GPS backup

[Larry in TN]

OK....but if GPS goes out for everyone and nobody else in the air has a backup, how many aircraft does ATC have the bandwidth to support?

You are only looking at this from a GA perspective. The FAA looks at it from a national airspace system perspective and most of the users of the NAS have a backup to GPS--DME/DME/IRU.

The minimum VOR network is designed to support DME/DME/IRU.

 

[Skip Miller]

If you really want an on-board backup besides your compass+DG, I’ll offer three words: inertial navigation system.

I’ll offer a one word solution: Omega.

I used it while in the Navy. But it has its quirks, too.

-Skip

 

[RussR]

You are only looking at this from a GA perspective. The FAA looks at it from a national airspace system perspective and most of the users of the NAS have a backup to GPS--DME/DME/IRU.

The minimum VOR network is designed to support DME/DME/IRU.

Thank you. I always wonder why this isn't brought up in these sorts of discussions.

A backup network for GPS already exists, it's DME/DME (with or without IRU). As VORTACs and VOR/DMEs and even TACANs are decommissioned, very often the DME portion of the facility is left in place, specifically to support DME/DME operations.

Since the technology for DME/DME positioning already exists in thousands of airliners and other turbine equipment (even smaller stuff like the King Air I fly), I think it would be a pretty simple matter for it to be adapted for light GA. I don't see why there couldn't be a GPS receiver with built-in DME/DME backup. It's certainly technologically possible - companies literally already make them. Just adapt the form factor.

 

[Cap'n Jack]

You are only looking at this from a GA perspective. The FAA looks at it from a national airspace system perspective and most of the users of the NAS have a backup to GPS--DME/DME/IRU.

The minimum VOR network is designed to support DME/DME/IRU.

As alluded to in this post, VORs aren't completely going away (yet).
The FAA is going to keep some just in case of GPS issues. They intend to keep a "Minimum Operational Network" VOR MON

https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gbng/vormon

https://www.faa.gov/about/officeorg/headquartersoffices/ato/navigation-programs/vor-candidate-retention-list

 

[TCABM]

I think the idea is that the "new" NDB would not have the pilot interact with it. It would just be a signal generator the Nav unit (GTN, etc.) would pick up, do the calculations, and then present you with the results as it does now with the GPS signals.

That’s pretty much how a DME/DME FMS works.

 

[Half Fast]

As alluded to in this post, VORs aren't completely going away (yet).
The FAA is going to keep some just in case of GPS issues. They intend to keep a "Minimum Operational Network" VOR MON

Yes, but I believe they're focusing on decommissioning the low altitude VORs and that the MON will be primarily high altitude stations. Since piston GA flies at lower altitudes, I think the impact on little gasoline burners will be more significant than the overall hit.

 

[Half Fast]

That’s pretty much how a DME/DME FMS works.

https://www.faa.gov/sites/faa.gov/files/FAA%20Navigation%20Administration%20Programs%20Update%20by%20Deborah%20Lawrence_Sept2022_0.pdf

"NEXTGEN DME program." Slide 16 and following.

 

[Cap'n Jack]

Yes, but I believe they're focusing on decommissioning the low altitude VORs and that the MON will be primarily high altitude stations. Since piston GA flies at lower altitudes, I think the impact on little gasoline burners will be more significant than the overall hit.

Don't the high-altitude VORs have the same service volume at piston altitudes as the low-altitude ones?

https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap1_section_1.html

Alternatively, are you stating that the removal of low-altitude VORs will leave gaps at lower altitudes? Note that the reference suggests new service volumes above 5000 feet will be 70 nm rather than the 40 we are used to using.

 

[Half Fast]

Alternatively, are you stating that the removal of low-altitude VORs will leave gaps at lower altitudes?

Yes, exactly. I missed the 70 mile statement, but that’s still pretty short.

 

[TCABM]

https://www.faa.gov/sites/faa.gov/files/FAA%20Navigation%20Administration%20Programs%20Update%20by%20Deborah%20Lawrence_Sept2022_0.pdf

"NEXTGEN DME program." Slide 16 and following.

The way I read that is strategy decision in 2 yrs and implementation period going out to 2047.

I’ll likely hang up my spurs before there’s an RNAV/DME/DME box in my panel.

The more I think about it though, Pilotage/DR is still foundational enough that it needs to be a solid skill for the GA pilot in a glass panel + EFB no-gps environment but we’ll end up with grandchildren of the magenta line for the most part.

