Future of VOR and Instrument?

[WDD]

Odd question number 2: Why not build out a multitude of new WAAS ground based units, more than they have now? You would have so many independent transmitters you'd be covered?

Odd question number 3: Will there be a day when the primary altimeter will be GPS driven, and the back up will be the air pressure driven one?

 

[WDD]

Jamming exercises are usually NOTAM'd in advance. The challenge will come when we're facing the real thing.

For example, imagine some terror group launching a 20-drone attack on US public targets. We're already seeing extensive use of cheap, Chinese-built attack drones in Libya and elsewhere (including the refinery attack in Saudi Arabia). If the US government believes 20+ drones are heading for major public targets, they'll just shut down the GPS satellite transmitters with no warning, probably for a week or more, and also turn on widespread jamming for GLONOST.

They would anticipate that and switch their drones to use the VOR / DME / other backup navigation systems in place. ????

 

[Dr. O]

".... getting unjammable, Military Grade GPS units ...."
Might want to check with the Iranians on that "unjammable military grade"

I go way back to needle ball airspeed days using an FAA chart and a watch for navigation
My dad went back to (actually, came from) an open biplane while using a Sunoco road map and reading the town names on the water-towers

 

[Half Fast]

Why wouldn’t the attackers use GLONAAS?

 

[Dr. O]

"switch their drones to use the VOR / DME / other backup"

Very likely - even radio broadcast towers, cell towers, yadda yadda
A state actor has unlimited funds to acquire technology and sneak it into the terror cells in this country

 

[Half Fast]

".... getting unjammable, Military Grade GPS units ...."
Might want to check with the Iranians on that "unjammable military grade"

I go way back to needle ball airspeed days using an FAA chart and a watch for navigation
My dad went back to (actually, came from) an open biplane while using a Sunoco road map and reading the town names on the water-towers

IIRC, the Iranians didn’t jam, they spoofed. Why the drone didn’t have a SASAAM receiver is a mystery to me. It’s typically required for critical systems like missiles. Seems like the drone should have been equipped if it were to be flown in hostile airspace.

 

[David Megginson]

They would anticipate that and switch their drones to use the VOR / DME / other backup navigation systems in place. ????

I'm not talking major state-sponsored actors here with big R&D teams; I'm talking groups using cheap, off-the-shelf Chinese drones, like is happening in Libya and happened in Saudi Arabia.

 

[David Megginson]

Why wouldn’t the attackers use GLONAAS?

You must have missed my mention of GLONOSS in my original post. I assume that's why the US military is running so many satnav jamming exercises, because they see how things are shaping up in conflict zones overseas, with increasing use of cheap, off-the-shelf autonomous hardware. And if we know anything about the government, it's that they're capable of a massive overreaction to a small threat.

 

[Larry in TN]

Odd question number 2: Why not build out a multitude of new WAAS ground based units, more than they have now? You would have so many independent transmitters you'd be covered?

WAAS doesn't replace the GPS satellites, it monitors accuracy and provides corrections to it. You still need a good signal, and good geometry, from the GPS satellites.

From Wikipedia's WAAS page:
WAAS uses a network of ground-based reference stations, in North America and Hawaii, to measure small variations in the GPS satellites' signals in the western hemisphere. Measurements from the reference stations are routed to master stations, which queue the received Deviation Correction (DC) and send the correction messages to geostationary WAAS satellites in a timely manner (every 5 seconds or better). Those satellites broadcast the correction messages back to Earth, where WAAS-enabled GPS receivers use the corrections while computing their positions to improve accuracy.

 

[David Megginson]

Hypotheticals aside, one of the basic safety principles in aviation is redundancy: two ignition systems with two plugs in every cylinder, gyros powered by two different sources, an alternate static air source for IFR flight, etc. So It doesn't make sense not to have an alternate source of navigation in IMC, whatever that turns out to be.

 

[John Myers]

The FAA has been studying what makes the most sense as a backup for many years under the Alternative Position, Navigation, and Timing (APNT) effort. VOR MON is really left in place for GA– transport category will use IRU and DME-DME in the short term and other solutions are being looked at to augment the coverage and accuracy of DME-DME or replace it outright. The GPS constellation is also slowly being replaced with GPS III satellites which are much more jam-resistant.

