GPS Anomolies

[+GeoGeeBee]

We've had threads here before about why a consumer-grade GPS receiver cannot provide centimeter-level accuracy. Yesterday I experienced a strange example of this.

 

I got a notification of a new cache in my area, and since I was about to leave work anyway, I went to search for it. It is a very well camo'd micro hidden on a tree.

 

I got to ground zero, fairly quickly determined which tree was the target, and started visually searching for the cache. Did I mention that it's well camo'd? I had been there about 10 minutes when another cacher showed up. His GPSr agreed with mine that it was in that particular tree.

 

Every few minutes I would glance at my GPS screen. It did the normal bit of bouncing, showing at various times that I was 3, 4, or sometimes up to 6 feet away from the cache. But one time I glanced down and saw an indication that I was now 90 feet west of the cache! My first thought was that the GPSr, which was hanging from my neck on a lanyard, had been shielded from the sky by my body and was giving an erroneous reading because of that. But I mentioned it to the other cacher, and he verified that his GPSr was showing the same thing! Over the next three or four minutes, the distance slowly dropped until it was again showing me several feet from the cache... even though I had not moved. This was in an open area, under clear skies, and two different GPS receivers showed the same error.

 

Can you explain what happened?

 

Have you ever had a similar unexplained discrepancy?

[+Caped Crusader]

The only thing I can think of that would affect both GPSrs is that maybe one or more satellites moved out of view.

[+StarBrand]

Something moving nearby might have caused a signal to bounce temporarily. I've seen weird readings like that for a few moments from time to time.

[Motorcycle_Mama]

Moving to the GPS and Technology forum.

[+hzoi]

Sunspots?

 

Signal interference?

 

Aliens?

[+user13371]

Ref'd elsewhere in these forums, but see also:

 

https://www.faasafety.gov/files/notices/2011/Jan/GPS_Flight_Advisory_CSFTL11-01_Rel.pdf

 

Released

January 19, 2011

FEDERAL AVIATION ADMINISTRATION

FLIGHT ADVISORY

GPS Testing

CSFTL GPS 11-01

January 20-February 11, 2011

February 15-February 22, 2011

Brunswick, GA

The Department of Defense will conduct GPS tests on January 20th through February

22nd, 2011. During testing, the GPS signal may be unreliable or unavailable.

A. Location: Centered at 304906N/0802811W or the location know as 105.25

degrees and 52.1 NM from the SSI VOR.

B. Dates and times: GPS testing is scheduled on January 20th through February 11th,

2011 from 0000-0245 UTC daily and February 15th through February 22nd, 2011

from 0000-0245 UTC daily. These tests may not be concurrent so pilots are

advised to check NOTAMS frequently for possible changes prior to operating in the

area.

C. Duration: Several test events will be conducted and will be active for 45 minutes

followed by 15 minutes of off time.

D. During testing, GPS will be unreliable and may be unavailable with in a circle with

a radius of 370NM and centered at 304906N/0802811W or the location known as

105.25 degrees and 52.1 NM from the SSI VOR at FL400; decreasing in area with

a decrease in altitude to a circle with a radius of 325NM at FL250; a circle with a

radius of 260NM at 10,000FT MSL and a circle with a radius of 215NM at 4,000FT

AGL.

E. Pilots are highly recommended to report anomalies during testing to the

appropriate ARTCC to assist in the determination of the extent of GPS degradation

during tests.

The NOTAMs discussed in this advisory may change with little or no notice.

Pilots are advised to check NOTAMs frequently for possible changes prior to

operations in the area. NOTAMs will be published at least 24 hours in advance

of any GPS tests.

Edited January 21, 2011 by Portland Cyclist

[+ecanderson]

 

Have you ever had a similar unexplained discrepancy?

See the post directly above. You are within the area impacted by this stuff until 22 February. Per the advisory, this will be taking place for as much as 45 minutes out of every hour. You may well be the first "victim" to have posted here.

 

Details of your experience (how long such errors last and how frequently) would be of real interest to cachers in your area. It would be of use if a few folks in your area would set up tracks to record once per minute and leave your units on for on the order of 6 hours and report back.

[+dfx]

I did get into a GPS black hole once. We parked the car at the trail entrance and started taking the bikes off, just to find the Oregon suddenly losing signal 5 minutes later. It was out in the open, no hills or buildings around, blue sky above our heads. First reaction was to reboot the Oregon, but after that it still wouldn't get a signal, at which point I started panicking, thinking that maybe the antenna broke or something. I turned the car GPS back on, which had worked just before as we used it to get us to the trail entrance, and ... no signal either! Total weirdness. We crossed our fingers and headed down the trail anyway, and not even 100 meters down the Oregon would suddenly pick up signal again, from one second to another back to full strength.

 

Once we were finished and got back to the car the black hole had disappeared of course.

[Dj Storm]

Many GPSr users are unaware of the multipath effect. Multipath occurs when your GPSr, instead of using the signal that traveled directly from the satellite, picks up a signal that was reflected of a nearby surface. You can read more about this here: http://gpsinformation.net/multipath.htm

Once I was hiking through some mountains, signal was less than adequate, and after one hour my GPSr showed a trip 320 km (200 miles) long.

Another time I took a reading from the balcony of a building, and the reading was 100m (330 feet) off.

 

Since both your GPSrs showed the same anomaly, I guess that one GPS satellite moved behind a structure. The GPSrs lost the direct view of that satellite and picked up the signal reflected off a nearby surface.

[+user13371]

GeoGeeBee, could you let us know the cache you were looking for, along with the date & time? That might help identify which (if any) of the suggested causes here were really in play for your incident.