> --- Original message --- > from: "Tom Clark, K3IO (ex W3IWI)" <[EMAIL PROTECTED]> > to: time-nuts@febo.com > concerning: Re: [time-nuts] Rain, Water, etc > Date: Mon, 06 Mar 2006 15:28:24 -0500 > > > [EMAIL PROTECTED] wrote: > > David and Tom opined: > > Rain should have an effect on the timing of the signal, since the > propagation speed of radio waves through water is different from that > through air. It will also attenuate the signal, causing worse S/N > ratio which would cause the lower-elevation satellites to not be seen. > > > David, > > Maybe we can figure this out. First, the refractive > index of water is about 1.3. So I think this means > the propagation speed of radio waves in water is > down to about 0.75 c, right? > > Then, how much water are the GPS signals traveling > through? Let's assume the typical amount of rain in > a heavy storm is a couple of inches. ... > Thanks, > /tvb > > > If it is in fact raining, rain will act as an absorber, decreasing > the SNR. If you don't believe this, take a small "hockey puck" GPS > antenna and drop it into a dish with an inch or 2 of water in it and > watch the signals go away. This is why GPS doesn't work for swimmers. > The real "wet" effect comes from water vapor in the atmosphere. The > index of refraction of water departs from unity by about 300 parts in > 10e6. Plug this into the fact that the water vapor in in the bottom > ~2km of the atmosphere and you will see that the number works out to > be < 1 meter of path delay change in the zenith, and it will increase > towards the horizon as secant(zenith angle) [strictly speaking, this > is true only for a plane parallel atmosphere, but it gives an order > of magnitude idea].... > > 73, Tom
Hello Tom and the group, very interesting this issue. Perhaps it is the case that the reason for the commented timing jitter is to be identified on ground as antenna or transmission-line problem etc., I would check for it. But I believe as well, the behaviour of heavy saturated and moving clouds, growing up to 12 km over ground, extending up to some kms in thickness and containing ice particles and water drops, creating electrical charges, currents and discharges due to heavy turbulences inside, cannot solely be expressed in a pure delay and attenuation number of the signal on the path. I think, out of the precipitation amount on a local site I cannot get any correct number of the amount of water mass in the cloud above nor any value about the thickness of the cloud, so I do not know how I can judge the dielectric equivalent of perhaps several thousand meters path of above conditions? Is it really just the equivalent of a few inches of pure water? As The L1 carrier has a wavelength of 19 cm, such big and saturated cumuli clouds must act as a or several lenses, continuously changing their parameters, thus creating bent, distorted, attenuated, delayed and even timewise eliminated signals. The result is hard to predict I believe. Can a GPS-RX really drag such situation reliable? Perhaps the jitter of hundrets of nanoseconds is not the real jitter on the path but the processor result due to the distorted signal? Finally I am not fully convinced that all this could result in such heavy time-jitter after math-processing in the GPS-receiver, but why not? Eg. having even a oversized big antenna dish for digital sat-tv with big gain margin, I did observe (seldom) some total blackouts for most channels for several seconds due to very heavy thunderstorm clouds or very heavy snow loaded clouds in the propagation path.... I could not find better and detailed information about, but I found a in general very interesting report giving a lot of informations: from the DANISH METEOROLOGICAL INSTITUTE SCIENTIFIC REPORT Atmospheric Water Vapour Detection using Satellite GPS Profiling by Jakob Grove-Rasmussen Copenhagen University, Faculty of Science Niels Bohr Institute for Astronomy, Physics and Geophysics http://www.dmi.dk/dmi/sr03-05.pdf If one is interested on a summary of sometimes even amusing EMC effects, here I found another document, worth to be read: "Banana Skins" Compendium anecdotes to help you to identify interference problems http://www.cherryclough.com/Downloads/Compendium%20of%20Banana%20Skins,%207%20Oct%2005.doc © Cherry Clough Consultants Oct 2005 Enjoy reading it. Curious to get finally the information of the real reason of this mystical effect. Take care of the time, you don't get another. 73 Arnold, DK2WT -- "Feel free" mit GMX FreeMail! Monat für Monat 10 FreeSMS inklusive! http://www.gmx.net _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
