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Author Topic:   GPS on Cloudy Days
jimh posted 11-11-2014 10:55 AM ET (US)   Profile for jimh   Send Email to jimh  
Why Cloudy Days Don't Stop GPS from Working

I was quite bemused to read, on a website associated with angling for a specific species of fish, some rather crazy advice--being given with a claim of expert knowledge and with the imprimatur of two rather respected marine electronics companies--that reception of signals from the USA Air Force's NAVSTAR Global Positioning System satellites could be blocked by clouds. This is such nonsense that I am compelled to explain the real effects of clouds, rain, moisture, fog, and other atmospheric effects on reception of signals from orbiting navigation satellites.

The simplest explanation why the Global Positioning System (GPS) works in all weather is just this: it was carefully designed to do so. GPS has now been fullly opertional for about thirty years. It is a remarkable global system, and, with no dispute, it provides the greatest general benefit to mankind on a global basis that has ever been achieved by any space project. It has done this by a combination of great engineering, design, fabrication, expertise, and administration by the Air Force and many scientists and researchers. It is utterly silly to think that an investment of many billion dollars would have been made to create a navigation system that could be foiled by a cloud in the sky. The experience of about three billion users of GPS contradicts the claims made about clouds, but, nevertheless, let's examine the real science behind this.

Signals from orbiting satellites in a global radio satellite navigation system must pass through the Earth's atmosphere to reach users on the ground. There are two layers of the atmosphere which can affect radio waves: the ionosphere and the troposphere. Both layers can block radio waves, but these effects vary with the frequency of the radio waves. The effects can be described very simply.

The ionospheric layers can become reflectors for radio waves, but this effect is limited in most cases to signals whose frequency is below 100-MHz. On occasion, ionospheric reflection can occur at higher frequencies, up to 300-MHz, but these occurrences are very sporadic. These effects are widely known, and need little explanation. In any case, clouds are far below the ionosphere and are not part of ionospheric effects.

The tropospheric layer, which extends to about ten miles in altitude, can also affect radio waves, and these effects are not as well known. The troposphere can absorb, attenuate, or scatter radio waves when the wavelength of the radio waves is close to the resonance of water molecules and oxygen or nitrogen molecules. Again, this effect is frequency dependent (because the size of the molecules is variable and the wavelength of the radio waves depends on frequency). The effect is most pronounced at frequencies above 10-GHz.

The radio signals from GPS transmitters operate in a frequency band known as L-band. The signal used by boaters and their GPS receivers is called the L1 C/A signal, and is sent at 1575.42-MHz. This frequency was chosen precisely for its ability to pass through the ionosphere and the troposphere without being reflected, absorbed, attenuated, or scattered to any significant effect. Several experts confirm this.

In a scientific paper titled "Tropospheric Effects on GNSS", Dr. El-Arini writes: "The effects of water vapor, rain, and nitrogen attenuation at L-band frequencies are negligible."

http://www.icao.int/SAM/Documents/2008/IONOSFERASEMINAR/ Tropospheric%20Effect%20on%20GNSS.pdf

A propagation tutorial written by Mike Willis includes a graph showing the attenuation of oxygen and water vapor on radio signals as a function of frequency:

Chart showing attenuation as a function of frequency for oxygen molecules and water vapor.

Source: http://www.mike-willis.com/Tutorial/PF5.htm

For the GPS L1 C/A signal the attenuation due to water vapor is below 0.001-dB/km. The troposphere is only about 20-km thick, so the attenuation due to water vapor would be less than 0.020-dB, an infinitesimal amount of attenuation. Note that attenuation due to water vapor increases with frequency, and in the region around 20-GHz is more than 100-times greater than at 1.5-GHz. This is the region used for home satellite receivers.

In a treatise "Global Positioning System: Theory and Applications, Volume 1," James J. Spiker writes:

"Atmospheric attenuation in the 1—2 GHz frequency band is dominated by oxygen attenuation, but even this effect normally is small. The attenuation is on the order of 0.035-dB for a satellite at zenith....The effects of water vapor, rain, and nitrogen attenuation at frequencies in the GPS frequency bands are negligible."

Source: Google Book Link to source material

I hope that readers will give some consideration to the information provided by the three (real) experts on propagation and GPS signals I have mentioned above, and will ignore the advice being passed off on boating websites that a cloudy day can stop GPS from working.

jimh posted 11-11-2014 01:01 PM ET (US)     Profile for jimh  Send Email to jimh     
It must also be recognized that the influence of Earth's atmosphere on the radio signals of the GPS was considered in the design of the system. The GPS tries to provide ground users with signals at a certain minimum strength, typically around -130-dBm. In accomplishing this, the designers considered all factors which would affect the received signal strength. They allotted loss in the atmosphere as having perhaps as much as 2-dB of attenuation in their design. In actual realization of the system, atmospheric loss has been found to be about 0.5-dB. Because the actual losses are lower than the designed-for loss, the GPS signal is typically stronger than the design goal by 1.5-dB. The point here is that there was already a consideration given to atmospheric loss, and sufficient signal power or antenna gain was provided in the system to compensate. This further negates any argument being made that a cloudy day could knock out GPS reception. Signal loss in the atmosphere was already planned for, and a very ample margin was provided.
acseatsri posted 11-11-2014 05:51 PM ET (US)     Profile for acseatsri  Send Email to acseatsri     
Myself and others here appreciate the work you do running this website and all the useful info and experiences of others.
jimh posted 11-11-2014 11:35 PM ET (US)     Profile for jimh  Send Email to jimh     
Thanks for the kind comments.

Re the attenuation of radio waves from GPS in the atmosphere, it is also significant to note that the attenuation from Oxygen molecules is much greater than from water vapor in L-band. (See graph above.) There is really no way you can take Oxygen out of the atmosphere--it is there all the time--so it seems rather pointless to worry about the influence of water vapor that might show up in a cloud from time to time which has negligible effect.

I should also point out the vertical and horizontal axis of the graph are plotted with logarithmic scales. This means that attenuation due to water vapor is much greater (0.2-dB/km compared to less than 0.001-dB/km) at frequencies around 20-GHz (where the Ku-band is used for satellite television transmission) than it is at 1-GHz or L-band frequencies used for GPS.

jimh posted 12-30-2014 09:25 AM ET (US)     Profile for jimh  Send Email to jimh     
Leaving the technical realm of global satellite navigation system (GNSS) operation with respect to clouds, one can make a very reasonable inference about the ability of GPS to work on cloudy days just from the success of the system. The annual sales of devices that can receive GPS is now estimated to be about $100-billion. It is hard to imagine the pubic spending $100-billion every year on a device that will not operate if there are clouds in the sky. Would consumers spend $100-billion on GPS receivers that stopped working if there were clouds in the sky?

Another reasonable inference can be drawn from the investment in GPS by the government of the USA. GPS was begun in 1973. GPS is now over 40-year-old. The USA has spent over $35-billion on GPS satellites, launches, ground stations, research, and operation. Now, in it 41th year, could a lone fisherman discover it does not work on cloudy days? Could this single fellow have found a critical flaw that has gone unnoticed for four decades of scientific research, military implemenation, and global use? Hardly.

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