Performance analysis and integrity aspects of Tight Optical Integration (TOI) with GPS

This paper evaluates the performance of the tight integration of GPS and optical sensors such as digital vision cameras. The paper furthermore addresses certain integrity aspects of the proposed Tight Optical Integration (TOI) method. Target applications for the proposed system include the operation of UAVs in an urban environment for both civil and military applications. A regular unaided GPS position solution requires at least four satellites. However, in an urban canyon it is likely to have fewer than four GPS satellites at certain locations due to satellite signal blockage and degradation. Augmentation of GPS with measurements from other sensors is thus required for a reliable navigation capability in an urban environment. TOI integrates range measurements from the available GPS satellites (usually less than four) with feature data from a regular camera to form position estimates. If a building blocks a GPS satellite, markers or features on that building can be located and used as an “optical pseudolite.” Unlike pseudolite or laser range scanner measurements, no range information is required from the optical sensor. Instead, the TOI algorithm takes the estimated azimuth and elevation of the marker to form an equivalent direction cosine for this marker. Then, these direction cosines are combined with range measurements from GPS satellites in a weighted least square solution. The current implementation of the TOI method forms linearized camera equations together with the linearized GPS equations and solves for the solution iteratively similarly to a regular GPS only solution. Inertial sensors are used to transform direction cosines from the camera frame to the GPS frame.