2000
SPOT2, SPOT4 satellites
1998
1999
2000
2 periods : 1997 and 2000
2000 with a high solar and geomagnetical activity
1997 with a weaker solar and geomagnetical activity
alt.
alt. 860 km, incl.
incl. 98
heliosynchronous orbit (20H30 local time at ascending node)
node)
TOPEX/POSEIDON
TOPEX/POSEIDON results
results
TOPEX/POSEIDON satellite
alt.
alt. 1335 km, incl.
incl. 66°
2h local time drift of the orbit ascending node every 10 days
SPOT 2 ratio of lost data : 1% in 1997, 3% in 2000 : figures not critical for the system
SPOT4 : lower ratio (youngest instrument) but with the same signatures
TOPEX ratio of lost data :
0.6 % in 1997, 1.5 % in 2000 : not critical for the system
The main caracteristics of the ionospheric scintillations can be found on the maps and stations plots :
- a concentration around the geomagnetic equator and the auroral zones,
- a sensitivity to the sunspot cycle (year 2000 is near the maximum),
- a seasonal variation (see Kourou,
Kourou, Dakar, Terre Adélie).
Adélie).
Time of the day dependence ?
With Topex/Poseidon
Topex/Poseidon satellite, the correlation of signal fades with the ionospheric
scintillations is not so clear than with SPOT.
A maximum effect is expected during the sunrise and after the sunset. But as shown in the
SPOT2 plots of local time data distribution (left) this situation is infrequent.
ADEA (SPOT2)
1200
1000
1997
3
2000
200
0
1
2
21
19
17
9
15
23
3
2000
400
5
4
3,5
Local time
3
5
7
9
11
13
15
17
19
21
23
Ratio of losses
2,5
2
1997
1,5
2000
1
1
0,5
23
21
19
17
15
9
13
0
7
Local time
11
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
5
0
3
23
21
19
17
15
9
13
7
5
3
1
11
Local time
Ratio of losses
6
0
1997
600
1
100000
Topex data losses ratio versus local time
0
800
13
Spot 2 data losses ratio versus local time
7
2000
50000
7
2000
200000
The distribution map of lost data show lots of aligning points along the satellite tracks .
They correspond to internal system jamming. It is more intense with Topex compare to
Spot, due to the higher altitude of the orbit. In addition and for the same reason, Topex
is more sensitive to external jamming resulting of human activity (radars
(radars,, …). The
effects are particularly intense in Europe.
100000
11
1997
300000
1997
150000
1
400000
200000
5
500000
DORIS data lost at Terre Adélie (Antarctica)
Antarctica)
250000
3
Local time SPOT2 data distribution
600000
Number of measurements
300000
Most of the losses occur around 23h local time of the sub- ionospheric point along the ray
supposed to be at 400 km altitude (above). It is known that the scintillations are very intense
around midnight in the auroral oval region. It is verified with the DORIS 400 MHz data lost at
Spitsberg or Terre Adélie stations (right). The high latitude stations contribute a lot to the
amount of observations because of the polar orbit.
700000
Number of measurements
Local time TOPEX/POSEIDON data distribution