View Full Version : What degrades ANP in an FMS?

15th Sep 2007, 16:23
I tend not to make procedural approaches using FMS having been presented with gross navigation suggestions typically once every three years I don't trust it.
What is the view in your company?
I suspect temporary DME stations can cause problems but what else interferes with our normally hyper accurate Precision RNAV kit?

15th Sep 2007, 21:28
GNSS link failing. Flying into variously the Channel Islands, Norfolk or leaving the ground without a GNSS signal (box can go a bit funny as it starts picking up Nav Aids) makes SIDS interesting...

stator vane
16th Sep 2007, 09:26
we used to fly a 737-300 that after a few minutes in cruise, one of the nav radios would drop off.

we suspected the cold temp contraction would break the connection on one of the antenna wires or something along that line.

wrote it up after a few flights and it returned to normal.

FE Hoppy
16th Sep 2007, 14:13
What degrades ANP in an FMS?

1)GNSS RAIM yes or no
2)Ground based NAV aid availability and geometry when GNSS RAIM is no
3)IRS Drift rate when GNSS RAIM is no and ground based nav aids are not available.

17th Sep 2007, 15:05

My aircraft has a decent IRS (low drift rate) so does this mean if there is a RAIM and ground based nav aid issue the IRS will still provide good navigation performance?

FE Hoppy
18th Sep 2007, 07:20
I'm not familiar with your type but in general an FMS system will blend position sensors to compute the FMS position. The accuracy of the position ANP(EPU) depends on the sensors available and the priority that the FMS uses. If RAIM is yes then the FMS will probably use 100% GPS position so the accuracy of the IRS position is irrelevant.
The normal sensor priority would be:
as this reflects the accuracy of the various nav systems.

Although you may have a good IRS even the best can drift at 2 nm/h.
Some systems monitor irs error when other sensors are being used so that when the irs is the only sensor it will keep the last known offset from irs position and not jump(map shift) to the raw irs position.

18th Sep 2007, 16:02
FE Hoppy,

You obviously know what you are talking about.

Will Solar storms etc affect RAIM...if not what are the key issues likely to cause RAIM problems.

Cheque is in the post.


19th Sep 2007, 01:50
Well, as ionospheric effects are one of the sources or GPS position error, I would say it's possible; Assumptions are made about the signal path through the ionosphere (and troposphere) which would be locally invalidated.

However, the fundamental cause of loss of RAIM must surely be the loss of sighting of 5 or more satellites (or SVs: Space Vehicles)? You need 4 SVs to compute a position, and 5 to give redundency. This is not always assured, hence the fact that you should use the GPS almanac to predict RAIM coverage at the ETA for your destination (if required).

Another cause of RAIM loss would be clock error in one of the SVs, which would then be rejected by the FMS, without actually degrading the accuracy of the derived position.

The whole point of RAIM, though, is that it allows assured GPS accuracy without other sensor inputs (Receiver Autonomous), so if you have DME/DME input, I wouldn't have thought RAIM was required.

19th Sep 2007, 02:09
That is correct, in an intergrated nav solution such as that produced by an FMS, the integrity function can also be provided by other sensors, or a combination of them.

FE Hoppy
19th Sep 2007, 15:10
Nothing to add to the 2 posts above.
I do think that there is a lack of training on these subjects as technology is being introduced much faster than training requirements are updated and I see day in day out a real lack of basic knowledge on this subject.
RMC: these comments are not directed at you and I'm very happy to see someone take an active roll in improving their knowledge.

19th Sep 2007, 23:24

I would like to add the following comments:

Airbus aircraft (equipped with Litton/Northrop Grumman LTN-101) use AIME (Autonomous Integrity Monitored Extrapolation) in their Global Navigation Air Data Inertial Reference System (GNADIRS).

Quote from the manufacturer's website:
"If the data’s integrity is compromised, AIME® automatically uses the aircraft’s position history to maintain accuracy and integrity."

That means RAIM is not required, for example when you're doing a RNAV (GNSS) approach (for example with RNP 0.3).

As part of the pre-planning the pilot should check that more than 22 satellites are available from the expected time of departure time until the expected time of arrival. Applies to a flight were GNSS is required.

The US Coast Guard and the European AUGUR-Website http://augur.ecacnav.com/ provide prediction tools.

Even if GNSS and Radio Update are not available, the IRS provides the pilot with good accuracy. Here's an example for the reliabilty of modern IRS:

From FAA Order 8400.12A (applies to RNP-10 airspace).
In case of a flight through the RNP-10 airspace you can fly up to 6.2 hours after the IRS system has been placed in the NAV-mode. This time can be extended depending on the IRS installed and the last time a Radio Updated was done.



FE Hoppy
20th Sep 2007, 09:23
That means RAIM is not required, for example when you're doing a RNAV (GNSS) approach

I would be very interested to see where this is stated in the AFM.

I'm not familiar with the type but where the nav aid is stated in the title of the procedure i think you MUST use the stated navaid for position update. Where no navaid is stated i.e. RNAV RNP.3 then i would agree no need for RAIM as long as ANP(EPU) is good.

22nd Sep 2007, 16:14

FE Hoppy, I think my posting wasn't precise enough, what I tried to say is the follwing:

If you are doing a GNSS based RNAV Non-Precision Approach with an aircraft that has GNSS-Equipment using RAIM, then you will need a RAIM prediction for you ETA. As far as I can remember this is not necessary if you have GNSS-Equipment using AIME.

Is that the answer to your question?

Please correct me if I'm wrong, as RNAV & GNSS are not easy subjects.