Adv. Geosci., 50, 87–100, 2020 https://doi.org/10.5194/adgeo-50-87-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Copernicus Sentinel-1 satellites: sensitivity of antenna offset estimation to orbit and observation modelling Heike Peter1, Jaime Fernández2, and Pierre Féménias3 1PosiTim UG, Seeheim-Jugenheim, Germany 2GMV AD, Tres Cantos, Spain 3ESA/ESRIN, Frascati, Italy Correspondence: Heike Peter ([email protected]) Received: 13 June 2019 – Revised: 9 January 2020 – Accepted: 29 February 2020 – Published: 27 March 2020 Abstract. The SAR (Synthetic Aperture Radar) Copernicus 1 Introduction Sentinel-1 satellites require a high orbit accuracy of 5 cm in 3D in comparison to external processing facilities. The offi- cial orbit products delivered by the Copernicus POD (Precise The European Copernicus Programme (Aschbacher and Orbit Determination) Service fulfil this requirement. Nev- Milagro-Pérez, 2012) has been set up to establish European ertheless, analyses have shown discrepancies in the orbit capacity for Earth Observations. The core of the programme results for the two satellites Sentinel-1A and Sentinel-1B. are Earth observation satellites. The Copernicus Sentinel Since the satellites are identical in construction estimated or- missions are dedicated satellites for specific needs within the bit parameters like the scale factor for the radiation pressure programme. are expected to be at the same magnitude, which is not the The Synthetic Aperture Radar (SAR) satellite Sentinel- case. Estimation of GPS antenna offsets leads to differences 1A (S-1A, Fletcher, 2012; Torres et al., 2012) is the first between the two satellites, which might explain the discrep- satellite of the Copernicus programme. It was launched on ancies in the estimated orbit parameters. Such offset estima- 3 April 2014 from Kourou, French Guiana. The second satel- tions are, however, very sensitive to orbit and observation lite of the mission, Sentinel-1B (S-1B), was launched from modelling. It has to be assured that the results are not bi- the same place about two years later on 25 April 2016. ased by insufficient models. First of all, stabilisation of the Figure1 shows an artist’s impression of the Sentinel-1 antenna offset estimation is achieved by improving the obser- satellite. The C-Band SAR is the main instrument accom- vation modelling by applying single receiver ambiguity reso- panied by telemetry antennas, three star trackers for attitude lution. The Copernicus Sentinel-1 satellites have a very com- determination and two (main and redundant) eight-channel plex shape with the long SAR antenna and the two large solar Global Positioning System (GPS) units for precise orbit de- arrays. Antenna offset estimation based on different satellite termination (POD). The orbit accuracy requirement for Non- models may give results which differ by up to 1.5 cm. The Time Critical (NTC) POD products is given as a maximum dispersion of the estimates is quite large depending also on of 5 cm in 3D in the comparison to external processing facil- eclipse and non-eclipse periods. Consideration of simple as- ities (GMES Sentinel-1 Team, 2006). The Copernicus POD sumptions on satellite self-shadowing effects improves the Service (CPOD Service, Fernández et al., 2014, 2015) is satellite model and also the results of the antenna offset esti- in charge of providing the orbital and auxiliary products mation. Finally, more consistent results for the two Sentinel- needed by the Processing Data Ground Segment (PDGS) of 1 satellites are achieved by applying the antenna offset esti- the satellites. The orbit determination processing is done with mates. NAPEOS (NAvigation Package for Earth Orbiting Satellites, Springer et al., 2011) used for POD of other Earth observa- tion satellites as well, e.g., Jason-2 (Flohrer et al., 2011). GPS is the only POD observation technique available for Sentinel-1. Assessment of the absolute orbit accuracy is, Published by Copernicus Publications on behalf of the European Geosciences Union. 88 H. Peter et al.: Sentinel-1: sensitivity of antenna offset estimation Figure 1. Artist’s impression of the S-1 satellite; © ESA. Figure 2. Sun angle above the orbital plane (β), eclipse period be- tween lines in the middle of the year. therefore, difficult. Radar measurements have already suc- If the same orbit and observation models are applied cessfully been applied for orbit validation of TerraSAR- for precise orbit determination of the two satellites dif- X and TanDEM-X (Hackel et al., 2018). Schmidt et al. ferent results for the estimated orbit parameters might be (2018) indirectly validated the Sentinel-1 NTC orbits with caused by erroneous satellite geometry, namely center-of- radar measurements but did not reach the level of accu- mass (COM) coordinates, GPS ARP coordinates or GPS an- racy necessary for absolute orbit validation. The valida- tenna PCO C PCVs (phase center variations). Antenna off- tion of the orbit accuracy can, therefore, only be done set estimation is a common method to sort out inconsisten- by orbit overlap analysis and by comparison to other or- cies between satellite geometry, observation and orbit mod- bits generated from different institutions. The Coperni- els. Luthcke et al.