IFD 5 Series & IFD 4 Series Touch Screen GPS/NAV/COM - IFD

IFD - Questions re GPS Status

Printed From: Avidyne
Category: Avidyne General
Forum Name: IFD 5 Series & IFD 4 Series Touch Screen GPS/NAV/COM
Forum Description: Topics on Avidyne's IFD 5 Series and IFD 4 Series Touch Screen GPS/NAV/COM
URL: http://forums.avidyne.com/forum_posts.asp?TID=2487
Printed Date: 09 May 2025 at 8:17am
Software Version: Web Wiz Forums 12.01 - http://www.webwizforums.com

Topic: IFD - Questions re GPS Status

Posted By: PhJebens
Subject: IFD - Questions re GPS Status
Date Posted: 15 Mar 2023 at 8:49am

Hi, trying to wrap my head around the Status-GPS-page and some of the terminology. I would appreciate your help:

Are the numbers for HPL and VPL calculated by the IFD, and does the calculation reflect the status of "SBAS Nav" vs "FDE Nav"? Or do those numbers come from the SBAS-satellite?
If the numbers are calculated, what is the cylcle? 15 secs?
Why are some of the satellite vehicles shown with a solid green bar and others with only green outlines?
Am I right in assuming that the system is showing "SBAS Nav" whenever it receives valid SBAS-signals, and in this case GPS integrity is based on HPL vs. HAL?
If no valid SBAS-signal is received, does the system automatically revert to "FDE Nav"?
Does the HFOM play any role in determining whether "GPS integrity is lost" or is this CAS tied to the relationship between HAL and HPL?
I understand that the FMS switches to the appropriate Approach mode (LPV, LNAV/VNAV and LNAV) on vtf with the FAF as the active waypoint. When is the integrity checked? I understand that the Garmin boxes run the check 60 secs prior to reaching the FAF. Is this similar with the IFDs?

Thanks a lot!

Philipp

Replies:

Posted By: AviSteve
Date Posted: 15 Mar 2023 at 9:53pm

FDE (Fault Detection and Exclusion) uses redundant range measurements to monitor the integrity of the satellite signals. When it finds a range measurement with an unacceptably large position error, it excludes that source from the solution. SBAS (Satellite Based Augmentation System) uses a ground station network to monitor the integrity of GPS and SBAS geostationary satellite signals.

HPL/VPL are computed by the GPS receiver. HPL/VPL(FDE) is computed differently from HPL/VPL(SBAS), but they are both statistical measurements that provide an indication of the solution accuracy. It represents the radius of a circle around the true position which is assured to contain the indicated position.

A hollow bar means that the satellite is being received but is not used as part of the solution. A solid bar means that the satellite is being used in the solution. I thought this was in the Pilot’s Guide but I couldn’t find it. I’ll make a note of that.

The IFD will show the integrity monitoring scheme that is currently being used to determine the position. Generally, if you’re getting SBAS corrections it will be SBAS Nav. GPS integrity is lost when the HPL exceeds the HAL (Horizontal Alert Limit). FOM is not used for that determination. Integrity is constantly being checked.

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Steve Lindsley
Avidyne Engineering

Posted By: PhJebens
Date Posted: 19 Mar 2023 at 11:26am

Thanks for your help!