How can GPS work?

It does not use earth-based points to determine your position; it uses at least 2-3 satellites that
triangulate your position to within 3 meters ( depending on signal strength, etc ). As long as the
GPS unit maintains signal with 2 or more satellites, your "position" is updated in real-time.

Make more sense now?

Bill

"dave b" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Seems to me that those gps units can't be very accurate, unless you run in

a

Quoted message said:

straight/level line.

If it measures the dist between two points in space and time, how does it know if you took a less
than straight (longer) route?

Am I missing something?

Dave in Plano

"dave b" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Seems to me that those gps units can't be very accurate, unless you run in

a

Quoted message said:

straight/level line.

If it measures the dist between two points in space and time, how does it know if you took a less
than straight (longer) route?

Am I missing something?

Dave in Plano

That would be true if it only measured the start and finish positions, or if the sample rate were
very low. The Timex Speed + Distance, for example, according to their web site, computes data once
per second, and transmits it to the watch every 3.57 seconds. That should provide reasonable
accuracy for all but the most spasmodic runners. They claim 0.1 mph accuracy with 95% probability.

Joseph (not a Timex S+D owner)

In article said:

Seems to me that those gps units can't be very accurate, unless you run in a straight/level line.

If it measures the dist between two points in space and time, how does it know if you took a less
than straight (longer) route?

Am I missing something?

Dave in Plano

It knows where you are on Earth at all times. It doesn't matter which direction you go. If you take
a long route, it just totals up the distance that you're moving. It tells you how fast you're
moving, how far you've gone, how long it's taken, etc. I have one. It's fairly accurate. At times,
it will lose a signal, but when it doesn't, it's it's dead-on.

Art

--
Connection closed.
------------------------------------------------------
Art Gary - [email hidden] Systems Administrator - Software Engineering/CSBU

From Garmin's Website:

------
GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information
to earth. GPS receivers take this information and use triangulation to calculate the user's exact
location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite
with the time it was received. The time difference tells the GPS receiver how far away the satellite
is. Now, with distance measurements from a few more satellites, the receiver can determine the
user's position and display it on the unit's electronic map.

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D
position (latitude and longitude) and track movement. With four or more satellites in view, the
receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's
position has been determined, the GPS unit can calculate other information, such as speed, bearing,
track, trip distance, distance to destination, sunrise and sunset time and more.
------

Also note that most GPS units require an initial altitude settings so that it can approximate
distance given changes from that initial altitude setting ( in the absence of 4 or more
satellites ).

Garmin has an excellent "GPS Buide for Beginners" at garmin.comgps4beg.pdfOpen ↗

Bill

"dave b" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Seems to me that those gps units can't be very accurate, unless you run in

a

Quoted message said:

straight/level line.

If it measures the dist between two points in space and time, how does it know if you took a less
than straight (longer) route?

Am I missing something?

Dave in Plano

First it samples often, I believe the once per second is close. Mine does just fine on a 1 mile
circle route. They also know if you gain or loose altitude and can compute that as well.

--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math

"dave b" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Seems to me that those gps units can't be very accurate, unless you run in

a

Quoted message said:

straight/level line.

If it measures the dist between two points in space and time, how does it know if you took a less
than straight (longer) route?

Am I missing something?

Dave in Plano

But won't it short you mileage when you gain or lose altitude?

Scott

jthvedt said:

"dave b" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Seems to me that those gps units can't be very accurate, unless you run in

a

Quoted message said:

straight/level line.

If it measures the dist between two points in space and time, how does it know if you took a less
than straight (longer) route?

Am I missing something?

Dave in Plano

That would be true if it only measured the start and finish positions, or if the sample rate were
very low. The Timex Speed + Distance, for example, according to their web site, computes data once
per second, and transmits it to the watch every 3.57 seconds. That should provide reasonable
accuracy for all but the most spasmodic runners. They claim 0.1 mph accuracy with 95% probability.

Joseph (not a Timex S+D owner)

I run with Magellan 315 and it accurately track my mileage and mph. It also layout a diagram of
where you ran. Its accurate enough to tell when ran 3 feet to the right and then back to avoid a
slower runner!

Rob "Joseph Meehan" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

First it samples often, I believe the once per second is close. Mine does just fine on a 1
mile circle route. They also know if you gain or loose altitude and can compute that as well.

--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math

"dave b" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Seems to me that those gps units can't be very accurate, unless you run

in

Quoted message said:

a

Quoted message said:

straight/level line.

If it measures the dist between two points in space and time, how does

it

Quoted message said:

Quoted message said:

know if you took a less than straight (longer) route?

Am I missing something?

Dave in Plano

In 'jthvedt said:

(snip) ... That should provide reasonable accuracy for all but the most spasmodic runners. They
claim 0.1 mph accuracy with 95% probability.

I have a Timex SPD watch, and have posted before about accuracy issues. Maybe it's just my watch,
but on a long evenly paced run, my "best" and "worst" paces reportedly vary between 5:30/mile and
9:45/mile. Clearly this is complete rubbish as I've never run that fast, or that slow.

I find the watch is often totally off in the first 1/2 mile or so, and then seems to settle down.
And yes, I don't move around at all until it has locked into a satellite signal, so that is not the
problem. The only thing these watches are good for is total distance and average pace, and even then
only on a course of at least a few miles.

In article said:

Seems to me that those gps units can't be very accurate, unless you run in a straight/level line.

If it measures the dist between two points in space and time, how does it know if you took a less
than straight (longer) route?

Am I missing something?

Depends on how often it takes sample, the precision of samples, if you are running fairly stright
etc. For twisty routes in the woods, it seems to be up to 10% short, and other routes up to 5%. This
is with normal sampling on the garmin etrex vista.

I can set it to 1 sample per second, and I can see the errors in there, actually making the route
longer. But normally, it will tell which side of the road you can on etc. And with WAAS/EGNOS it can
show your position with 95% confidence within less than 5 ft.

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Cool. Thanks.

bb said:

From Garmin's Website:

<good stuff

Oh great. Another jogger with the desire to know where his place in the cosmos, is. I'm still
waiting for technology to give me a hand held device which tells me of nearby joggers, and safely
points me in the opposite direction. I'm sure it would be odor activated........or maybe triggered
by arrogance.