Strong Signal

[Guest Paul Lamble]

More help for a newbie, please!

 

This morning I will turn on my GPS outside and plot in the coordinates of the 3 caches I'll be looking for today.

 

First Question:

When I turn on the GPS and it has found a sufficient number of satellites, it tells me it is accurate within so many feet (anywhere from 19 to 37 feet I've seen so far). If I get a message saying it is reading within 37 feet, would it be better for me to turn it off, go somewhere with more view of the sky, and turn it on again trying to get more accuracy?

 

Would this get me closer to caches (and give me more accurate coordinates for caches I place), or is that range too small to bother with?

 

Second question:

I read that GPS accuracy falters sometimes under leaf cover and also amidst tall buildings. A friend who was in the survey unit in the National Guard said this is a fallacy (assuming the leaves or buildings are not generating magnetic fields, that is). He said those have no effect on the receipt of a signal. (I've also wondered about cloud cover.) Of course, my friend was using the best government-issued equipment, and this was during the time of selective availability. But he insists that leaf cover interference is only a myth.

 

What's a newbie to do???

[Guest fairbank]

The accuracy of your GPS changes depending mostly on satellite geometry. This is affected primarily by your GPS's "view" of the sky. Your friend is totally wrong. The more the signal is blocked by foilage, buildings, mountains, your body...etc. the worse the satellite geometry and the worse your accuracy. The best thing to do when your are trying to pinpoint a location, is to view the "satellite" screen of your GPS and try to orient your receiver to get the highest number of sats with high SNR. Also, some GPS's do position averaging, if yours does, then the longer you leave it in the same position the more accurate the indicated position becomes over time.

 

Eric

Topo Map help page,

http://www.geocities.com/fairbank56

[Guest opie]

Also, early GPS receivers were much less sensitive than the current crop. My first unit was a 1 channel Magellan Trailblazer. It would not acquire from inside a vehicle, and frequently lost acquisition when in the woods. My GPS 315 acquires from inside my house, and never has a problem under foliage.

 

As to the 17-37 ft estimated error, not sure which unit you are using, but the 315 provides distance measurements to .01 mile resolution. That is 52 feet. If you use the actual lat/lon to zero in, you can get better resolution (but it isn't as easy as the distance readout). Anyway, the estimated error is well within 52 feet of resolution, so it is not likely to make much difference in actual usage.

[Guest Paul Lamble]

Here's an update. My military friend uses a govt issue GPS and receives a much more accurate signal. I was talking to a USGS man yesterday about the same thing. He surveys and says he can get accuracy down to the centimeter when he needs it. He also said that using his $15,000 GPS he is not affected by leaf cover or tall buildings, but using a hand held GPS (like my Etrex Summit) he is just like civilians.

 

Perhaps both points are correct?

[Guest jeremy]

An area with lots of trees do affect the GPS unit, as well as sitting in a valley or anyplace where there are some obstructions between you and the satellites.

 

The difference between an eTrex and the $15,000 GPS unit (other than the obvious cost) is that the more expensive unit uses differential GPS and has a much stronger antenna. Much weaker signals reach the more expensive unit. The eTrex does not have its own external antenna attachment so the signal has to be much stronger to be received.

 

FYI differential GPS uses signals from fixed positions on the earth that help to locate you to within a centimeter (and eventually less). It is more precise since satellite signals tend to get skewed, redirected, etc when passing through the atmosphere.

 

Jeremy

[Guest fairbank]

The $15,000 unit is a Survey GPS Receiver. Just about all GPS's on the market today can receive corrections from a differential receiver. This will improve your accuracy to less than 5 meters (1-5 meters). The survey receivers are carrier tracking receivers. They start their timing measurements using the pseudo random code as all other GPS navigation receivers do and then they switch to measurements based on carrier frequency to obtain sub centimeter accuracy.

[Guest ScottJ]

Just some FYI from a technogeek:

 

GPS signals are microwaves. The reason leaves and foliage affect the signal is that leaves (and most other living plant appendages) contain water. Water readily absorbs microwaves, for reasons related to the shape of the water molecule. (That's how microwave ovens work...they heat the water in your food.)

 

Scott