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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 ?

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    \$\begingroup\$ You try to reinvent DGPS, I would recommend reading this en.wikipedia.org/wiki/Differential_GPS but for 1 cm accuracy, phase differences of the carrier wave may be necessary. \$\endgroup\$ – Uwe May 26 '17 at 9:29
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    \$\begingroup\$ @FakeMoustache 3mm + 0.1 ppm RMS accuracy in high precision static survey mode. trl.trimble.com/docushare/dsweb/Get/Document-625158/… Oh and GPS L1 is 1.57542 GHz not 1.8 GHz, other bands are lower frequency. Glonass G1 is the highest any GNSS system uses at around 1.61 GHz at the high end of the band. \$\endgroup\$ – Andrew May 26 '17 at 10:54
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    \$\begingroup\$ Simple question is why? \$\endgroup\$ – Andy aka May 26 '17 at 11:22
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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.

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