RAIM

[Fuji Abound]

Can someone please explain non baro aided outages?

[Deano777]

Receiver Autonomous Integrity Monitoring


To receive a 3D fix your receiver would normally track 4 satellites, every 20 seconds a fifth satellite replaces the data of one of the 4. If your position "changes" then the 5th satellite or the one that it "disregarded" is in error, so by continuously replacing one of the 4 with a 5th a faulty satellite can be identified.
So your normal 3D fix would require 4 satellites, but if you are using RAIM monitoring then you need the 5th.
Let's say an error is found; we will need another satellite for RAIM to continue to function, so RAIM now uses a 6th satellite for redundancy.

3D fixing with RAIM requires 6 satellites

Basic explanation I know, but this is all I remember

I hope it helps

Dean

[411A]

It has to do with WAAS, wide area augmentation system, in use now in the USA for non-precision approaches, with added vertical guideance.
I don't have one of these new WAAS compatable units in my private airplane, but I wll attempt to ask on another forum about this, and if an answer is forthcoming, will post it here.
However there has been considerable difficulty with some of these WAAS aircraft installations lately and some folks that have the newer WAAS compatable GPS units are not able to use all functions.
It appears to be a software issue, apparently.

[Fuji Abound]

4 AUGUR has been developed by EUROCONTROL and is available at http://augur.ecacnav.com/

It comes from the new AIP supp published by the CAA regarding the recently approved GPS approaches in the UK.

If you look at the site it seems to me to include predicted non baro outages.

I ddnt think this was to do with WAAS becasue as far as I am aware we have no WAAS augumented approaches.

[IO540]

WAAS (EGNOS in Europe) is a means of improving GPS accuracy to the point where it can be used for a synthetic glideslope on a GPS approach. The GPS signal suffers from variable atmospheric delays which place an upper limit on the 3D accuracy. With WAAS/EGNOS you have a ground GPS receiving station which obviously knows its exact position, and it receives the GPS signal and works out the required correction, which is then transmitted back up to a satellite which sends it back down to all compatible receivers. These then use the correction to produce a very accurate 3D solution, within (of the order of) a metre or so.

I doubt there are any such approaches operating in Europe. However I am not in touch with currently ongoing experiments or proposals.

WAAS/EGNOS is not really relevant to enroute navigation, or lateral navigation on a GPS approach which is already plenty accurate enough.

It's worth having I suppose; my Garmin 496 picks up the EGNOS signal.

As to what a "non baro aided outage" means, I haven't got a clue. The way RAIM works is that if you are receiving less than (I think) 4 satellites, the IFR GPS's baro input (from an encoding altimeter) is used to replace the extra satellite. As soon as that extra satellite is received OK, the baro input is ignored. It might be something to do with that.

[slim_slag]

I know what 'baro aided' means - or I think I do - it's where the vertical navigation part of an approach comes from atmospheric pressure, i.e altimeter.

So extending that with some lateral thinking or perhaps guesswork.

'baro aided outage' must mean some failure of this pressure based method of vertical navigation.

'non baro aided outage' must mean some failure which isn't related to the pressure based method of vertical navigation.

Maybe it means you are shooting a GPS approach, using your altimeter for altitude, and the GPS bit fails. A roundabout way of just saying your GPS horizontal navigation has failed, but you still know your altitude. It is a bit like eats shoots and leaves, confusing.

[High Wing Drifter]

I presume it means that there are only enough standby sats for a reliable 2D fix meaning that you only have lateral references from the GPS and no altitude reference.

If so then it is probably only related to you specific position in that not enough suitable sats available rather than anything to do with the equipment.

[Another_CFI]

The following extract from a French AIC may help.

“In order to know whether to look at "baro-aided" or "non baro-aided" predictions, it is necessary to know whether a barometric altitude input is provided or not to the installation aboard the aircraft of interest.

RAIM availability demands that a minimum number of satellites be received. Barometric aiding reduces this number by one.”

That is why the RAIM prediction tools show different results for baro-aided and non baro-aided. Using the Augur prediction tool for my home airfield often shows short non baro-aided outages for RAIM prediction but so far as not shown an outage for baro-aided.