Some of the DME-DME improvement/replacement solutions are pretty creative, and include a hybrid DME-DME and "pseudolite" system, which are ground stations that act similar to GPS satellites. IIRC just improving the signal shape and using modern tolerances should get DME to 30-40 meters. 0.3nm is about 500 meters. So of all the options available I think an improved DME-DME network is very likely to be what we will have as a backup.

I don't know how much a three channel DME RNAV receiver would be, I would imagine not much more than two VLOC receivers. The current VOR network costs something like $100,000,000 a year to maintain, the airlines don't need it, and it's not very accurate. Writing is on the wall, but at the rate things move I think we'll all be retired from flying before they completely shut it down.

 

[David Megginson]

Too bad so many people threw out their ADFs. NDBs are dirt-cheap to maintain and operate, have potentially long range at low altitude (not limited to line-of-sight), and could be supplemented by commercial AM radio stations. They're not super accurate, but they'd get you to the ILS if there were a major GPS outage, and the FAA could have knocked a couple of zeros off their annual costs.

 

[John Myers]

Too bad so many people threw out their ADFs. NDBs are dirt-cheap to maintain and operate, have potentially long range at low altitude (not limited to line-of-sight), and could be supplemented by commercial AM radio stations. They're not super accurate, but they'd get you to the ILS if there were a major GPS outage, and the FAA could have knocked a couple of zeros off their annual costs.

NDBs can be off as much as 5 degrees, which is an error of 6nm (31,000 feet) 60nm out. DME could be tweaked to be less than 100 ft, two orders of magnitude more accurate, and with only one NDB you don’t know how far away you are or really where you are. NDB is also subject to many difference sources of interference. I don't think it could serve as a realistic backup to GPS.

 

[LongRoadBob]

NDBs can be off as much as 5 degrees, which is an error of 6nm (31,000 feet) 60nm out. DME is less than 100 ft, two orders of magnitude. NDB is also subject to many difference sources of interference. I don't think it would be accurate enough to serve as a viable backup to GPS.

Doesn’t it increase accuracy when you can home in on two different ones though?

 

[John Myers]

Doesn’t it increase accuracy when you can home in on two different ones though?

If my geometry is right, if you have two in range that are both exactly 60nm apart you’d be able to tell where you are in a diamond shaped polygon something like 36 square miles in size, or potentially much worse if the geometry was poor or there was interference, or less depending on how far away the transmitters are. You could probably get close to that with ded reckoning that some GPS units do for you. Not really a meaningful backup to GPS.

 

[David Megginson]

NDBs can be off as much as 5 degrees, which is an error of 6nm (31,000 feet) 60nm out. DME could be tweaked to be less than 100 ft, two orders of magnitude more accurate, and with only one NDB you don’t know how far away you are or really where you are. NDB is also subject to many difference sources of interference. I don't think it could serve as a realistic backup to GPS.

But the thing is, we're talking about just an emergency backup, so that's OK. That's why we made airways so wide far from the navaids. The only purpose of a backup is to make it possible for aircraft to transition from enroute to an airport environment for emergency landing in a major GPS outage, not to give us WAAS-like razor accuracy. It's been only 2½ years since I flew my last for-real NDB approach in IMC (just before I finally installed an IFR GPS), so it's not like we don't have 60+ years of proven experience with NDB-based IFR navigation.

Anyway, this is just wishful thinking. The majority of small-aircraft owners have already ditched their ADFs, so it's too late to go for the cheap-and-easy solution. DME is line of sight, so not only would we all have to install expensive new equipment for DME-based RNAV, but the FAA would have to install a lot more transmitters (especially in mountainous terrain), so it's a lose-lose now. And when you get up to Canada, where we have 1/10 of the population but a lot more land area, DME-based RNAV would get really expensive fast, probably well beyond Nav Canada's financial capability.

 

[David Megginson]

If my geometry is right, if you have two in range that are both exactly 60nm apart you’d be able to tell where you are in a diamond shaped polygon something like 36 square miles in size, or potentially much worse if the geometry was poor or there was interference, or less depending on how far away the transmitters are. You could probably get close to that with ded reckoning that some GPS units do for you. Not really a meaningful backup to GPS.

