Abstract's details
Leveraging tandem satellite missions to optimize altimeter system calibration with ground based external reflectors.
Event: 2026 Sentinel-6 Validation Team meeting
Session: S6VT plenary session
Presentation type: Oral
External and independent calibration and validation (Cal/Val) techniques are widely employed to assess the stability and performance of satellite altimeter systems [1, 2, 3], providing a complementary method to cross-over analysis and in-flight characterization, which are routinely performed throughout a mission's lifetime. Among these techniques, transponders (TRPs) and corner reflectors (CRs) currently offer some of the highest performing solutions for altimeter calibration, achieving range bias precisions on the order of ±1-2 cm [4, 5, 6]. Despite their effectiveness, existing TRP and CR approaches do not yet deliver range-bias uncertainties low enough to fully satisfy the stringent altimeter range drift requirements of the future Sentinel-6NG mission. Furthermore, significant knowledge gaps remain in the uncertainty budgets of TRP and CR measurements, particularly regarding the temporal and spatial dependencies of these uncertainties, key aspects for accurately predicting the performance of a future Cal/Val network designed to meet Sentinel 6 NG specifications [7].
The dispersion observed in TRP and CR range-bias measurements is typically interpreted as white noise formed by the combination of all contributing error sources in the calibration chain [8]. Improving the characterization of this overall uncertainty, especially isolating the contribution of the satellite altimeter system, remains challenging. This difficulty is due to the limited temporal sampling of satellite overpasses at Cal/Val sites and from the heterogeneity of existing calibration facilities, which currently hinders robust spatiotemporal analyses of range bias measurements.
In this study, we exploit the Sentinel 6A/Sentinel 6B tandem phase, during which two independent altimeter observations are acquired only 30 seconds apart over the Crete and Gavdos TRPs (for both ascending and descending passes). This configuration enables the assessment of correlations between the two measurements, providing new insight into the underlying error budget. Unlike the Sentinel 3A/Sentinel 3B tandem phase, where only a few cycles were available, yielding indicative but limited results, the Sentinel 6 tandem phase offers a substantially larger dataset, allowing for statistically more robust calibration analyses. We are also preparing analogous investigations for the upcoming Sentinel 3 tandem phases, with particular focus on the CNES CR at Calern in the southern Alps. This site benefits from collocated supporting instrumentation (GNSS, laser ranging, DORIS), which helps reduce non altimetry error contributions. It is already overflown by Sentinel 3A and Sentinel 3B, enabling relevant inter altimeter comparisons, and will soon benefit from the addition of Sentinel 3C, further expanding the constellation and enhancing our ability to characterize the accuracy and uncertainty of range bias measurements.
We will present the preliminary results of this study, highlighting the added value of tandem-orbit satellite missions for altimeter calibration using ground based external reflectors.
References:
[1] Bonnefond, P. et al. (2021). Corsica: A 20-yr multi-mission absolute altimeter calibration site. Advances in Space Research, 68(2), 1171–1186
[2] Mertikas, S. et al. (2024). The ESA Permanent Facility for Altimetry Calibration in Crete: Range, sigma0, and sea-surface calibration with transponders, corner reflectors and coastal reference sites, 30 Years of Progress in Radar Altimetry Symposium, Montpellier, France
[3] Garcia-Mondéjar, A. et al., (2024). 30 years of calibration with transponders, 30 Years of Progress in Radar Altimetry Symposium, Montpellier, France
[4] Mertikas, S. et al. (2024). The ESA Permanent Facility for Altimetry Calibration: Monitoring Performance of Radar Altimeters for Sentinel-3A, Sentinel-3B and Jason-3 Using Transponder and Sea-Surface Calibrations with FRM Standards, Remote Sens, 12, 2642
[5] Gibert, F. et al. (2023). A Trihedral Corner Reflector for Radar Altimeter Calibration, IEEE Transactions on Geoscience and Remote Sensing, vol. 61, pp. 1-8
[6] Maraldi, C. et al. (2023). Calibration of Sentinel-3 altimeter data using a corner reflector, OSTST San Juan, Porto Rico
[7] Prandi, P. et al. (2025). Design elements for an external verification system of S6NG stability requirements, Living Planet Symposium, Vienna, Austria
[8] Mertikas, S. et al. (2018). Fifteen Years of Cal/Val Service to Reference Altimetry Missions: Calibration of Satellite Altimetry at the Permanent Facilities in Gavdos and Crete, Greece. Remote Sens, 10, 1557.
