## 3 Calibration Methodologies & Dataset Used
### Transponder Calibration  
In satellite altimetry, the actual observation is the two-way travel time of the altimeter's signal: transmission by the altimeter, reflection by the Earth's surface, and recording by the altimeter. Subsequently, given that the speed of light is constant, it is the range (signal propagation time) between the satellite and the Earth's surface that defines the primary measurement in altimetry. Thus, in order to claim direct and absolute calibration of satellite altimeters it is the range bias that has to be examined meticulously by external and independent means on the ground. Microwave transponders are the apparatus to provide the absolute direct range bias of the satellite altimeter with no averaging of different reflection points and of the dynamic conditions at sea during satellite overpass [(13)]. In this Cal/Val technique, a transponder on the ground receives the transmitted data by the altimeter signal as the satellite overflies its location. Then, the transponder amplifies it and transmits it back to the satellite. The amplification procedure at the transponder is fully controlled, stable, and designed to ensure that the point target response, such as the one produced by a transponder, is clearly detectable in the recorded signal at the altimeter and certainly distinguishable among other reflections from the surrounding setting on the ground. Absolute range calibration of altimeters using transponders have been carried out for Envisat [(14)], Jason-2 [(15)], CryoSat-2 [(16)], and Jason-3 [(17)]. Sentinel-3A transponder calibration results have been presented in [(17; 18)] using the transponder operating at the CDN1 Cal/Val site in West Crete. The technical characteristics of this transponder instrument have been presented elsewhere [(8)]. The Sentinel-3A altimeter operates under two radar modes: (a) the Low-Resolution Mode (LRM) with the traditional pulse-limited signal transmitted on a 3 Ku-band/1 C-band/3 Ku-band patterns and (b) the Synthetic Aperture Radar (SAR) mode, with a high resolution along its track. SAR mode is made of bursts, each of which contains 64 Ku-band pulses surrounded by two C-band pulses. The SAR mode processing makes use of these 64 echoes within each burst, and it allows narrowing the along track resolution from a few km to about 300 m [(19)]. The reader can refer to other publications for further details on this the SAR mode altimetry [(20; 21; 22)]. Transponder calibration of Sentinel-3A in the SAR mode requires extensive processing of individual pulses within each burst as these were first received and retransmitted by the transponder and later received and recorded by the satellite altimeter in its orbit. In-house software has been developed to process all these data for transponder calibration. It relies upon raw Sentinel-3 Level-0 products along with raw telemetry data. A comprehensive block diagram of the essential and sequential steps involved in the processing of Level-0 products of Sentinel-3A for transponder calibration is given in Figure 7.  
Specifically, transponder calibration in the SAR mode consists of the following steps:  
1. _Data retrieval_. The data necessary for the implementation of the transponder calibration processing are: (a) Sentinel-3A Level-0 binary products; (b) altimeter internal calibration files; (c) satellite orbit navigation and attitude data; (d) auxiliary data, including Centre of Gravity (COG) and Ultra Stable Oscillator (USO) data; (e) accurate and absolute transponder's measuring point coordinates in relation to a reference coordinate system (i.e., ITRF 2014); (f) atmospheric delays (i.e., wet and dry troposphere, ionosphere) of altimeter signals and geophysical Level-2 corrections; (g) the transponder's internal path delay. 2. _Waveform Calibration_.