This paper presents the development of a land vehicle navigation system that provides accurate and uninterrupted positioning. Ground speed Doppler radar and one MEMS gyroscope are used to augment differential GPS (DGPS) and provide accurate navigation during DGPS outages. Using Doppler radar has advantages of easy assembling and lowcost maintenance compared to wheel encoders. The Doppler radar and gyro are calibrated when DGPS is available. Loosely coupled Kalman filter gives optimally tuned navigation solution. Field tests were carried out to evaluate the performance of the system. The results show that position accuracy of 1.5 meters can be achieved during 15 seconds DGPS outages.