APM/Pixhawk Won't Arm - PreArm: High GPS HDOP

[Hands0n]

While working on a new APM 2.6 build I came across this issue whereby the APM will not arm because of a reported GPS HDOP of greater than 2.  Occasionally, re-powering the multi-rotor will see the HDOP below 2 and so arming is possible. However, that has not been consistent.

Tonight I have been researching this High GPS HDOP issue that we have been seeing.  And I think that I have some answers now ...

Trigger
The PreArm error message only triggers when attempting to arm the APM when a GPS-dependent function is enabled. That would be Loiter, Pos Hold, Auto (mission) or Geofence (I had Geofence enabled). 

In my case, disabling Geofence and setting flight mode to Stabilize sees the FC arm as normal and consistently with a reported HDOP of greater than 2 (but less than 3).

Mitigation
In Mission Planner, Config/Tuning, Full Parameter List, search for GPS_HDOP_GOOD and change it to 300 (from its default 200).  This not such a bad thing to do. See below for why.

Seeing as I want to use Geofence, which when enabled will require GPS before arming, I have set my FC';s parameter to 300 (3).

Root Cause
A longstanding bug exists in the Arducopter code that has been there for a couple of years and not fixed. It is where the Arducopter code is reading the PDOP value from the GPS and misrepresenting it as HDOP. Of course, there can be very many good reasons why PDOP might be higher. 

There is a fairly good explanation of it in a blog post here: https://www.cloud-surfer.net/2014/09/12/high-hdop-values-when-flying-iris/

If this is true then we will have to wait for the bug to be fixed, but the mitigation of setting the value to 300 (3) is benign.