IMC with GPS, few Q's

[Cessna 210 Heavy]

Hi all,

I have just got my hands on the Saratoga, and lovin it. The thing is I was wondering how someone could pass the IMC test in it. The aircraft is equiped with 530 and 430, flight director, 3 axis autopilot and other 'bells and whistles' with the ADF down by my left knee and VOR 2 just above it. So I was thinking, if on my renewal or test the examiner asked my to maintain say '230 track on the NDB' could I just punch this into the GPS, select OBS, and follow the line? Me thinks not, but it would be a lot easier. But, how about if I also tuned and idented the NDB and had it on the ADF, and used this to check the GPS, so not using the GPS as the primary nav equipment? So in effect I could fly the director, cross checking to the ADF.

I know you can use the GPS for VOR, just like a normal VOR but it seems a waste of equipment in the cockpit. A friend of mine once got a mark against him in his IR test for not using 'all avialable equipment' and not usung the GPS for his ground speed.

Sorry for the dumb questions, but I tend to go OTT!

[IO540]

You need to ask some instructors as to the latest procedures but IIRC in the IMC test all the nice kit will be switched off, leaving you with just a VOR, DME, ADF.

However, in any FAA checkride, the examiner will ask you to show that you can use all of it. It may not be to the full degree; for example in the PPL he may not ask you to load a flight plan into the GPS. But he will want the autopilot usage shown.

I heard that in the JAA IR they are now testing GPS and autopilot usage.

I think the CAA should require knowledge of all installed equipment to be demonstrated.

[scooter boy]

IO, I agree with your experience of the FAA attitude. During my last FAA IPC I was allowed to use all the gadgets - it was more about being signed off as a safe bet not to crash than ticking boxes on a checklist.

Makes a lot of sense.

SB

[S-Works]

When I did my IR test I had the GNS430 switched to config page and flew the trip using steam guages and dead reckoning to the beacon and the used the ADF inside the promulgumated distance. After the test the examiner asked my why I did not just punch in a DCT to the beacon and use the GPS......

You still have to use the ADF in raw data mode as with the other instruments but if the other kit is installed and cetified for use you can use it.

You are right seeting the OBS on an NDB is indeed more accurate and you would be surprised just how many of us fly our approaches this way!!

[Cessna 210 Heavy]

Thanks for the replies guuys, the thing is that if I turned off all the nice kit I would have no NAV 1 or 2 or Comms!
This is what I have to play with;
http://img.photobucket.com/albums/v321/vrteng/DSCI0013.jpg

[Fuji Abound]

I suspect it will depend on the instructor. I doubt there are many who would insist on your flying en route using the VOR for an IMCR

It will also depend on you.

You could chose to use the VOR to satisfy yourself you can still do so.
I suppose it begs the questions as to whether the examiner should be happy with your flying an approach using GPS. There are none approved in the UK so strictly the primary aid for approach navigation will not be GPS and therefore presumably the instructor will want to be satisfied that you are primarily relying on somethng other than the GPS!! Is it still the case that each renewal a different procedure should be demonstrated (?) and if so it might be the turn of the NDB DME. Flying an NDB DME procedure he might even want you to "switch off" the GPS (always assuming you are not actually in IMC :) ).

Thanks for the replies guuys, the thing is that if I turned off all the nice kit I would have no NAV 1 or 2 or Comms!

Just turn down the brightness on the 430 and 530 having selected the approach frequency!

Interestingly it would be more difficult with the G1000 setup. I suppose you could turn down the brightness on the map screen which if you have selected VORs, NDBs or DMEs would leave you with only these as your points of reference.

How many of us check the RAIM prediction before flying a "GPS approach?".

[S-Works]

Good try Cessna210, but you can easily switch to the GPS data or someother non nav mode while still having access to the steam nav.

You are not the only one with that set-up!!!

[Cessna 210 Heavy]

BOSE X I thought I had to urn the whole system off, not just switch to VLOC, makes sense now :ugh:

[Grummanaa5]

Nice machine, should you not think of changing your login name to PA32 'Heavy'! - I can't talk!!!

