GPS vs NDB (does NDB really meet RNP)?

[mm_flynn]

I was reading a recent CAA publication which outlined the RNP for approval as a primary approach aid - and all of the reasons why the current GPS system does not meet these requirements. After monitoring the NDB 20 approach into Shoreham (with 35 knots declining to 20 on the surface from 280 and very gusty) on the GNS430 I thought - does this NDB approach meet the RNP requirements.

With the impact of shore line and mountain refraction, random swings from some nearby CBs and other interference and reasonably complex pilot interpretation to reflect the shifting winds (at least complex compared to the GPS or a VOR) does this system really deliver the RNP standards (and certainly is the GPS less reliable)

Any thoughts on the question.

[Say again s l o w l y]

The main reason the CAA don't like GPS, is that they don't have any direct control over it.

GPS is a 1000X more useful than an NDB as long as the Yanks don't mess with the signal!

[bar shaker]

Mr Flynn

You may be unaware of it, but you are only allowed a maximum of 3 TLAs in any thread title.

;)

[IO540]

The Shoreham 20 NDB/DME AIP is atrocious in the NDB error; it is a good 20 degrees out at 6D, straightening out by the time one gets to about 3D.

SAS

I am not sure that is the reason they don't like it. They don't like all sorts of things. Look at the Cirrus BRS - there is no rational argument against it, or against any other feature which might save lives. I think one might get an interesting angle on it if one looked at the sort of aircraft which the PPL holders working at the CAA GA Dept fly themselves...

[DFC]

An NDB meets the RNP for an NDB be it used to define the centerline of an airway or as an approach aid.

When constructing the approach procedure, allowance is made for the inaccuracies of the NDB and the obstacle clearance heights reflect the errors inherent in the system.

GPS can be more accurate than NDB and if an approach procedure was to be constructed, it's minima would reflect that accuracy.

However, when completing the NDB approach everyone constantly monitors the ident because there is no warning of failure. GPS does not have any ident or warning of failure that the CAA find as acceptable for instrument approaches. Thus there are not any GPS approaches yet.

The CAA is not overly worried that the Yanks can turn off the Navstar system at any time it chooses because the owner of the NDB at Shoreham is similarly entitled to act with their property.

The CAA is worried that errors and jamming can affect the Navstar system in unpredicatable and possibly unexpected ways possibly without any warning to the user. It is illegal for the owner of the NDB at Shoreham to mess with the signal from the NDB.

What are people using to assess how well the NDB is showing the approach centerline?

If it is the runway centerline then remember that the NDB is not on the runway and thus an angular difference exists between the runway centerline and the NDB approach. This lateral displacement gets further away from the runway centerline with distance. Remember that as long as it is within 30 degrees of the runway it counts as a straight in approach.

Also remember that any error is the result of the total system error i.e. the receiver error and the indicated error as well as pilot error.

Insted of worrying about coastal refraction which is not a significat factor on the 20 approach, pilots should remember the effects of quadrantal error especially with a strong westerly wind.

There is however one inherent system failure in the UK - NDBs are not reguluarly flight checked in the same way as other approach aids.

Regards,

DFC

[TonyR]

I just love my ADF, no matter where you are in the world you can always get the BBC

I did hear that some people still use them to find airports, not sure if it's true though.

Tony

[Flyin'Dutch']

DFC,

Approved GPSs have RAIM warning, which is akin to the warning flag on your VOR/ILS display.

That is one of the key differences between handhelds and installed approved GPSs.

You can read up more about this here (http://www.faa.gov/ats/hhrafss/gps_raim.htm).

The CAA does approve sets for en-route navigation but not approaches, so not sure if they are too worried about the technological side of the matter.

Could it be the time money and effort required to get the GPS approaches sorted.

The UK system does not allow IAPs without an approach controller. These creatures are fairly thinly spread over the places that do have IAPs. Places that could be most interested in GPS approaches are those that don't currently have any approach and may be unlikely be able to afford the bells and whistles required to set up the entire entourage for an IAP.

I suspect there is more to it than meets the eye from our perspective.

FD

[Chuck Ellsworth]

My, my I do get a charge out of these discussions. :D

Maybe we should discuss the pros and cons of the horseless carriage and the reliability of transport by horse?

Anyhow gang I still get a kick reading all this hand wringing about GPS.

Cheers.... :ok:

Chuck

[redsnail]

GPS NPA approaches are quite common in Australia. Approach charts are designed and issued.
The GPS must be a fixed mounted IFR approved unit with RAIM and an updatable (and current) database. The pilot must also be checked out on NPA's as well.

