Fun fact: (most) civilian GPS receivers stop working when they're moving faster than 515m/s, are more than 18,000m above sea level, or are accelerating faster than 4g. (39.2m/s^2)

If you're thinking that these operational constraints sound a lot like the performance envelope of a guided missile, then gosh, you're right. IIRC, unlocked GPS recievers are classified as MTCR category II dual-use missile related components, with the usual suite of import restrictions.

EDIT: Looked it up[1] for... some reason, and yep, category II, item 11, section A.3:

  b. Designed or modified for airborne applications and having any of the following: 
    1. Capable of providing navigation information at speeds in excess of 600 m/s; 
    2. Employing decryption, designed or modified for military or 
    governmental services, to gain access to GNSS secure signal/data; or
    3. Being specially designed to employ anti-jam features (e.g. null steering 
    antenna or electronically steerable antenna) to function in an 
    environment of active or passive countermeasures. 

So, in fact, it's just the speed restriction. The numbers I have are probably from some American arms-control law, not a MTCR restriction.

1: http://www.mtcr.info/english/MTCR-April2011-Technical-Annex....