Satgen 212 Doppler Part 6 by GM4IHJ 17th April 93 BID of this msg is SGEN212 Please use this BID if you retransmit the msg Previous satgens on this topic have concentrated on the way that doppler shift of received frequency affects satellite communications. This bulletin will discuss other situations where doppler is useful to us. 1. Deep Space Navigation . It is common practice to navigate Interplanetary spacecraft using the doppler shift on the signals they return to Earth. Voyager, Pioneer, Ulysees and the Mars Observer presently use this technique. If we know the original spacecraft transmit frequency, we can measure the frequency coming from it in deep space and calculate the spacecraft's velocity along our line of sight to it, and hence the distance it has moved in a given time, all from doppler shift. We then correct these relative figures, for Earth rotation and Earth orbital motion effects, and we can use continuous doppler checks day after day to plot the spacecraft's track in space. If the spacecraft approaches another planet we get a double check , using the change in doppler induced by the planet's gravity , plus video pictures of the view from the satellite of the adjacent planet versus the stars in the sky behind it. Pioneer's use 8410.925 or 8412.283; Voyager's use 8420.432097 and 8415.00; Ulysees uses 8408.209876 and Mars Observer uses 8417.71605 MHz. 2. Terrestrial Navigation. If we know a satellite's exact position in space , we can measure the doppler shift curve of its signal as it goes past us, and compute our position on the earth's surface . Theoretically this might seem to produce a problem whereby it is difficult to decide on which side of our station the satellite passed? But this is easily resolved, because Earth rotation ensure that the passes on either side are not the same shape of curve, depending whether the Earth is turning us towards or away from the satellite track. Two families of satellites centred on 150 MHz and 400 MHz approx ( each sat broadcast simultaneously on both bands ), provide this doppler navigation service. 3. SARSAT Sea Air Rescue Satellites, employ a variation of the terrestrial Navigation technique. The man in the liferaft , or wrecked aircraft, operates his distress beacon. The next satellite to fly in range of his position. hears the rescue beacon and plots a doppler curve of its signal as it flies past. The received doppler curve and known satellite track are then compared to compute the location of the emergency. This location data is then transmitted to a ground station by the satellite and the ground station alerts the nearest emergency resue group. All NOAA and most Meteor American and Russian Weather satellites now carry this extra SARSAT capability. Indeed one NOAA satellite recently picked up a rescue beacon which had automatically switched on when the parent aircraft crashed in a wilderness area in Northern Canada. The satellite alerted rescue personnel and the badly disabled occupants of the aircraft were quickly located and got into hospital. The interesting thing about this rescue was that the NOAA sat concerned had only been launched a few days before and had not yet completed its post launch tests. 73 de GM4IHJ @ GB7SAN