What your describing sounds alot like GeoCaching.

An already-established sport which I have participated in.
Finally, most if not all of the participants in this sport
also seem to be a younger and/or a college educated
clientele.

More about it at the link(s) below:

http://www.geocaching.com/

http://www.mdgps.net/

http://gpsinformation.net/

"Scott in Aztlan" wrote in message
...
Suppose you were participating in a search for a hidden radio
transmitter, only
instead of the usual radio receiver with a directional antenna you have a
receiver with an omnidirectional antenna and a GPS receiver. As you
wander
around, you collect positions from the GPS and signal strength values at
those
positions from the radio; your goal is to crunch these data points into
an
estimate of the transmitter's location.

Clearly you could use trilateration with any 3 of the data points and get
an
estimate, but how would you make use of the fact that you have an
arbitrary
number of data points? Isn't there some algorithm which, the more data
points it
is given, the better its estimate gets (similar to the way you can
average a
series of position readings taken from a stationary GPS receiver to
compensate
for the effects of SA)?

Can someone point me in the right direction?

Thanks!

--
Friends don't let friends shop at Best Buy.

"Scott in Aztlan" wrote in message
...
Suppose you were participating in a search for a hidden radio transmitter,
only
instead of the usual radio receiver with a directional antenna you have a
receiver with an omnidirectional antenna and a GPS receiver. As you wander
---snip---

Can someone point me in the right direction?

Just follow the strongest radio signal. ;-)

Scott in Aztlan wrote:

Suppose you were participating in a search for a hidden radio transmitter, only
instead of the usual radio receiver with a directional antenna you have a
receiver with an omnidirectional antenna and a GPS receiver. As you wander
around, you collect positions from the GPS and signal strength values at those
positions from the radio; your goal is to crunch these data points into an
estimate of the transmitter's location.

Clearly you could use trilateration with any 3 of the data points and get an
estimate, but how would you make use of the fact that you have an arbitrary
number of data points?

If I understand clearly you are interested in locating an RF source which
has nothing to do with GPS by using RF receivers and nondirectional antennae.
Trilateration is only of value if one can establish accurate timing distances,
synchronized between all receivers.

If you used directional RF antennae, then, in conjunction with receiver positions
determined by GPS receivers, you could simply triangulate to estimate the RF
source position.

Isn't there some algorithm which, the more data points it
is given, the better its estimate gets (similar to the way you can average a
series of position readings taken from a stationary GPS receiver to compensate
for the effects of SA)?

Of course SA was permanently discontinued more than three year ago.

Interagency GPS Executive Board (IGEB) - Special Statement
http://www.ostp.gov/NSTC/html/pdd6.html
http://www.igeb.gov/sa.shtml

Presidential Policy & PRESIDENTIAL DECISION DIRECTIVE NSTC-6
http://gps.faa.gov/gpsbasics/PresPolicy-text.htm
http://www.peterson.af.mil/GPS_Suppo...ts/gps_pdd.htm

Selective Availability
http://gps.faa.gov/gpsbasics/SA-text.htm

Joint Program Office
http://gps.faa.gov/gpsbasics/JPO-text.htm

Scott in Aztlan wrote:

Suppose you were participating in a search for a hidden radio transmitter, only
instead of the usual radio receiver with a directional antenna you have a
receiver with an omnidirectional antenna and a GPS receiver. As you wander
around, you collect positions from the GPS and signal strength values at those
positions from the radio; your goal is to crunch these data points into an
estimate of the transmitter's location.

Clearly you could use trilateration with any 3 of the data points and get an
estimate, but how would you make use of the fact that you have an arbitrary
number of data points? Isn't there some algorithm which, the more data points it
is given, the better its estimate gets (similar to the way you can average a
series of position readings taken from a stationary GPS receiver to compensate
for the effects of SA)?

Can someone point me in the right direction?

A system for integrating GPS with direction finding has been out for a
few years now. Utilizing Doppler Direction finding equipment, the
heading of the Doppler array, along with the Lat Long of the monitoring
station is sent in a packet burst and superimposed on a map. When you
have 2-3 of these stations submitting this data in real time, you get a
practically instantaneous triangulation on the location of the transmitter.

The Agrelo Doppler units that were available several years ago, had this
capability.

Andy
WD4KDN



--
An armed man is a citizen, an unarmed man is a subject.
An armed society is a polite society.

In Scott in Aztlan
wrote:

Clearly you could use trilateration with any 3 of the data points and
get an estimate, ...

Since I'm not familiar with the term "trilateration", I'm guessing that
you mean "triangulation"?

But without a bearing to the transmitter from your location, how can you
do triangulation? That's why simple DF techniques use directional
antennas.

If you knew the output power of the transmitter, knew the radiation
pattern of the transmitter's antenna and knew the signal propagation
characteristics at the instant you took your signal strength reading, you
might be able to make a guess as to your current distance from the
transmitter, and draw a circle of that radius around your location.

Repeat this for enough points, look at all the places that all the circles
interesect and you might find your transmitter.

Other than being able to mark your location on a map and then navigating
to the possible transmitter location(s), I don't see a GPS receiver being
much direct use in this application.

--
Bert Hyman St. Paul, MN

Your scheme is doomed to failure.

Although signal strength is proportional to disrtance it
is also affected to too many other things as well.

For your scheme to work both transmit and receive
antennas must be truly omni-directional with no
lumps in the polar diagram (not easy espicially
mobile). The terain must not cause any disturbance
to the signal (impossible). There should be no
reflections. The transmit power or receive gain
must not change. I am sure there are a few more
that I have not thought of off the top of my head.

