First off there is a very great deal of difference between being followed by a human and having your vehicle or other possession tagged by a GPS tracking unit.
Importantly the human should be able to testify that "the person" being followed was in the vehicle or not, where as the GPS can only log where radio signals tell it, it's antenna is in three dimensional space and time, if and only if it can receive the signals directly from the satellites.
Thus the GPS provides NO EVIDENCE of a persons movments only where the GPS "RESOLVES" it's self to be. As we know GPS can be "spoofed" and it can be "relocated" onto another object.
The prosecution would therefore have to prove that,
1, The GPS had not been jammed or spoofed.
2, The GPS had not been relocated.
3, The GPS log was from the right device.
4, The GPS log was within "chain of custody" rules.
5, That the person was in/with the tagged object.
If the defence can show doubt on any of those points, then in an honest judicial process the evidence is not "beyond reasonable doubt" and it should not be used.
Now it is fairly easy to throw a bucket load of doubt into any system using either civil or military GPS alone...
As many will know GPSs does not work in many places such as "garages in/under houses", "multistory and underground car parks", "under some overhead power lines", "in some built up city/industrial areas"... Basicaly any where were the GPS suffers "Loss Of Signal" (LOS).
We also know that a GPS unit cannot tell the difference between LOS and "jamming" with Wideband Gausian Noise (WGN).
Thus, a switched on criminal could simply drive to some place known to cause LOS such as a carpark and whilst there turn on a GPS jammer and drive off. Provided they return to the same carpark before the jammer is turned off then the log shows the vehicle entering the carpark causing LOS, some time later the log shows AOS as the car leaves the same carpark.
So secondly, the GPS alone is not sufficient to say very much, it needs to be augmented in some way. And provided the criminal is aware of what the augments are they can take appropriate measures. The simplest being to just leave the vehicle in the car park and go catch some other form of transport whilst an accomplice goes "shopping" etc. However they also need to leave their phone etc with the accomplice, and remembering all this is the difficult bit...
Now the question arises as to how the criminal knows there is a GPS tag on any objects they associate with. All I will say (having gone trough it in the past on this blog) is look up a "Grid Dip Meter" or "Grid Dip Oscillator" it shows the prescence of "tuned circuits" at the selected frequency, and nearly all antennas for GPS units are effectivly "on frequency tuned circuits".
Thirdly and importantly the GPS does not provide any evidence as to what the person in/with the tagged object is doing. In the UK there was a case of a member of a local council who the police objected to joining the "oversight committee". After much back and forth the poice finally admitted that their objection was that his car registration had been recorded on successive Wednesday nights in a known red light district, therefore he was consorting with prostitutes. As it turned out he was having dinner with his sister who had the missfortune to have the road she lived in used by sex workers and their associates.
Humans unlike unminded objects are not always bound by Occam's razzor. In fact the opposit is true, a sutibly minded person can "game the system" the technolgy uses.
In London we have already seen this with gangs swapping clothing, mobile phones, travel cards etc. The simple fact is two people approximately the same hight wearing the same baseball cap, hoodie and sunglasses etc are very difficult if not impossible to tell appart reliably these days even with reasonable CCTV footage.
Importantly a gang also allows "ghosting" if you have six or seven people milling around at the bus travel (Oyster) card touch in point, it is quite possible for one of them to touch in twice with different travel cards thus one person has a "ghost companion" traveling with them. The person can also touch in for real and then under the cover of the gang slip out of the doors without it being recorded by the CCTV.
So GPS like most other "technical surveillance" measures is not realy of a lot of use against an aware and intelligent target, and can infact provide a clever criminal with support for an alibi...