# Impact of Satellite Attitude on Altimetry Calibration with Microwave Transponders
## 3 Satellite Attitude and Transponder Calibration  
### Determination of the Satellite Attitude  
In this Section, the Jason-3 satellite will be used as an example to demonstrate the applied methodology of attitude determination and its impact on transponder calibration. The Jason-3 spacecraft was designed based on a \"box-wing\" structural model and consists of the spacecraft bus (box) and the solar panel arrays (wings). The main scientific payload of Jason-3 comprises the Poseidon-3B (P3B) dual frequency (13.575 GHz and 5.3 GHz) nadir-looking radar altimeter, a microwave radiometer, a precise orbit determination system (PODS) and an attitude determination and control system (ADCS) [37]. The PODS is responsible for the determination of the satellite's position in space and consists of a Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) receiver, a GNSS receiver and a laser retroreflector array. The ADCS provides the satellite's three-dimensional orientation via the combined analysis of measurements coming from star trackers, magnetometers, sun sensors, magnetic torquers and gyroscopes. The star trackers have optical sensors that capture snapshots of the sky to calculate the positions of specific stars. Comparison of the observed position of stars with the corresponding one from the on-board ephemeris results in the accurate estimation of spacecraft attitude. The instantaneous orientation of the satellite is used by an attitude actuation mechanism, consisting of reaction wheels, to perform attitude adjustments. The two attitude criteria for a reliable operation of the satellite are as follows. Firstly, the altimeter boresight should point downwards and perpendicularly to the reference ellipsoid (nadir-pointing). This criterion is necessary for the altimeter to receive reliable return waveforms, following their reflection on the ocean surface. Secondly, the solar panels direction should be perpendicular to the Sun arrays in order to provide and store adequate power to the satellite. The above requirements were firstly realized in the TOPEX/Poseidon altimeter using the algorithms described in [38]. The same algorithm principles are also used for the adjustment of Jason-3 attitude. The following reference systems need to be defined for the estimation of satellite attitude and the post-processing of altimetric products [39]:  
* The inertial reference system (IRS; [O: \(UVW\)]), with its origin at the geocenter O. It is a system that does not rotate with the Earth, and its axes are parallel to the barycentric system whose origin is at the Sun. The \(U\) axis passes from the equatorial plane with direction towards the vernal equinox \(\upgamma\). The \(W\) axis coincides with the Earth's mean rotation axis, and the \(V\) axis is perpendicular to the \(U\) axis to form a right-hand orthogonal reference system. This system is also referred to as \"pseudo-inertial\" because of the acceleration arising from the Earth's revolution around the Sun. * The Earth-fixed reference system (EFRS; [O: \(XYZ\)]), with its origin at the geocenter O. It is a reference system that rotates with the Earth's angular velocity. Its origin O also coincides with the center of the reference ellipsoid, i.e., the WGS84 ellipsoid for the latest processing baseline (F) of Jason-3 products. The \(X\) and \(Y\) axes are located at the equatorial plane with directions towards the prime meridian (\(\lambda=0^{\circ}\)) and perpendicular to it (\(\lambda=90^{\circ}\)), respectively. The \(Z\) axis coincides with the Earth's rotation axis with direction towards the North Pole. * The satellite body reference system (SAT; [Q: \(x_{B}y_{B}z_{B}\)]), with its origin at the center \(Q\) of the launcher attachment ring on the satellite. The \(x_{B}\) axis is parallel to the satellite primary direction and towards the radiometer antenna. The \(y_{B}\) axis is parallel to the rotation axis of the solar panels with direction towards the right solar panel array.