Solar Radiation Pressure (SRP) modeling for TDRS spacecraft in HPOP
TDRS spacecraft are geosynchronous communications satellites. They have a large solar array panel to collect power that nominally points toward the apparent position of the Sun. They have smaller antennas that are used for communicating down to the Earth, and so are nominally Earth pointing. The main body of the spacecraft does not have pointing constraints.
Solar radiation pressure is a significant force perturbation that must be modeled for the TDRS spacecraft when numerically propagating an orbit. HPOP provides a generic SRP force model that depends on the size of the area of the spacecraft exposed to solar radiation, and you can only specify a constant value for area-to-mass ratio in the user interface. While for many applications this modeling suffices, for high-fidelity applications (e.g., orbit determination), the model should consider the fact that the area exposed to solar radiation pressure changes over time, due to pointing requirements for appendages.
Of course, each spacecraft will have its own set of modeling issues. In many instances, the generic modeling provided by HPOP will suffice for producing the accuracy required. In cases where the generic models are inadequate, you can create an HPOP Force Model Plugin to implement the higher-fidelity model and have ODTK use it for HPOP during the normal propagation process. Moreover, you can upgrade the plugin component as models improve, without having to wait for an upgrade to HPOP.
Numerical propagation of ephemeris involves many interacting software components: a numerical integrator that solves the ordinary differential equations, a force model that evaluates the environmental forces on the satellite, coordinate frame transformations, ephemeris generation for planetary objects, etc. The force model itself is comprised of several independent constituents: a gravity field, a solid tide model, an ocean tide model, drag, solar radiation pressure, general relativistic effects, etc. These in turn may require further computation of atmospheric density, solar intensity, and other quantities. HPOP provides all of these capabilites (and more) already as a COTS tool.
For the TDRS spacecraft, only the generic SRP model provided by HPOP needs customized modeling. All the other aspects of HPOP's numerical propagation are sufficient for high fidelity. You can create an HPOP Force Model plugin that implements just the SRP computation. In fact, a TDRS SRP model is one of the provided samples. It accounts for the attitude of appendages and even specular and diffuse reflectivity effects.