Did you never do NDB navigation for real? I did quite a bit of it ~10 years ago, because the MEAs were often much lower than on the Victor airways, so it would let me get under icing layers. It was less accurate, but it worked fine. It's only been a couple of years since Nav Canada decommissioned the Romeo airways (NDB-based) near my airport.

 

[John Myers]

But the thing is, we're talking about just an emergency backup, so that's OK. That's why we made airways so wide far from the navaids. The only purpose of a backup is to make it possible for aircraft to transition from enroute to an airport environment.

Anyway, this is just wishful thinking. The majority of small-aircraft owners have already ditched their ADFs, so it's too late to go for the cheap-and-easy solution. DME is line of sight, so not only would we all have to install expensive new equipment for DME-based RNAV, but the FAA would have to install a lot more transmitters (especially in mountainous terrain), so it's a lose-lose now. And when you get up to Canada, where we have 1/10 of the population but a lot more land area, DME-based RNAV would get really expensive fast, probably well beyond Nav Canada's financial capability.

NDB doesn't make sense even if we had them. It's not accurate enough to use as RNAV, so you'd have to install thousands of transmitters, and even then it's not accurate. There is no case for it. The DME network already covers most of the US and is used by transport category airplanes already. The idea is DME-DME would replace VORs, so you would have a large net reduction in cost and equipment.

This is an example of the current, not future, DME coverage in Florida, for example, to support .3nm accuracy (vastly better than NDB on a good day). Only three additional transmitters would be needed to close those green areas so DME could be used for a non-precision RNAV approach anywhere in the state.

In Canada (and Alaska), I can see NDB still being useful, as you could use the cheap transmitters to get back to the few major airports there are in an emergency from relatively far away, and the RNAV capability isn't as important. If you are out of range of an NDB you could still use ded reconing to at least get within range of an NDB and then home in to the station from there.

 

[John Myers]

Did you never do NDB navigation for real? I did quite a bit of it ~10 years ago, because the MEAs were often much lower than on the Victor airways, so it would let me get under icing layers. It was less accurate, but it worked fine. It's only been a couple of years since Nav Canada decommissioned the Romeo airways (NDB-based) near my airport.

Not really, just for training and practice NDB approaches, although not in awhile. There are still some Romeo airways in Alaska, I don't think there are any in the lower 48.

 

[David Megginson]

NDB doesn't make sense even if we had them. It's not accurate enough to use as RNAV, so you'd have to install thousands of transmitters, and even then it's not accurate. There is no case for it. The DME network already covers most of the US and is used by transport category airplanes already. The idea is DME-DME would replace VORs, so you would have a large net reduction in cost and equipment.

This is an example of the current, not future, DME coverage in Florida, for example, to support .3nm accuracy (vastly better than NDB on a good day). Only three additional transmitters would be needed to close those green areas so DME could be used for a non-precision RNAV approach anywhere in the state.

In Canada (and Alaska), I can see NDB still being useful, as you could use the cheap transmitters to get back to the few major airports there are in an emergency from relatively far away, and the RNAV capability isn't as important. If you are out of range of an NDB you could still use ded reconing to at least get within range of an NDB and then home in to the station from there.

For the airlines with their turbines, I see your point, but given that line-of-sight MRAs even in the US can be 8,000–10,000 ft MSL on the airways in flatter areas (and presumably higher off them in more-sparsely-populated areas), those of us with little piston planes would be left with no option but to climb into an icing layer or try to dead-reckon around terrain.

But again, I'm not arguing that we should turn back the clock and reinstall ADFs (though I'm keeping mine personally); just pointing out that we missed an opportunity for a much-cheaper emergency backup network.

 

[John Myers]

For the airlines with their turbines, I see your point, but given that line-of-sight MRAs even in the US can be 8,000–10,000 ft MSL on the airways in flatter areas (and presumably higher off them in more-sparsely-populated areas), those of us with little piston planes would be left with no option but to climb into an icing layer or try to dead-reckon around terrain.

