AnalysIR brought up some good points. Here are a few additonal considerations.
- What is your required accuracy?
- Are there going to be multiple vehicles (or whatever) on the track?
The answer to these questions will determine whether or not the use of GPS and IR/laser sensor will be sufficfient.
If there is a single vehicle, an IR/laser will be sufficient, and will provide better accuracy than GPS, which will then be unnecessary.
If more than one vehicle, you are restricted to the accuracy of the GPS. Why? Because you won't know if another vehicle broke the beam to the light sensor. The consequence of this is that the light sensor becomes useless for anything other than detecting a "start" condition, and then only if the beam can be pointed in such a way as to detect ONLY the vehicle being measured.
So, let's look at GPS as a measuring device for this purpose. You speak of trigglering only by one coordinate. This would be fine if the track is not a closed loop, and if other parts of the track do not cross the starting value of the coordinate being measured. (oval, circle, 'S'-shaped, etc.)
So, you will need to check two coordinates. One can be single coordinate, and the other will need to be a range of coordinates. Assuming your coordinated are, for example, a single longitude (say 82.00000), and a range of latitudes (say 42.01344 to 42.01339), this will give you a position somewhere on the start/finish part of the track( assuming that falls within the latitude range), at a particular longitude. You can, of course, use a single latitude and a range of longitudes. Alternatives could be (1) a single coordinate that you can test for being between the start/finish line and any other part of the track, or (2) a continuous measure of speed, and calculated distance traveled, which could be used to tell if the vehicle is approaching the line.
The next place you run into problems is if the start/finish line is not oriented exactly north-south or exactly east-west. So in this case, you would need to calculate the arrival at a box or a single line that defines the finish line.
As for GPS accuracy, a single, fixed GPS that is near the start/finish line and that can provide error signals to the vehicle or to the receiving station, would considerably improve the accuracy. After all, you don't need absolute position, only positions relative to the start/finish line.
Sounds like an interesting project, and is definitely challenging.