DME maximum ranging

[handbag]

The FAR/AIMS states that maximum DME range is 199 miles. Other info tells me that line of sight is max, ie 300NM plus. Being a bottom feeding heli pilot, I never get the chance to get high enough to find out. Are US DME stations restricted to prevent a readout of >199NM, hence the quote in the AIM? Sorry if it's been posted before, but didn't have any luck with the search function

[Intruder]

I've seen DMEs above 260 NM.

[kenparry]

Years ago I saw a reading over 300 nm on an analogue DME display on a B737-200. We had just departed Porto Santo after a fuel stop on the way to Tenerife, before the Madeira islands had any VOR or DME, so we tuned Tenerife North VORDME before takeoff. At about 5000 ft the DME came in, a good lock with ident, and we verified that the reading was about right from checking the airways charts. Obviously not line of sight - there must have been some ducting at work.

The ground station cannot "know" how far away the interrogating aircraft is - it just sends a response to the interrogation. The timing & range calculation are done on the aircraft, so surely any range limit would come from the aircraft kit?

[Wodrick]

The proof of this is lost in the mists of time as it is 40 years ago now but the range limitation comes from the PRF (pulse repartition frequency) of the Aircraft equipment. The DME seeks a response to a pulse pair, when the new pair is transmitted the first one is ditched. This is really scraping the cell but I seem to remember search prf of about 100 or so dropping to about 25 when locked, this may well be wrong and have changed considerably with technology since I did DME in 1970

[Old Smokey]

I think that you'll find that the FAR/AIMS 200 mile limit applies to the maximum distance that DME may be used for position fixing. DME may well be functional beyond 200 miles (I've seen much larger than 200 nm distances also), but not "legally" useable for position fixing.

The Australian regulations state 180 nm as the maximum DME for position fixing, so, in this case, the Regulatory Authority has spelled out the reason why. (They, too, accept that DME may well be functional beyond this limit).

Regards,

Old Smokey