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Gps Reception In Extreme Fog?


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It was extremely foggy in many parts of the New York metro area today and still is tonight. Curiously, I tried my GPS both around lunchtime and tonight as I always wondered how it would behave in this weather, I know that normal cloudy days do not really affect it and it worked fine in a lesser fog I tried it in last fall, but I imagined that fog this thick might provide a "blockage" the way trees or underpasses do, especially with lower-end units.


It definitely took longer to get and hold a fix though I generally had about 3-5 birds, but it worked better than my "blockage" theory would've thought, I thought maybe it would only see satellites directly overhead if at all. It wasn't as accurate though, when I got home it showed me about 200 feet away, for example (at the time 4 birds were locked on).


Anyone else with any thoughts/knowledge on this?

Edited by hairymon
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I've used GPSes in absolutely terrible weather and the only time I noticed there being a reception problem was when I had my old Street Pilot ColorMap on the dash of my car during a torrential rainstorm. And even then it was just the metal windshield wipers periodically interfering with the view out the front. I turned the wipers off for a moment (not recommended while actually moving :laughing:) and it kept a solid lock on the satellites.

Edited by Rotareneg
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Although other problems or nearby objects may affect reception.......(Like standing under a high voltage highline) the fog or clouds should not affect it at all. I know the military uses a different channel or spectrum as civilians, but this system was designed to be an all weather system. It guides our military aircraft and missles with awesome precision in all kinds of weather. :laughing:

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Thanks, that's what I thought. I have used it in rainstorms before with no problem except the wipers as you've noted.


I was charging my cell phone so I was using the batteries instead of my car adapter, so that may have been the issue too, the batteries were getting low and I've had problems when that happens in the past.

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I don't make a habit of using my GPSr for navigation on a vehicle but I do use it when the fog gets bad. I've never had a fog related problem with it and have found that I can rely on it to tell me where I am when I can't tell by looking out the window. It's especially helpful when nearing intersections - the map lets me know when I'm approaching one. Any errors that may have occured were not outside of the norm of the unit. I would suspect that a 200 foot error was due to only having four satellites locked, but I doubt that the fog attributed to any lack of, or loss of lock.


In my part of the world (San Joaquin Valley, California) we have some of the worst fog in the world (from what I understand) called Tule Fog. It's a very common occurance in the winter nights, particularly in the rural areas, and visibility can easily be less than 50 feet. Sometimes much less, like a zero-visibility snowstorm. There are times when literally you can be lucky to see even one dash of the dashed center line on the road you're driving. If any kind of fog would mess with a GPSr, it would be this stuff, and I've yet to see a problem.


- Kewaneh

Edited by Kewaneh & Shark
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The weather does not have anything to do with the performance of your GPSr. The satellites are 20.000km (roughly 12.400 miles) from earth, and clouds/weather are usually about 1km (0,6 miles) above our heads. Though it's unusual, on a "bad GPS day" the cloud/weather can be 9km (5,6 miles) up. Even then it does not have any effect on the performance. The "layer of weather" is simply to thin.

If you have a microwave radio link the weather can be a problem. But the radio waves of such a device travels only a couple of 100 feet in the air, and therefore only travels through weather.


Did I make any sense or do I have to take an english class? :P

Edited by Dr_Dajom
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Did I make any sense or do I have to take an english class?

I agree with you that the weather has almost no direct effect on GPS reception (the attenuation is only a couple dB at most which is pretty negligible), and the english is fine.


But I wouldn't argue that the effects are small *only* because of the thinness of the weather layer. Afterall, when I place a flat dish with about 3 mm of water in it directly over my receiver it blocks the signals and a much thinner layer of aluminum foil will do so as well. Even though a thin sheet of water absorbs the signals, the small droplets in fog or rain don't have that effect since the signals are able to diffract (bend) around them.

OTOH, foggy or rainy days can have indirect effects on reception by forming sheets of water on overhanging foliage, on windows, or possibly even causing some condensation inside the antenna housing of your receiver.

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I understand your point, but if someone throws a large ball of cotton on you, it doesn't hurt, but an equally sized or even smaller piece of steel will. You can't compare apples and pears.


The attenuation is not even a couple of dB. A 3dB loss would mean that the signal loss were about 50%. That would be a serious issue and if that was the case the weather would have a very large impact on the performance.

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