by Chuck Husick
- Current as of May 2009
The GPS consists of
earth based command and control stations and a constellation of polar
orbit satellites. The system is operated by the U.S. Department of Defense.
Each satellite transmits information describing its precise orbital position.
All satellites use the same frequency, allowing the use of a fixed tuned
receiver. Using position information received from at least three satellites,
a GPS receiver / navigator can calculate its two dimensional position
on the earth's surface, its latitude and longitude. Signals from four
or satellites provide improved accuracy, including the ability to calculate
height above sea level (3D position fixing). Although altitude capability
is clearly not of great significance for marine use, the stability of
altitude information can be a useful check on the overall accuracy of
the system. Large altitude changes at times when you are not hauling your
boat up or down a mountain may be reason to wonder a bit about the accuracy
of the displayed position.
At one time, the number
of satellites a receiver could track was a useable evaluation criteria
in selecting a GPS. Most of today's GPS receivers are capable of tracking 12 or more satellites. In most circumstances a GPS will
use data only from those satellites at a reasonable elevation above the
user's horizon. It is rare for more than 8 satellites to be sufficiently
above the users horizon for use by the GPS receiver, therefore virtually
any GPS capable of dealing with 8 at one time will likely provide all
the performance you require.
In order to work properly
the GPS antenna must have a relatively clear view of the sky. In general,
any position on or near the deck of a boat will be satisfactory. The presence
of the mast, boom and rigging of a sailboat at least a few feet distant
won't create noticeable problems. The antenna should not be located close
to and at the same height as a radar antenna. Locations more than a few
feet from a VHF or SSB antenna are usually satisfactory. It is not advisable
to mount the GPS antenna high on a sailboat's mast. The antenna motion
created by the roll and pitch of the vessel will complicate life for the
GPS while offering no compensating advantage.