# Calibrated Semi-Empirical Solar Radiation Pressure Models for Altimeter Satellites

**CoAuthors**

**Event: **2014 Ocean Surface Topography Science Team Meeting

**Session: **Precision Orbit Determination

**Presentation type: **Type Oral

The solar radiation pressure models used in present POD processing (Jason, Cryosat, Altika, HY2A...) are not perfect, and remaining errors are compensated by empiricals 1/rev terms in radial and normal axes, and also partially constant along track terms which are anyway necessary to deal with drag model uncertainties.

The analysis of the phase and amplitude of these empirical terms as functions of solar angle and orbit angle shows remarkable systematic effects which are clearly related to solar radiation pressure.

For satellites in Yaw Steering attitude mode (Jason, and also GPS satellites), it is very interesting to analyze the radiation accelerations in the solar array frame (theoretical pointing). In a first pass, an equivalent harmonic model in these axes is generated to represent the nominal model. In a second pass, small perturbations are adjusted, corresponding to the structure of the possible errors in the radiation model. The achieved values lead to small remaining empirical forces.

For Jason, a specific adjustment has been added for the solar array, because there are non negligible pointing errors w.r.t. normal pointing (these errors are known if the measured attitude data is used). Also this effect is significant in fixed yaw mode where the nominal pointing difference can reach 15 degrees.

The harmonic model is extended to the fixed yaw case, but the behavior is much complex, and limits the efficiency of the empirical model.

Results are shown for the achieved altimetry performance (crossover residuals). These models for Jason 1 and Jason 2 satellites will be used in the future GDR-E products.

The empirical accelerations found on Cryosat and Altika are also discussed. These accelerations show that an important part of the empirical accelerations cannot be represented by a model depending only on the solar angle and orbit angle (as would be the case for a solar radiation pressure effect).

The analysis of the phase and amplitude of these empirical terms as functions of solar angle and orbit angle shows remarkable systematic effects which are clearly related to solar radiation pressure.

For satellites in Yaw Steering attitude mode (Jason, and also GPS satellites), it is very interesting to analyze the radiation accelerations in the solar array frame (theoretical pointing). In a first pass, an equivalent harmonic model in these axes is generated to represent the nominal model. In a second pass, small perturbations are adjusted, corresponding to the structure of the possible errors in the radiation model. The achieved values lead to small remaining empirical forces.

For Jason, a specific adjustment has been added for the solar array, because there are non negligible pointing errors w.r.t. normal pointing (these errors are known if the measured attitude data is used). Also this effect is significant in fixed yaw mode where the nominal pointing difference can reach 15 degrees.

The harmonic model is extended to the fixed yaw case, but the behavior is much complex, and limits the efficiency of the empirical model.

Results are shown for the achieved altimetry performance (crossover residuals). These models for Jason 1 and Jason 2 satellites will be used in the future GDR-E products.

The empirical accelerations found on Cryosat and Altika are also discussed. These accelerations show that an important part of the empirical accelerations cannot be represented by a model depending only on the solar angle and orbit angle (as would be the case for a solar radiation pressure effect).