I have exactly the same problem, just become the proud owner of an Arrow with Avidyne Flightmax with two (HD ready:-) screens and little else. There is an ADF somewhere down by the fuel selector I think, but no standby VOR.

The Cirrus and Cessna G1000 boys are going to be in a similar situation.

[Sir George Cayley]

..with apologies to Mr Dylan.

Those nice people at the CAA ran a trial last year on GPS Non-Precision approaches around the country. Can't remember all the locations but those of us who flew the EGKT one found it a doddle, compared to chasing a wayward ADF needle all over the sky.

I've spoken to a person in "the know" who thinks it likely that they will give the go ahead sometime later this year. Apparently, the big issue is training.

Some countries insist on difference training but the UK wants to make it voluntary. There might be some internal wrangling over this though.

I'm also told that the requirement to carry equipment to receive NDB signals is under review at the same time.

All in all if true the futures bright, the futures Garmin;)


Sir George Cayley

[IO540]

When I did my IR test I had the GNS430 switched to config page and flew the trip using steam guages and dead reckoning to the beacon and the used the ADF inside the promulgumated distance. After the test the examiner asked my why I did not just punch in a DCT to the beacon and use the GPS......

That's really funny... in my FAA PPL checkride, I drove the examiner nuts by being a "good CAA boy" and switching off all the extras. I didn't realise he wanted quite the opposite - he wanted me to demonstrate that I knew how to use it all.

This was despite the FAA CFI telling me about this beforehand. It shows how strong the "CAA "make it hard" mindset" is, and helps to explain the sometimes vicious anti-GPS attitude held by so many UK pilots.

This was a VFR checkride of course, so having the kit on or off doesn't really make any difference to one's flying. But it makes it much easier to keep outside of CAS - something which is the pilot's responsibility, under FAA.

I would, I was told, have a problem on the IR checkride, because I have an RMI, which makes partial panel NDB holds (with timed turns) rather hard to do "properly", because the rotating outside ring cannot really be covered up. In the end I did the IR in the USA in an old dog of a PA28...

I've spoken to a person in "the know" who thinks it likely that they will give the go ahead sometime later this year. Apparently, the big issue is training

I wonder how they reached that conclusion based on the tiny sample participating; tiny as predicted because most pilots who have the required kit are flying N-reg. Also, note that everybody who has ever been within 10nm of Gatwick or Kingsway is "in the know" and is spreading their favourite rumours ;)

I flew one of the approaches, manually, on the flight director, and fully coupled. Being N-reg prevented me from descending below 2000ft agl (and from reporting it on the website) but it was a doddle and highly accurate.

It's all irrelevant anyway; GPS approaches won't do much for the UK until the CAA removes the need for ATC for an IAP, and that is a much harder nut to crack than just drawing up some approaches.

[S-Works]

I wonder how they reached that conclusion based on the tiny sample participating; tiny as predicted because most pilots who have the required kit are flying N-reg
The turnout in the end for the trials was actually quite large and enough for the figures to be valid. We will be getting GPS approaches this summer by the looks of it, initially the approaches that were available during the trial and then spreading to other airfields.
I suspect as IO says it will be a long time before the need to haveATC to fly these approaches vanishes but don't discount the validity and the impact they will have. Given the choice of a GPS approach over an ndb I will of course take the GPS anytime, accurate and reliable. Even holding with a gps is easy, OBS the inbound track, wind correct the outbound track using the visual reference of the magenta line and the wind aloft and use the HSI to intercept the inbound. A perfect racetrack every time.

[IO540]

We've been here before, bose-x, of course, but unless the GPS approach offers a lower MDH than the conventional approach (and as far as I can see all the places that will be getting GPS approaches already have NDB or VOR or ILS ones) there is no actual advantage to the pilot, because the overlays for the conventional approach have been in the GPS database for years and the inbound leg of these can be flown (using the GPS) just as well as the inbound leg of a GPS approach.

Well almost; the GPS won't auto-select the 0.3nm full-scale mode on the overlay approach, but that is really relevant only to coupled autopilot operation, and in any case 0.3nm FS can always be selected manually.

The key "mission capability" advantage comes from a lower MDH figure. When we get ~ 400ft DH I will be happy.

