GPS and the WEATHER

[+The 4 Childs]

Has anyone noticed a difference when they cache on a sunny day and then on a cloudy day? We tested this on a cloudy day with out Meridian Gold and the coordinates were so off, that we could not find the cache.

I am wondering what effect the weather has on geocaching?

[+GOT GPS?]

Thunder storms and snow storms can effect the signal. It maybe wet trees, or time of day also. The positions of the satellights in the sky are alway different, since they orbit twice a day at about 12,000 miles up.

 

Your best bet is to leave the unit turned on for about ten minutes, before starting out for the day, also keep the unit still during that time. Once that is out of the way the GPS should have all the information needed to determin which satellites to use and which ones to avoid. It takes about 10 to 15 minute to download the latest Satellite almanac data to the GPS. Almanac data includes the health of the satellites also. You would not be aware of the almanac data itself, but if you turn on the unit and wait several minutes, you with see the EPE(Accuracy improve a bit).

 

This may seem long winded, but Ive tried many GPS units over the past 5 years.

 

Supporting links:

http://www.gmat.unsw.edu.au/snap/gps/gps_survey/chap5/532.htm

 

http://www.goldenessays.com/free_essays/2/engineering/global-positioning-system.shtml

 

http://www.precisionag.ncsu.edu/scripts/faq/index.php

 

http://www.mohawkc.on.ca/dept/disted/ol/CV630.htm

 

 

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My home page about GPS units and information

 

[This message was edited by GOT GPS? on February 19, 2003 at 12:19 PM.]

[peter]

Weather does not have any significant direct effect on reception in the GPS frequency band (1.5 GHz). However there can be indirect effects such as rain causing a continuous layer of water to form over the antenna or on a windshield which attenuates the signals. Water buildup on foliage can also reduce the signal strength and under very humid conditions there could be condensation forming inside the GPS receiver that affects its operation.

[jfitzpat]

There was another thread about this and many folks were at pains to point out that the frequency selected was picked expressly to minimize weather impact, etc.

 

Supposedly, conditions in the troposphere (pretty much where all the weather we experience is) only effect accuracy to about the tune of 1 meter. Clocking error combined with variation in the ionosphere reportedly account for something like 6 or 7 times as much.

 

But, all this does miss one point. We are talking not just about signals and theoretical errors. We are also talking about low cost, lower power, consumer grade receivers. My personal observation is that the handheld receviers are definately effected by cold, humidity, and EMI, perhaps to the tune of 10-15 meters.

 

That said, I usually find that terrain and sat. position plays a much bigger role. When you use a GPS to make an instrument approach in a plane, it is not unheard of for there not to be enough birds in sight for the receiver to give an adaquate lock (the receivers have to support something called RAIM). It is a big enough deal that you are encouraged to check in advance on sat positioning for a particular date, time, and location.

 

-jjf

[+EraSeek]

Believe what you will, rain, wind, cloud, snow, hail, has little effect on the signal as stated here:

http://www.gpsinformation.net/gpsclouds.htm

However, Peter's post is correct.

And yes, cold will affect things but that is because it is affecting the power of the batteries, not the signal. I have been in several whiteouts, and many rainstorms, and had good signal. The satellite cofiguration you are getting is ever changing, as is ionospheric conditions. These are the things that will affect your GPS. Perhaps you are seeing the weather change and think ''Ah Huh!'' and are attributing your GPS's performance to the wrong variable.

 

[jfitzpat]

Eraseek, it is a lot more than batteries. The electrical properties of many components change with temperature and humidity. The idea that it is the receiver, and not the signal was precisely my point.

 

Although I did note that sat. pos. and terrain is usually the culprit, weather is not always benign. High level inversions (non standard lapse rate) in the troposphere can have a pretty significant impact on signal accuracy. There is actually an article on this at the JPL web site.

 

I'm not sure what your definition of a whiteout is. I've seen lots of units crap out between 0 and freezing, so severe winter conditions probably aren't a valid test for many receivers. Both Bendix King and Garmin aviation units are clearly effected somewhat by precipitation static and structural icing, but again, those issues probably are not applicable to handhelds.

