This project is concerned with determining the orientation of small Unmanned Aerial Vehicles (UAVs). To make commercial use of these aircraft in aerial surveying markets their attitude needs to be determined accurately and precisely throughout a survey flight. Traditionally inertial sensors have been used on larger aircraft to estimate both position and orientation in combination with Global Navigation Satellite Systems (GNSS). High quality inertial sensors have many downsides when used on the small UAV. They are expensive, power hungry and often heavy. Inertial sensors are vulnerable to vibration, high acceleration, high rotation rate and jerk. All of these are present on the small UAV. This work identifies GNSS attitude determination as a potentially suitable alternative to inertial techniques. It develops a novel method which uses this a priori baseline measurement to validate estimates of the carrier phase ambiguities.