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When I Googled it seems it is not common to use Vortac like systems from UAVs as a backup to GNSS outages, like in manned aircraft. And also I haven't seen any equipment designed for just UAV platforms, it's all designed for in-cockpit use.

Why do UAVs not use ground-based navaids?

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  • $\begingroup$ There are at least three reasons. First, UAVs don’t fly on airways, so VORs aren’t particularly useful. Second, is size. GPS receivers and antennas are smaller than a pack of gum. A VOR antenna is at least 12 inches long, and you need two branches. Plus the receivers weigh a lot. Third is programmability. GPS receivers contain location, altitude, and speed data in digital form that can be transmitted to the controller or used for autonomous control. $\endgroup$
    – JScarry
    Oct 3, 2017 at 17:28
  • $\begingroup$ TACAN is a military version of VOR. It also transmits a DME signal that is used by both military and civilian pilots. I don’t know anything about the antenna size and receiver weight but I’d imagine that they are similar to VORs. $\endgroup$
    – JScarry
    Oct 3, 2017 at 18:05
  • $\begingroup$ @JScarry, a VOR antenna is normally a 1/4 wave horizontally polarized antenna. In most aircraft each 1/4 wave is mounted on either side of the vertical stabilizer. But with one VOR receiver, one would only need one antenna. The actual VOR electronics can be rather small, but it has not gone through the mass production and miniaturization that GPS has, and the production volumes are low, so it won't get much more enhancement. $\endgroup$
    – mongo
    Oct 4, 2017 at 14:45
  • $\begingroup$ @mongo Now that I think about it, I do have a handheld aviation radio that has a VOR receiver and display in it. With the battery it weighs less than a pound (.4 kg) and the antenna is about 6 inches (150 mm) long. So someone probably could make a VOR receiver for UAVs if there was any demand for them. $\endgroup$
    – JScarry
    Oct 4, 2017 at 16:00
  • $\begingroup$ @JScarry, as indicated in my answer, the service volume, the spatial resolution and the repeatability of VORTACs is inconsistent with the mission of domestic sUAS. I can buy GPS with antenna, USB interface and WAAS, GLONASS, etc. for about $18 in small quantities. The VOR handheld receiver separated out would be much more, and it doesn't come close to meeting the use cases for a GPS in a sUAS. The handheld antenna is likely a wound 1/4 wave, which results in some loss. Do you get a VOR signal on the surface from where you would launch a sUAS? Probably not. $\endgroup$
    – mongo
    Oct 4, 2017 at 16:34

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For UAS, the aircraft may be below the service volume of a VORTAC most of the time. Furthermore in the US sUAS are normally line of sight to the pilot so in theory if GNSS is unavailable, the pilot will control visually. Finally the variability and uncertainty of VORTAC is high enough so as to make it impractical for UAS stability, drift and position control

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