 

[Half Fast]

The more I think about it though, Pilotage/DR is still foundational enough that it needs to be a solid skill for the GA pilot....

Amen and amen, and again I say amen!

 

[WDD]

The more I think about it though, Pilotage/DR is still foundational enough that it needs to be a solid skill for the GA pilot in a glass panel + EFB no-gps environment but we’ll end up with grandchildren of the magenta line for the most part.

Yep - we all still need to know how to look out the window.

 

[Brad W]

ok, so I'm learnin' stuff here....

So I have follow-on questions.
So what ya'll are saying is that in many (some/all?) cases they are maintaining those big 'ol conehead buildings out there for use, at least in part, as DME stations.

How big or significant is the DME portion of that structure?

I mean if in the future we want to modernize/update and get rid of these buildings to eliminate all the no longer used or needed VOR stuff.

tear down these big old buildings that are probably suffering from leaks, infestations, and whatever else old unoccupied buildings suffer from...
And install a network of stations

How big is the DME "box" and antenna?
and how expensive is it?
and how much power does it need?

and how would an equivalent modern NDB transmitter compare in size, cost, power demands, etc..?

 

[WDD]

I think if we’re creating a modern back up to GPS we’d start from scratch and not just make a faster horse.

 

[MauleSkinner]

I’ll offer a one word solution: Omega.

I used it while in the Navy. But it has its quirks, too.

-Skip

I got to play with Omega before it went away…we got to dead reckon on our proving runs with the FAA on board.

 

[Pinecone]

This has come up before.

My suggestion is for the GPS nav companies to add a DME receiver to the GPS and VORs. This would allow DME/DME and VOR/DME nav capability.

Add in one of the micro INS and we would have a very robust system.

DME units are small foot print and easy to set up all over.

NDBs are low frequency and need very large antennas.

 

[Half Fast]

Yep - we all still need to know how to look out the window.

You might want to mention that to the pilots at Southwest.....

 

[WDD]

I’ll offer a one word solution: Skynet

What could go wrong?

-Skip

 

[TCABM]

ok, so I'm learnin' stuff here....

So I have follow-on questions.
So what ya'll are saying is that in many (some/all?) cases they are maintaining those big 'ol conehead buildings out there for use, at least in part, as DME stations.

Seriously? First, a VOR facility ain’t that big. Second, to rip it down and leave the DME requires money, which in .gov terms is time bound in units of slothiness.

…
How big is the DME "box" and antenna?
and how expensive is it?
and how much power does it need?

Cost? Budget dust.

Power? Less than a VOR and for most of the locations the power is already there.

…and how would an equivalent modern NDB transmitter compare in size, cost, power demands, etc..?

That’s irrelevant; the technology is not being considered for adoption. But, Donkey Hotey told me if you have a few billion dollars to donate to a variety of causes over a long time that you may be able to finally tilt a windmill.

 

[Larry in TN]

You might want to mention that to the pilots at Southwest.....

If my best guess turns out to be correct, it was spending too much time looking out the window that got them in trouble in OKC and TPA.

 

[Brad W]

Seriously? First, a VOR facility ain’t that big. Second, to rip it down and leave the DME requires money, which in .gov terms is time bound in units of slothiness.



Cost? Budget dust.

Power? Less than a VOR and for most of the locations the power is already there.


That’s irrelevant; the technology is not being considered for adoption. But, Donkey Hotey told me if you have a few billion dollars to donate to a variety of causes over a long time that you may be able to finally tilt a windmill.

A VOR building isn't that big? Well true, compared to an office building or a Wal-Mart.... but it sure is bigger than I'm imagining a small low-powered NDB transmitter might be.... as in maybe the size of a shoebox or certainly no bigger than a small refrigerator.

And besides, I wasn't saying rip it down just because.... but if you've got this building that's maybe pushing 50-60 years old or more, built to house a bunch of omnidirectional beacon stuff that is INOP, and is now only being used for the DME....there probably comes a time when it doesn't make sense to keep repairing the roof and keeping the exterminator on staff...when the whole thing could be replaced with some small little thing on the roof of the FBO

and NDB is irrelevant? maybe so...ahh, except for the OP of this thread. It in fact what this thread is about...or at least the basis for it.

and what does that last sentence even say? I take it it was meant as a dig towards me. I'm sorry, your wit was missed. Try again because I don't get it.