But again, I'm not arguing that we should turn back the clock and reinstall ADFs (though I'm keeping mine personally); just pointing out that we missed an opportunity for a much-cheaper emergency backup network.

The DME network would replace VORs, so I’m sure MRAs would drop, although I don’t know of an airway with a 10k MRA in a flat area.

NDBs can’t be used for RNAV, so if the goal of GPS jamming is to cripple our air infrastructure, they would succeed with only an NDB backup network.

Don’t get me wrong, I think the simplicity of NDBs are great. I thought about putting an old KNS-80 in the TXi panel for fun.

 

[David Megginson]

The DME network would replace VORs, so I’m sure MRAs would drop, although I don’t know of an airway with a 10k MRA in a flat area.

NDBs can’t be used for RNAV, so if the goal of GPS jamming is to cripple our air infrastructure, they would succeed with only an NDB backup network.

Don’t get me wrong, I think the simplicity of NDBs are great. I thought about putting an old KNS-80 in the TXi panel for fun.

I don't think we're too far off—fun discussion.

The problem with reception range is that DMEs are in the UHF band, and VHF are UHF are line-of-sight, so the VOR service volumes apply (also considering that DME is bidirectional, so the aircraft's transmitter power is also limiting). So we're probably looking at VOR-like service volumes:



My experience flying IFR with VOR/DME for 14 years, before I installed my GTN 650, is that I'd usually pick up the VOR 10–20 nm before I could establish a good DME connection. So the FAA would have to install a lot more DMEs than there currently are VORs,VOR-DMEs, or VORTACs to provide full DME-based RNAV coverage below 10,000 ft, even in CONUS.

 

[John Myers]

I don't think we're too far off—fun discussion.

The problem with reception range is that DMEs are in the UHF band, and VHF are UHF are line-of-sight, so the VOR service volumes apply (also considering that DME is bidirectional, so the aircraft's transmitter power is also limiting). So we're probably looking at VOR-like service volumes:

View attachment 80502

My experience flying IFR with VOR/DME for 14 years, before I installed my GTN 650, is that I'd usually pick up the VOR 10–20 nm before I could establish a good DME connection. So the FAA would have to install a lot more DMEs than there currently are VORs,VOR-DMEs, or VORTACs to provide full DME-based RNAV coverage below 10,000 ft, even in CONUS.

They are increasing the VOR service volumes for the remaining MON VORs out to 70nm down to 5,000 ft (low and high), and similarly they would enhance the existing DME network by decreasing tolerances, changing the signal shape, adding more DME stations, and possibly increasing signal strength in certain areas. It's pretty neat that using an updated 1960s technology we can get in the same ballpark as the pre-WAAS GPS accuracy with a ground network.

I wonder if they could put the new DMEs where the ADS-B towers are as presumably those were sited in locations for good coverage also, and about the same frequency. The tower is there and the land is already leased/purchased with a service road and power. The DME shed by our hangar isn't very big, other than the amplifier I wouldn't think this stuff costs very much with modern equipment (probably from China...).

 

[Kenny Phillips]

I have a Loran unit that works fine, and it was working fine the day they turned the Loran signal off. I recall exactly Pres Obama when asked about it said, "Everybody has gps anyway." No i dont and I even knew how to work that Loran unit.

Ha, same here, except that I had GPS. So I would nav with GPS, Loran, Omni, ADF, and a map on my lap. I've never been lost.

 

[John Myers]

There’s always the Sextant. Doesn’t work so well in IMC though.

 

[David Megginson]

There’s always the Sextant. Doesn’t work so well in IMC though.

True, but if you were keeping track of your heading and ground speed before the GPS outage, those will probably be good for a while, so you can use your watch, pencil, and a paper chart to dead reckon for the next 50-100 nm with tolerable accuracy (as long as you don't have to change course, and you're not flying through a front). The goal is just to get in range of a VOR and not to run into any cumulus granitus while you're getting there. If you're well above terrain and wrote down the upper winds before you started, the reliable old E6B will even get you through a course change or two in your hunt for that elusive radio navaid.

