Undifferenced Ambiguity Resolution Applied to RTK

The Phase Lock Loops of a GNSS receiver can provide very precise carrier phase measurements that can potentially be used for positioning. However these measurements are inherently ambiguous since they include an unknown integer number of carrier cycles referred to as the carrier-phase ambiguity. To estimate this ambiguity, important biases have to be removed from carrier phase measurements such as atmospheric delays, satellite clocks and orbits errors... There are two ways to do this: (1) to difference the observables from the user receiver (or rover) with the measurements from a reference receiver that is spatially close in order to remove common biases, (2) to remove the biases directly by either using a linear combination between observables, or estimating them or obtaining their values from an external source. The first technique is the basis for Real-Time Kinematic (RTK) that uses at least 2 receivers to estimate the differenced carrier-phase ambiguities. The second technique is the basis for Precise Point Positioning (PPP) that estimate the receiver coordinates, the zenith tropospheric delay and the carrier-phase ambiguities from an ionosphere-free carrier phase combination using precise ephemeris. Ambiguities can be estimated either directly as integers if the residual measurement errors are small compared to the carrier wavelength or as floats if this is not the case. Once the ambiguities are estimated correctly, carrier phase measurements can be used as unambiguous measurements and the position can be determined with a very high precision, usually at centimeter-level. In this paper, it will be shown that estimating undifferenced carrier-phase integer ambiguity using a PPP filter on the reference station can help RTK positioning of a road user. First, a new technique allowing a seamless switch from RTK positioning to PPP positioning will be presented. The capacity of this technique to keep sub-meter accuracy when the communication link required by RTK technique is no longer available will be underlined for both single-frequency and dual-frequency users. Secondly, it will be shown that estimating ambiguities on the reference receiver and broadcasting them to the rover can be used to improve the accuracy of the RTK float ambiguity solution, resulting in a higher ambiguity resolution success rate.