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Electronics and Sensors

101

4.5 GPS

Accurate positioning is crucial to the operation of an autonomous outdoor robot. The

mechatron must have knowledge of its position relative to the goal location or

positioning of known obstacles in order for it to carry out tasks. One form of sensor

available to achieve reliable, absolute positioning is the Global Positioning System

(GPS). GPS is a popular localization tool for navigation of mobile robots due to the

constellation of orbiting satellites providing world-wide coverage. Relative

positioning using odometry alone produces poor results accurate to only 5 – 10 m for

straight line motion. The errors accumulate due to track slippage and uneven terrain

requiring the implementation of an absolute positioning system such as GPS.

4.5.1 Background

GPS is an outdoor positioning system designed, financed and operated by the U.S.

Department of Defence (DoD). The system was developed to provide continuous,

worldwide 3D positioning to the full range of military platforms at sea, land and air.

Civilian use of GPS currently far exceeds military use despite the project origins

however, as it is still operated by the DoD they have the ability to restrict performance

at any time. Despite this limitation there is substantial innovation in civilian GPS

technology and applications some of which are aimed at overcoming some of the

current constraints applied by the operators.

GPS positioning works on the mathematical principle of 3-dimensional trilateration.

This method uses the distance between the receiver and satellites to create an

imaginary system of spheres around the satellites. The intersection points of the

spheres represent the receiver location therefore at least 3-sattelites are required for a

3D fix. The receiver determines the locations of the satellites and distance to them by

analyzing high frequency, low power radio signals sent by the satellites. A long

digital pattern called a pseudo-random code is transmitted by the satellite which is

compared to an identical signal generated by the receiver. The phase shift represents

the signal travel time and therefore the distance (by multiplying by the speed of light).

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Recon Outdoors Talon IR Spécifications Page 121