Back to GG Proposal to ESA, F2&F3 CompetitionGALILEO
GALILEI" (GG)
A Small Satellite to Test the
Equivalence Principle of Galileo, Newton and Einstein
Proposal to ESA, F2&F3 Competition, January 31 2000
6. Mission Operation, Ground Control, Management of Scientific Data, Impact on the Media and the General Public
The GG mission is devoted to a single experiment that, once initialized, runs uninterrupted to the end of the scientific data collection (6 months after the end of the set-up and first calibration). There are no maneuvers, either orbital changes or attitude slews, during the scientific mission. The processing of scientific data is done in bulk, therefore no scientific quick-look is required. All scientific operations are autonomous, executed on the basis of time-tagged operation sequences that are loaded at least one day in advance. Given the high level of autonomy, the tasks of the ground control are essentially limited to generation and transmission of command sequences and parameters, and analysis of satellite data to establish that the satellite is operating correctly. The mission requires an equatorial orbit and therefore an equatorial station (e.g., Malindi) is ideally suited. Because of the low-inclination orbit, a regular pattern of ground passes with almost constant duration can be assumed. Support by other stations in the early orbit phase is assumed as a standard service. After the nominal attitude has been achieved no other attitude maneuvers are needed throughout the life of the mission.
As it is customary, the ground segment will include, besides the ground station, an Operational Control Center (OCC), responsible for the execution of the mission operations, and an Operational Scientific Center (OSC), responsible for the generation of the scientific operation sequences. There is no special requirement for real-time interaction between the on-board payload and the OSC, or, in general, between the satellite and the OCC.
The main operational modes of the satellite (after commissioning at the beginning of life) are:
The experiment set-up includes the balancing of the test masses and the mechanical balancing of the capacitance read-out sensors. Both operations need to be repeated at regular intervals, estimated as 20 days for the balancing of the test masses and 15 days for the mechanical balancing of the capacitance bridge. Automatic procedures for such operations will be elaborated, possibly with some interaction with the ground control.
In the science measurements phase, the operation will be essentially autonomous. The Normal Mode is characterized by the drag-free control, executed by the FEEP electric min-thrusters. However, the survival of the mission does not depend on the drag-free control, since the stability of the operational attitude is guaranteed by the gyroscopic stability. In case of malfunctions, the scientific operations will be put on hold and housekeeping data will be collected and transmitted to ground on the next station passes; resumption of the operations will be commanded by the ground. Generally, the command and parameter sequences of the Normal mode will need to be updated on a time basis of several weeks, except in the set-up phase when the frequency will be higher (some hours).
The scientific data are sent to ground after demodulation, and the telemetry rate is generally small. Exceptionally, it may be necessary to transmit to Earth the raw (non demodulated) data, for special checkout, parameter identification, and troubleshooting. Because of the nature of the experiment, the duration of such high rate data collection periods will not exceed about 10minutes
. Therefore, the telemetry capacity of the telecommunication links is not exceeded. The scientific data comprise the position of the test masses relative to each other and the "laboratory" (PGB), the time, the spin reference signal and ancillary data such as the temperature, the attitude of the spin axis and the phase difference between the PGB and the spacecraft’s outer vessel. The scientific signals are demodulated on board at the spin frequency and only demodulated data (i.e., the data that contain the putative Equivalence Principle violation signal at the orbit frequency) are sent to ground. The only exception is the spin reference signal, used for the demodulation, that is sampled 15 times per spin period of 0.5sec, that is at 30Hz, and is sent to the ground without further elaboration. The scientific data collection rate is small, about 1.5kbit/s, and the total telemetry rate is well below the limit data rate (1Mbps) of the ESA S-band ground stations, including Malindi, even in the worst case of 24-hour autonomy from the ground. In normal circumstances, we assume the data are downloaded to ground at each orbital pass. Tracking with a normal accuracy of several km along-track is sufficient for the purposes of the scientific mission.The minimum integration time of the experiment is determined by the thermal noise and is about 7 days. Hence, examination of the scientific data at shorter intervals is, strictly speaking, not significant. Therefore, quick look procedures are not needed and the scientific data can be routed to the Scientific Data Center within a couple of days of reception. On the other hand, for the purposes of checking the health of the scientific payload and the correct execution of the measurement procedures, shorter reaction times may be desirable. Tests based on consistency checks, threshold parameter values etc. will be elaborated and implemented in automatic self-check procedures that can be run periodically by the payload computer, and can be used to alert the ground control of any non-nominal state of the scientific payload. Data affected by anomalies of any sort will be rejected on post-processing and will have no effect but a shortening of the data collection period (which could be made up for by a corresponding extension of the mission lifetime).
The tasks of the Operational Control Centre will comprise, besides the normal spacecraft operations (mission planning, monitoring and control; orbit and attitude determination), also the execution of the operations required by the scientific measurements. The OCC will be responsible for routing of the payload telemetry to the OSC, and processing of the telecommand requests from the OSC. Co-location of experimenter staff at the OCC, particularly during the early set-up phase, when interaction with the payload on board is more frequent, may be considered.
The data set resulting from the mission will be archived on CDROM and put at the disposal of the scientific community. The complete data set will include raw data, calibrated data and support data (housekeeping, tracking and attitude). The complete data set is expected to comprise about 26 Gbit.
A small satellite based on fine technology and simple mechanical principles, and capable to test to an unprecedented level of accuracy a physical principle that three fathers of modern science have regarded as fundamental, would have an enormous impact on the general public. The mission will be presented primarily through a Web Site with different levels of sophistication aimed at Universities, School and the general public, which will provide material on the history and development of the theory of gravitation from Galileo and Newton, to Einstein and beyond, including a description of general relativity (at levels appropriate for the different audiences). This will place the Equivalence Principle in its historic context, describing past experiments as well as the GG mission in space. Additionally we will seek to present information on the experiment and the scientific results through popular articles and broadcasts. Galileo, Newton and Einstein are well known to the public and attract the attention and interest of the media, both press and television. Even though the project is not yet approved for flight we have already had the opportunity to bring the subject to the attention of national TV programmes and newspapers. Images of the GGG payload prototype were broadcast by a major Italian TV channel conveying to a very large audience the simple message that small satellites can be designed by ordinary scientists, they contribute to the advance of human knowledge and help develop new technology. It was never a difficulty to explain what GG is aiming to, why its goal is so important, why only space technology can provide a crucial scientific result to be reported in textbooks for a long time to come.