Where am I? Sensors and Methods for Mobile Robot Positioning

80 Part I Sensors for Mobile Robot Positioning
Table 3.6: Summary of data analyzed (Courtesy of [Byrne, 1993].)
Receiver
Magellan
Magnavox GPS Engine
Rockwell NavCore V
Magnavox 6400
Trimble Placer
Data Gathered
Latitude, longitude.
Number of satellites used - implies navigation mode (none, 2-D, or 3-D).
Latitude, longitude.
Navigation Mode (none, 2-D, or 3-D).
Latitude, longitude, navigation mode (none, 2-D, or 3-D).
Number of satellites tracked also available from raw data.
Latitude, longitude
Number of satellites tracked.
Latitude, longitude.
Navigation Mode (none, 2-D, or 3-D).
Other performance factors include the amount of filtering in a GPS receiver. Excessive filtering
reduces the amount of variance in the position and velocity data, but also slows the response of the
receiver. Excessive filtering will cause a receiver to output incorrect positions when starting,
stopping, or turning sharply. In applications where the GPS data is processed off board and needs
to be transmitted via RF-link to a central computer, this type of error is not very important because
the delay introduced by the communication link will probably be much greater than the delay
introduced by filtering in the receiver.
Parameters that were not analyzed in the Sandia experiments are velocity and heading accuracy,
because in Sandia's application (and many other typical mobile robot navigation tasks) accurate
velocity information was already available from odometry. Heading information that would be
required for dead reckoning is not needed while GPS is functional.
Another easy-to-measure performance criterion is static position accuracy. This parameter was
measured by placing the GPS receivers at a surveyed location and taking data for approximately 24
hours. Although in typical application the receivers are moving most of the time, the static accuracy
does give a good idea of the receivers' position accuracy capabilities. The parameters measured and
the performance insights gained from these measurements are summarized in Table 3.7.
In summary, the GPS testing performed for this project consisted of storing position and
navigation mode data from five different GPS receivers for both static and dynamic tests. The static
testing provides information about the static position accuracy as well as the sensitivity of the
receiver and antenna if DOP switching is taken into account. The dynamic testing mostly provides
information about the receiver/antenna sensitivity and the receiver's ability to recover from
temporary obstructions (taking into account DOP switching). The dynamic testing also provides
some qualitative information about position accuracy by comparing plots of the data points from the
various receivers.