On ELT User and Standard Location protocols

Wondering if anyone has had to travel this learning curve?

I have found that….

Regarding installing an ELT which has inbuilt GPS or can be fed GPS data (like the Artex 345). Avoid having the unit programmed to use a "User" protocol. Use a Standard protocol instead. You can tell which from the very first bit of the 15 digit hex ID. If this bit is 1 then a User protocol (i.e. the first hex digit 8-F), if 0 then a Standard protocol (first hex digit 0-7).

The problem is the User (location) protocol conveys GPS positional data only to 4 minutes of precision (4nm at the equator), which is not much better than a totally non-GPS enhanced beacon. By contrast the Standard protocols convey GPS positional data to 4 seconds resolution, just a few 10s metres, doing much more justice to the GPS enhancement.

This is such a contrast in performance (60x) that in the Australian context and maybe elsewhere the registration of such User protocol programmed beacons "with GPS enabled" is actually blocked, at least with the beacon model I was registering (Artex 345 - GPS enabled). The registration would proceed if I chose the beacon model to be Artex 345 - non GPS. The reason for the blockage is not made clear in any documentation, but from the above the reason for doing this is clear.

I have confirmed with the AMSA (Australian Maritime Safety Authority) that the local COSPAS-SARSAT receive infrastructure supports all protocal formats. I take this to mean that even the lower resolution GPS information in the User protocol transmission is collected and used - forwarded to SAR personnel - even if I were to select the beacon model to "Artex 345 - non GPS". AMSA stated that any information recorded in the registration is only used for "additional intelligence".

In any event I am now using the Standard location protocol variant that holds the 24 bit aircraft (transponder) address. I was able to correct this issue before my ELT unit was shipped out from the UK, where it was programmed. In Australia the hex ID starts with 3EE... The previous problem hex ID went as BEE...

The attached images are from the document.

"SPECIFICATION FOR COSPAS-SARSAT 406 MHz DISTRESS BEACONS C/S T.001 Issue 4 – Revision 7 March 2021"

The very first bit of the Hex ID (here called bit 26 due to preamble) indicates User (Hex ID begins with 8...F) or Standard (Hex ID begins with 0...7) protocols.