ADS-B Out and DME

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MRC01

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MRC01
ADSB Out transmits on 978 or 1090 MHz, both of which are in the range of DME frequencies. Can they conflict with each other?

My airplane has a Narco model DME890. It always worked reliably even at long distance (> 50 nm) except for the past few years, where it has started to get intermittent and only works within about 30 miles of the ground station. I've troubleshot the tray, power, and antenna, all of which test good. I've replaced it a couple of times with used known-good units from avionics shops, which perform the same as the original. The problem isn't the unit itself, nor is it the installation.

Looking back through my logs, I realize that the problems started back in late 2019 which is also when I equipped for ADS-B Out. Coincidence?

Has anyone else experienced or read about conflicts between DME and ADS-B out?

PS: the DME890 has a 25 W transmitter. The King units were 50 W and 100W. So the DME890 has always been a less powerful DME, which could make it more prone to being blocked / jammed. Also, DME ground stations can only handle a limited number of DME aircraft at a time, since the ground station has to transmit separate replies to each individual aircraft, replicating its unique pulse timing/jitter. When the ground stations get saturated, they prioritize the strongest DME signals, so weaker DMEs like mine can get ignored. This jives with my experience, since my DME works less reliably with stations like SEA (class B) and more reliably with stations like OLM (class D) and HQM (rural), and it always works on ILS that have DME.
 
MRC01 said:
ADSB Out transmits on 978 or 1090 MHz, both of which are in the range of DME frequencies. Can they conflict with each other?

My airplane has a Narco model DME890. It always worked reliably even at long distance (> 50 nm) except for the past few years, where it has started to get intermittent and only works within about 30 miles of the ground station. I've troubleshot the tray, power, and antenna, all of which test good. I've replaced it a couple of times with used known-good units from avionics shops, which perform the same as the original. The problem isn't the unit itself, nor is it the installation.

Looking back through my logs, I realize that the problems started back in late 2019 which is also when I equipped for ADS-B Out. Coincidence?

Has anyone else experienced or read about conflicts between DME and ADS-B out?

PS: the DME890 has a 25 W transmitter. The King units were 50 W and 100W. So the DME890 has always been a less powerful DME, which could make it more prone to being blocked / jammed. Also, DME ground stations can only handle a limited number of DME aircraft at a time, since the ground station has to transmit separate replies to each individual aircraft, replicating its unique pulse timing/jitter. When the ground stations get saturated, they prioritize the strongest DME signals, so weaker DMEs like mine can get ignored. This jives with my experience, since my DME works less reliably with stations like SEA (class B) and more reliably with stations like OLM (class D) and HQM (rural), and it always works on ILS that have DME.
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PS: if that is true, why didn't the transponder conflict with the DME, long before ADS-B was installed? Transponders also use frequencies that overlap with DME. And the transponder's fin antenna is located only a few feet away from the DME antenna underneath the airplane.
 
BTW, trying to answer my own question, I did find some relevant search results: https://duckduckgo.com/?t=ffab&q=transponder+dme+interference

The bottom line is yes, DME, transponders, ADS-B out and GPS all use near or overlapping frequencies and can interfere with each other. Some DME units (Narco, King, etc.) are better than others at dealing with this interference. It shouldn't be a problem most of the time, except for worst-case scenarios such as being near a very busy DME ground station.
 
You did not mention checking/replacing the cable.
 
I think a bigger question is whether to fix the DME or get a GPS like a Garmin 175. It will give you distance and bearing to any airport, fix, waypoint you wish.
 
WDD said:
I think a bigger question is whether to fix the DME or get a GPS like a Garmin 175. It will give you distance and bearing to any airport, fix, waypoint you wish.
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I don't plan to fix it. In fact, I'm not even sure it's broken or could be fixed. From what I read, the limitations I'm experiencing may be typical or expected for any low power DME. My OP question is more about whether this is the case - are others also finding that DME has become less reliable since "everyone" has ADS-B and that frequency range is more congested?

