GPS

[Garrett Wollman]

<<On Thu, 16 Dec 2004 11:23:23 -0500, Sid Whitaker <sid.whitaker@unh.edu> said:

> Shawn is right about the military's use of GPS. My understanding is that
> civilian GPS is bascially the same as military GPS, with something called
> "selective accuracy" (SA) thrown in.

Actually, it's called "selective availability".  GPS offers three
different services of varying accuracy.

- SPS with SA ("Standard Positioning Service with Selective
Availability") is the lowest-level service.  The signals broadcast in
this mode have an artificial, pseudorandom noise added to them, which
degrades the positioning accuracy to about 100 m on all axes.  In the
current fleet of NavStar satellites, the amplitude of the SA noise can
be varied depending on where the satellite is currently beaming, so
service can be degraded in trouble spots while allowing high-quality
solutions over the US mainland.  (This is the principal reason why the
Russians have their own positioning system and the European Space
Agency is about to launch its own.)  For a stationary observer, the
noise introduced by SA is uniform and can be removed by averaging over
a long time.

- SPS without SA is the mid-level service.  This service has two modes
of operation: either SA is disabled in the satellite, or (in trouble
spots) the key to the pseudorandom sequence is broadcast periodically,
encrypted so that it may only be decoded by authorized receivers.
This service has positioning accuracy of about 10 m.

- PPS ("Precise Positioning Service") adds an encrypted timecode on a
second downlink frequency, which allows receivers to precisely
determine the propagation delay between the satellite and their
position, giving a better position fix.

It is possible to get near-PPS accuracy using a technique called
"differential GPS", wherein a radio transmitter is precisely located
using traditional surveying.  The station broadcasts the difference
between the current GPS position fix and its true location; receivers
located nearby can then use this signal to correct their own
independent fixes.  (This works because all of the receivers in a
given area will be observing the same satellites at the same time.)
The US Coast Guard operates DGPS stations in major harbors as aids to
navigation.

Now, all of these positioning services rely on the distribution of
very precise timing information.  Each NavStar satellite has on board
an atomic clock which is synchronized with the US Naval Observatory's
hydrogen maser ensemble to provide very accurate and very stable time.
This makes GPS an important resource for precise time and frequency
measurements.  All modern radio and television exciters use GPS as a
frequency reference (most will also accept a standard frequency input
from an atomic frequency standard).  Qualcomm's CDMA
cellular-telephone technology (used by Verizon Wireless and Sprint
PCS) requires all cell sites and all handsets to be synchronized to
GPS time within a few microseconds.  Computer network operators depend
on GPS for distribution of accurate time signals across a wide-area
network to accurately determine one-way delays, and many networking
protocols require some measure of clock synchronization among
participants.  Of course, all sorts of navigational activities, from
hiking to aviation, depend on the positioning service.

This makes it highly unlikely that GPS would ever be "turned off".  It
might well be degraded by enabling Selective Availability again; this
would have little impact on most timing applications (which already do
the averaging I mentioned) and civilian navigation can usually survive
with 100 m accuracy.

-GAWollman