(2003) and Haines et al.(2004) for in- cus POD Quality Working Group (QWG) is part of the stance did an offset estimation for Jason-1 by estimating the CPOD Service. The member institutions of the QWG de- GPS antenna PCO together with the phase center variations liver independent orbit solutions for all Sentinel satellites (PCV). Radial orbit accuracy and post-fit observation resid- using different software packages and different reduced- uals could significantly be improved. Peter et al.(2017) dis- dynamic orbit determination (Wu et al., 1991) approaches. cussed the interdependency of PCO and PCVs in detail for These alternative orbit solutions are compared in four- S-1A. They claimed the necessity of having PCVs available, month batches to the operational CPOD solutions. The which do not induce offsets, in particular for satellite mis- comparisons are published in the Regular Service Review sions where such auxiliary data is provided to users with dif- (RSR) reports (https://sentinel.esa.int/web/sentinel/missions/ ferent software packages and different reduced-dynamic or sentinel-1/ground-segment/pod/documentation, last access: even kinematic orbit determination strategies. 16 March 2020). Based on these RSR comparisons and fol- In the study presented here antenna offset estimation lowing detailed investigations (Peter et al., 2017) an erro- means the estimation of the vector from COM of the satellite neous information in the Sentinel-1 satellite geometry could to the ARP, in most cases the physical mounting point of the be identified. The Up-component of the GPS antenna phase GPS antenna. Although a potential error cannot be assigned center offset (PCO) w.r.t. the antenna reference point (ARP) either to the COM or the ARP or PCO/PCV, the antenna PCO has, therefore, been corrected by 29 mm. The consistency be- and PCVs are handled separately and are not included into tween the CPOD and POD QWG orbit solutions could sig- this term in this case. nificantly be improved although systematic differences were A main topic of this research is the sensitivity of such an- still present in particular during the eclipse period from May tenna offset estimations to orbit and observation modelling to August. for the special case of the Sentinel-1 mission. The two satel- S-1A and S-1B satellites are identical in construction. lites A and B are flying in the same sun-synchronous dawn- Therefore, estimated orbit parameters allow to indirectly val- dusk orbit (inclination 98.18◦, altitude 693 km) but 180◦ idate the orbit solutions by comparing the estimated param- apart. This orbit implies a sun angle over the orbital plane eters such as solar radiation pressure (SRP) coefficient or between 58 and 87◦ with an eclipse period of less than 100 d empirical cycle-per-revolution (CPR) parameters with each between May and August (see Fig.2). other. They are expected not to be exactly the same but they The solar panels are fixed to an angle of 30◦ with respect should have the same magnitude. to the satellite’s z-axis (perpendicular to the SAR antenna Adv. Geosci., 50, 87–100, 2020 www.adv-geosci.net/50/87/2020/ H. Peter et al.: Sentinel-1: sensitivity of antenna offset estimation 89 face direction and positive in direction of radiation) in the The sensitivity analysis of the S-1 antenna offset estima- satellite-fixed reference frame (SRF). The xSRF-axis approx- tion is composed as follows. Section2 shows the differences imately points into the flight direction and the ySRF-direction in the orbit determination results of the two S-1 satellites. is the anti-sun direction. In nominal attitude mode (for details Section3 describes the different orbit and observation mod- see Peter et al., 2017; Fiedler et al., 2005; Miranda, 2005) els used, in particular a detailed description of the macro- the satellite bus is tilted approximately by 30◦ with respect model update including assumptions on the self-shadowing. to the nadir direction as it is implied in Fig.1. The GPS an- The results of the different antenna offset estimations based tennas are also tilted w.r.t. the satellite bus (Peter et al., 2017) on the different orbit and observation models are summarised to have the antenna boresight close to zenith direction all the and analysed in Sect.4. The article is closed with conclusions time. The non-gravitational force modelling is done based on in Sect.5. a satellite macromodel (box-wing model). The satellite and orbit design implies a strong correlation between SRP mod- elling and the cross-track component of the orbit. This fact 2 S-1A and S-1B orbit determination results impacts the selection of the orbit parameter setup and also plays an important role when analysing sensitivity of the S-1 Part of motivation for this study is that the estimated orbit antenna offset estimation to orbit and observation modelling.