[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. All that water is either puddles already on the ground, drops on their way down, or moisture still in the clouds waiting to come down. The total amount of water in a cross section column of the atmosphere that the GPS signals travel though is thus a couple of inches total, max. Let's assume a worst case -- 6 inches. So, those GPS signals go through 20,000 km of empty space and atmosphere containing a total of 6 inches of water; in which it slows down by 30%. At a ns/foot, this comes to 25 ps per inch of water content in the air; a total of 150 ps in my worst-case example above. My conclusion is that rain or snow, light or heavy, has no effect, even at the ns level. Can someone who really knows double check this back of the envelope calculation? 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 ~2 km 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]. Secant(60º)=2, so the number at 30º elevation is twice what it is in the zenith [assuming that the weather is the same in both directions]. To show the range it can be, at -60° F (or C, it makes little difference!) in the dead of winter in Fairbanks, I have seen under 1 mm of zenith path delay. Similarly, in the middle of the Alticama desert at 13,000ft altitude, it can also be ~1mm (this is why so many radio & optical telescopes are located in Chile). The other extreme is in more tropical regions. In St Croix & Miami, I have seen as much as 2M in the summer afternoon. As an aside, geodetic quality, dual frequency GPS receivers at fixed locations are used to measure precipitable water. The ionospheric delays are measured using 2 frequency observations. The stations are assumed to be fixed. Precision orbits are extracted from a global network of GPS sites, and what's left is the water vapor contribution. In "Tornado Alley" in the Oklahoma-Kansas area, a network of many types of sensors (including GPS) are used for research attempting to "nowcast" tornadoes. You can glean some more info on these GPS factoids at the IGS web site: [2]http://igscb.jpl.nasa.gov/ While I am on such topics, we can't forget the ionosphere. While the troposphere & water vapor are only in the bottom ~2 km of the atmosphere, the ionosphere sits at ~300 km altitude. Ionospheric effects vary with time-of-day, solar cycle, solar flux and location in a complex manner. While the water vapor contributions at all radio frequencies are equal, the ionospheric contributions scale as 1/frequency². While the water vapor zenith path delay is ~50cm-2M, it just so happens that the typical ionospheric range over one day is also typically 50cm-2M at S-band (2.3 GHz). Noting that (2.2/1.57)²=2.15, we would expect the "typical" zenith value to be in the 1-4M range for GPS in the zenith. At a more oblique angle the effect is larger so we can expect the GPS "signature" to be something like 5-6M. And it turns out that it is not unusual to see daily timing variations at the ±10 nsec level. Back in 2002, in conjunction with an effort to calibrate the "DC" timing offset of the Motorola GT+ timing receiver, Rick Hambly and I happened to catch a ~50 nsec ionospheric excursion due to a solar flare which then also produced a spectacular aurora. You can see the "Critical Evaluation ..." paper at [3]http://gpstime.com/. In the paper, you will see the interesting result that the ~50 nsec timing "glitch" reproduced perfectly with receivers separated 22 km. And you can see some pretty pictures (including a "movie") of the Aurora as seen at 39º latitude in Maryland on my photo web site at [4]http://www.pbase.com/tomcat/aurora. 73, Tom References 1. mailto:[EMAIL PROTECTED] 2. http://igscb.jpl.nasa.gov/ 3. http://gpstime.com/ 4. http://www.pbase.com/tomcat/aurora _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts