That device is marine Loran location device for use on fishing boats.   It could get to you about 200' absolute accuracy, but as long as you kept "nudging" it, it was much better than trying to guess which 2.5 acre grid we were in.   

In 1988, GPS was only available about 12 hours per day, in 2D mode.   We lied to the DJ controllers by intercepting ground speed and generating a relatively different ground speed into it to force a rate change.  

In '90 or '91, we got one of Rawson's first variable rate seed controllers.