Alert Limits for GPS??

[uwochris]

Hey guys.

I got a question regarding GPS.

From the "Aeronautical Information Publication"- section Com 3.16.3:

" The level of integrity for each phase of flight is expressed in terms of horizontal and vertical alert limits... if the satellites in view do not support the applicable alert limit (2 NM enroute, 1 NM terminal, .3 NM non-precision approach), the avionics RAIM function will alert the pilot, but will continue prividing a navigation solution.... pilots must discontinue using GPS for nav when such an alert occurs."
So the question is- what exactly is an alert limit? I have no clue what the significance of the 2nm, 1 nm, or .3nm is. Does anyone have an explanation? I havent found any info in other sources.

Thanks in advance,

Chris.

 

[JediNein]

In plain English is means if the GPS gives you a "RAIM Failure" or "INTEG" or "Approach not active", you do the missed approach.

In technical English, if the GPS can't get the position error down within the limits for the mode it is in, then it will giving a warning message of some sort. The position error is the field usually shown on the satellite screen that says how accurate the unit really is, 99% of the time that error is less than 10 feet.

Fly SAFE!
Jedi Nein

 

[HMR]

So the question is- what exactly is an alert limit? I have no clue what the significance of the 2nm, 1 nm, or .3nm is. Does anyone have an explanation? I havent found any info in other sources.

Thanks in advance,

Chris.

This refers to GPS's that are "approach certified". All approach certified GPS's have three modes of sensitivity. Full scale deflection on your CDI will equal 5nm ("enroute" mode when more than 30nm from destination airport), 1nm ("Approach Armed" or "Terminal" mode when within 30nm of destination) and .3nm ("Approach" mode at approx. 2nm miles from Final Approach Waypoint) depending on where you are in relation to the approach.

The receiver can do a RAIM check at anytime but has to do it by 2nm from the Final Approach Waypoint. If there is a problem with RAIM you will get an alert.