GPS affected by snow?

[Contacttower]

I'm sure I heard on the news recently with all the snow that drivers were finding their sat navs stopped working when it got very heavy.

I was very surprised to here this - I had always assumed that the frequency that GPS operates on, aprox 1200MHz and 1575MHz if I recall, should not be affected by/reflect off precipitation of any sort. Is this the case though? Could really large snow flakes in the air disturb the signal?

I wondered if perhaps it was physical snow accumulations on the car itself that were causing the signal to be lost.

[zerozero]

I don't mean to be sarcastic, but wouldn't your question be better suited on a CAR forum?

I mean, airplanes use wheels and brakes too, but if my expensive BMW (yeah, right!) isn't braking properly I'm surely not going to ask around here for advice!

:8

[one dot right]

From what I remember of Radio theory, higher frequencies suffer more attenuation than lower ones. Why this should be worse with snow than rain I haven't a clue. :(

Try and ignore people with nothing better to do than make (to themselves at least) clever remarks.

[dcoded]

That the signals are not effected by atmospheric conditions is not true.

How does your weather radar work? By reflection of precipitation, right?

In the GPS we have two channels. One the L1 is used for civil use.

So why two channels?

The L1 contains the Precision code and the C/A (which is for civil use).
Both codes contains a model of the atmosphere. But the C/A time code is dithered to degrade the accuracy.

Anyways. The Military will use the L1 and the L2.

The L1 and L2 contains both the Precision code.

Since the channels have different frequencies the refraction in the atmosphere will be different. So the two P-codes will arrive at the receiver at different times. Because of the propagation in the atmosphere.

So the UHF band is affected by precipitation to some extent.

But that the receiver will stop working all together is something I have not heard of..

[dcoded]

one dot right: You are correct about your statement.

frequencies around 10GHz are very much affected by precipitation. Hence, this frequency is used in weather radars. The attenuation can be as much as 1dB/km!

Snow also affect frequencies around 10GHz but not as much as rain.
Snow will have greatest attenuation around 30GHz

[Contacttower]

I mean, airplanes use wheels and brakes too, but if my expensive BMW (yeah, right!) isn't braking properly I'm surely not going to ask around here for advice!

Well they both likely have GPS...:ugh:

I guess I could have left out the car reference to be honest and just asked 'can GPS be affected by snow?' It's a legitimate question that has relevance to aviation.

[Contacttower]

Snow will have greatest attenuation around 30GHz

Thanks, wondering how on earth that it would stop GPS then? Unless the report was just wrong.

[ampclamp]

You put enough of anything between the tx and rx antennae and you will lose your signal.
The satellites put out pretty low power (50 watts according to online source)
From an online GPS info blurb..

Due to their high frequency, GPS signals cannot permeate stone or water. Even a thick foliation in forests may attenuate the signal to an extent that some (mainly older) GPS receivers have difficulties in receiving the signal. However, GPS works in any weather including a thick cloud cover. Problems may only arise in very heavy snowfall.

[Dg800]

The L1 contains the Precision code and the C/A (which is for civil use).
Both codes contains a model of the atmosphere. But the C/A time code is dithered to degrade the accuracy.

Your information is a bit out of date. The Commander in Chief put an end to that in 2000. :ok:

[Dg800]

From what I remember of Radio theory, higher frequencies suffer more attenuation than lower ones. Why this should be worse with snow than rain I haven't a clue

I'm guessing this is possibly related to the fact that snow falls a lot more slowly than rain, causing a higher density of water per unit volume of atmosphere that the signal has to traverse to get to the receiver, hence the greater attenuation.

Ciao,

Dg800

[Groundloop]

Your information is a bit out of date. The Commander in Chief put an end to that in 2000.

No, he didn't. He ruled that Selective Availability would not be used in future. SA was a system that reduced civil GPS precision even further when activated. The civil C/A signal is still deliberately degraded compared to the military P-code.

[Dg800]

And your source is?

[Groundloop]

And your source is?

20 years of teaching GPS. But also here, if you don't believe me :-

GNSS Frequently Asked Questions - GPS (http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/faq/gps/#5):-

[A Squared]

Ground loop is correct. I also spent a lot of time both teaching GPS applications and using it in the field for precise surveying. Selective availability and C/A code are 2 different things Selective availability has indeed gone away, but Civilian receivers still use the C/A code for positioning which is less accurate that the encrypted code available to the military.

[ion_berkley]

Just to be clear about this, "Less accurate" and "deliberately degraded" are not the same thing. L1 C/A is at this point is running at it's full potential accuracy. It's true to say it provides less accuracy than the L1 and L2 P(Y) codes (and the newer SPS and PPS signals), but that is by design, not because it's being "degraded".

Oh and to the original question, attenuation and scattering due to (falling) atmospheric precipitation should have a negligible effect on the GPS link budget. However significant deposits on the antenna radome could indeed have a debilitating effect.

[Basil]

According to my Audi car manual, snow on the GPS aerial can affect reception. Sounds intuitively logical.

[Dg800]

Just to be clear about this, "Less accurate" and "deliberately degraded" are not the same thing.

Couldn't have said it better myself. If the U.S. Navy were still purposefully injecting truly random errors to degrade accuracy the whole WAAS shenanigan would be completely useless, as you cannot compensate for a truly random error (being truly random, it is absolutely unpredictable, even short-term).

Ciao,

Dg800