Poul-Henning Kamp asked: Could you explain one little detail for me ? What is the mechanism that makes air pressure affect antenna height ? Is it simply the weight of the air on the ground ? Or is it because it affects the speed of light in the atmosphere ?
The radio waves from outside (this applies to both VLBI and GPS) must travel thru the atmosphere to reach the antenna. The index of refraction for the "dry" (i.e. Oxygen + Nitrogen) is about 3 parts in 10e4 larger than one (i.e. for vacuum). At low altitudes, this means that the "dry" troposphere adds about 2 meters to the zenith path delay (ZPD), and this correction is constant with frequency. A barometer is merely a scale that weighs the atmosphere, and 1 mbar change corresponds to about 2mm change in ZPD. But neither VLBI nor GPS observes very much directly overhead, so a correction needs to be applied; if the earth were flat (i.e. ignoring curvature), the correction away from the zenith would then need to be ZPD * secant (Zenith Angle). This means that at ~30 degrees elevation, the ~2M ZPD would become ~4M. When you factor in curvature and the vertical structure of the atmosphere, the numbers are slightly different but you get the idea. In addition to the ~2M ZPD that can be calibrated pretty well with a barometer, water vapor in the earth's atmosphere adds a variable ranging from 0 to about 50 cm ZPD. I have seen zero in Fairbanks in the dead of winter at -40 (makes no difference C or F -- it's cold!). I have seen as much as 50-60 cm in Miami and the Virgin Islands. Here in the Washington DC area today, I'd guess it's about 40 cm. In fact, with both GPS and VLBI, you can use the barometric calibration of the "dry" part along with precise radio observations and turn the problem around to measure the integrated water vapor content. In the USA in Oklahoma and Kansas, where severe summer storms spawn devastating tornadoes, a network of high quality GPS receivers is used (along with radars) to provide warnings of severe storms. In addition to the tropospheric components that are constant with frequency, the earth's ionosphere also perturbs the path delay. At S-band (~2.3 GHz, right where 802.11b/g operate), the ZPD is typically ~2M. It scales as 1/f**2, so at GPS (L1=1575 MHz) the ZPD is more like 4M. GPS uses two frequencies (L1=1575, L2=1226) to calibrate this bias. In VLBI we have typically used 2.3 GHz & 8.5 GHz, with the longer "lever arm" being one reason for VLBI's exquisite measurement precision! Another manifestation of the ionosphere of possible interest to time-nuts is that single frequency GPS timing receivers exhibit about 20 nsec of daily noise. This shows up especially well in Figure 7 of the [1]M12+ timing receiver paper on gpstime.com. This plot is interesting because it also show a ~50 nsec timing "glitch" due to an [2]ionospheric storm that later caused an aurora visible here in MD. One last factoid to throw into this discussion. The "wet" bias arises primarily in the first 1-2 km of altitude, the "dry" atmosphere is more like 5-10 km in depth, while the dominant ionospheric effects arise at 200-400 km altitude. Since the earth isn't flat, this makes the zenith scaling laws different for the 3 atmospheric biases. That fact alone has accounted for a half-dozen PhD thesis! Back to Poul-Hennings question, there is actually another small effect. The earth is an elastic sphere. The crust is ~20 km thick and "floats" on mush below. Atmosphere pressure changes actually do have a small local effect raising and lowering the antenna at the 1 cm level. This is small compared with the changes in altitude caused by the tidal response of the earth to the gravitation of the sun & moon. You don't normally notice the ~20 cm height changes because everything near you is moving at the same time unless you have a very precise accelerometer or you make observations with networks like VLBI and GPS that span the whole earth. Now, I'm certain that is much more than you wanted as an answer to a simple question! 73, Tom References Visible links 1. ftp://ftp.cnssys.com/pub/PTTI-2002/PTTI_2002_CNS_Testbed.pdf 2. http://www.pbase.com/tomcat/aurora Hidden links: 3. http://www.pbase.com/tomcat/aurora _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