 

[MikeNY]

an area where GPS coverage was interrupted

If useful as heads up,
FLIGHT ADVISORY - GPS Interference Testing - Patuxent River (29 July-16 August)
(weekdays, 7am-11am lcl)
"testing ... may result in unreliable or unavailable GPS signal"

https://www.faasafety.gov/files/notices/2024/Jul/PAXRVR_24-03_GPS_Flight_Advisory.pdf

"This notice is being sent to you because you selected "Selected ATC Notices" in your preferences on FAASafety.gov."
[faasafety.gov > My Preferences and Profile > Email Notification Preferences]
... or full list under Resources > Notices

Addition to checking NOTAMs, RAIM check, etc., 'email notification' for Flight Advisory, (VIP TFRs, etc.) can be helpful as ... heads up.

 

[Pinecone]

I’ll likely hang up my spurs before there’s an RNAV/DME/DME box in my panel.

The more I think about it though, Pilotage/DR is still foundational enough that it needs to be a solid skill for the GA pilot in a glass panel + EFB no-gps environment but we’ll end up with grandchildren of the magenta line for the most part.

Hard to do when IMC.

DME/DME could be implemented with a LRU and software changes to later model GPSs.

 

[Pinecone]

A VOR building isn't that big? Well true, compared to an office building or a Wal-Mart.... but it sure is bigger than I'm imagining a small low-powered NDB transmitter might be.... as in maybe the size of a shoebox or certainly no bigger than a small refrigerator.

The problem is not the size of the transmitter, it is the size of the antenna required to get decent range.

 

[TCABM]

Hard to do when IMC.

DME/DME could be implemented with a LRU and software changes to later model GPSs.

Eh, DR can be done IMC for the en route portion, it’s can be a lot more workload though if you’ve got a lot of fixes to hit. Direct? Fly the last known heading for the remaining ETE.

But I agree, implementation shouldn’t be that hard to do; certification is what would likely drive the cost and time to market up though.

 

[dtuuri]

and NDB is irrelevant? maybe so...ahh, except for the OP of this thread. It in fact what this thread is about...or at least the basis for it.

Yeah, it didn't take long for the digit heads to ruin a simple concept by making it complex, modern and expensive. DME/DME is line of sight, so not seeing how that does much good for atmospheric bottom-feeders in the GA fleet — rooting around in the valleys and below the horizons of DME stations. Not gonna work on the ground or close to it, like during departure. Standard AM broadcast stations are a free and existing infrastructure, though, and the signal hugs the earth. IMO, not backed by science.

 

[Salty]

The more I think about it though, Pilotage/DR is still foundational enough that it needs to be a solid skill for the GA pilot in a glass panel + EFB no-gps environment but we’ll end up with grandchildren of the magenta line for the most part.

This is how I used to always respond to this topic. During my last cross country deep into area I'd never been before, I started wondering how I'd find my little podunk destination if I lost GPS. Pretty sure I could have done it, but it sure would have been a LOT more stressful, and I probably would have had to zig-zag around a bit.

Anywhere in Florida, easy peasy. A new place without a big river or mountain or something very obvious, it certainly is going to take a lot more effort than if you had an electronic means.

 

[Salty]

Oh, relative to the above, I don't have VOR or DME equipped. Thanks to this thread, I'm probably going change my mind about adding a nav radio, not for ILS as much as for this scenario.

 

[Half Fast]

Yeah, it didn't take long for the digit heads to ruin a simple concept by making it complex, modern and expensive.

Standard AM broadcast stations are a free and existing infrastructure, though, and the signal hugs the earth.

The OP very specifically said:

......what if it had happened during IMC conditions or during an IFR approach?

So please explain how that AM broadcast station will let you fly an IFR approach, unless it happens to have its broadcast tower located on the airport runway, which seems unlikely.

Can you triangulate between multiple broadcast stations? Yes, if you have some sort of steerable antenna, but the accuracy probably won’t be sufficient for an IFR approach, and you will want some automation that doesn’t exist yet. If you have to develop something new, maybe it makes sense to use a better method.

So yes, the geeks are indeed “ruining” a concept that doesn’t work and suggesting concepts that might.

 

[midlifeflyer]

Eh, DR can be done IMC for the en route portion,

I disagree with you and explain why in Dead Reckoning in IFR (no paywall).

 

[dtuuri]

The OP very specifically said:



So please explain how that AM broadcast station will let you fly an IFR approach, unless it happens to have its broadcast tower located on the airport runway, which seems unlikely.

