Absolute or relative precise orbit determination (POD) is essential for many low Earth orbit (LEO) missions and typically relies on onboard global navigation satellite system (GNSS) measurements. Absolute POD is commonly based on an ionosphere-free combination, while integer ambiguity resolution (IAR) requires external GNSS satellite phase bias products. Relative POD generally uses double-differenced observations, which amplify observation noise and eliminate parameters on which dynamic constraints could otherwise be imposed. Based on undifferenced and uncombined (UDUC) observations, this contribution proposes a model for both absolute and relative POD. Ambiguities of common-view satellites are constructed in double-differenced form, so IAR can be achieved without external satellite phase bias products; multi-frequency scenarios and frequency-separated residuals are also supported. Evaluation using onboard GPS observations from T-A and T-B formation-flying satellites shows 2.8-3.8 cm three-dimensional consistency with reference orbits, reductions of 16.3% and 10.6% over ionosphere-free POD for T-A and T-B, and relative-orbit consistency of 1.1-1.5 mm.