Rob,

Sounds like you've delved into a lot of details on more than one AHRS.  You mentioned the Crossbow 500 uses both GPS and airdata aiding. Crossbow's datasheet states that the 500 operates as an independent device without the need for GPS or airdata aiding.  It does state velocity aiding is optional on models "-220" through "-227".  The velocity aiding packet shown in the Operators Manual is for this optional feature. Models "-200" through "-207" do not use this feature  The Crossbow 510 can also operate standalone or with optional GPS and airdata aiding.

Tom Gourley