[IO540]

DFC

Is your post really the official view of the CAA? Do you work there?

"However, when completing the NDB approach everyone constantly monitors the ident because there is no warning of failure"

Actually, receiving the audio ident means nothing, zero, zilch, nowt, nothing whatever about whether the NDB is indicating anything meaningful whatsoever. All it tells you is that the the part of the circuitry that receives and demodulates the AM ident works, and the unit is tuned to the right frequency.

Same incidentally applies to a VOR. I've flown with dead VOR receivers (in "VFR" school aircraft) which idented just fine.

Whereas a GPS will detect if it isn't getting a solution from the received signals. There is also RAIM.

If what you wrote is really the official view of the CAA, they need some education on how radio works :O

It is true GPS can be jammed. Then one would not do that approach. Same with DME packing up; one cannot do a "DME mandatory" IAP. Any gizmo can pack up, anytime. Loads of GA planes have duff DMEs or duff ADFs. So instrument flying should be banned unless one has two of everything.

Finally, does the CAA know something the FAA don't?

[mm_flynn]

DFC you wrote -

"An NDB meets the RNP for an NDB be it used to define the centerline of an airway or as an approach aid."


The question that was in my mind was - does the actual accuracy, reliability, precision, time to alert on various failure modes (i.e. the RNP) for an NDB achieve the level the CAA argue GPS (with RAIM) would need to achieve (but in the CAA view doesn't) to be used as a primary non-precision approach device.

[Say again s l o w l y]

Personally, when doing an NDB approach I always tend to use the GPS more than the NDB. I find it a far simpler and more accurate method than a needle on an RMI which then requires you to move your view to cross reference the DME.

This isn't on a bug smasher, this in two crew airline ops and seems to be standard practice amongst the crews in the company.
I've heard some pretty harrowing stories about nav aids that ident properly and seem to be working, but in reality are far from fully functional. The worst concerns an A320 of Bmed going into somewhere in Africa where the VOR radials themselves were rotating slowly causing the EGPWS to believe it was in a different place (no GPS input). The a/c narrowly missed piling into a ridge line (by about 70' I believe) only due to the dilligence of the crew.
How did Bmed respond? By putting a GPS card into the EGPWS.
Says it all really. If the technology is there, use it.

[skydriller]

I cannot help but think that the whole GPS Nav/Approach issue in Europe has to do with the fact that its american and free...

I am willing to bet that as soon as the European version (Galileo?) becomes operational it will be approved overnight...but then we will have to pay through the nose for it too......

But then I am getting really cynical the more I see how Europe operates...

Regards, SD.

[Timothy]

Here is a picture of the 02 approach at Shoreham:
http://www.artifax.net/misc/shoreham.jpg

I have flown that approach at night, with the needle bang centred on the RMI, and popped out of cloud over the coast about 2nm West of the airfield. That looks to be about 35deg off course.:uhoh:

Shoreham is a particularly bad example, because of the combination of night effect and refraction, but give me GPS every time!

[map5623]

I am unfortuneatley not in the exalted ranks of the IR/IMC/Night brigade so cannot comment from that point of view. However I am sorry to say that I class Politicians, Salesmen and government agencies and experts in the same class, the truth modified by all the gigary pokery that has to be applied because we the people are "just to ignorant to understand or use our brains".
Dare I mention Iraq, oops!
Mike

[IO540]

I find the 02 IAP not as bad; usually I end up east of the track but not too much. Tracking an inbound of 035 on the RMI would work a lot better. One would expect a lot of coastal effect on that one, however.

It is the 20 IAP which is potentially worse because of the terrain.

[Timothy]

just to ignoranthas a certain ironic resonant twist, does it not?:p

[Warped Factor]

This CAA Paper (http://www.caa.co.uk/docs/33/CAPAP2003_09.pdf) may, or may not, be of interest.

WF.

[IO540]

That CAA paper is a fair compendium of the traditional GPS issues, but it doesn't say why the CAA is afraid of allowing people to use it for real.

[reynoldsno1]

RNP requires RNAV. RNAV systems do not use NDBs. QED

[DFC]

Anyone heard about RAIM holes.

We check for them every day as I am sure every other pilots using approved GPS for BRNAV does of course!!

IAP's do require app controllers. However, cloudbreak procedures which are not published do not. How do people think that we get into places like Blackbushe on IFR flights during those dark winter evenings? :)

GPS is great for providing situational awareness. However, if there is a serious discrepency between the NDB and the GPS we go-arround because the NDB is the primary aid and we can't tell for sure which is giving the false info.

