The Doppler effect the sliding frequency A low satellite comes closer, passes, moves away: its frequency goes up, then down. With Doppler and Kepler, see why, hear it, and learn to correct it. All tutorials
Pick a level 1 The sliding sound A pass seen from the side, the S curve, TCA, the radial speed. ☆☆☆ 2 Keep it tuned Correct the tuning by hand through a whole pass. ☆☆☆ 3 145 or 437? Same pass, two bands: Doppler follows the frequency. ☆☆☆ 4 High or grazing A pass at the zenith, another skimming the horizon. ☆☆☆ 5 Who corrects what? Uplink, downlink, an FM repeater's memories, a linear transponder. ☆☆☆ 6 Final quiz Ten questions to go over it all. ☆☆☆ The tutorial needs JavaScript.
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The Doppler effect in three lines Formula : f heard = f (1 − v_r / c), that is Δf = −f · v_r / c. v_r is the radial speed, positive as the satellite moves away; c = 299,792.458 km/s.The S curve : positive while approaching, zero at TCA, negative while receding; steepest at TCA, all the more so on a high pass.Proportional to the frequency : about ±3.5 kHz on 145.8 MHz, ±10.5 kHz on 437.8 MHz for a satellite around 420 km up.Who corrects what Receiving, you follow the frequency heard: higher at rise, lower at set. Transmitting, you do the opposite, in advance, so that the satellite hears the right frequency. By hand, you correct the higher of the two frequencies; tracking software can correct both.
The model A circular orbit 420 km up, a spherical Earth of 6,371 km held still during the pass: its turning changes the figures by a few per cent, not the shape of the curve.
The sources The tutorial's figures and advice rest on these references:
GM4IHJ, AMSAT Satellite Gen 199 — zero Doppler at closest approach, the fastest change at TCAAMSAT, Satellite Gen 255 — orders of magnitude: about 3 kHz on 145 MHz and 9 kHz on 435 MHz for an average pass, pass durationsAMSAT, « Working Satellites » (W2JV, WD9EWK) — SO-50's memories from 436.810 to 436.785 MHz, tune the higher of the two frequencies by handKB5MU (Paul Williamson), « The One True Rule for Doppler Tuning », AMSAT — the « One True Rule »: keep the frequency at the satellite fixed by correcting both uplink and downlinkESA, « Types of orbits » — low orbit: below 2,000 km, about 90 minutes, about 7.8 km/sNASA, « Space Station Facts and Figures » — the ISS makes 16 orbits in 24 hours, about one every 90 minutes