RSI: Radio Science Investigation
The Radio Science Investigation (RSI) makes use of the communication system that the Rosetta spacecraft uses to communicate with the ground stations on Earth. Either one-way or two-way radio links can be used for the investigations. In the one-way case, a signal generated by an ultra-stable oscillator on the spacecraft is received on earth for analysis. In the two way case, a signal transmitted from the ground station is transmitted back to Earth by the spacecraft. In either case, the downlink may be performed in either X-band or both X-band and S-band.
RSI will investigate the nondispersive frequency shifts (classical Doppler) and dispersive frequency shifts (due to the ionised propagation medium), the signal power and the polarization of the radio carrier waves. Variations in these parameters will yield information on the motion of the spacecraft, the perturbing forces acting on the spacecraft and the propagation medium.
Doppler data provide time-resolved measurements of the spacecraft motion and the plasma state and thus may be used for physical investigation of the nucleus and the inner coma of the comet. In particular, the following scientific objectives may be addressed by an analysis of dual-frequency one-way or two-way radiometric tracking data, together with information provided by other Rosetta experiments, for example the remote imaging system (OSIRIS):
- Gravity Field and Dynamics
|- ||Cometary mass and bulk density|
|- ||Cometary gravity field coefficients |
|- ||Cometary moments of inertia and spin state|
|- ||Cometary orbit, light shift, thermal properties of the nucleus|
|- ||Asteroid mass and bulk density|
- Cometary Nucleus
|- ||Size and shape (from spacecraft occultation observations)|
|- || |
Internal structure (from nucleus sounding)
|- ||Dielectric constant and roughness of the surface (from bistatic radar experiment)|
|- ||Rotation, precession and nutation rates (from bistatic radar)|
- Cometary Coma
|- ||Distribution of mm - dm size particles (from coma sounding)|
|- || |
Plasma content of the inner coma (from coma sounding)
|- ||Gas and dust mass flux (from non-gravitational perturbations of the spacecraft) |
- Solar Corona Science
|- ||Electron content of the inner corona, solar wind acceleration, search for coronal mass ejections, turbulence|
The two-way radio link is established by transmitting an uplink radio signal either at S-band or X-band to the spacecraft. the received uplink carrier frequency is transponded to downlinks at X-band and S-band upon multiplying by the constant transponder ratios 240/221 and 880/221, respectively, in order that the ratio of the two downlinks is 880/240 = 11/3. This radio mode takes advantage of the superior frequency stability inherent to the hydrogen maser in the ground station on Earth. This mode is used for all RSI gravity science applications, routine tracking observations when in orbit during the escort phase, and for the sounding of the solar corona. The one-way radio link is used only during an occultation of the spacecraft by the nucleus as seen from Earth. This enables radio sounding of the immediate vicinity of the nucleus and perhaps even the nucleus itself, should the solid cometary body prove to be penetrable by microwaves. These one-way occultation experiments require an Ultra-Stable Oscillator (USO) added to the radio subsystem. The prime purpose of the USO is to serve as a phase-coherent frequency reference for the simultaneous one-way downlink transmissions at S-band and X-band. The required stability (Allan variance) of the USO is about Δf/f ≈ 10-13 at 10-1000 seconds integration time. The one-way radio link can be transmitted either while receiving a non-coherent uplink or without any uplink contact at all.
Ground stations include antennas, associated equipment and operating systems in the tracking complexes of Perth (ESA, 35 m), Australia, and the Deep Space Network (NASA, 34 m) in California, Spain and Australia. A tracking pass consists of typically eight to ten hours of visibility. Measurements of the spacecraft range and carrier Doppler shift can be obtained whenever the spacecraft is visible. In the two-way mode the ground station transmits an uplink radio signal at S-band (if the spacecraft receiver operates at S-band) or at X-band and receives the dual-frequency simultaneous downlink at X-band and S-band. The information about signal amplitude, received frequency and polarization is extracted and stored as a function of ground receive time.
||RPC: Rosetta Plasma Consortium
||VIRTIS: Visible and Infrared Thermal Imaging Spectrometer
Last Update: 11 September 2013