Jump to content

GPS Myths


Recommended Posts

While reading a different post by Centex in the GPS 101 thread it made me wonder, what myths are there about GPS units?

 

Being new to the entire GPS/geocaching world i don't know what the myths are and i am curious as to what the community has come across and how these myths got started. I assume most myths are due to lack of experience with GPS units and navigation in general.

Link to comment

While reading a different post by Centex in the GPS 101 thread it made me wonder, what myths are there about GPS units?

 

Being new to the entire GPS/geocaching world i don't know what the myths are and i am curious as to what the community has come across and how these myths got started. I assume most myths are due to lack of experience with GPS units and navigation in general.

 

Well, since I was mentioned I will start:

 

I think the biggest myth I have heard about GPS is that the Government is tracking your every movement through your handheld GPS.

 

Another one along that line is that an off the shelf handheld GPS can be used relay your position to authorities such as search and rescue. (similar to a PLB)

Link to comment

Another one that I have heard (this was from an obviously non-GPS using co-worker).

 

He bought a Garmin handheld, walked his property and was dissapointed that the unit was showing him 6 or 8 feet from his starting point (and this was under some tree cover). He thought, and had always heard, that a GPSr should bring you back to the "exact same spot". That lead to a discussion of how you define the "exact same spot". He thought it should be within inches. I told him he could get that kind of accuracy--for about $5000!

Link to comment

Another one that I have heard (this was from an obviously non-GPS using co-worker).

 

He bought a Garmin handheld, walked his property and was dissapointed that the unit was showing him 6 or 8 feet from his starting point (and this was under some tree cover). He thought, and had always heard, that a GPSr should bring you back to the "exact same spot". That lead to a discussion of how you define the "exact same spot". He thought it should be within inches. I told him he could get that kind of accuracy--for about $5000!

 

That is a good one for a newbie. Specifically, if the hider of the cache's GPSr is +/- 20 feet and you GPSr is +/- 20 feet that means that the cache could be +/- 40 feet from where your GPS says is ground zero (GZ)

Link to comment

Another myth is that rain or clouds will affect your GPS accuracy. In truth, the frequency used for the GPS signal was chosen in part because it is not affected by weather (at least not so far as you can tell without very precise instrumentation).

 

For more information, see http://www.gpsinformation.net/gpsclouds.htm.

Ha! Thanks for this little tid-bit of info. I always thought that rain or clouds did affect receptivity. Good to know the facts!

Link to comment

 

Well, since I was mentioned I will start:

 

I think the biggest myth I have heard about GPS is that the Government is tracking your every movement through your handheld GPS.

 

Another one along that line is that an off the shelf handheld GPS can be used relay your position to authorities such as search and rescue. (similar to a PLB)

 

Hey Dodger,

actually..........................FYI, the off the shelf Garmin Rinos will send your position to another compatible Garmin Rino GPSr. Our local GSAR team has some of these Radio/GPSr units, and they are in the trial stages with these units to see if they will do what they want them to do!

 

Here is a quote taken from Garmin's web-site:

 

Find Your Friends

 

Like the rest of the Rino series, Rino 520 offers a Position Reporting feature so you can send your exact location to other Rino users in your group so they can see your position on the map page. And, because Rino is a standard FRS/GMRS radio, you can use it to communicate with any other conventional FRS/GMRS radio around.

 

https://buy.garmin.com/shop/shop.do?cID=146&pID=283#

Edited by Xlobsterman
Link to comment

 

Well, since I was mentioned I will start:

 

I think the biggest myth I have heard about GPS is that the Government is tracking your every movement through your handheld GPS.

 

Another one along that line is that an off the shelf handheld GPS can be used relay your position to authorities such as search and rescue. (similar to a PLB)

 

Hey Dodger,

actually..........................FYI, the off the shelf Garmin Rinos will send your position to another compatible Garmin Rino GPSr. Our local GSAR team has some of these Radio/GPSr units, and they are in the trial stages with these units to see if they will do what they want them to do!

 

Here is a quote taken from Garmin's web-site:

 

Find Your Friends

 

Like the rest of the Rino series, Rino 520 offers a Position Reporting feature so you can send your exact location to other Rino users in your group so they can see your position on the map page. And, because Rino is a standard FRS/GMRS radio, you can use it to communicate with any other conventional FRS/GMRS radio around.

 

https://buy.garmin.com/shop/shop.do?cID=146&pID=283#

 

Yeah, I thought about mentioning the RINO in my original post. I was referring to non radio units such as the e-trex or 60 series. I should have been more clear.

Link to comment

The first question I get asked always seems to be "How much do you have to pay to get GPS signals?"

 

Hmmmm.....I can make a MINT selling subscriptions to GPS signals!!!!

 

$200 lifetime subscription or $19.99/month! [:blink:]

 

$200 for a lifetime subscription!