IO540,

The NDB is different from the VOR. As long as you get the ident then there is a signal for the ADF to point at. Unlike the VOR it is non directional (as the name suggests) and thus can not have any error. To check your aircraft's system you select "test" and move the pointer away from the indicated direction before releasing the selector and checking that the pointer returns to the expected bearing.

Perhaps you should think about my previous comments regarding the siting of the NDB and quadrantal error in westerly winds at Shoreham and then comment further on some radio theory.

----

People who get caught out on NDB approaches are the ones to track inbound to the NDB from a VOR using VOR or even GPS without noting that the NDB pointer is constantly in a place that it shouldn't be.

-----

PANS OPS has the procedures for GPS approaches and we have a few drawn up ready to use ( we have tried some out in VMC). But while the accuracy is great the cloudbreak procedure minimum altitudes are no different from those we use with the NDB and DME.

Places like Shoreham would require very little work to publish GPS approaches - simply draw up some waypoints to describe the existing NDB procedure and make it a GPS procedure with the same minima.

regards,

DFC

[reynoldsno1]

simply draw up some waypoints to describe the existing NDB procedure and make it a GPS procedure
Doesn't work quite like that, I'm afraid - what you're describing is an overlay procedure, and they are a pain. Much better to design the procedure from scratch, ensuring the waypoint properties are correctly described in the database.
Those countries that have been using GPS approaches for 10+ years abandoned overlays some time ago.....
Anyone heard about RAIM holes
Nothing new. Get AIS to set up a RAIM prediction service (not using your receiver)... works fine.

[Aussie Andy]

Hi all, check out this thread on the Dunnunda & Godzone forum: http://www.pprune.org/forums/showthread.php?s=&threadid=137070

Andy

[Timothy]

Unlike the VOR it is non directional (as the name suggests) and thus can not have any error. To check your aircraft's system you select "test" and move the pointer away from the indicated direction before releasing the selector and checking that the pointer returns to the expected bearing. ...hmmmm...

A dangerous thought for those that do not understand how MW signals can be bent.

Sure, there are failure conditions in VORs which do not produce a fail flag (not least the famous time when Alan Mann Avionics wired up my OBS to give reverse indications on both VOR and ILS) but at least some failures are trapped either by the TST ident or the fail flag.

On the ADF there is simply no way of telling that the direction from which the signal being received is the straight line to the NDB.

I am not going to regurgitate ATPL radio theory, but night effects, coastal diffraction, mountain effect, dip, thunderstorms, reflections/multi-path, quadrantal error all add up to an alarmingly inaccurate and unreliable aid.

It seems a shame that, because GPS is demonstrably imperfect, the CAA are chucking it out with the bathwater. It is much better than NDB/ADF.

The CAA polices by consent. When they take a position so radically opposed to that of many pilots who (in possession of all the facts) do prefer GPS over ADF, they risk their ability to regulate and control, in the same way that 20mph speed limits are ignored.

[reynoldsno1]

NDBs to be phased out in favour of GPS NPAs in Australia!?
'Tis already happening/happened in NZ (in consultation with operators). Some will be retained to fulfil current GNSS operating rules regarding nominated alternates - particularly for rotary operators. But in the long run, all the NDB's will disappear eventually....:D

[Aussie Andy]

Ah, civilisation at last!

Andy :ok:

[IO540]

It's a curious observation that an NDB must be a very cheap thing to run (could even be solar powered), while the receiving bit, the ADF, is relatively expensive.

The typical ADF, even brand new, is a 1970s design. I've just seen the innards of some in an avionics shop. There is no reason why it should cost £3000. One could also implement it by signal processing and measuring reception delays, i.e. without the present directional aerial system.

A VOR transmitter in comparison must be hugely expensive to install and maintain.

So an NDB ought to be a reasonable navigation fallback choice for remote locations. Inland, with no thunderstorms about, they are pretty accurate.

[Aussie Andy]

an NDB must be a very cheap thing to run If you read the D&G thread referenced in the link above it refers to some AirServices Australia information here (http://www.airservicesaustralia.com/pilotcentre/forums/RAPAC/rapacnsw040617.pdf) (scroll down to item 6.2) which indicates that the cost of maintaining aging NDB installations is becoming prohibitive due to lack of spares and end-of-service-life issues. Replacement cost is estimated at AU$100,000 each (about £40k) according to this doc. Doesn't seem too much in isolation, but in Australia this appears to have led to the decision that it is not worth maintaining several hundred NDBs across the continent and the intention is to phase out most of these. (I guess this is why NDBs at e.g. Sywell and Compton Abbas are NOTAM'd as U/S forever... too expensive to maintain and capex not available to replace!?)...

