SAP accuracy

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billfear
Posts: 17
Joined: Wed Mar 13, 2013 1:12 pm

SAP accuracy

Post by billfear »

I was out practicing some coupled ILS approaches this weekend (SL30 to my HS/WS system) and noticed that the centered needles were putting me about 50 yards left of center at DH and below at KSVH. I then turned on the SAP and noticed that it was right on the button. I realize the SAP is GPS derived position and GRT database runway information.

Just wondered if others have noticed this where the GPS data seems to be better than the good old ILS?
Bobturner
Posts: 444
Joined: Mon Mar 11, 2013 6:34 pm

Re: SAP accuracy

Post by Bobturner »

Something is wrong. The ILS spec is full course width (full left to full right needle) of 700' at the threshold. So at the DA it's about 1000' for a typical runway, or 500' half width. If you were 150' (50 yards) off the needle should have been about 1/3 out of center.
billfear
Posts: 17
Joined: Wed Mar 13, 2013 1:12 pm

Re: SAP accuracy

Post by billfear »

Bob, I agree. But I am sure that the needles were centered on the GRT as the air was still and the TT was doing a nice job, This ILS has had some GS issues over the last couple years but the identfier was coming in loud and clear and it seemed to be working fine. One other thing that is funny, when the GRT auto sets the inbound course for this approach, it sets 284 versus the 282 on the chart. I wonder if that has anything to do with it. I do not have another CDI in the plane but i could potential check the electronic CDI on the SL30 to see if it matches up with the GRT display.

Maybe Jeff or Ben can comment. the airport is KSVH and the approach is ILS or LOC/DME RWY 28.

Next time I go up, i will go to another local airport with an ILS and see what results I get there (in VFR conditions).
Bobturner
Posts: 444
Joined: Mon Mar 11, 2013 6:34 pm

Re: SAP accuracy

Post by Bobturner »

I would check (1) the "moving dots" localizer display on the SL30, make sure it too was centered; and (2) another airport, to see if it is your airport or the SL30. You can test (1) on the ground, just taxi onto the runway.
Were there any aircraft in the run up area? Some airports have ILS critical areas, although that is usually for the GS. Anyone taxiing at the far end, near the localizer antenna?
Don't worry about the 2 degree difference in course settings. That helps out the autopilot but does not affect the cdi needles. What's going on is the gps runs on true course data, which doesn't change. It has very up to date true-to-magnetic conversion data. Its data is more up to date than the chart data. As you know, magnetic north moves around. You'll see the same thing on the airways - the VORs are only corrected for changes in the magnetic north pole every five years or so.
billfear
Posts: 17
Joined: Wed Mar 13, 2013 1:12 pm

Re: SAP accuracy

Post by billfear »

good input, I will do the tests and advise next time I am up.

No, there were no other planes around the runway when I noticed this issue.

Do you know how the GRT flags a localizer or GS failure. I use the yellow needles and I assume they just go away if the EFIS senses a signal error? I think I have seen them do this before.
Bobturner
Posts: 444
Joined: Mon Mar 11, 2013 6:34 pm

Re: SAP accuracy

Post by Bobturner »

Yes, if the SL30 sends a 'flag' (which would drop the warning flag on a conventional cdi) signal, the needles go away.
billfear
Posts: 17
Joined: Wed Mar 13, 2013 1:12 pm

Re: SAP accuracy

Post by billfear »

Flew a couple more ILS approaches last night and the localizer seemed to be right on. This included one at the airport where I had noticed the issue the other day. Only thing I noticed this time, on both approaches I lost the GS needle for about 10 - 15 secs around the time I was capturing it at the FAF. It came back and was fine down to DH. On one of the approaches, there was a Falcon jet landing the other way on the same runway (uncontrolled field, we were talking), wonder if that could affect the signal?

It sure is nice to have the SAP boxes up there as a secondary reference. My autopilot does not do vertical, so I have to manage the GS portion by hand.
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