Precise kinematic positioning using single frequency GPS receivers and an integer ambiguity constraint

In this work a system is proposed which will provide carrier phase accuracy positioning and two axis attitude measurements using only three low cost, single frequency, GPS receivers. Such a system will find application on platforms where minimal cost, weight and power consumption is critical; for example the mini-unmanned aerial vehicle (UAV). The six state measurements provided by the system may be used in the georeferencing of other sensor data or in the navigation of the platform itself. This contribution aims to increase the success rate of the integer ambiguity resolution process when positioning the platform by utilising knowledge of the double differenced integer ambiguities obtained in an attitude determination system. The robustness and 'time to solution' of the system will thus be improved. It is well known that the use of baseline length or geometry constraints in the attitude determination environment may significantly increase the integer ambiguity success rate, particularly where low cost, single frequency receivers are used. It has been shown that such constraints may increase the success rate to a level approaching 100%. Such constraints have been proven to allow the low cost GPS receiver to become an effective attitude determination system. In this work a two receiver attitude determination system is used with a third base receiver to provide both two axis attitude and position measurement. Knowledge of the double differenced integer ambiguities from the attitude determination system is used to reduce the number of candidates during the search for the integer ambiguities arising when the third receiver is included. When these ambiguities are resolved the unknown baselines between the roving (attitude) receivers and base receiver may be determined and the relative position obtained. The constraint in the positioning stage reduces the effect of uncorrelated errors between the two roving receivers. The benefits of this are observed primarily when using low quality receivers with considerable Gaussian noise exacerbated by exposure to highly dynamic environments. It is likely however that in most applications the baseline length between the two roving receivers will not allow the decorrelation of the multipath error and consequently this will still have a detrimental effect. Real world performance of the algorithm is examined by comparing the low cost system to a high grade Novatel SPAN integrated GPS/INS installed on a vehicle test bed. Tests are performed in an area with good sky view and within 3 km of the base station.