sources and provide real-time image transfer to system. Various components of the

imaging board are shown in Figure 3-3. The imaging board features twelve software

selectable input channels to switch between twelve composite video signals. It accepts an

external trigger output and can operate in next valid frame/field mode. The imaging board

provides an auxiliary power supply unit that could be used to provide power for a camera.

The imaging board features 4 Mbytes of video transfer memory for temporary frame

storage to minimize loss of data during long bus-access latencies found in heavily loaded

computer systems.

3.2.2 DGPS Receiver and Encoder

 A Differential Global Positioning System (DGPS) receiver (model: AgGPS 132,

Trimble Inc., Sunnyvale, CA) with a Coast Guard Beacon antenna was used to locate the

position of an image in the grove. The DGPS receiver was configured for a 5 Hz

refreshing rate. An incremental encoder (model: CI20, Stegmann, Dayton, OH) was

attached to the small frame to measure the amount of distance traveled in the grove

(Figure 3-4). The encoder has a resolution of 4048 pulses per revolution. A 12-bit

multifunction PCMCIA I/O board (model: DAQCard-Al-16E-4, National Instruments,

Austin, TX) was used to acquire and count pulses from the encoder.

 The encoder was calibrated before the actual experiment in the citrus grove. Pulses

from the encoder were read for known distances three times and the average of those

values were taken to be the encoder output for that particular distance. Two channels

were read from the encoder and the phase between the channels helped to identify

whether the wheel was moving in the forward direction or in reverse direction.