Cristina Pérez Hernández, CDTI; Katia Maria Cáceres Mosqueira, CDTI; Cassien Jobic, CNES; Francesc Vilardell Sallés, GMV; Alfredo M. Antón, GMV; David Moreno-Espinosa, GMV
Keywords: sensor characterization, calibration, data quality, Space Traffic Management, Galileo
Abstract:
European Space Surveillance and Tracking (EUSST) services rely on a worldwide network of sensors capable to follow Resident Space Objects (RSOs) in all orbital regimes. The network is composed by ranging lasers, radars, and optical telescopes (with tracking and survey capabilities). However, the data provided by this network must be accurate and with enough quality to ensure the best possible provision of the EUSST services. To evaluate the correct performance of all sensors, EUSST has established a mandatory calibration procedure for all sensors within the network, both for new sensors and those already operational. The performance revealed during the calibration process is used to determine the category of each sensor within the EUSST network and, hence, its usefulness for the provision of data to EUSST.
During the period available to submit calibration observations (called calibration campaigns), the sensors must observe a list of well-defined RSOs with accurate orbits. Depending on the operational regime of the sensor, the observed objects are either GNSS satellites (usually Galileo) or LEO satellites suitable for calibration. Only the data obtained during the calibration campaigns (and labelled for calibration) are currently used to calibrate the sensors.
Once the calibration observations are obtained, they are processed by two independent national centers (S3TOC and COO). The calibration procedure evaluates the performance of the sensors by comparing the provided observations with the well-known position of the objects selected for calibration. During the comparison process, the data provided by some of the sensors need to be corrected by relativistic effects (annual and diurnal aberration). In addition, using state of the art observation measurement models, the reference orbits are used to compute the signal travel time, the tropospheric influence, and the ionospheric effect (for ranging related measurements). The residuals with respect to the reference positions are then computed, and the process is iterated using a weighted sigma-clipping rejection algorithm to remove any possible outliers. The results of the calibration are finally compared between the two independent national centers and, in those rare occasions when results differ, the underlying causes are investigated and resolved.
The resulting biases (e.g., time bias or range bias) and RMS (Root Mean Squared) of the data are then evaluated and compared with previously determined thresholds . The threshold values are set according to the service provision needs. Only those sensors providing data within the thresholds for a certain period (at least two calibration campaigns) are categorized as operational and considered suitable for the provision of data to EUSST. New sensors fulfilling all the calibration thresholds in a calibration campaign are considered temporarily operational and also allowed to provide data to the EUSST.
The calibration procedure provides a robust method to categorize the sensor capability to provide quality data to the EUSST. This procedure is continuously reviewed and improved when possible. For example, the calibration thresholds may be evaluated and updated between consecutive calibration campaigns, if considered necessary. In some specific cases, the sensors are allowed to provide calibration data to perform a fast calibration and change its operational status without the need to wait for a new calibration campaign. Finally, several additional studies are undergoing to further improve the EUSST calibration procedure. The possibility to use GEO satellites for calibration may provide a better characterization of the sensors observing these RSOs. Another possibility would be to continuously monitor the performance of the sensors between two calibration campaigns, allowing the detection any possible deviations due to maintenance operations, or degradation of the sensor capabilities. In this sense, a preliminary near-real time continuous calibration tool is being implemented, and qualification tests are on-going.
To summarize, the EUSST sensor calibration procedure provides a well-tested method to evaluate and ensure the quality of the data provided by the global network of sensors. This method has already been applied in nine calibration campaigns, proving to be a valuable tool to ensure the quality of the provided EUSST services.
Date of Conference: September 19-22, 2023
Track: SDA Systems & Instrumentation