I am searching for a simplified model of EGM2008 that can be used for rapid, repetitive computations. It needs to be valid only up to GPS altitudes. I'm thinking such a model already exists but cannot find it and would prefer not to reinvent the wheel. I'm aware of the math, so I could make such a model if I had to do so but would prefer to use a community model. Suggestions?
We recently developed a web API that provides access to the EGM2008 model, as implemented by the open-source GeographicLib Gravity library.
Geoid undulation values calculated using the API agree with corresponding values generated by the official EGM2008 harmonic synthesis program to within 1 mm. The API is open and available here in case that satisfies your needs.