 

[John Myers]

True, but if you were keeping track of your heading and ground speed before the GPS outage, those will probably be good for a while, so you can use your watch, pencil, and a paper chart to dead reckon for the next 50-100 nm with tolerable accuracy (as long as you don't have to change course, and you're not flying through a front). The goal is just to get in range of a VOR and not to run into any cumulus granitus while you're getting there. If you're well above terrain and wrote down the upper winds before you started, the reliable old E6B will even get you through a course change or two in your hunt for that elusive radio navaid.

Most GPS units (at the least the Garmin units) have a dead reckoning mode if GPS signal is lost to do the math for you. I was kidding about the Sextant but early airliners had a hole in the cockpit ceiling for just that.

 

[bobmrg]

Odd question number 2: Why not build out a multitude of new WAAS ground based units, more than they have now? You would have so many independent transmitters you'd be covered?

Odd question number 3: Will there be a day when the primary altimeter will be GPS driven, and the back up will be the air pressure driven one?

Answer number 2: WAAS ground stations rebroadcast signals from the GPS satellites. You could have a WAAS ground station on every street corner and they would be useless without operating satellies.

Answer number 3: GPS accuracy is great laterally, miserable vertically. Your GPS altitude is not measured above sea level or above ground level but from the GPS ellipsoid, something you cannot see or feel because it is derived mathematically. GPS and barometric altitude can vary by as much as 600 feet. GPS altitude will never replace barometric altitude because there are places in this world where the GPS ellipsoid surface is lower than the mountain tops.

https://eos-gnss.com/elevation-for-beginners

 

[John Myers]

Answer number 2: WAAS ground stations rebroadcast signals from the GPS satellites. You could have a WAAS ground station on every street corner and they would be useless without operating satellies.

Answer number 3: GPS accuracy is great laterally, miserable vertically. Your GPS altitude is not measured above sea level or above ground level but from the GPS ellipsoid, something you cannot see or feel because it is derived mathematically. GPS and barometric altitude can vary by as much as 600 feet. GPS altitude will never replace barometric altitude because there are places in this world where the GPS ellipsoid surface is lower than the mountain tops.

https://eos-gnss.com/elevation-for-beginners

WAAS ground reference stations simply measure the error between where they are and where GPS says they are. This correction is then send via a ground network to the master stations on each coast, which relay these corrections to the stationary WAAS satellites (there are currently 3), which in turn downlink these corrections to WAAS receivers. The ground reference stations are usually located on the roof of the ARTCC buildings.

WAAS GPS altitude is far more accurate than your baro altimeter, and all modern GPS units reference the geoid and not the GPS ellipsoid, this was only true for the early units. This is a somewhat complicated topic but the end result is a WAAS GPS navigator will usually have your altitude to within a meter or two, which is much more accurate than your baro altimeter. On a cold day, your baro altimeter may place you 30 feet below minimums while showing you at minimums, for example, which is OK because the procedure designers build in this error (or mark it as requiring cold temp adjustment). Keep in mind for LPV approaches your aircraft already uses GPS altitude to get to 200ft off of the ground.

You can test this while lining up for takeoff. The TDZE should be very close to what your GPS reports, any error is likely mostly due to the position of your antenna from where the survey point was.

Ground based satellites have been studied: https://en.wikipedia.org/wiki/Pseudolite

All things considered, a DME-DME network makes the most sense, or so the research suggests.

To get an idea just how accurate WAAS is today, you can look up your nearest major city here: https://www.nstb.tc.faa.gov/reports/waaspan69.pdf

In Denver, it's within a couple FEET. Amazing. We benefit from being near ZDV up in Longmont where the reference antenna is.

 

[airheadpenguin]

A quick version of why keep VORs around, I was up last Sunday with a buddy flying approaches around KCON. We were on an IFR flight plan, and as we were coming back into Nashua there was a thin layer of clouds from about 1200 - 2000 which were below freezing. They were rolling in west to east, if you look at the RNAV32 and RNAV14 at KASH they both would have been in the clouds (and icing) at low altitude but the VOR-A which starts to the east and ducks down to 900 MSL was clear as can be, we never entered IMC.

So why keep the VOR approaches? Because options ....