[S-Works]

The key "mission capability" advantage comes from a lower MDH figure. When we get ~ 400ft DH I will be happy.
No you won't!!! :p

[tmmorris]

And from GPS approaches without ATC, of course, when they will become really useful. Imagine approaches at Kemble, Shobdon, Thruxton, or wherever takes your fancy...

Tim

[Fuji Abound]

The key "mission capability" advantage comes from a lower MDH figure. When we get ~ 400ft DH I will be happy.

Yes, but what is the "safe" theoretical MDH for GPS without WAAS? Altitude is unreliable without WAAS so the procedure must rely on DME or GPS step downs. Without WAAS can the final stepdown be any lower because GPS is providing great accuracy in its 2D position?

[IO540]

I don't know the IAP design criteria (PAN/OPS or TERPS) but a GPS IAP should be no worse than a VOR/DME one.

Currently, if you look at e.g. Shoreham, the GPS MDH is identical to the (close to lethal, if at minimums) NDB/DME 20 IAP there. That NDB procedure is absolutely hopeless due to the NDB errors and obstacle clearance relies almost entirely on the DME stepdowns. This is fair enough (not following stepdowns will get you killed in most places) but with a 30 degree NDB error you will not be in a position to make a safe landing from 800ft. Yet, the GPS procedure (which will take you over the runway within a few yards) does not offer a more useful MDH.

Consequently, Shoreham is closed during most "warm front" type of weather, except to pilots with a serious can-do attitude.

I think WAAS/EGNOS is really for creating a synthetic glideslope which you fly like an ILS. That's ultra slick, if you have the latest kit, but is quite some years away anywhere in Europe, IMHO. They have it in the USA, of course.

[Fuji Abound]

I think WAAS/EGNOS is really for creating a synthetic glideslope which you fly like an ILS. That's ultra slick, if you have the latest kit, but is quite some years away anywhere in Europe, IMHO. They have it in the USA, of course.

Yes, it will give you a 3D accuracy of better than 30 feet I believe. Does it need to be supported by an adequate RAIM prediction - that I dont know, but I guess it would?

I don't know the IAP design criteria (PAN/OPS or TERPS) but a GPS IAP should be no worse than a VOR/DME one.

Yes, I would also have thought so, so realistically the best one could hope for is that airports with only an NDB DME (eg EGKA) with a PGPS the MDH could be expected to fall in line with a VOR DME procedure. However it is interesting how many pilots would not accept an approach down to and with a cloud break at minima on the 20 approach even though legal because of the proximity of the close ground. Has this got more to do with the concerns over sheer and turbulence close to high ground on the approach or the implied greater inaccuracy of the present NDB DME procedure and how much more comfortable would pilots be with either a GPS or localiser procedure so far as the MDH is concernded.

However it does mean at airports with less demanding terrain on the approach presumably an MDH the same as for a VOR DME procedure at the same airport is theoretically equally as safe. Is that in fact what we will see or do the procedure writers have a hang up that the horizontal reliability of GPS is more suspect than a VOR?

[denhamflyer]

IIRC when using WAAS/EGNOS RAIM is no longer pertinent since the WAAS system compensates for the satellite positions in the local area. Thus making the whole process even more robust (accurate for a higher percentage of the time). Hence the lower MDA

[S-Works]

Not sure you are correct on that one Denham. RAIM is run to check satelite availability not accuracy. So if you are planning an IFR flight WAAS or not you still need to run RAIM to check that the satelites will be available for the entire trip. If the satelites are available it makes little difference to how much more accurate WAAS/EGNOS would make it........

[denhamflyer]

In principle I agree - but all RAIM does is monitor and potential predict where the standard satellites will be (the monitor is essential) but the prediction does not take into account the WAAS. Extract from the new G1000 manual

"In G1000 systems with WAAS enabled, performing a RAIM prediction will not be necessary in most cases.
However, in some cases, the selected approach may be outside the WAAS coverage area and it may be
necessary to perform a RAIM prediction for the intended approach. "

I did read somewhere why this is the case - but I may well be wrong. But I think had something to do with the WAAS signal also provide a backup to the original satellites. Bear in mind the prediction doesnt know about failures..

