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The Pilot Controller Glossary (and other places) define MOCAs as:

MINIMUM OBSTRUCTION CLEARANCE ALTITUDE (MOCA)− The lowest published altitude in effect between radio fixes on VOR airways, off-airway routes, or route segments which meets obstacle clearance requirements for the entire route segment and which assures acceptable navigational signal coverage only within 25 statute (22 nautical) miles of a VOR.

VORs aren’t used in GPS navigation so I don’t understand why there would be a GPS MOCA on a T-Route or Victor Airway.

Even the example given in the Aeronautical Chart Users Guide (p56) has a GPS MOCA below the MEA.

GPS MOCA Here’s a segment of a Victor Airway in Florida with a MOCA of 1300 but the MEA for GPS is 3000.

GPS MOCA on Victor Airway

I would think the GPS MEA would be the MOCA or even lower, since VOR reception isn’t required in the first 22 miles. If you fly at the MEA it guarantees that a normal climb will keep you above obstacles when entering the next segment with a higher MEA. But I would think that the MOCA would do the same thing unless there was an MCA. So that doesn’t seem to be a reason.

There are even T-Routes in Florida with a GPS altitude and a lower GPS MOCA. That makes even less sense to me.

Does the GPS MOCA also take into account communication reception—but in that case why isn’t that mentioned in the regular MOCA definition?

If anyone could shed some light on how they come up the GPS MOCAs it would be appreciated.

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  • $\begingroup$ As far as I know, the only thing an MEA provides that a MOCA doesn’t is navigation reception past 22 NM. If it provides more than that, I’d be interested in knowing what else. $\endgroup$ – JScarry Mar 2 '17 at 22:35
  • $\begingroup$ It seems to me that you are asking the wrong question. MOCAs, as you note, are determined by obstacle clearance requirements. MEAs also provide this clearance, as well as meeting additional requirements, such as airspace and communication requirements (especially for RNAV routes). Shouldn't your question be more along the lines of, "how are MEAs determined?". $\endgroup$ – J Walters Mar 2 '17 at 23:42
  • $\begingroup$ @JonathanWalters As you noted, I’m confused. Why does the MOCA differ from a GPS MEA if the only thing different is the navigation signal. Maybe there are other things that determine the MOCA other than navigation signal coverage? $\endgroup$ – JScarry Mar 3 '17 at 1:39
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MOCAs provide minimum obstacle clearance, as you note; MEAs also provide this—and more.

MOCAs are determined essentially the same across the board. The question that I think you should ask is why is the MEA higher than the MOCA in some cases where you might not expect it, namely in the case of some GNSS MEAs.

Firstly, all MEAs take into account multiple factors, including those described in the following. Note the mention of airspace buffers and communications requirements.

From the FAA's Instrument Procedures Handbook, on p. 2-35:

The MEA is the lowest published altitude between radio fixes that assures acceptable navigational signal coverage and meets obstacle clearance requirements between those fixes. The MEA prescribed for a Federal airway or segment, RNAV low or high route, or other direct route applies to the entire width of the airway, segment, or route between the radio fixes defining the airway, segment, or route. MEAs for routes wholly contained within controlled airspace normally provide a buffer above the floor of controlled airspace consisting of at least 300 feet within transition areas and 500 feet within control areas. MEAs are established based upon obstacle clearance over terrain and manmade objects, adequacy of navigation facility performance, and communications requirements.

Earlier, on p. 2-16, the requirement for communications reception in the case of GNSS MEAs (at least those on RNAV T-routes) is explicitly stated. This makes sense given that these two factors are the primary factors needed in flight guided by GNSS.

A GNSS MEA for each segment is established to ensure obstacle clearance and communications reception.

I'm not sure that this answers your stated questions, but I do think it should ultimately answer what you are wondering.

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