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Tennessee 811 offers free safety meetings at your
office or work site.
We don't restrict the hours in which to have a
presentation. We work around what works for you! We
can easily have a meeting before your crews go out in
the morning, during lunch or when they arrive back in
the afternoon. We can also tailor the time frame of
the presentation to suit your meeting needs.
Call or email now and ask for Holly Austin or Scott
Holder and we'll be happy to schedule a meeting for
you:
Holly - haustin@tnonecall.com - 615-367-1110 (x7102) Scott - sholder@tnonecall.com - 615-367-1110 (x7140) "Call Before You Dig" is a great safety topic for any
meeting. Even your inside personnel can benefit.
Remember - there is no cost involved to you, and it's a
service we offer. The better educated your employees
are, the safer they will be!
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We thought it might be interesting to run a series about how GPS works. The content for this series comes directly from the Discovery channel's "How Stuff Works" programs via their website. So, sit back and enjoy the read. This article begins the 4th topic in the
series.
GPS Calculations We saw that a GPS receiver calculates the distance to
GPS satellites by timing a signal's journey from
satellite to receiver. As it turns out, this is a fairly
elaborate process.
At a particular time (let's say midnight), the satellite
begins transmitting a long, digital pattern called a
pseudo-random code. The receiver begins running
the same digital pattern also exactly at midnight.
When the satellite's signal reaches the receiver, its
transmission of the pattern will lag a bit behind the
receiver's playing of the pattern.
The length of the delay is equal to the signal's travel
time. The receiver multiplies this time by the speed of
light to determine how far the signal traveled.
Assuming the signal traveled in a straight line, this is
the distance from receiver to satellite.
In order to make this measurement, the receiver and
satellite both need clocks that can be synchronized
down to the nanosecond. To make a satellite
positioning system using only synchronized clocks,
you would need to have atomic clocks not only on all
the satellites, but also in the receiver itself. But atomic
clocks cost somewhere between $50,000 and
$100,000, which makes them a just a bit too
expensive for everyday consumer use.
The Global Positioning System has a clever, effective
solution to this problem. Every satellite contains an
expensive atomic clock, but the receiver itself uses an
ordinary quartz clock, which it constantly resets. In a
nutshell, the receiver looks at incoming signals from
four or more satellites and gauges its own inaccuracy.
In other words, there is only one value for the "current
time" that the receiver can use.
The correct time value will cause all of the signals that
the receiver is receiving to align at a single point in
space. That time value is the time value held by the
atomic clocks in all of the satellites. So the receiver
sets its clock to that time value, and it then has the
same time value that all the atomic clocks in all of the
satellites have. The GPS receiver gets atomic clock
accuracy "for free."
When you measure the distance to four located
satellites, you can draw four spheres that all intersect
at one point. Three spheres will intersect even if your
numbers are way off, but four spheres will not
intersect at one point if you've measured incorrectly.
Since the receiver makes all its distance
measurements using its own built-in clock, the
distances will all be proportionally incorrect.
In order for the distance information to be of any use,
the receiver also has to know where the satellites
actually are. This isn't particularly difficult because the
satellites travel in very high and predictable orbits. The
GPS receiver simply stores an almanac that tells it
where every satellite should be at any given time.
Things like the pull of the moon and the sun do
change the satellites' orbits very slightly, but the
Department of Defense constantly monitors their exact
positions and transmits any adjustments to all GPS
receivers as part of the satellites' signals.
Next time, we'll look at errors that may occur and see how the GPS receiver corrects them. Brain, Marshall, and Tom Harris. "How GPS
Receivers Work." 25 September 2006.
HowStuffWorks.com. Photo courtesy U.S. Department of Defense Artist's concept of the GPS satellite constellation |
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Tennessee 811
email:
tnocs@tnonecall.com
phone:
(615) 367-1110
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