Airborne or unmanned aerial vehicle (UAV)-borne interferometric synthetic aperture radar (InSAR) can offer topography and forest measurement with high accuracy and high resolution. In these measurements, the conversion between height and interferometric phase is frequently involved. The existing conversion approximations may be more suitable for the space-borne InSAR. However, for the airborne or UAV-borne InSAR in a low flight altitude, these conversion formulas cannot work well for topography and forest height measurement. To achieve high-quality InSAR measurements, we derive a refined conversion formula based on the Taylor expansion, i.e. Firsec-order conversion formula. Differing from existing studies contributed by first-order derivative, the Firsec-order conversion formula expands a second-order term. During refined phase-to-height (PHAtoH) conversion, an external low-precision digital elevation model (DEM) is introduced to calculate and then remove the second-order phase from the original interferometric phase. Thus, the high-precision DEM is reconstructed based on the refined phase rather than the differential phase. In addition, this article investigates the sensitivity of the second-order term to different baseline lengths and flight altitudes with respect to two common baseline configurations in airborne or UAV-borne InSAR. The results show that for the horizontal baseline configuration, the second-order term must be considered. In addition, the longer the baseline and the lower the flight altitude, the larger the second-order phase, especially for the UAV-borne InSAR geometries with altitudes of under 2 km. However, for the vertical baseline configuration, the adoption of the refined formula depends on the actual situation. The performance test shows that the Firsec-order conversion formula has much higher conversion accuracies than traditional ones.
