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I am trying to take two drones in an open field and have them know precise and accurate relative positioning of each other. I don't need them to actually know where they are. My plan is to use some cheap Allystar TAU1308 RTK GPS modules. Would the lack of a need for absolute location and only relative positioning that I can do without a base station and still achieve the TAU1308's RTK accuracy?

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  • \$\begingroup\$ The drones would need to communicate directly with each other. \$\endgroup\$
    – Andy aka
    Oct 19, 2023 at 19:57

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Yes, "moving baseline RTK" is "a thing", and is commonly used for exactly the purpose you describe.

Here is a link from Trimble on the subject.

Essentially, as with normal RTK, one of the devices runs in base station mode transmitting carrier phase info (typically in RTCM format), and the other runs in rover mode receiving the phase info, and using it to correct its own solution.

The key difference is the contents of the RTCM 1005 messages, which give the base station location in ECEF cartesian coordinates.

  • In the normal case, the base station periodically transmits the precisely surveyed fixed position (typically generated with PPP).

  • In the mobile base station case, it sends its current best location estimate, derived using normal GPS. These messages are typically at a much higher rate, as the base station location is constantly changing.

To a first order, both cases give you a precise geographic offset between the two devices, but the accuracy of the absolute locations are clearly dependent on the accuracy of the base station reference position.

It is also worth noting that for maximum precision:

  • the rover RTK logic needs to be aware that the base station is mobile, so it can interpolate base station location between 1005 messages.
  • you need a dual frequency L1/L2 antenna.

Finally, if you have more than a couple of drones, it may be better to use a fixed (but only roughly surveyed) base station transmitting corrections to all the drones, rather than having one of the drones acting as a base station. This allows the base station to have a high quality (but larger and heavier) GNSS antenna, a more powerful transmitter, and minimizes the single point of failure if the "master" drone is lost or its antenna is obscured.

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Yes. This is how dual antenna Marine GPS Compass works.

TAU1308 only supports L1, Single L1 RTK is hard in real-world and not reliable. GPS module need to see 5+ satellites with signal to noise better than 38 db.

TAU1302 is slightly more expensive but support L1/L2 or L1/L5, is much better.

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