Back to the list of abstractThe dispersion observed in TRP and CR range-bias measurements is typically interpreted as white noise formed by the combination of all contributing error sources in the calibration chain [8]. Improving the characterization of this overall uncertainty, especially isolating the contribution of the satellite altimeter system, remains challenging. This difficulty is due to the limited temporal sampling of satellite overpasses at Cal/Val sites and from the heterogeneity of existing calibration facilities, which currently hinders robust spatiotemporal analyses of range bias measurements.
In this study, we exploit the Sentinel 6A/Sentinel 6B tandem phase, during which two independent altimeter observations are acquired only 30 seconds apart over the Crete and Gavdos TRPs (for both ascending and descending passes). This configuration enables the assessment of correlations between the two measurements, providing new insight into the underlying error budget. Unlike the Sentinel 3A/Sentinel 3B tandem phase, where only a few cycles were available, yielding indicative but limited results, the Sentinel 6 tandem phase offers a substantially larger dataset, allowing for statistically more robust calibration analyses. We are also preparing analogous investigations for the upcoming Sentinel 3 tandem phases, with particular focus on the CNES CR at Calern in the southern Alps. This site benefits from collocated supporting instrumentation (GNSS, laser ranging, DORIS), which helps reduce non altimetry error contributions. It is already overflown by Sentinel 3A and Sentinel 3B, enabling relevant inter altimeter comparisons, and will soon benefit from the addition of Sentinel 3C, further expanding the constellation and enhancing our ability to characterize the accuracy and uncertainty of range bias measurements.
We will present the preliminary results of this study, highlighting the added value of tandem-orbit satellite missions for altimeter calibration using ground based external reflectors.
References:
[1] Bonnefond, P. et al. (2021). Corsica: A 20-yr multi-mission absolute altimeter calibration site. Advances in Space Research, 68(2), 1171–1186
[2] Mertikas, S. et al. (2024). The ESA Permanent Facility for Altimetry Calibration in Crete: Range, sigma0, and sea-surface calibration with transponders, corner reflectors and coastal reference sites, 30 Years of Progress in Radar Altimetry Symposium, Montpellier, France
[3] Garcia-Mondéjar, A. et al., (2024). 30 years of calibration with transponders, 30 Years of Progress in Radar Altimetry Symposium, Montpellier, France
[4] Mertikas, S. et al. (2024). The ESA Permanent Facility for Altimetry Calibration: Monitoring Performance of Radar Altimeters for Sentinel-3A, Sentinel-3B and Jason-3 Using Transponder and Sea-Surface Calibrations with FRM Standards, Remote Sens, 12, 2642
[5] Gibert, F. et al. (2023). A Trihedral Corner Reflector for Radar Altimeter Calibration, IEEE Transactions on Geoscience and Remote Sensing, vol. 61, pp. 1-8
[6] Maraldi, C. et al. (2023). Calibration of Sentinel-3 altimeter data using a corner reflector, OSTST San Juan, Porto Rico
[7] Prandi, P. et al. (2025). Design elements for an external verification system of S6NG stability requirements, Living Planet Symposium, Vienna, Austria
[8] Mertikas, S. et al. (2018). Fifteen Years of Cal/Val Service to Reference Altimetry Missions: Calibration of Satellite Altimetry at the Permanent Facilities in Gavdos and Crete, Greece. Remote Sens, 10, 1557.