Method and apparatus for reducing the contributions of ionospheric propagation time delay error, multipath signal error and receiver noise error from signals received in a Satellite Positioning System (SATPS) from one or more SATPS satellites by formation and appropriate filtering of differences...http://www.google.co.uk/patents/US5796773?utm_source=gb-gplus-sharePatent US5796773 - Compensating for multipath errors, ionospheric time delay and noise errors in differential satellite positioning systems 
