TheUS Global Positioning System (GPS) consists of
a constellation of 24 satellites that emit radio signals
for reception by specially designed devices.
If signals from 4 or more of these satellites are
picked up by a GPS device, it can determine its location
with high, reliable accuracy.
The orbits are arranged such that there are,
in fact, always at least 4 satellites visible
from any point on the surface of the earth.
See tutorials below
for GPS details.
President Clinton announced at the
beginning of May 2000 that the GPS signals would
no longer be degraded for civilian receivers. Now
these civilian devices can obtain location accuracy
of 15m (~50ft) horizontally (100m previously)
and 100m (328ft) vertically.
GPS devices are now available at prices as low
as $100. The low end devices provide basic features
such as current position, distance and direction to
a set point, current speed, trip distances, track
logs, etc.
More expensive devices provide map features such
as showing your current location on a map display.
Also, they can guide you to a given address. They
also have bigger, more sensitive antennas for faster
response. Some portable units can also be connected
to cigaretter lighters for power.
Comment: I did not include
GPS as a HobbySpace topic initially because, like Direct-to-Home
TV, I thought the space aspect of GPS was too hidden
and irrelevant to the user. However, after using and
enjoying my own handheld GPS navigator, I changed my
mind.
When I first turn on my navigator, for example, the
display shows the expected positions of GPS satellites
in range and then gradually indicates which ones have
been detected. Until a minimum number are found, it
cannot find its position. As I move around I must avoid
obstructing the view of the sky if the device is to
keep the position updated. I continually return to the
display to see how many satellites are currently detected.
So my GPS navigator makes me quite aware of the satellites
above me. This is squarely in the HobbySpace
spirit - promoting those activites that involve us,
i.e. the general public, with the space infrastructure
and make us aware of its benefits .
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