I wish to get to high precision distance between 2 not moving points.
e.g. 1 cm accuracy for distance 1 km
I think if I will catch same GPS signal in two points then distance between resulting coordinates would be highly precise even if coordinates would not be precise.
Not sure how to catch same signal frame on both devices.
May be GNSS-SDR?
I remember there was project syncing 2 ublox GPS receivers but can't find either.
Any idea how to measure distance precisely ?
How to get 1 cm relative accuracy with GPS:
Buy a 1 cm accurate GPS receiver.
For that level of accuracy you need an RTK (Real Time Kinematic) receiver. A good system will give you 1 cm accuracies without any averaging. It will also give you a very large bill to pay, those systems aren't cheap.
Best bang for you buck would probably be something like the ublox NEO-M8P, that datasheet gives an accuracy of 2.5 cm + 1 ppm (that's 1 ppm of the distance between the base and the rover) so around 2.6 cm total for your distance.
If you average that for a while the average should be getting down to your 1 cm target accuracy.
