Membership Services General Info Financial Info Activities Awards Coordinators Director's Info Members' Info Policies Forms Publications Official Publications Director's Publications Ask Dr. SETI ® Fiction NonFiction Reviews Reading Lists Technical Support Systems Antennas Amplifiers Receivers Accessories Hardware Software Press Relations Fact Sheets Local Contacts Editorials Press Releases Photo Gallery Newsletters Internet Svcs 
I've run into a problem where math and intuition seem to be giving different answers. When that has happened in the past, either I was looking at the problem wrong, or I had done the math wrong. Here is the intuition part: if I live right on the equator and point my antenna to the Eastern horizon (90 degrees azimuth at zero degrees of elevation), that point should give me maximum positive Doppler, right? Then, if I keep the antenna locked onto that position, the point being monitored will rise in the sky until it is at its zenith. My antenna should now be pointed up at 90 degrees elevation, and Doppler should be at a minimum. Then, as the point starts to set in the West, and I track it with my antenna, the Doppler shift should go negative, and reach its maximum negative just as the radiant sets  right? So, the Doppler should go through a onehalf wave of a sine from Max positive to Max negative, and the slope of the Doppler curve should always be negative. Here is the math part: using the Excel spreadsheet for GeoRotational Doppler, it never seems to go negative. I can set the hour angle at 6 hrs (point at the Eastern horizon) or +6 hrs (point at the Western horizon) and the Doppler sign does not change.
What have I missed?
The Doctor Responds: I believe what the spreadsheet may be calculating is not absolute received frequency change (Doppler shift), but rather the first time derivative of Doppler shift (the rate of change of frequency over time). In other words, you are seeing the slope of the Doppler curve, which is nearly zero at AOS, maximum at closest approach (zenith), and nearly zero at LOS. By the way, as you suspected, the sign of this slope is always negative (frequency continuously decreasing), and can never go positive. I suspect the spreadsheet is merely displaying the magnitude of the rate of change of Doppler shift, since the sign is constant (therefore irrelevant). That is to say, if the Doppler rate (first time derivative of frequency) is "100 Hz per minute", it goes without saying that this is a change in frequency which is negative over time.

email the Webmaster  entire website copyright © The SETI League, Inc.; Maintained by Microcomm this page last updated 3 March 2007 
Top of Page 