 

I've seen Magellan and Garmin handhelds lose lock in heavy upslope fog several times. But, in mountain terrain, lock can be pretty tenuous to begin with, so it is hard to say, definately, that the fog was specifically to blame.

 

One thing to keep in mind is that, although they are resistant to certain types of attenuation and interference, GPS signals are still pretty weak. Anything that brings up the noise floor in or around the receiver can potentially have an effect.

 

-jjf

[+Renegade Knight]

Water blocks GPS signals.

 

That's some people get reception in their house (roof is dry dead wood) but not under tree cover.

 

On a recent foggy and cloudy day my GPS was much more prone to wander about, and in general the caches I found were 'further' from ground zero than normal. Locks took longer to achieve when I turned the unit on and off.

 

Wherever you go there you are.

[+EraSeek]

"I'm not sure what your definition of a whiteout is."

 

Blowing sleet and fog at 10,000' on Mt Rainier. Visibility at about 10'. Glad I had my unit with me to find my way. I'll check out the JPL site. Thanks. Always willing to learn more.

 

[+Searching_ut]

I've been carrying a gps receiver with me on hikes since about 95, and haven't ever noticed any reception problems I'd attribute to weather. Not even with my old Magellan 2000 which would loose lock when a butterfly flew over. I have however noticed that broad leaf trees will interfere with reception more when the leaves are wet, and that pines are murder when they're covered with snow.

 

Some weather related things I have noticed is that the dislay will go black if it gets to hot, and the unit will shut down shortly therafter. If it gets really cold the display will quit working as well. If you're running lithium batteries though I know my eTrex will keep on recording tracks at -30F, even though the screen looses it at around -10 or so. I haven't ever noticed any accuracy problems I would attribute to temperature either.

 

For what it's worth

 

Jeff

[+The 4 Childs]

Thanks all, after reading all that...whew lots of info. Are you all "closet weathermen"? The day this all happened we were using our recharchable batteries. That might have had a effect I guess, maybee recharchables are too touchy.

[+chris-mouse]

rechargable batteries are very, very touchy in cold weather. I've had a NiCD battery pack for a videocamera that was supposed to last two hours die in only 20 minutes at -10c.

I've also had NiMH batteries in my GPS die in a fraction of the time they were supposed to last. My GPS would simply shut down without warning until I put the batteries inside my coat for a while to warm up.

As a general rule, I've gone to alcaline batteries for everything that will be exposed to the cold, and keep a spare set of batteries in an inside pocket so I can do a quick swap.

[+EraSeek]

quote:

---

High level inversions (non standard lapse rate) in the troposphere can have a pretty significant impact on signal accuracy. There is actually an article on this at the JPL web site.

 

-jjf

---

 

I didn't find it on JLP, but I did find this elsewhere. Looks like units have some inbuilt program to deal with tropophere error. :

 

''In addition another significant error source was troposphere errors. To attack these error sources the WAAS system sends clock corrections, ephemeris corrections, and ionospheric corrections. It cannot compute tropospheric corrections due to the localized nature of this error but it does remove the tropospheric error component from the data it computes so that the local receiver can apply its own corrections based on an atmospheric model that is based on the current sky location of the SV

 

[jfitzpat]

Eraseek,

 

If you are interested in some of the articles, I'll dig them up. There is quite a bit of stuff on atmospheric errors on the FAA sites as well. The FAA's primary interest in WAAS is vertical accuracy, hopefully allowing GPS receivers to be used for IFR precision approaches, but some interesting material has been collected while non-precision RNAV (GPS) approaches are established and test flown.

 

As for -30 on a Garmin, I've never been able to get one to operate at all reliably below about 0 to -5, even with Lithium batteries. -30 is well outside the rated spec for many of the components inside, so I suspect that solid operation at such temperatures is the exception, not the rule.

 

-jjf