Design me a Product - Differential GPS

We require a differential GPS solution that has got 2 components: 1. A base station that is placed at a fix position. It contains a GPS unit and a 915MHz radio. The base station transmits correction messages constantly within at least a 10km radius. The base station is powered by battery and/or solar panel. 2. One or many GPS units mounted on vehicles (rovers) that run within the 10km radius of the base station. They contain a GPS unit and a 915MHz radio that receives the correction messages from the base station and passes on to the GPS unit. GPS receiver is connected to a computer which does not need to be supplied. Requirements: - Rovers achieve at least 10cm accuracy, ie. repeatability (repeatability means that a rover placed at a position today giving GPS coordinates, and then placed at exactly the same position in the future, these GPS coordinates will be within 10cm of the original GPS coordinates - Rover GPS produce an NMEA output stream via USB - Rover GPS / radio antenna have got a magnetic mount and a cable length of at least 3 metres - Base station surveys its position within 15 minutes after power up. We have an option to manually set the GPS coordinates of the base station, instead of the survey option. - The environment in which the system operates is open field / open sky with few obstructions, eg. some trees - Any hardware exposed to weather must be weather proof, ie. GPS and radio antennas. Rover GPS receiver can be in the vehicle and Base station GPS receiver / battery can be housed in a box. - Hardware must be easily sourced on the open market, eg. uBlox GPS units. - Hardware costs is to be less than $700 USD. Detailed requirements can be finalised upon contractor engagement. NOTE: If you are a consultant or agent who wants to act as a middleman, please do not apply. We are only interested in contractors who have the required experience and skills, and who can design the system themselves.