Looking for good papers and open-source codes for ephemeris prediction (long-term orbit determination)

Hi there,

We would like to predict the ephemeris for a relatively longer period,
say 3 days or 5 days. We expect within this period the accuracy of the
predicted ephemeris should be better than Almanac, close to the
broadcast ephemeris. Could anybody please recommend us some papers?
Open-source codes are highly appreciated as well.

Thank you.

Johnsonn