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Encode Mode
On power-up the VED begins searching for either GPS input or edge-coded video input
to determine whether to enter encode or decode mode. If GPS data is detected the VED enters
the encode mode and begins inserting alpha-numeric data and edge-code data into the video.
Once the encode mode is entered, removal of GPS input will cause the warning "NO GPS IN" to
be displayed. If GPS input is present but the receiver is not tracking, the warning
"ACQUIRING" will be displayed. If neither warning is displayed, the GPS data is valid.
As a minimum, the VED requires the $GPRMC message in RS-232 form (4800 baud, 8
data bits, 1 stop bit, no parity) to be input from an external GPS receiver. If the $GPGGA
message is also present, receiver status and altitude will be added to the display. If a pulse-per-
second is available from the GPS receiver, precision time will be displayed to the millisecond,
otherwise time will be displayed to the second. When millisecond time is displayed, it is
sampled at vertical sync and updated on each video field.
Assuming that the $GPRMC and $GPGGA messages are input and the receiver is
tracking, the display will be as follows:
Bottom line from left; serial number, latitude, longitude, speed, course.
Top line from left; date, time, hour offset, receiver status, altitude.
The receiver status consists of the number of satellites separated by a "/" or a "*" from the
horizontal dilution of precision (HDOP). If more than 9 satellites are tracked the number of
satellites tracked remains at 9. A "/" indicates normal track, and a "*" indicates differential
track. The HDOP ranges from 00 to 99 and is an indication of the quality of the position fix
based on geometry. Smaller numbers indicate higher quality, with 00 or 01 being excellent.
To access the internal switches and adjustment, disconnect power and remove the two
screws from the rear connector panel. Slide the rear connector panel and attached board out a
few inches to gain access to the switches. The potentiometer adjustment is factory set and
determines the width of the lines of displayed data. Adjustment is correct when the lower line is
fully visible and centered horizontally on the video monitor in normal scan. The four internal
switches control display options as follows:
Switch 1 determines the polarity of the hour offset. In the on position, hour offsets of -12 to 0 to
+3 are obtained by using the rotary switch. In the off position, hour offsets of 12 to 0 to -3 are
obtained.
Switch 2 in the on position displays data at the bottom of the video picture. In the off position
data is displayed at the top of the picture.
Switch 3 in the on position displays altitude in feet. In the off position altitude display is in
meters.
Switch 4 in the on position displays speed in MPH if switch 3 is on, and Km/H if switch 3 is off.
In the off position speed is displayed in knots regardless of the position of switch 3.
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