Just to be clear RAIM Monitoring is essential since it is a validation of the actual signals. However prediction is less clear since WAAS coverage seems to imply it will always say yes.

[Fuji Abound]

My local airfield had a:
1) VOR MDA 680'
2) RNAV GPS Y (non WAAS) MDA of 620'
3) RNAV GPS Z (WAAS) MDA of 327'

Well there we have it - that would suggest in FAA land without WAAS GPS is going to give you minima around the same as for a VOR DME approach - everything else being equal.

Bose

If the satelites are available it makes little difference to how much more accurate WAAS/EGNOS would make it........

I dont understand - I thought WAAS gave you the vertical accuracy lacking with full 3D coverage? Maybe I am wrong - can you expand please?

WAAS corrects for GPS signal errors caused by ionospheric disturbances, timing, and satellite orbit errors, and it provides vital integrity information regarding the health of each GPS satellite.

From Garmin.

I think I understand that by the use of ground based references stations the positions of which are known with extreme accuracy, WAAS is able to promulgate two additional signals that enable receivers to "adjust" for any inaccuracy caused by the problems Garmin identify.

I dont see how it provides each receiver with integrity information? Perhaps what Garmin mean is it better enables the network to be monitored for any problems that may arise and the appropriate corrective action taken?

Finally if that is correct presumably it would seem one of two satellites transmit that information but once agian presumably if the RAIM predicted coverage is for some reason inaccurate even with WAAS navigation may be degraded?

[denhamflyer]

Further clarification RAIM is a measurment of the ACCURACY within a protection limit not the number of satellites. The key requirements being :-
2.0 nm for oceanic and enroute,
1.0 nm for terminal,
and 0.3 nm for non-precision approaches
This is actually a function of the number satellites that will be available - but with WAAS improving the accuracy then I believe that RAIM prediction becomes near 100% to these levels. (This is not the same as actual signals - subject to all other failure modes).
The other factor is that WAAS corrects a number of errors not only improving 2D accuracy but also 3D for vertical navigation.
In all cases it is the accuracy that is improving to defined levels of reliability and safety.

PS. I believe there are a number of ONLINE WAAS/RAIM prediction programs that will actually take into account the serviceability of the satellites (among other factors) and I can see that in the future it will become a planning requirement to use such information as per NOTAMs, when solely navigating on GPS.

[S-Works]

Fuji et al.

If the satelites are available it makes little difference to how much more accurate WAAS/EGNOS would make it........

Should actuall have read:

If the satelites are NOT available it makes little difference to how much more accurate WAAS/EGNOS would make it........

[denhamflyer]

There will always be satellites available (excluding real time failure...) to provide a GPS fix around the globe. The key is - are enough satellites going to be available in the correct positions to provide the required accuracy for the kind of flying (see above post).

I agree that ultimately if there are not enough for a basic location then WAAS cannot improve upon that. But it can improve upon basic degradation - wich is what RAIM prediction is predicting.

The minimum of 5 satellites (6 for corruption detection) is a minimum for actual monitoring. The RAIM prediction/measurment requires a significantly higher standard than this to say "yes".

[Fuji Abound]

I still dont understand why the FAA will (or are likely to) certify GPS with WAAS for precision approaches?

I think I understand how the greater accuracy works and I think I understand that with the appropriate RAIM prediction you can be confident sufficient satellites will in range whilst conducting the approach, however I dont see that the system even with RAIM provides any means of integrity checking?

Presumably if there was an error in the system then with WAAS the additional information transmitted by the WAAS network would correct for this error but how would you know that you are receiving the WAAS information and is that in itself sufficient to provide the integrity check?

[mm_flynn]

The FAA have authorised a large number of WAAS approaches (about 1000 with VNAV, target to increase by 500/year).

I believe the integrity checking is continuous and checks - a certain minimum number of valid signals are being received, the WAAS ground receivers are receiving internally consistent data from all of the satellites (any problem is up linked to shut down the effected satellites or flag to the WAAS enabled device that there is a problem), your onboard kit has determined less than some allowable size variance in the position determined by each of the combinations of satellites it has available.

As such, this is has significantly more integrity checking than an ADF or VOR and probably about the same as an ILS.