[Fuji Abound]

To hopefully illustrate the point being debated this si what you get for Gatwick for example:

Augur NPA RAIM prediction


Scenario Information
Scenario Start UTC 26/07/2007 23:00:00
Scenario End UTC 29/07/2007 23:00:00
Receiver Mask Angle 5.00
Scenario Almanac Week: 413 TOA: 589824
Active NANUs 2007078
Request Time UTC 27/07/2007 10:05:17



Gatwick (EGKK)
Baro Aided Outages
No RAIM Outages
Non Baro Aided Outages
27/07/2007 07:01:30 until 27/07/2007 07:12:30
27/07/2007 10:39:30 until 27/07/2007 10:45:30
28/07/2007 06:56:30 until 28/07/2007 07:07:30
28/07/2007 10:34:30 until 28/07/2007 10:41:30
29/07/2007 06:52:30 until 29/07/2007 07:03:30
29/07/2007 10:30:30 until 29/07/2007 10:37:30

[slim_slag]

[email protected]

[IO540]

I think Another CFI has it right. You are more likely to get a loss of RAIM if your IFR GPS installation does not have the GPS's baro input connected up. Such installations need one more satellite to get RAIM coverage.

[Deano777]

Fuji

Sorry for the confusion, I merely thought you were enquiring about RAIM itself, sorry for my ignorance :ok:

[Fuji Abound]

No problem Deano.

I am still not sure we have got to the bottom of this one.

Maybe it is my lacking of understanding of the detail of how the G1000 works - and of GPS for that matter.

I wasnt aware in the UK at any rate that GPS is providing vertical guidance?

[IO540]

There is no official context I am aware of, in the UK, where vertical guidance is obtained from a GPS, for the purpose of flight or a landing.

Now, if I was flying some NDB approach somewhere, the altimeter was telling me I am at 2000ft and the Garmin 496 (with EGNOS) was telling me I am at 1520ft, the KLN94 was telling me I am at 1525ft, and my bluetooth GPS (which has the SIRF2 chipset with its +200ft c0ckup) was telling me I am at 1733ft, I know where I would go. And it wouldn't be that NDB approach ;) It would be an ILS.

[Fuji Abound]

IO - yes I agree, but the AIC has this to say

Autonomous Integrity Monitor (RAIM) availability prediction should take into account the GPS constellation predicted for the duration of the flight, NOTAM and avionics architecture eg, Baro aiding input.

my bold.

That sort of suggests that RAIM prediction and in particular baro outages may be important to me as I am steaming down my approach.

It still leaves me worrying how and why and in what way the G1000 is interperolating its GPS information with baro information in arriving at its RAIM predictions and in what way something different is happening in the States for example.

The CAA says I must ask my qualified flight instructor - that should be an interesting conversation.

Perhaps I should ask the CAA - they obvioulsy know something I dont (which is hardly surprising) but I wish they would tell me. :) :)

[IO540]

I think is just the standard requirement for valid RAIM during the GPS approach.

And valid RAIM will require the baro input if you short of that one satellite.

This is how it works in the USA, AIUI.

Whether you need RAIM for enroute (where BRNAV is a requirement i.e. FL095+ airways) I don't know, but I very much doubt it.

In none of these situations is the altitude part of the GPS solution used to suggest where the plane should be vertically.

A RAIM prediction is done purely on satellite constellation geometry, taking into account any duff satellites, and your location. Atmospheric pressure doesn't come into it, AIUI. There are even websites which will do it for you.

[JP1]

Fuji,

"non baro aided outages" means outages based purely on satellite constellation data. Eg the prediction algorithm has not used barometric data.

The use of barometric data will reduce periods of outages.

GPS receivers run something called a Kalman filter. For the vertical channel a GPS can use barometric altitude as an input to the kalman filter. Theoretically this will give a smoothed and more accurate calculation of altitude. Also if a satellite is lost (4 to 3) a 3D solution will still be possible when the GPS is baro-aided although it will be a degraded position/velocity solution.

Cheers
JP1

[Fuji Abound]

Brilliant - that was the sort of explanation I was hoping for. Thank you.

Now I dont understand what the RAIM predictor is doing in my G1000.

Is it using barometric pressure when the system is predicting non baro outages and if it is how do I know it is? Is there any indication from the inbuilt RAIM prediction. Presumably during the period the of predicted non baro outages the aircraft bound system will report reliance on only three satellites?

[JP1]

Sorry can't help with the specifics of a particular system. If the exact implementation of the RAIM prediction model is not explained in the manual, maybe an email to Garmin will provide the info.