Outside interference of GPS signal

[+timk54]

Sometimes my Smart Phone GPS is right on. I use the CGEO free app and I am happy with it. Yesterday the wife and I found 23 caches in a rural area and on most I was within 10 feet of the cache, some as close as 5 to 6 feet. Today I wanted to grab a quick cache near work. It was near the front of a department store. My compass on the same smart phone never got closer than 35 feet and then would swing around and around. I have experienced similar results near those big green electrical boxes where COs hide so many Micros and sometimes near a cell towers. Do structures and electrical equipment cause distortion in the GPS results.

Thanks in Advance

Timk54

Edited May 15, 2012 by timk54

[+Glenn]

It's been documented that interference from power lines behaves like multipath propagation. Depending on how your GPSr handles these multipath signals it can have an negative effect on accuracy.

[knowschad]

The key is most likely this: "near the front of a department store" Brick building, no doubt? You were probably experiencing signal "bounce" (aka multipath propogation that Glenn mentioned), Cliffs, deep ravines and narrow valleys, brick buildings... these can all cause poor readings caused by signal reflection.

[+ras_oscar]

I have also experienced similar problems in areas with significant rock outcroppings.

Edited May 15, 2012 by ras_oscar

[+Viajero Perdido]

Perhaps a delivery truck was parked nearby, with a GPS jammer on the dashboard. Somebody who doesn't like their employer tracking their travels. Highly illegal, but it happens.

[+farrtom]

I doubt this was your case, but I heard (and have not confirmed) that the military can have some of the satellites in an area they are training shut down which can effect gps devices. I was told this by a surveyor that had a crew in a remote area not able to work for a day because their gps equipment stopped working. The next day it was working again and they could continue on.

[+jellis]

I have an iphone. Me it's also if it is affected by if you have Wifi On or Off.

The way I work around it.... I have a real GPS.

[+The A-Team]

I doubt this was your case, but I heard (and have not confirmed) that the military can have some of the satellites in an area they are training shut down which can effect gps devices.

This doesn't sound right to me. When was this? Was it before the removal of Selective Availability on May 2, 2000? If they shut down satellites these days, it would negatively impact a huge area, affecting everything from regular consumers to aircraft. Here's a site that shows the view of the earth (and therefore the coverage area) of one GPS satellite. Shutting down GPS satellites to blank out an area would blank out or otherwise impact a massive area. A localized GPS jammer seems much more likely to me for these exercises.

[Pup Patrol]

I doubt this was your case, but I heard (and have not confirmed) that the military can have some of the satellites in an area they are training shut down which can effect gps devices.

This doesn't sound right to me. When was this? Was it before the removal of Selective Availability on May 2, 2000? If they shut down satellites these days, it would negatively impact a huge area, affecting everything from regular consumers to aircraft. Here's a site that shows the view of the earth (and therefore the coverage area) of one GPS satellite. Shutting down GPS satellites to blank out an area would blank out or otherwise impact a massive area. A localized GPS jammer seems much more likely to me for these exercises.

 

From the UK forum (all of them for 2012):

 

"NOTIFICATION OF GPS JAMMING EXERCISES"

 

http://forums.Groundspeak.com/GC/index.php?showtopic=292693

 

http://forums.Groundspeak.com/GC/index.php?showtopic=291801

 

http://forums.Groundspeak.com/GC/index.php?showtopic=291641

 

http://forums.Groundspeak.com/GC/index.php?showtopic=292691

 

http://forums.Groundspeak.com/GC/index.php?showtopic=292977

 

 

B.

[+timk54]

I doubt this was your case, but I heard (and have not confirmed) that the military can have some of the satellites in an area they are training shut down which can effect gps devices. I was told this by a surveyor that had a crew in a remote area not able to work for a day because their gps equipment stopped working. The next day it was working again and they could continue on.

 

The biggest Marine Corp. base in the country is ten miles to the west of my location. the cache was near the front of the store and it has a brick facade which another person commented may be the problem with signal bounce.

Thanks for your input.

timk54

[+timk54]

I have an iphone. Me it's also if it is affected by if you have Wifi On or Off.

The way I work around it.... I have a real GPS.

I use an android phone but I will try to cache with the wi-fi off and see if that's a help.

thank you for your comment

timk54

[+ras_oscar]

I have an iphone. Me it's also if it is affected by if you have Wifi On or Off.

The way I work around it.... I have a real GPS.

 

I also have a real GPS. There are multipath signal distortions that can affect the reception. Never heard about the truck with illegal GPS jammer. I know there are commercial devices that can block cel coverage, allegedly used at places such as churches. I assume the technology is available. I usually have 10-12 birds my GPSR is tracking at any particular time, so military exercises blanking out a coverage area would most likely be highly difficult without publication. (Think air traffic control)The simplest explanation is usually the correct one, I vote for multipath interference from buildings.

Edited May 15, 2012 by ras_oscar

[7rxc]

There are also many sources of strong electromagnetic discharges. Anything that does that creates a broadband rf signal that can block reception of the weak satellite transmissions. They can range from very localized sources to very wide area sources. Lightning is one major wide area source. On some bands you can hear one crackling thousands of miles away (in fact you hear all of them on AM broadcast bands). Similarly you can have local power grids doing arcs, sparks, coronal discharges, and more. Sometimes you get signals being rectified by rusty metal to metal contacts and rebroacasting into the air. Anyway there are lots of sources. Most of which are short lived or fairly weak. I can easily go with the multipath situation, given the speed of light (and RF) a reflection of a signal off of a cliff several miles away, can easily be stronger than the direct path or other paths. Since the system uses the time of reception against the time of transmission to locate you, a very short lengthening of the route can throw off the calculations. Up here in the mountains it is common, so we tend to triangulate with compasses to confirm the proper area roughly.

Especially in trees and gullies.

 

Of course there seem to be times when NO satellites are in useful positions at all, being blocked by mountains completely or directly over head. For those times just wait a bit for new birds to gather or come back another time.

 

Doug 7rxc

[+JesandTodd]

My moods seem to cause the most interference..,.

 

 

 

I use an android phone but I will try to cache with the wi-fi off and see if that's a help.

thank you for your comment

timk54

 

It helps if the wifi is ON..

Edited May 16, 2012 by JesandTodd

[+Glenn]

Perhaps a delivery truck was parked nearby, with a GPS jammer on the dashboard. Somebody who doesn't like their employer tracking their travels. Highly illegal, but it happens.

 

If this happens you will lost all or almost all of your locks on the satellites. I know from personal experience.

[+Glenn]

I doubt this was your case, but I heard (and have not confirmed) that the military can have some of the satellites in an area they are training shut down which can effect gps devices. I was told this by a surveyor that had a crew in a remote area not able to work for a day because their gps equipment stopped working. The next day it was working again and they could continue on.

 

The military doesn't "shut down" satellites because GPS is relied upon for many safety of life operations. The military can make adjustments with the configuration that reduces the accuracy in certain locations by introducing errors in to the system. In the same way that the GPS system can correct for localized errors created by ionospheric interference the same procedure can introduce localized errors. Think of it like SA but localized to small area instead of system wide.