There's a lot of literature describing how a precise clock can
mitigate the strong correlation between the clock and vertical
position states, hence improve VDOP, improve integrity monitoring,
etc. Pratap Misra has written quite a bit on the topic.

An early paper on clock coasting (there were earlier experiments):

Sturza, Mark A., "GPS Navigation Using Tbree Satellites and a Precise
Clock", NAVIGATION, Vol. 30, No. 2, Summer 1983, pp. 146-156,
http://www.3csysco.com/Pubs/GPS%20Na...se%20Clock.pdf

Can't find an on-line copy of this one:

“The Role of the Clock in a GPS Receiver” by P.N. Misra in GPS World,
Vol. 7, No. 4, April 1996, pp. 60–66.

A dissertation that involved flight testing with a Boeing 767 at the
FAA's Atlantic City test site:

Kline, Paul A., "Atomic Clock Augmentation For Receivers Using the
Global Positioning System," Ph.D. Dissertation, Virginia Polytechnic
University, 1997, http://scholar.lib.vt.edu/theses/ava...2516142975720/

A thesis sponsored by Misra:

Sean G. Bednarz, "Adaptive Modeling of GPS Receiver Clock for
Integrity Monitoring During Precision Approaches," M.S. Thesis,
Massachusetts Institute of Technology, 2004, http://dspace.mit.edu/handle/1721.1/17756

A GPS World Innovations column from 2007:

http://tf.boulder.nist.gov/general/pdf/2267.pdf