A GPS-175 would certainly be more useful but I'm not sure the value I would get from it is worth the price, which is the equivalent of more than 100 hours of flying around with the equipment I have. The limitations of my current equipment don't affect my flying preferences and missions much at all.
 
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I’d get the free GTN IPad simulator and try it out. You might find it a good addition. Or not. At least you would have “test drove it” to help in your choice.
 
MRC01 said:
ADSB Out transmits on 978 or 1090 MHz, both of which are in the range of DME frequencies. Can they conflict with each other?
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There is a RANGE and then there are SPECIFIC Frequencies. I couldn't find a complete list of DME Frequencies, but I'm assuming non of them actually use 978 MHz or 1090 MHz.

Think of the FM radio you have in your car. The FM Radio Frequency Range is 88.0 MHz to 108.0 MHz, but you tune in individual stations within that Frequency Range.
 
M20K said:
There is a RANGE and then there are SPECIFIC Frequencies. I couldn't find a complete list of DME Frequencies, but I'm assuming non of them actually use 978 MHz or 1090 MHz.

Think of the FM radio you have in your car. The FM Radio Frequency Range is 88.0 MHz to 108.0 MHz, but you tune in individual stations within that Frequency Range.
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I thought the same, so I looked them up the DME frequencies that are actually used and paired to the VOR frequencies that you enter into the DME. Some do actually conflict, others are very close.

When you tune a DME to 108 MHz, the ground reply is 978 MHz. The DME also uses 979 and up. When you tune to 115.95, the reply is 1087. When you tune to 112.2 and 112.3, the DME transmit is on 1083 and 1094 respectively.

So it does seem plausible that there could be interference. But I don't know for sure, which is why I'm asking.
 
PS: another fact that made me think interference between DME, xponders and ADS-B is plausible relates to comm radios and GPS. Comm radios operate around 110 MHz, about 10x lower frequency than GPS. But sometimes they can jam GPS reception when transmitting because the radios have distortion, and the 8-12th harmonics are right in the GPS range.
 
MRC01 said:
I thought the same, so I looked them up the DME frequencies that are actually used and paired to the VOR frequencies that you enter into the DME. Some do actually conflict, others are very close.
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Where did you find the list? And is it current or old as they may have phased out any DME that used to be on 978MHz and 1090MHz.

And yes, there can always be issues with interference from any RF source. Usually the voice transceivers cause the most problems with the antennas are too close together. But again, not that any RF source couldn't create an issue.
 
usually the powers in charge pay attention to the specific DME frequencies in use in a particular area and the other uses of those frequencies in the area.
 
M20K said:
Where did you find the list? And is it current or old as they may have phased out any DME that used to be on 978MHz and 1090MHz.

And yes, there can always be issues with interference from any RF source. Usually the voice transceivers cause the most problems with the antennas are too close together. But again, not that any RF source couldn't create an issue.
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The list of frequencies is in the installation manual for my DME-890. It's a look-up table that all DME devices must implement internally. Some of those frequencies are still in use, for example the TOU Vortac is 112.2, so the DME is on 1083 and 1020. Incidentally, the TOU Vortac is one that actually does work with my DME. It fits the profile of the ones that do work: it's in a rural area having very low traffic.

PS: the RBG Vortac in Oregon uses 108.2, where the DME uses 1043 and 980 MHz.
 
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MRC01 said:
The list of frequencies is in the installation manual for my DME-890. It's a look-up table that all DME devices must implement internally. Some of those frequencies are still in use, for example the TOU Vortac is 112.2, so the DME is on 1083 and 1020. Incidentally, the TOU Vortac is one that actually does work with my DME. It fits the profile of the ones that do work: it's in a rural area having very low traffic.

PS: the RBG Vortac in Oregon uses 108.2, where the DME uses 1043 and 980 MHz.
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I believe they use 1MHz isolation. So the frequencies you just listed are within specs and are not using the ADS-B frequencies.
 
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