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During autumn and spring 2001, 2002, 2003, 2004 and the following years, strong link degradations were observed on the Cluster spacecraft, using the ground stations Villafranca and Maspalomas. Sudden variations of the received RF signal power were recorded on the spacecraft as well as on the ground. The duration of these disturbances ranged from 10 minutes to 5 hours. The anomaly coincides with the local evening hours and depends on geographical location of the ground station. Problems with the used communications equipment on ground and on-board could be ruled out.

A multi-disciplinary working group at ESOC with support from scientists at ESTEC, Noordwijk, The Netherlands, was set up in January 2003 to investigate and could identify ionospheric scintillations as source of these anomalies. Scintillations occur around the geomagnetic equator, preferably during the local evening hours. Figure 1 shows an example and demonstrates how all four Cluster spacecraft were serially affected on 17/18 October 2002. The up/downlink signal power is monitored with the Automatic Gain Control (AGC). During the observed events the AGC displayed strong and fast variations in signal strength of more than 10 dB peak-to-peak.


Figure 1: AGC fluctuations affecting serially all four Cluster spacecraft on Villafranca and Maspalomas tracking

These signal fluctuations were primarily observed when the ground station antennae were tracking the Cluster spacecraft below an elevation of 60 degrees and under azimuths between 90 – 240 degrees, i.e. when the antennae, located on the northern hemisphere ground sites, were looking southward through the equatorial ionosphere to the Cluster satellites, and that during the evening hours. By an analysis of the ground station – Cluster orbit geometry it was found that this southward tracking/evening hours ground station – satellite constellation mainly occurs during the September – December time period.

International GNSS Service (IGS) TEC maps were used to identify those regions in the ionosphere where the Cluster-ground station links did cross the ionospheric shell during the time in question and could in this way significantly contribute to the identification of the relationship between the observed phenomena and potential scintillation regions. Figure 2 presents an example (now in spring): SC4 was tracked from Maspalomas, and from 20:40 – 21:00 UT fluctuations were observed on SC4. SC1, SC2 and SC3 were tracked from Villafranca with stable links. The penetration points of the RF links with the ionospheric shell are marked with white spots in the corresponding IGS TEC maps in Figure 2. It can clearly be recognized that in the case of SC4 (bottom right in Figure 2) the white dotted intersection point is located in the suspicious region of enhanced scintillation appearance, while the intersection points of SC1, SC2 and SC3 are quite distant from that region.


Figure 2: Penetration point of the RF link with the ionosphere, Cluster, 19/03/2003, SC1 (top left), SC2 (top right), SC3 (bottom left), all tracked from VIL, and SC4 (bottom right) tracked from MSP, anomalies were only on SC4

RF link anomalies observed on other spacecraft, namely MSG-1 and XMM, could in the same way be related to ionospheric scintillation.

Based on the knowledge earned, the working group came up with recommendations for future missions and operations design in terms of selection of frequency bands, selection of ground stations, foreseeing of backup stations in case scintillations are recorded at a nominal tracking site, sufficient dump rate margin and on-board storage capabilities, and others. For more details about the AGC Fluctuations Working Group see Refs. [1] and [2].


[1] Feltens, J., J.M.Dow, G.Billig, D.Fornarelli, S.Pallaschke, B.Smeds, H.-J.Volpp, P.Escoubet, H.Laakso (2004a): „Usage of IGS TEC Maps to explain RF Link Degradations by  Spread-F, observed on Cluster and other ESA Spacecraft”, IGS Presentation, in Proceedings of the 2004 IGS Workshop & Symposium, AIUB, Berne, Switzerland,    March 1-5, 2004.

[2] Feltens, J., J. Dow, G. Billig, D. Fornarelli, S. Pallaschke, B. Smeds, H.-J. Volpp, P. Escoubet, H. Laakso (2004b): “Relation of RF Link Degradations Observed on Cluster and Other Spacecraft to Spread-F with the Aid of IGS TEC Maps”, Invited Presentation, in Proceedings of the 2nd ESA Workshop on Satellite Navigation User Equipment Technologies NAVITEC ‘2004, ESA/ESTEC, Noordwijk, The Netherlands, December 8-10, 2004.