Can you triangulate between multiple broadcast stations? Yes, if you have some sort of steerable antenna, but the accuracy probably won’t be sufficient for an IFR approach, and you will want some automation that doesn’t exist yet. If you have to develop something new, maybe it makes sense to use a better method.

So yes, the geeks are indeed “ruining” a concept that doesn’t work and suggesting concepts that might.

Well, I'm not the one seeking IFR-approval for a GPS backup device. I just don't want to hit anything because I can't tell where I am. If continuous triangulations using numerous AM stations are feasible, it just seems to me that an algorithm could smooth out the aircraft's location with a reasonable degree of accuracy despite the inaccuracy of any single bearing. To me, you guys just seemed to jump right into INS chips and DME/DME/IRU before explaining why existing AM stations can't possibly work. Probably a case of I don't know what I don't know, but how is an INS chip helpful where hundreds of AM stations are not?

 

[WDD]

If you loose GPS during an approach in IMC …….

Is there a cheap way to have independent ground based local GPS emitters that could be used instead? They would be close to the airport, not needling towers. Or maybe put them in existing cell towers.

 

[TCABM]

I disagree with you and explain why in Dead Reckoning in IFR (no paywall).

I agree in principle with your article; I think the context I did not provide is that given IMC, a GPS outage, and no other ground based navigation, we end up with vectors if in communication with ATC or lost comm procedures if not in contact with IMC.

My statement really was worst case scenario; lost GPS, lost comms, no other ground-based nav capability. Since landing is not optional, we have to get there somehow in this unplanned emergency. I recognize that’s a pretty narrow scenario, and our true goal is to get to VMC to land in VMC.

I think where my knowledge gap lies is with the performance of a TSO’d IFR GPS navigator in the absence of a valid GPS signal; i.e., how quickly and badly does the provided course guidance degrade to the point of being unusable.

Good food for thought.

 

[WDD]

Well, I'm not the one seeking IFR-approval for a GPS backup device. I just don't want to hit anything because I can't tell where I am.

If you’re in IMC and can’t tell where you are then you’ll need an IFR level / approved backup not to hit anything.

Agree that I don’t see why you can’t use AM radio stations as a data source. And maybe cell towers as well since they are everywhere. And maybe an inertial guidance chip is cheap enough today to make that viable. Lots of options. Just feed all of that data into the computer and make it tell you where you are.

 

[Salty]

If you’re in IMC and can’t tell where you are then you’ll need an IFR level / approved backup not to hit anything.

Agree that I don’t see why you can’t use AM radio stations as a data source. And maybe cell towers as well since they are everywhere. And maybe an inertial guidance chip is cheap enough today to make that viable. Lots of options. Just feed all of that data into the computer and make it tell you where you are.

 

[RussR]

Just feed all of that data into the computer and make it tell you where you are.

You've just described an FMS.

 

[RussR]

If you loose GPS during an approach in IMC …….

Is there a cheap way to have independent ground based local GPS emitters that could be used instead? They would be close to the airport, not needling towers. Or maybe put them in existing cell towers.

Conceptually, I don't see a problem there, after all you're basically combining aspects of LAAS and DME/DME.

Practically, I don't know how/if you could program existing GPS receivers to look for and include these additional "satellites".

 

[kontiki]

This subject came up when the ADSB final rules were in the docket back in 2007-2008. The subject has come up many times since then at all levels. GPS is a US military asset. In my opinion, it's always going to be a target. Signal power for civilian use was below the background noise floor as designed. I assume it still is. I know it is jammed intentionally in some regions of the world. The US Government could conceivably turn selective availability back on at any time to keep an adversary from using it.

I'm strictly a hobby pilot. I practice pilotage a lot. The pad with the magenta line usually remains on a vacant seat. My main concern is that the military is on top of the situation and has it set up so signals they use tactically always get through and that the US military has a backup system no one knows about for the day we really need it. I expect the commercial operators make adjustments and file complaints as required.

 

[dtuuri]

If you’re in IMC and can’t tell where you are then you’ll need an IFR level / approved backup not to hit anything.

Isn't WAAS stand alone? Then no "approved backup" is required, and anything is better than nothing.

 

[midlifeflyer]

I agree in principle with your article; I think the context I did not provide is that given IMC, a GPS outage, and no other ground based navigation, we end up with vectors if in communication with ATC or lost comm procedures if not in contact with IMC.

Sure, If you have nothing else, all bets are off and the rules don't matter.