[Fuji Abound]

I believe the integrity checking is continuous and checks - a certain minimum number of valid signals are being received, the WAAS ground receivers are receiving internally consistent data from all of the satellites (any problem is up linked to shut down the effected satellites or flag to the WAAS enabled device that there is a problem), your onboard kit has determined less than some allowable size variance in the position determined by each of the combinations of satellites it has available.

I see.

So what I think you are suggesting is that in an extreme case if for some reason a number of satellites were not transmitting correct data the WAAS satellite would "instruct" your receiver to make those satellites invisible to your receiver. If the number of satellites then fell below that required for 3 D navigation you would receive a warning.

Not having used an installation with WAAS enabled I still dont understand how the unit would "know" it was no longer receiving WAAS information - is this monitored and annuciated in some way?

[denhamflyer]

The integrity is provided in a number of ways one of the key ones being that the ground stations actually receive the real GPS signals and verify them against known facts (i.e. where they are) the results / variations and status are then set out over WAAS signal so the GPS receivers can adjust themselves accordingly.
There is also significant additional information for the receiver to verify all the facts (and detect errors) as best it can. The detail of this is a hugely complex subject and apparently isnt perfect - hence why the FAA and other put limitations on its use.
I look forward to when LAAS arrives and the autopilot can land for us:E
This site may be of some help
http://gps.faa.gov
Edited: Sorry for duplication posts overlapped.

Edited: Yes WAAS is monitored and in the same way you get a RAIM alert. It will alert you and I believe not activate approaches that rely on it.

[High Wing Drifter]

The key is - are enough satellites going to be available in the correct positions to provide the required accuracy for the kind of flying (see above post).I think you are referring to the Dilution of Precision errors (geometric position of the satellites). I think the main problem is that the UHF signals are extremely weak compared to other radio aids. It is my understanding that it takes very little to jam a GPS signal such that jamming is usually inadvertent. The Dilution of Precision errors can degrade accuracy, but not to the degree that would cause significant problems for en-route navigation, up to about 70 meters I believe. If the satellite is low on the horizon atmospherics can degrade accuracy by up to 4 meters. Plus some other bits and bob, say about 80m possible inaccuracy.

I have once seen my non RAIM GPS place me 5nm from where I knew I was. I think this would be impossible with a RAIM set without a warning.

[Fuji Abound]

The integrity is provided in a number of ways one of the key ones being that the ground stations actually receive the real GPS signals and verify them against known facts (i.e. where they are) the results / variations and status are then set out over WAAS signal so the GPS receivers can adjust themselves accordingly.

Yes but that is not integrity of the on board kit which is the issue.

With the localiser in the event the signal is compromised either the ransmission is discontinued or or the navi and ident components are removed from the carrier. So far as the pilot is concerned the warning flag will immedaitely confirm the integrity of the signal has been compromised.

I am still interested how or if WAAS provides any additional form of integrity check. For example, and I can understand that if the WAAS signal were to fail for any reason the onbaord set might flag the failure (and it probably does) - however not having used a WAAS enabled system I was interested to learn if this is so.

[denhamflyer]

No I was refering to the combination of The Estimated Position Uncertainty (EPU), Dilution of Precision (DOP), and horizontal and vertical figures of merit (HFOM and VFOM) general accuracy based upon visibility of satellites and reliability of the timing of the signal that can be seen in an area. RAIM is continuously trying to calculate these and whenever it goes below the "protection limit" then a RAIM alert will be raised. RAIM is also looking to failures/inconsistencies that directly affect the above items.
RAIM prediction is more about satellite geometry at the destination at a given time.

I agree with your point about jamming etc. But I would suggest that in practice it is less susceptible to interference than an ILS/VOR - metal buildings planes crossing the path etc.etc.. (Malicious interference would be a different matter). The difference is that the PILOT gets told in a RAIM environment but suffers (unless ATC notice and inform him) in the other.

Extract from the FAA website:-
"The WAAS message improves the accuracy, availability, and integrity (safety) of GPS-derived position information. Using WAAS, GPS signal accuracy is improved from 20 meters to approximately 1.5 - 2 meters in both the horizontal and vertical dimensions. Availability is increased through the addition of the WAAS satellites providing additional course correction. WAAS also eliminates the requirement to conduct receiver autonomous integrity monitoring (RAIM) predictions. In terms of safety, WAAS provides pilot alerting within 6 to 8 seconds (depending on the airborne equipment) anytime the input signal for positioning becomes unusable."

