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Q: GPS station technology/health safety ( Answered,   1 Comment )
Subject: GPS station technology/health safety
Category: Science > Technology
Asked by: mike521-ga
List Price: $20.00
Posted: 14 Feb 2005 10:58 PST
Expires: 16 Mar 2005 10:58 PST
Question ID: 474421
Hi.  A company is interested in setting up business on the farm I live
on, and to do this they would like to put some kind of GPS "station"
up in the middle of the farm.  I'm not sure what kind of
radiation/information it will be transmitting/receiving, but I was
wondering if it is likely going to be safe to live near or not.  It is
used to track employees and equipment using GPS technology, but that
is all I know about it.  Thank you for your help.
Subject: Re: GPS station technology/health safety
Answered By: siliconsamurai-ga on 14 Feb 2005 12:08 PST
Hi, thank you for submitting your question to Answers.Google, I hope I
can provide the information you are seeking.

The bottom line is that if you live at least several yards from the
unit in question there should be far less danger than is already
generated by your computer screen. (Which is also extremely small.)

GPS, the federal government?s global positioning system, is based on
the reception of radio signals from a number of satellites (26 in
2000) and, as such, a GPS ground station is about as innocuous as you
can get. It is essentially just a radio receiver and the satellite TV
signals you are constantly bombarded with are far, far more powerful,
although they are less powerful than the electro magnetic field
generated by a lawn mower.

What you are probably referring to is a GPS differential ground
station such as that operated at many airports.

Standard GPS, now that the military has released the encryption locks,
are generally accurate to about 3 meters. In practice I have found it
to be even more accurate in the Eastern U.S., always being able to
tell which side of a small two-lane road I was on. By comparing the
satellite data with a known-fixed ground transmitter, this accuracy
can be increased to sub-meter, that is, two or three foot accuracy.

For surveying purposes, you can reduce this to a matter of inches with
little difficulty but I suspect from your question that you are

Ground stations are the official reference points coordinated with GPS
signals, but what you are referring to is a fixed, known position
station which transmits relatively low-power signals which advanced
GPS systems can use to increase the accuracy of the satellite signals.

You are probably looking at signals in the 1755 to 1780 Megahertz range.

See also:

There is a photo of a differential gps ground system unit at


You will find a lot of HAM-oriented technical details at

GPS differential explained:

Thank you again for turning to Answers.Google for help. The bottom
line for you is that a GPS differential station can have a variety of
power signatures, most of which would be completely innocuous. You
should speak with the company in question and simply ask them about
the power output of the station they wish to install. With the limited
intention of being able to precisely locate workers on a ranch, or
even within a few dozen miles, should be in the 1 watt range or so
which would be virtually risk-free at even a few feet distance from
the unit. Your cell phone, if you have one, or a FRS radio or other
walkie-talkie unit, would be much more dangerous.

Since I have provided a conclusion to assist you, I should point out
that I have a background in physics and served for many years as a
state certified radiologic-monitor for my region.

The bottom line is that if you live at least several yards from the
unit in question there should be far less danger than is already
generated by your computer screen. (Which is also extremely small.)

Request for Answer Clarification by mike521-ga on 14 Feb 2005 13:45 PST
Hi.  Thanks so much for your extremely fast and helpful response.  It
is also very encouraging to know that we are talking about a fairly
harmless GPS station here.
One thing I did want to clarify about my question (and make sure it
doesn't change the answer) is that the company has their headquarters
about 200 miles from the farm where I live.  It seems that the tower
is a differential ground station just as you mentioned, but is it at
all possible that they're sending the coordinate information collected
at the farm to the headquarters through this tower?  I doubt it, but
if they're not, are they sending the info through standard telephone
I'm not really worried if it's just a receiver, but if it's
transmitting EM waves then that could be worse, right?  I have a bit
of physics knowledge as well, so you can use some physics talk if that
helps.  Basically I'm just wondering what all is happening with the
whole system, which consists of handheld GPS units, a tower, and a
company that is monitoring from 200 miles away.  Thanks so much for
your help, I really appreciate it!

Clarification of Answer by siliconsamurai-ga on 15 Feb 2005 04:35 PST
Without knowing where you are located, what kind of company it is, and
what they intend to use the station for I just can't provide any
further information. It isn't a question of getting too technical in
the answer.

There are just so many things they could be doing but it is very
unlikely that any plans they have include siting a powerful radio
transmitter on the property. Of couse that is always possible but they
are probably doing something in your area which requires their
vehicles or employees to have sub-meter GPS accuracy and you are in a
"dead" area from a commercial airport which is where such differential
beacons are normally located. They probably aren't sending any
information back to that distant HQ, although they may have recording
devices in their vehicles which let them download details later.

Not knowing the kind of company it is I just can't narrow down the
guesses but GPS use if very common in farming country to guide
equipment in the fields where, obviously, landmarks keep getting
plowed under every spring.

Why not just ask them? I'll be happy to try and interpret the answers
they give you.

If they won't provide any information and you can't compel them to,
you can get some clues from things such as seeing whether this will be
powered by solar cells or line current. If solar cells then the power
can't be very great. Even if they want to run a power line it may not
mean anything but if not then you have an answer right there.

I wouldn't want to live right under a differential station but the
power levels from any transmitter, unless it is directional, drops off
very quickly.

You can see the specs for a typical commercial GPS and differential
beacon receiver at

Bear in mind that you can also fit a sub-meter receiver in your pocket
? one which attaches to a laptop ? there are a lot of options.

On the technical side there is, of course, the inverse-square law
which applies unless there is a directional antenna. If there is a
directional antenna for some purpose then the side lobe radiation
intensity is much less than that resulting from the inverse-square

Consider cell phones which some people have claimed can?t possibly be
dangerous because they only use a few watts ? they forget that the few
watts are coming out of an antenna only inches from the brain which
makes them a lot more powerful than, for instance, living a half mile
from a 50,000 watt radio station transmitter.

In your case you haven?t specified the distance from the antenna so I
can?t do any calculations but I suspect it isn?t being built right on
top of your home so even a 50-100 watt transmitter would probably be
less potentially dangerous than using a cell phone, FRS walki-talkie,
or other radiation source.
Subject: Re: GPS station technology/health safety
From: siliconsamurai-ga on 15 Feb 2005 04:42 PST
This detailed description of some government stations might be useful
but the fact that they are using 1.5KW transmitters isn't any
indication that the unit you are concered about will have anything
like that output - the coast guard is obviously trying to cover a very
large area which has no place to put stations.

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