## 2 Missions and Spacecraft Payload  
The life-time of the RO satellite missions selected for the present study of POD for climate applications span the entire RO era, starting with first continuous long-term RO measurements  
Figure 1: Schematic overview of parts of Wegener Center for Climate and Global Change (WEGC)’s Reference Occultation Processing System (rOPS) relevant to this study, comprising the input data for the precise orbit determination (Input Data; orange box), the daily system modeling (DSM; green box), and the occultation data processing (ODP; red box). For spelling of institution acronyms (in Input Data and DSM boxes) and further description see Section 3. The ODP box outlines the main level 1 and level 2 retrieval steps (L1a to L2b) from excess phase to essential climate variables (ECVs). obtained by CHAllenging Minisatellite Payload (CHAMP) in February 2001. The Gravity And Climate Experiment (GRACE) mission provided RO measurements since 2006, although it has been in space since 2002 and previously activated the RO receiver for testing [34]. Meteorological operational satellite A (Metop-A), the first of a series of three meteorological satellites developed in cooperation by the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT) and the European Space Agency (ESA), switched on its RO instrument 8 days after its launch on 19 October, 2006, followed by Metop-B on 17 December, 2012, before the series was completed with the launch of Metop-C from Guiana Space Centre, Kourou on 7 November, 2018 [35]. The CHAMP satellite was launched on July 15, 2000 into a near-polar but non sun-synchronous orbit of 87.3\({}^{\circ}\) inclination and a mean initial orbit altitude of 454 km which decreased over time despite intermediate orbit raise maneuvers, to 325 km approaching the decay. The German Research Centre for Geosciences (GFZ) managed mission provided measurements for atmospheric, magnetic and gravity field research before its end-of-lifetime after over 10 years in space [36]. The onboard _BlackJack_ Global Positioning System (GPS) receiver [37], manufactured by the National Aeronautics and Space Administration's (NASA) Jet Propulsion Laboratory (JPL), is connected to a multi-antenna system including a zenith-looking choke ring antenna for navigation tracking of inter-satellite links at a rate of 0.1 Hz, an anti-velocity facing antenna array for occultation tracking at 50 Hz, and a nadir looking antenna for altimetry measurements. The receiver front-end and electronics support dual-frequency tracking in 16 \(\times\) 3 channels for C/A, P1, and P2 code respectively. Out of all channels, 12 are allocated for POD (effectively only a maximum of 10 GPS satellites were tracked simultaneously [37; 38]) and the remaining 4 channels are reserved for operation in occultation mode for atmospheric sounding [27; 39] or alternatively for altimetry measurements. The satellite's orientation relative to the inertial reference frame is recorded in quaternions (1 Hz) by two dual-headed Advanced Star Camera (ASC) assemblies developed by the Technical University of Denmark (DTU) [40; 41]. Also onboard, a Laser Retroreflector (LRR), consisting of four cube corner prisms designed by GFZ [42; 43], is mounted on the platform. The LRR passively reflects short laser pulses from an emitting ground station on Earth, thus allowing for the calculation of satellite laser ranges [44]. The GRACE twin satellite mission was a joint mission between NASA and the German Aerospace Center (DLR) and launched into similar near-circular orbits of 89.5\({}^{\circ}\) inclination and about 500 km altitude on 17 March, 2002. The two identical satellites were separated approximately 220 km in nominal orbit for the primary mission goal of measuring climate-relevant variations of the Earth's gravity field [45; 46]. As a follow-on mission to CHAMP, GRACE features a similar design and payload, with an identical ASC and LRR mounted on the platform.