Edited: post crossed again. The WAAS signal in effect gives a further cross check that the GPS can use to very the whole picture "consistency check". Unfortunately it may just result in a "I dont trust what I see" - most systems will "dead reckon" for a short period until the signal integrity is re-establish - hence the requirement for 95% of the time. The 6-8 seconds is important so one can perform a missed approach if it occurs. I think bigger/faster aircraft have a faster detection time requirement.

[IO540]

It's very difficult to jam a GPS signal without it being very obvious.

If you can find the Volpe GPS vulnerability report (I have a copy if you can't find it online) that is very interesting reading on how to jam GPS.

Whereas one can bend a VOR/LOC signal quite easily.

[mm_flynn]

Yes but that is not integrity of the on board kit which is the issue.
With the localiser in the event the signal is compromised either the ransmission is discontinued or or the navi and ident components are removed from the carrier. So far as the pilot is concerned the warning flag will immedaitely confirm the integrity of the signal has been compromised.

As others have said, the WAAS signal provides the equivalent of the removal of the ident. That is the Navaid notifying you that it is unreliable.

On all of your other onboard kit there is very little additional integrity checking. If you are receiving a signal with an ident and there is no fundamental problem with the box you will not see a Nav flag. Your ILS doesn't do a DME/DME position cross check or radar altimeter check to confirm that the beams have not been bent by a piece of equipment encroaching on the transmitters. On the other hand the WAAS receiver gets its integrity/correction signal plus has a number of satellites to make multiple calculations on its position + some level of trend monitoring so that if it unreasonably jumps in position it can notice this. In my view, significantly greater integrity than other GA navaids.

On an 'amusing' note - I had my ADF indicator fail inbound to the ILS hold at Antwerp. I had the Ident and a remarkably stable needle pointing directly at the beacon (as verified by the moving map picture). It was only when I over flew the beacon and the needle still pointed straight ahead that I realised the ADF indicator had packed it in!! So the only 'Real' ADF hold I have ever had to do wound up as a GPS hold.

[IO540]

The morse ident does nothing at all in reality - except confirming that you have the right frequency tuned. It says nothing about whether the navaid or your receiver is actually working.

An ILS is monitored and AIUI the ident is suppressed if a failure is detected but that is only at the transmitter. I used to fly a PA28 Archer which had a duff VOR which idented and the flag was away, but it was dead half the time.

[Fuji Abound]

Your ILS doesn't do a DME/DME position cross check or radar altimeter check to confirm that the beams have not been bent by a piece of equipment encroaching on the transmitters.

It doesnt, but my understanding was that the test equipment on the approach for Cat II and III would identify the integrity of the localiser had been compromised and fail the ident carrier or the nav. Subject of course to the receiver working there is a fail safe loop.

It was my understanding that as reliable as we all know and love GPS to be one of its shortcomings is that it does not have that fail safe loop.
I appreciate nor any more than VORs or NDBs or for that matter DMEs but that is why they are not used for precision approaches.

This debated moved on to whether WAAP enabled GPS makes precsion approaches safe and the the same minima as a localiser with glide slope.
It seems to me if the receiver "tells you" the integrity of the WAAP encoded signal has failed you are on to a pretty good winner but if it does not I cant see how the system is any more fail safe hence my question.

[mm_flynn]

IO,
If the ident is TST rather than what you expected or not there then it means the NAVAID (or its operators) think there is a problem.

you are correct in the sense that the ident's presence doesn't prove it is working
---------------------
Lack of the WAAS signal does cause the receiver effectively to 'Flag' the display. In addition, even the non-WAAS IFR approved GPSs will flag if they detect a posiiton inconsistency based on the multiple satellites that they use.
AIUI the monitoring of a CAT I ILS is only in the very immediate area of the transmitters. It will not notice if the metal object is outside of the immediate area. I believe CAT II/III has much more extensive monitoring and can detect more but not all disturbances.