I have worked with a company for the past year that implemented a tracker device on their service vehicles.
Within 3 months, several engineers had left, Not because they were mediocre or slackers but because of the managements attitude and sudden lack of give and take.
What they did was print out the tracker report every morning ( and still do ) then see if it corresponded with the timesheets.
This did not allow for any rounding off to the quarte hour or show that lunch was not taken.
Engineers were then subjected to questions relating to time it took to get to site, actual time that the customer signed the job sheet to the van leaving the site and even stopping to go for a bodily function.

I personally have nothing against a GPS if it is used correctly and with a bit of logic but I have found that a lot of companies promote the least productive engineers into management positions where they then look at everyone else as being work shy like they were as an engineer.

This normally results in the most productive, packing tools and leaving.