years ago, a commercial airplane crashed in the Atlantic, just off Long
Island. A multi-agency task force was created and had responsibility for
recovering victims, and later, the wreckage. The technology used in this
effort was breath taking. Several square miles of ocean floor, 120 feet
deep, were mapped with SONAR and Laser Line Scanning devices. The
resulting images revealed a huge field of debris along with the exact
latitude and longitude for individual pieces. Next, dive teams were
deployed to recover these targets. The challenge that remained was how
to find these exact points on Earth, without any visual landmarks, and
place divers precisely enough to avoid lengthy searches?
answer came from NAVSTAR GPS. Short for Navigation
System with Time And Ranging Global
Positioning System, this technology is operated by the U.S. Air
Force. It consists of a constellation of 24 artificial satellites in high
Earth orbit, each transmitting data to land based GPS instruments. The
receiver/computer (R/C) GPS instruments used by the recovery dive teams
were able to determine
position to within a meter of accuracy. These devices guided the teams to
their target, where they dropped an anchor connected with a descent line
to a surface float and launched their divers. In this fashion many of the
victims, and much of the wreckage was ultimately recovered.
Originally created for national defence, GPS is now available to all of
us. Unlike any other navigation system or technique, it works 24 hours a
day, in any weather, in any visibility, and to high levels of accuracy.
Inexpensive and dependable instruments are available and they have
revolutionized navigation. With minimal training, users and find their
exact positions, determine course and speed, local variation, time to
destination, and cross-track error (how far off course you are). They can
also keep a library of pre determined positions (way points), and can
determine the bearing and range between any two way points.
How Does it
Starting in 1978,
the U.S. Department of Defence began launching GPS satellites into orbit,
approx. 11,000 nm above the Earth. This network now has 24 operational
satellites plus several back ups. Each is assigned to one of six
different orbital paths which keep them to within 60° N and 60° S
latitudes. These flight paths, together with the high altitude of the
satellites, allow GPS signals to reach any point on Earth. Each satellite
is about 2000 pounds, is partially solar powered, and transmits at a
maximum of 50 watts.
So what are these
satellites telling us? Well each is assigned a unique identification code
called a pseudo-random number. This tells the land based GPS instrument
which satellite is transmitting, and this number (1-32) is displayed on
the R/C display. Next the satellite transmits date and time data as well
as orbital data for ALL of the NAVSTAR satellites.
To find position,
the GPS instrument takes the satellite data, determines the exact time and
calculates the exact predicted position of the satellite. Next it
measures the amount of time that the satellite's signal takes to reach the
instrument and calculates the distance to the satellite. By doing this
with at least three different satellites, a 2 dimensional fix (latitude
and longitude) can be triangulated by the GPS R/C. With data from 4 or
more satellites, a 3 dimensional fix can be obtained showing lat/long as
well as altitude.
Author's note: As
of midnight, 5/1/00, the U.S. government has set Selective Availability to
Zero. While reserving the right to degrade the signal if strategically
necessary, the Department of Defence will be transmitting accurate signals
for all GPS users. This section on SA will remain in the module for
(hopefully) historic reasons.
Oh boy, this is
great! But what's the catch? It's called Selective Availability
(SA). Uncle Sam didn't spend $10 billion bucks just so we could get to
our favourite fishing site. GPS was developed and is maintained primarily
for national defence, so the system is degraded slightly when used for
civilian purposes. This is so that hostile countries or organizations
don't have the precise accuracy available to the U.S. military.
transmits over three frequencies, one being assigned to civilian use.
Under Selective Availability, an intentional error is introduced to the
civilian signal, creating positional variation of from 0 to 300 feet. For
most navigational purposes this is good enough, but where greater
accuracy is needed, Differential GPS (DGPS) technology can correct
the SA error for an accuracy of within 3 meters or better. This system,
administered in selected areas by the U.S.C.G., uses a GPS
receiver/computer located at a precisely known position. This receiver
then determines the exact amount of SA error for that moment and transmits
correction data to DGPS equipped R/Cs within the coverage area. Some of
the limitations of DGPS are the added cost of the equipment and the
limited coverage area.
which affects the accuracy of GPS is satellite geometry. Remember in the
plotting module, when we talked about bearings? The most accurate fixes
were obtained when bearings were about 90° from each other. The same is
true for GPS "bearings." A fix from three satellites located in line with
each other will not be as accurate as one obtained from four satellites
spread out around our position. Because of the orbital paths that the
satellites take, this is usually not a problem unless the ship is sailing
in a river or fjord, where steep banks block a satellite's signal.
Buy a GPS Receiver/Computer
There are dozens of
different instruments available. How do you pick one? Well first ask
yourself several questions:
What do you need a
GPS R/C for? How much accuracy do you need? Obviously an inner
harbour tug boat would require a much greater
degree of accuracy than a family cruiser. Maybe you don't really need
one, but would like to have one aboard for fog, or emergencies.
What features do you
need? How many waypoints will you be keeping? What type of display
do you like? Is it easily visible at night? Does it have a map
display? Do you want the portability of a handheld or do you need the
large display size of a mounted model? Do you need to interface the GPS
with your RADAR or electronic course plotter?
How much do you want
to spend? Quality instruments start at under $200 and go up into
the thousands. I have used an inexpensive hand held Garmin 45 for
several years now, and it's never disappointed me.