Objectives  Considering the advantages of low cost, simple algorithm model and high accuracy, the ultra-long baseline positioning technology was widely used in the fields of crustal deformation monitoring and high-precision timing. However, there are still many factors that may reduce the accuracy of the ultra-long baseline solution, such as satellite orbit error, tropospheric delay error and solid earth tide, etc.

  Methods  In this contribution, characteristics of various errors and bias (such as broadcast ephemeris orbit error, tropospheric delay error, earth tide and phase windup) were analyzed based on ionosphere-free combination model. The rule of error changing with baseline length was analyzed using single difference observations of long(ultra-long) baselines (146-1 724 km).

  Results  The results show that broadcast ephemeris error could be neglected while baseline is less than 500 km. When baseline length exceeded 500 km, precise ephemeris was needed to reduce the influence of orbital errors on positioning. The accuracy of the zenith tropospheric delay calculated by GPT2+Saastamoninen model ranged from centimeter to decimetre, and the method of simultaneously estimating zenith tropospheric delays at both stations could provide accuracy of 1-2 cm. Compared with the horizontal direction, the vertical direction of long baseline was more significantly affected by solid Earth tides which must be corrected in the solution of long baseline. Phase wind-up error could be neglected while baseline is less than 2 000 km.

  Conclusions  The experimental results of five long(ultra-long) baselines (146 km, 491 km, 837 km, 1 043 km and 1 724 km) solutions were presented using estimating zenith tropospheric delays at both stations of baseline. While baseline is less than 500 km, the positioning precision based on broadcast ephemeris is better than 0.05 m and 0.08 m in horizontal and vertical direction, respectively. While baseline is less than 2 000 km, the positioning precision based on ultra-rapid precise ephemeris is better than 0.025 m and 0.055 m in horizontal and vertical directions, respectively. The initial convergence time of the baseline solution decreased as the lengths of baseline increased.