If interested in ADS-B, there is another AirServices Australia link which explains the programme for this in Australia here: http://www.airservicesaustralia.com/pilotcentre/projects/adsb/adsb.htm

Andy

[IO540]

At £40k somebody is making a mint. I would happily start up a business to deliver say 500 1kW 300-400kHz transmitters, at £10k :O

I suppose there are power costs too, not cheap for a remote location.

[Aussie Andy]

Good luck IO540, but don't plan to pay the mortgage with this plan! would happily start up a business to deliver say 500 1kW 300-400kHz transmitters, at £10kI'll bet you wouldn't: your costs will be much more than the sum of the parts, especially taking into account the regulatory requirements (FCC, ICAO, CE, etc.) you'd have to meet. Also bear in mind that the total turnkey purchase price for an aiport operator would have to include the cost of commissioning and testing not only the NDB Tx itself but also the antenna array. You'll also need to produce a matching ATU (impedance matching unit) as well as UPS and remote monitoring facilities if the installations are to be reliable enough for say an approach procedure. You would have to sink considerable R&D investment in order to address a globally limited and apparently shrinking market! I can't think of a seed investor around these parts who I know that would help you raise the capital for that project!

This business is already well served by large diversified groups, such as TV/Radio broadcast transmission equipment vendors such as Nautel, whose economies of scale enable them to re-use modules from other designs and to leverage the approvals process from their main product lines in order to viably produce a range of NDB transmission systems for nautical and aviation purposes, from 50W to 4kW... see http://www.nautel.com/products/index.php?action=displaygroup&group=7 for example. BElow is a picture of Nautel's 50W unit... just the housing may well cost £10k!?

I am surprised it can be done for average replacement cost of £40k!

Andy :ok:

http://members.lycos.co.uk/andyhardyuk/pprune/Nautel%2050W%20NDB.gif

[PhilD]

From the CAA document:
In the short to medium term ground-based augmentation systems (GBAS), aircraftbased augmentation systems (ABAS), space-based augmentation systems (SBAS)
and augmentation with stand-alone GNSS such as Galileo offer the possibility to achieve the RNP. This study has shown that GPS augmented with Galileo offers a user level monitoring capability through RAIM good enough to satisfy the integrity
requirements for en-route to precision approach phases of flight.

So there it is: GPS might be good enough for the FAA (and others), but we Europeans obviously know better and will stick with medium wave radios until we have our own toy that we can charge our users for.

[Aussie Andy]

Did you mean from the AirServices Australia document?

[PhilD]

No, that's a quote from the Executive Summary of CAA document 2003/9 (Issued April 2004), referenced earlier in this thread.

[IO540]

Last I heard was that the European GPS system (Galileo) will now be compatible with the American one, and that the basic Galileo signal (whose accuracy will be of the same order as the existing American one) will not be encrypted, i.e. it will be free.

Presumably the plans to charge for the higher-accuracy encrypted version will hinge on it being legally required for certain operations e.g. GPS approaches?

Now... how long will it be before the code is cracked? Won't be very long, considering how many "approach approved" GPS units will eventually be in free circulation. They will have to sell Sky-TV-type smartcards.

Makes one wonder why they want to bother charging for it, given that they will never get any significant money from the tiny group of GA "IR" pilots. The potentially big money would come from airliners and bizjets but they are doing just fine with ILS, and the major benefit of GPS approaches (getting an instant IAP into an airfield where there are no navaids) won't apply to them.

I wonder who wrote this business plan...

[Keef]

Prescott? His next triumph after the Strategic Rail Authority, perhaps.

All the CAA does by insisting on NDB rather than GPS is to reduce its own credibility. But I'm turning into an angry old man...

[bookworm]

I was reading a recent CAA publication which outlined the RNP for approval as a primary approach aid - and all of the reasons why the current GPS system does not meet these requirements. After monitoring the NDB 20 approach into Shoreham (with 35 knots declining to 20 on the surface from 280 and very gusty) on the GNS430 I thought - does this NDB approach meet the RNP requirements.

Assuming you're referring to the document WarpedFactor cited, it's worth reading carefully. The "RNP" are the Aviation Operational Performance Requirements for GNSS. In other words, they are not generic requirements, they only apply to sat nav.

Any conclusions drawn by the Civil Aviation Authority merely underline their position as the brightest national aviation authority in the UK. ;)