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Instruments

SCE

Solar Corona Experiment

The SCE performs coronal-sounding measurements of the Sun's corona during the spacecraft's solar conjunctions. These investigations exploit Ulysses' radio communications links with Earth to extract information on the structure of otherwise virtually inaccessible media throughout the heliosphere.

Spacecraft Radio Telecommunications Subsystem
The SCE instrumentation consists of the same equipment on the spacecraft and in the ground stations that is used for radio communications (command, telemetry, and navigation). During SCE data recording intervals the telecommunications subsystem is configured to receive an S-band signal from the ground station and transmit S- and X-band signals coherent with the received signal. The coherence ratio (transponder multiplier from S-band up to S-band down) is 240/221. The ratio of the two downlink frequencies, X-band to S-band, is 11/3. The signals are transmitted to the ground station via the dual-feed high gain antenna.

Ground stations: the DSN radio science system
With the exception of the 34-m station (DSS 12) in Goldstone, California, all DSN antennas are equipped with both closed-loop and open-loop receivers. These receivers are supplied with frequency and timing reference signals from a highly stable hydrogen maser frequency standard.

  • Closed loop
    As part of each station's receiver-exciter subsystem, the closed-loop receiver acquires and tracks the spacecraft carrier signal using a phase-locked loop feedback scheme. The tracking subsystem estimates and reports the Doppler shift after comparing the phase-locked loop frequency output with a reference from the station's frequency standard, which is also used to generate the uplink signal. In addition to the Doppler measurements, the spacecraft's range can be determined using the sequential ranging assembly. Modulated with range code, the S-band uplink signal is transmitted to the spacecraft where it is detected and transponded back to the station. The round-trip light time, directly related to the spacecraft's distance, is computed in the ground station by comparing the received code with the transmitted code
  • Open loop
    Used primarily for Radio Science experiments and VLBI, the open-loop receiver downconverts the filtered carrier signal to baseband where it is digitized and recorded. The receivers at the 70-m stations have four channels, thereby enabling simultaneous recording of both S- and X-band downlinks in both polarizations (left- and right-hand circularly polarized)

Summary of Objectives

The main science objective of SCE is to derive the plasma parameters of the solar atmosphere:

  • To derive the 3-D distribution of the coronal electron density from the SCE ranging and Doppler data products. The large-scale structure can be inferred from the total electron content I obtained from dual-frequency ranging
  • In addition to the quasi-static structure of the outer corona, the dual-frequency Doppler data will be used to characterize the level and spectral index of coronal turbulence
  • Use Multi-station observations to derive the plasma bulk velocity at solar distances where the solar wind is expected to undergo its greatest acceleration

In addition, it is anticipated that the interplanetary vestiges of a Coronal Mass Ejection will be occasionally detected as a significant perturbation in the ranging and Doppler data.

Profiting from the favourable geometry during the Jupiter encounter on 8 February 1992, the SCE was also used to measure the electron density of the Io Plasma Torus.


Last Update: 08 December 2006

For further information please contact: SciTech.editorial@esa.int

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