GPS backup

DME limit is about 100 aircraft per transmitter. 252 frequencies for DME.

Stations are not huge, would not be hard to place them at other (cell, repeater, etc) transmitter sites.

And DME/DME navigation has been around for a long time. So not inventing anything new.
 
Pinecone said:
DME limit is about 100 aircraft per transmitter. 252 frequencies for DME.

Stations are not huge, would not be hard to place them at other (cell, repeater, etc) transmitter sites.

And DME/DME navigation has been around for a long time. So not inventing anything new.
Click to expand...

Yeah I didn’t want to look up the limits but was responding to those who said “just add DME/DME RNAV to everything”… as a backup to GPS”.

That’ll overload the available DME ground stations quick in some areas. Not really viable. I knew there was a fairly low limit — just by the fact that it’s a transponder based system.

Thanks.
 
typically, it's viewed as about 100 aircraft...

a bit of trivia, consider that overlapping interrogations aren't as much of a problem as you'd think. Imagine a DME ground station receiving the interrogation pulse pairs from two aircraft at exactly the same time. So what... It'll just look like a stronger (probably somewhat misshapen) interrogation and transmit one reply pulse pair... both aircraft will get to use that one pulse pair. And they have to be able to handle overlapping interrogations (the first pulse of a second interrogation arriving before the second pulse of the prior interrogation).

I vaguely remember the specification for the DME/P portion of MMLS had to be able to transmits upto 4000 pulses (pulses, not pulse pairs)... but that DME/P was only intended to support terminal area operations.

How many aircraft are within line of sight of high density places like Chicago or LAX?
 
denverpilot said:
Yeah I didn’t want to look up the limits but was responding to those who said “just add DME/DME RNAV to everything”… as a backup to GPS”.

That’ll overload the available DME ground stations quick in some areas. Not really viable. I knew there was a fairly low limit — just by the fact that it’s a transponder based system.

Thanks.
Click to expand...
A hundred planes in the service volume of a single DME station is a LOT of planes. :D

And a DME/DME nav system would be using closer DMEs versus ones far away. O'Hare has about 78 operations per hour on average.
 
Last time I bothered looking at the sensors page in the FMS, it was tracking DMEs 50-100nm away. Use an average of 75nm and that's 18 thousand square nautical miles that could be covered by one DME.
That would be a plane for 180 square nm, or a square 13nm on each side. I could see some busy areas having their DMEs overwhelmed.
 
The question would also be, how often does a DME/DME have to ping the DME versus the rate the DME does so?

Especially for GA, we don't move that fast that we need an update every second or two.
 
You need a minimum number of pulses and replies to ensure the DME is replying to you and not someone else.
During the initial search phase, the interrogator sends 150 pulse pairs per second, dropping to 30 PPS for the tracking phase.
A DME usually transmits 2700 PPS, regardless of the number of aircraft interrogating it. It will continuously adjust the receiver gain to achieve that pulse rate. In a low traffic environment, that mesns it will reply to random receiver noise, while in a high traffic environment that will cause it to ignore the more distant (weaker) interrogations.
 
The MMLS DME/P will transmit a minimum of 1000 pulses per second (referred to as squitter pulses).

Actually, I don’t know if there are any operational MMLS units are out there anymore
 
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