Satgen291  Sat Problem No.4   Doppler Imperfect  by GM4IHJ       22 Oct 94
BID of this msg is SGEN291  Please use this BID if you retransmit this msg

The most useful feature of any satellite radio signal is its doppler shift
- The higher than transmitted frequency you receive on the ground when the
satellite is approaching your station, and the lower than transmitted
frequency you receive when the satellite is moving away from your station.
Some experimenters measure the gradual frequency change minute by minute ,
then manage to graph plot it as a beautiful symmetrical double curve whose
second half is a mirror image of the first half.

There is just one problem. In the real world, the curves are never
symmetrical. In fact , some important Navigation aids use the curve
asymmetry as an essential part of their calculations.

The cause of the asymmetry is the location of your station on a rotating
Earth. A station on the Equator is moving eastwards at about 0.46
Kms/second ie about 7% of the maximum possible satellite opening or
closing velocity of around 6 kms/second. Even at higher latitude European
locations the additional motion can be + or - 3% or so. Worse still the
stations motion relative to the satellite is not regular. At the start of
say a Mir pass over Europe the space station is approaching your station
from a roughly westerly direction , while your station is rotating away
from the space station in an easterly direction . But Mir eventually
overtakes your station and passes to your east , at which point Mir is
pulling away from your station whilst your station is moving towards Mir .
A situation where the sum of ground station and Mir velocities is addative
in the first part of the pass but subtractive in the second part of the
pass, for this direct orbit satellite. Please note for retrograde orbiters
such as the pacsats ( satellites with inclination angle greater than 90
degrees), the relative velocity is reversed , subtractive in the approach
part of the orbit, additive in the receding part of the orbit.

It should be clear therefore that anyone who gets a beautiful symmetrical
doppler curve is cheating . Hi.

Users of the Transit and Cosnav Navigation satellites on 149 and 400 MHz
need to be very familiar with this situation. If the asymmetry produced by
earth rotation did not occur, it would not be possible to tell from the
doppler curve alone, which side of your station the Navsat had tracked. So
the would be navigator would be left trying to decide which of two sets of
doppler curves was the correct one for his navigation fix and which was an
artificial product of the the inability of the system to resolve the
ambiguity. Fortunately for navigators the earth rotation removes the
ambiguity - but as intelligent readers will note, the ambiguity gets less
as your station gets nearer to the north or south rotation pole of the
earth. GPS and Glosnass navigation satellites do not use doppler , so this
situation does not apply to them.

73 de GM4IHJ @ GB7SAN