MiniTurtle:
I thought that the distance measurement was a built in feature in the RF Link device that I bought.
There are probably chips out there that can do this (smartphones can get a rough fix on their location using only cell towers, but not to 1m).
How would you suppost I could do this? Sonar?
Yes, sounds travels slow enough to measure with an Arduino. There are a variety of ultrasonic sensors on the market, such as the Parallax Ping sensor (among others). Dunno if you can get 20m range out of one, though -- they tend to be quite directional, so you'd have to aim it.
How about using a Raspberry Pi to do the calculations (with 700MHz to 1GHz)?
Hm. You'd probably have to boot the Pi into kernel mode (no operating system). But first you need to do some math: how long does it take light to travel one meter? The time your CPU takes to execute one instruction needs to be considerably less than that.
Accuarcy must be < 1m or it's not going to work for me.
If this is going to cost more then 200€ i'm going for another solution (not distance measurement).
Well DGPS (Differential GPS) can do it, but I think a DGPS transmitter is out of your budget.
One possible option is to use 2 or more light emitting (IR, visible, or UV) beacons and use a pair of directional sensors on your craft to get a bearing on each. (Each beacon will need to take turns and transmit a unique identifier.) Getting a bearing on something is much easier than time-of-flight. A little math can tell you how far away you are from the pair of beacons (if the beacons are a known distance apart).
You can add a third beacon and additional sensors on your craft to get a 2-dimensional fix (latitude and longitude). A 4th beacon up high could be added in order to get an altitude fix as well.
Most of the complexity of this solution is in the software -- your beacons just need to transmit a unique pulse at given intervals, and your sensors just need to be able to get a bearing using the relative intensity between sensor pairs. The rest is just math.