 

 

WHERE DO I SIGN UP :rolleyes: I've been asked the same thing. :blink:

Link to comment

While reading a different post by Centex in the GPS 101 thread it made me wonder, what myths are there about GPS units?

 

"Magellan users will will walk way past the cache and have to come back while the garmin folks will walk right up to it."

My experience with Magellan 200, 500 and SporTrak and Garmin Etrex Legend have been just the opposite.

If you just slow down a bit with the eXplorist it will take you right to the cache. If there is ANY kind of tree cover the legend will start spinning like a top when you get withing 30 feet or so.

Link to comment

Another myth is that rain or clouds will affect your GPS accuracy. In truth, the frequency used for the GPS signal was chosen in part because it is not affected by weather (at least not so far as you can tell without very precise instrumentation).

 

For more information, see http://www.gpsinformation.net/gpsclouds.htm.

 

:( On the other hand, collected water or wet surfaces will attenuate GPS signals. You can't receive GPS signals underwater and signal reception is degraded in wet forest canopy conditions compared to when the forest canopy is dry.

 

If there is water or a film of water on the case (or cloth) that covers a GPS receiver's antenna it can degrade GPS signal reception. Drying-off of the GPS case over the antenna will improve the reception.

Link to comment

Another myth is that rain or clouds will affect your GPS accuracy. In truth, the frequency used for the GPS signal was chosen in part because it is not affected by weather (at least not so far as you can tell without very precise instrumentation).

 

For more information, see http://www.gpsinformation.net/gpsclouds.htm.

 

B) On the other hand, collected water or wet surfaces will attenuate GPS signals. You can't receive GPS signals underwater and signal reception is degraded in wet forest canopy conditions compared to when the forest canopy is dry.

 

If there is water or a film of water on the case (or cloth) that covers a GPS receiver's antenna it can degrade GPS signal reception. Drying-off of the GPS case over the antenna will improve the reception.

 

There is a very clear contradiction in these two statements;

a ) Rain or clouds *will not* affect GPS accuracy and

b ) water on the case of a GPS receiver *will* degrade GPS signal reception (this implies enough to degrade accuracy).

 

Personally, based purely on my engineering and scietific background, I believe that *both* of these statements are false. I believe the first post that says the GPS frequencies were picked to be minimally absorbed by the atmosphere including water and water vapor. But the key word here is "minimally". That is not the same thing as zero. On a clear sunny day like today, there is only water vapor between my receiver and the GPS satellites and a limited amount at that. But on a rainy or cloudy or even just foggy day, there is water vapor in much higher concentrations and even water liquid in significant amounts. Certainly there is more water liquid in the path through a cloud than the small water drops lying on the receiver's antenna. On the other hand, a few mm of water should not present a signficant amount of signal degradation to a wet GPS receiver.

 

I did some searching using Google, but I did not find anything useful that would answer this question. That includes the link in the first of the two quoted posts. I read that page and do not believe a word of it since it states both a ) and b ) above which are contradictory. I followed a link this person cited at http://www.navcen.uscg.gov/pubs/gps/sigspec/default.htm. This simply is the page where you can download the GPS SPS Signal Specification, 2nd Edition (June 2, 1995). I did and could not find a single mention of absorption of the signal by water. So this is not a valid refernce in my opinion.

 

If anyone can find a valid reference, I would appreciate reading it. Until then, I will believe my own experience that for older receivers with less sensitivity (anything designed more than two or three years ago) they will be affected by rain and other significant moisture in the atmosphere. The newer receivers, which are designed to work *inside* of buildings where the signal is significantly attenuated, may continue to get clear enough signals that their accuracy will not be degraded even in a heavy rain.

 

:(

Link to comment

Another myth is that rain or clouds will affect your GPS accuracy. In truth, the frequency used for the GPS signal was chosen in part because it is not affected by weather (at least not so far as you can tell without very precise instrumentation).

 

For more information, see http://www.gpsinformation.net/gpsclouds.htm.

 

:( On the other hand, collected water or wet surfaces will attenuate GPS signals. You can't receive GPS signals underwater and signal reception is degraded in wet forest canopy conditions compared to when the forest canopy is dry.

 

If there is water or a film of water on the case (or cloth) that covers a GPS receiver's antenna it can degrade GPS signal reception. Drying-off of the GPS case over the antenna will improve the reception.

 

There is a very clear contradiction in these two statements;

a ) Rain or clouds *will not* affect GPS accuracy and

b ) water on the case of a GPS receiver *will* degrade GPS signal reception (this implies enough to degrade accuracy).

 

Personally, based purely on my engineering and scietific background, I believe that *both* of these statements are false. I believe the first post that says the GPS frequencies were picked to be minimally absorbed by the atmosphere including water and water vapor. But the key word here is "minimally". That is not the same thing as zero. On a clear sunny day like today, there is only water vapor between my receiver and the GPS satellites and a limited amount at that. But on a rainy or cloudy or even just foggy day, there is water vapor in much higher concentrations and even water liquid in significant amounts. Certainly there is more water liquid in the path through a cloud than the small water drops lying on the receiver's antenna. On the other hand, a few mm of water should not present a signficant amount of signal degradation to a wet GPS receiver.

 

I did some searching using Google, but I did not find anything useful that would answer this question. That includes the link in the first of the two quoted posts. I read that page and do not believe a word of it since it states both a ) and b ) above which are contradictory. I followed a link this person cited at http://www.navcen.uscg.gov/pubs/gps/sigspec/default.htm. This simply is the page where you can download the GPS SPS Signal Specification, 2nd Edition (June 2, 1995). I did and could not find a single mention of absorption of the signal by water. So this is not a valid refernce in my opinion.

 

If anyone can find a valid reference, I would appreciate reading it. Until then, I will believe my own experience that for older receivers with less sensitivity (anything designed more than two or three years ago) they will be affected by rain and other significant moisture in the atmosphere. The newer receivers, which are designed to work *inside* of buildings where the signal is significantly attenuated, may continue to get clear enough signals that their accuracy will not be degraded even in a heavy rain.

 

B)

 

:) No "contradiction", you merely fail to recognize a distinct difference. Water vapor in clouds and rain is not static as is the water collected on a GPSr or surface of wet leaves in a forest canopy. The GPS signal wavelength (1.5 GHz) is able to "pass through" the dispersed moving drops of water or ice crystals.

Edited by greengecko
Link to comment

Another myth is that rain or clouds will affect your GPS accuracy. In truth, the frequency used for the GPS signal was chosen in part because it is not affected by weather (at least not so far as you can tell without very precise instrumentation).

 

For more information, see http://www.gpsinformation.net/gpsclouds.htm.

 

:rolleyes: On the other hand, collected water or wet surfaces will attenuate GPS signals. You can't receive GPS signals underwater and signal reception is degraded in wet forest canopy conditions compared to when the forest canopy is dry.

 

If there is water or a film of water on the case (or cloth) that covers a GPS receiver's antenna it can degrade GPS signal reception. Drying-off of the GPS case over the antenna will improve the reception.

 

There is a very clear contradiction in these two statements;

a ) Rain or clouds *will not* affect GPS accuracy and

b ) water on the case of a GPS receiver *will* degrade GPS signal reception (this implies enough to degrade accuracy).

 

Personally, based purely on my engineering and scietific background, I believe that *both* of these statements are false. I believe the first post that says the GPS frequencies were picked to be minimally absorbed by the atmosphere including water and water vapor. But the key word here is "minimally". That is not the same thing as zero. On a clear sunny day like today, there is only water vapor between my receiver and the GPS satellites and a limited amount at that. But on a rainy or cloudy or even just foggy day, there is water vapor in much higher concentrations and even water liquid in significant amounts. Certainly there is more water liquid in the path through a cloud than the small water drops lying on the receiver's antenna. On the other hand, a few mm of water should not present a signficant amount of signal degradation to a wet GPS receiver.

 

I did some searching using Google, but I did not find anything useful that would answer this question. That includes the link in the first of the two quoted posts. I read that page and do not believe a word of it since it states both a ) and b ) above which are contradictory. I followed a link this person cited at http://www.navcen.uscg.gov/pubs/gps/sigspec/default.htm. This simply is the page where you can download the GPS SPS Signal Specification, 2nd Edition (June 2, 1995). I did and could not find a single mention of absorption of the signal by water. So this is not a valid refernce in my opinion.

 

If anyone can find a valid reference, I would appreciate reading it. Until then, I will believe my own experience that for older receivers with less sensitivity (anything designed more than two or three years ago) they will be affected by rain and other significant moisture in the atmosphere. The newer receivers, which are designed to work *inside* of buildings where the signal is significantly attenuated, may continue to get clear enough signals that their accuracy will not be degraded even in a heavy rain.

 

:laughing:

 

:rolleyes: No "contradiction", you merely fail to recognize a distinct difference. Water vapor in clouds and rain is not static as is the water collected on a GPSr or surface of wet leaves in a forest canopy. The GPS signal wavelength (1.5 GHz) is able to "pass through" the dispersed moving drops of water or ice crystals.

 

Could this have more to do with the size of the water and not the movement of the drops? The difference in speed between moving rain drops / water vapour and a static layer of water are pretty minimal compared to the propagation speed of the GPS signals.

 

On the other hand, the size of the water droplets/vapour in the atmosphere is quite a bit smaller than the wavelength of the GPS signal, meaning it wouldn't have much effect. But a layer of water (on the GPSr or on big leaves above it) is larger than the wavelength and would affect it more.

 

As for the original point of this topic, I've also talked to people who think a $200 GPSr should get you within inches of your desired location.

Link to comment

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...