White Paper Phase biases for ambiguity resolution: from an undifferenced to an uncombined formulation

Integer ambiguity fixing has been recently applied to undifferenced GPS phase measurements to achieve precise positioning or PPP-AR. This technique relies on the estimation of specific phase biases combination identification and application. It works for dual-frequency GPS receivers. However, this phase biases representation is not suitable for triple frequency receivers, as the number of possible combinations for the biases can be dramatically high. This paper presents a new uncombined formulation for phase biases, as proposed in the RTCM SSR standardization framework. The validity of such a representation for triple frequency receivers is demonstrated. In order to validate the proposed model, a technique to compute such phase biases is presented. Using real measurements, we show that the integer ambiguity nature of phase measurement is valid under the new uncombined formulation at least for all the ionosphere free combinations whose noise is small enough. 1) INTRODUCTION Integer ambiguity fixing is now routinely applied to undifferenced GPS phase measurements to achieve precise positioning. Some implementations are also available in real-time [5]. This implementation allows performing PPP with ambiguity resolution at the cm level. With the new modernized satellites capabilities, performing PPP with triple frequency measurements will be possible and therefore, the current dual-frequency formulation will not be applicable. There is also a need for a generalized formulation of phase biases for RTCM. In this RTCM framework, the definition of a standard is important to allow interoperability between the two components of a positioning system, i.e. the network side and the user one.