Abstract: An altitude aiding positioning algorithm not constrained by altitude was proposed for the regional navigation satellite system (RNSS). The algorithm does not require construction of the approximate ellipsoid equation or the preceding position information. The key point of the algorithm is iteratively improving the virtual range from the earth center to the estimated position in processing of Newton’s method. The virtual range is combined by the measured altitude with the approximate latitude and longitude.  In order to evaluate the performance of the proposed algorithm, a simulation analysis was performed based on the Chinese area positioning system (CAPS) in the verification phase. And the results show that the proposed algorithm can provide positioning at any altitude level from the ground to medium earth orbit (about 12 000 km).With the analysis of the dilution of precision matrix for the altitude aiding algorithm, formulas of the error ellipse’s axial length and direction were proposed. The simulation results show that the formulas can describe the error ellipse geometric characteristic accurately.  

Key words: three-satellite positioning, altitude aiding, positioning algorithm, Chinese area positioning system, error ellipse