How often have you gotten lost on a road trip, or tried to
guess what kind of weather was headed your way. Well, a fairly new technology called GPS
has made it possible to tackle some of lifes most ancient problems. GPS (Global
Positioning System) is a constellation of 24 satellites owned and operated by the
Department of Defense (DOD). Scientist and military personal have largely used the system,
but in recent years has become a way for ordinary people to pinpoint their geographic
location and make life a bit easier.
checks his distance and location with a GPS device after a long day
photo credit Kent Clegg
The location accuracy of GPS can be
anywhere from 100 to 10 meters depending on your equipment. Military-approved equipment
however can pinpoint accuracy within one meter. In Advanced forms of GPS you can make
measurements to better than a centimeter, literally giving each space on earth a specific
address. Currently, GPS satellites and 3 spares orbit above the earth about 10,600 miles.
Each satellite contains a computer, an atomic clock, and a radio. The satellite is enabled
to continually broadcast its changing position. Each day at a designated time, the GPS
satellites check their own systems with a ground station to make minor corrections. The
satellites are spaced in equal distance from each other in a way that four satellites will
always be above the horizon.
The Space Segment
The space segment consists of the GPS satellites. The system has 24 operational satellites
that orbit around the earth in about 12 hours. Depending upon satellite conditions, and
possible repair needs, additional satellites may be launched on a needed basis. There are
6 orbital planes, with 4 satellites per plane. They exist equally apart at 60 degrees, and
are inclined 55 degrees with respect to the plane. This way, from any point on earth,
there are always 5 to 8 satellites visible.
The Control Segment
The GPS consists of several ground control stations, with the main control station located
in Schriever Air Force Base in Colorado. Each individual monitor station measures signals
from the space vehicles (SVs) located inside each satellite. The main station in
Colorado uploads ephemeral and clock data to the SVs. The SVs then send
subsets of orbital data to PPS receivers. They use very precise radar to check each
satellite's exact altitude, position and speed. GPS satellites are so high up their orbits
is very predictable. The errors they're checking for are called "ephemeris
errors" because they affect the satellite's orbit or "ephemeris." These
errors are caused by gravitational pulls from the moon and sun and by the pressure of
solar radiation on the satellites. The errors are usually insignificant, but for the most
part, if you want to have good accuracy you will want to make sure you have no errors
coming in. Once the DoD has measured a satellite's exact position, they relay that
information back up to the satellite itself. The satellite then includes this new
corrected position information in the timing signals it's broadcasting. It also contains a
navigation message with ephemeris information as well.
DoD intentionally sends noise into each signal to create inaccuracy. The purpose of this
is to ensure no hostile force uses GPS as means to exploit a terrorist agenda. Military
personal can use access codes to decrypt the errors; therefore they are still able to use
GPS with great accuracy.
The User Segment
The user segment consists of the GPS receivers and the users themselves. Navigation is the
primary function of GPS. Navigation receivers are made for various ships, vehicles, and
people. Time and Frequency have also become a popular use for GPS, as laboratory standards
can be set to precise time signals, and within well-referenced points. With special
purpose GPS receivers, precise positioning is possible using receivers at reference
locations providing correction and relative positioning data for remote receivers.
Examples of this include surveying, geological studies, and plate tectonic studies.
GPS at Work
The first and most obvious application of GPS is the simple determination of a
"position" or location. GPS is the first positioning system to offer highly
precise location data for any point on the planet, in any weather. That alone would be
enough to qualify it as a major utility, but the accuracy of GPS and the creativity of its
users are pushing it into some surprising realms. Knowing the precise location of
something, or someone, is especially critical when the consequences of inaccurate data are
measured in human terms. For example, when a stranded motorist was lost on in South Dakota
for two days. GPS helped rescuers find her.
Sometimes an exact reference locator is needed for extremely precise scientific work. Just
getting to the world's tallest mountain was tricky, but GPS made measuring the growth of
Mount Everest easy. The data collected strengthened past work, but also revealed that the
mountain itself is getting taller.
On the Water
[an error occurred while processing this directive]
It's interesting that the sea, one of our oldest channels of transportation, has been
revolutionized by GPS, one of the newer navigation technologies. And as you would expect,
navigating the world's oceans and waterways is more precise than ever. Today, receivers
exist on vessels the world over, from hardworking fishing boats and long-haul container
ships, to elegant luxury cruise ships and recreational boaters. A New Zealand fishing
company uses GPS so they can return to their best fishing holes without wandering into the
wrong waters in the process.
In the Air
By providing more precise navigation tools and accurate landing systems, GPS not only
makes flying safer, but also more efficient. With precise point-to-point navigation, GPS
saves fuel and extend an aircraft's range by ensuring pilots don't stray from the most
direct routes to their destinations. GPS accuracy will also allow closer aircraft
separations on more direct routes, which in turn means more planes can occupy our limited
airspace. The moon and stars have always been a guiding light to many of navigators in the
past. Tools like the compass and good memory for landmarks helped you get from point A to
point B. However, the situation has never been perfect, and knowing ones location and path
have never been that accurate. Todays outdoorsman are using GPS to remedy the
age-old science of finding ones way.
On the Ground
Tracking is the process of monitoring the user as he or she moves from location to the
next. Commerce relies on fleets of vehicles to deliver goods and services either through
such areas as crowed cities. So, effective fleet management has direct bottom-line
implications, such as telling a customer when a package will arrive, spacing buses for the
best scheduled service, directing the nearest ambulance to an accident, or helping tankers
avoid hazards. GPS used in conjunction with communication links and computers can provide
the backbone for systems tailored to applications in agriculture, mass transit, urban
delivery, public safety, and vessel and vehicle tracking. Cities like Chicago have
developed systems using GPS that track down ambulances, making it easier to save lives in
some of todays fastest growing metropolitan areas. Many of todays emergency
units are now finding it easier to respond to 911 calls.
GPS technology has made surveying easier than ever as well. GPS can pinpoint positions and
routes. Mountains, rivers, forests, roads, routes, and city streets can all be surveyed
using GPS. The city of Modesto California improved their efficiency and job performance by
using GPS and mountain bikes to create a precise map of its network of water resources and
Timing is Everything
Although GPS is well known for navigation, tracking, and mapping, it's also used to
disseminate precise time, time intervals, and frequency. Time is a powerful commodity, and
exact time is even better. Knowing that a group of timed events is perfectly synchronized
is often very important. There are three fundamental ways we use time. As a universal
marker, time tells us when things happened or when they will. As a way to synchronize
people, events, even other types of signals, time helps keep the world on schedule. GPS
satellites carry highly accurate atomic clocks. And in order for the system to work, our
GPS receivers here on the ground synchronize themselves to these clocks. That means that
every GPS receiver is, in essence, an atomic accuracy clock. Astronomers, power companies,
computer networks, communications systems, banks, and radio and television stations can
benefit from this precise timing.