Re: GPS and security

[karn@qualcomm.com (Phil Karn)]

>The requirement, of course, is that you be someplace near a differential GPS
>station. These stations are maintained by the US Coast Guard and obviously
>exist only where there is navigable water (and certainly not everywhere,
>since they take time and money to build).

Well, yes, the USCG *is* building its first DGPS beacons along the
coasts, for obvious reasons. But there is precedent for the USCG
becoming involved in radionavigation coverage elsewhere in the
US. Several years ago, in conjunction with the FAA, the USCG built
several new Loran-C stations to fill in what had been known as the
"mid continent gap". Now there is good Loran-C coverage over the
entire continental US. If GPS becomes as popular for aviation as
Loran-C has become (at least for helicopter and general aviation, if
not commercial aviation) then I think it quite likely that the same
thing will be done with differential GPS stations.

The only catch is likely to be that if the primary "legitimate" user
of DGPS is aviation, then the DGPS beacons will be sited so as to give
good coverage to aircraft in flight and near airports, not necessarily
good coverage to any user anywhere on the ground. A similar situation
already exists with respect to VORs; get high enough and the entire
country is completely covered, but it's hard to hear one on the ground
unless you're very close to one.

The actual coverage of any DGPS beacon will depend on its frequency
and transmitter power. VHF or UHF would be the most natural places to
put these things for aviation use, and as everyone knows these tend to
be limited to near line of sight, just like VOR. The USCG is putting
its DGPS data on its existing LF (300 Khz range) marine
radiobeacons. These have wider coverage than VHF/UHF, but suffer from
noise and interference problems especially near major cities. Again,
these beacons are optimized for their intended users -- ships at sea,
not users on land.

I should note that the USCG isn't putting up DGPS beacons just to
defeat SA. Even without SA, plain C/A GPS isn't accurate enough for
precise harbor navigation. It just so happens that DGPS takes care of
SA as well as the natural errors they were originally worried about.

An alternative to the USCG-provided DGPS beacons is already provided
via FM broadcast subcarriers as a commercial service. The subscription
price is pretty high, though. They're probably trying to quickly
recoup their investment before the USCG beacons take much of their
market.

Phil