Having said that such a scheme could give a very
approximate location if the data was analysed and
anomolous readings ignored.

Regards
Jeff

Jeff wrote:

Your scheme is doomed to failure.

Although signal strength is proportional to disrtance it
is also affected to too many other things as well.

For your scheme to work both transmit and receive
antennas must be truly omni-directional with no
lumps in the polar diagram (not easy espicially
mobile). The terain must not cause any disturbance
to the signal (impossible). There should be no
reflections. The transmit power or receive gain
must not change. I am sure there are a few more
that I have not thought of off the top of my head.

Having said that such a scheme could give a very
approximate location if the data was analysed and
anomolous readings ignored.

Regards
Jeff

While it may be doomed to failure the value of the theory is sound and
it is one of the techniques that can be used to determine user location
of a cell phone except that the cell phone solution also have the time
similar to GPS so they can use time instead of field strength.

It is an interesting idea theoretically. You could plot circles on a map
from the relative signal strength value and centered on the GPS
location, then increase the circle sizes proportionally until they
intersect and this will provide a rough idea of the location. With more
locations it would be better and better, throwing out some of the
circles that don't fit. It will probably work if you live in one of the
plains states and have a very sensitive RF meter and lots of driving time.

Dale





--
_ _ Dale DePriest
/`) _ // http://users.cwnet.com/dalede
o/_/ (_(_X_(` For GPS and GPS/PDAs

Scott in Aztlan wrote.

all you have is a radio receiver with an
omnidirectional antenna which gives you a signal strength indication, the
assumption being that the RSSI is roughly proportional to the line-of-sight
distance between the transmiter and the receiver). The idea is to write a
computer program to take these position and RSSI values and prodce an
estimate
of the transmitter's location.


Dale DePriest wrote in message ...

It is an interesting idea theoretically. You could plot circles on a map
from the relative signal strength value and centered on the GPS
location, then increase the circle sizes proportionally until they
intersect and this will provide a rough idea of the location. With more
locations it would be better and better, throwing out some of the
circles that don't fit. It will probably work if you live in one of the
plains states and have a very sensitive RF meter and lots of driving time.

Dale

One problem is that when you get close to the transmitter the signal
strength meter hits the maximum and ceases to provide useful information. A
method for attenuating the signal is useful for this problem.

I like the intersecting circle idea. Remember to use the inverse square law
for the circle sizes. The power level of the signal will decrease by the
square of the distance from the transmitter. Its not linear.

A similar concept to the intersecting circles theory is that if you collect
a set of data along a straight line (by driving down a straight road) then
the transmitter is most likely on a perpendicular line crossing the road at
the point where you received the maximum signal. You can work around the
"maxing out the signal strength meter" problem by noting the two points on
the line where the signal shows maximum and then draw the perpendicular line
half way in-between.
This idea assumes the transmitter has an omni directional antenna as well.

Scott in Aztlan wrote.

all you have is a radio receiver with an
omnidirectional antenna which gives you a signal strength indication, the
assumption being that the RSSI is roughly proportional to the line-of-sight
distance between the transmiter and the receiver). The idea is to write a
computer program to take these position and RSSI values and prodce an
estimate
of the transmitter's location.


Dale DePriest wrote in message ...

It is an interesting idea theoretically. You could plot circles on a map
from the relative signal strength value and centered on the GPS
location, then increase the circle sizes proportionally until they
intersect and this will provide a rough idea of the location. With more
locations it would be better and better, throwing out some of the
circles that don't fit. It will probably work if you live in one of the
plains states and have a very sensitive RF meter and lots of driving time.

Dale

One problem is that when you get close to the transmitter the signal
strength meter hits the maximum and ceases to provide useful information. A
method for attenuating the signal is useful for this problem.

I like the intersecting circle idea. Remember to use the inverse square law
for the circle sizes. The power level of the signal will decrease by the
square of the distance from the transmitter. Its not linear.

A similar concept to the intersecting circles theory is that if you collect
a set of data along a straight line (by driving down a straight road) then
the transmitter is most likely on a perpendicular line crossing the road at
the point where you received the maximum signal. You can work around the
"maxing out the signal strength meter" problem by noting the two points on
the line where the signal shows maximum and then draw the perpendicular line
half way in-between.
This idea assumes the transmitter has an omni directional antenna as well.

Jeff wrote:

Your scheme is doomed to failure.

Although signal strength is proportional to disrtance it
is also affected to too many other things as well.

For your scheme to work both transmit and receive
antennas must be truly omni-directional with no
lumps in the polar diagram (not easy espicially
mobile). The terain must not cause any disturbance
to the signal (impossible). There should be no
reflections. The transmit power or receive gain
must not change. I am sure there are a few more
that I have not thought of off the top of my head.

Having said that such a scheme could give a very
approximate location if the data was analysed and
anomolous readings ignored.

Regards
Jeff

While it may be doomed to failure the value of the theory is sound and
it is one of the techniques that can be used to determine user location
of a cell phone except that the cell phone solution also have the time
similar to GPS so they can use time instead of field strength.

It is an interesting idea theoretically. You could plot circles on a map
from the relative signal strength value and centered on the GPS
location, then increase the circle sizes proportionally until they
intersect and this will provide a rough idea of the location. With more
locations it would be better and better, throwing out some of the
circles that don't fit. It will probably work if you live in one of the
plains states and have a very sensitive RF meter and lots of driving time.

Dale





--
_ _ Dale DePriest
/`) _ // http://users.cwnet.com/dalede
o/_/ (_(_X_(` For GPS and GPS/PDAs