DOPPLER SHIFT EFFECT.

Everyone has experienced Doppler Shift effect ... perhaps without realising it.

Imagine if you will, a motor-car approaching you. The sound of the engine has a certain pitch or frequency ... as the car passes and recedes, the tone of the engine sounds lower in pitch.

If on the other hand you were moving parallel to the motor car, at the same speed, the sound of the engine would remain at the same pitch. You have now learned the basics of the Doppler Shift effect.

When a satellite is moving towards you the frequency is HIGHER than the nominal beacon frequency and when it's moving away from you the frequency is LOWER than the nominal beacon frequency. The rate of Doppler Shiftdepends primarily on the frequency in use but also very much on the speed of the satellite relative to the observer and it's orbital height above the earth's surface.

Doppler Shift is proportional to the frequency in use.

For LEO's (low earth orbit) satellites at a height of about 1000 km the rate is approximately ...

28Mhz.................plus/minus 4 to 7Khz

145Mhz...............plus/minus 8 to 10Khz

435Mhz...............plus/minus 16 to 20Khz

1300Mhz.............plus/minus 40 to 60Khz

2400Mhz.............plus/minus 80 to 100Khz.

As can be seen ... the higher the frequency, the higher the rate of Doppler Shift.

The above Doppler Shift curve illustrates that found when using the AO-7 satellite in Mode U/V (mode-B) with a 70cm uplink and a 2m downlink.

The TCA (time of closest approach) denotes the actual beacon frequency.

Note how at AOS (acquisition of signal) the received signal is 145.976Mhz ... passing through 145.972 (the actual beacon frequency) then dropping to 145.970Mhz at the LOS (loss of signal).

The dots on the curve represent actual frequency measurements.

So, how does one compensate for Doppler Shift? Different operators use different techniques. In fact, some satellites use INVERTING transponders to help compensate for Doppler Shift. That is the say, they invert the signals they pass through the satellite’s pass-band so that the Doppler Shift effect is less severe.

Logically, the standard convention had previously been to compensate for Doppler Shift by "tuning" the higher of the two frequencies since this has the most Doppler Shift ... however, due to the varying distances involved, different stations located at different points on the globe will obviously have different "rates" of Doppler Shift from their QTH to the satellite ... so they will still be "wandering" across the Satellite’s pass-band using this technique.

Another convention much used is to leave the transmitter where it is and retune to the signal each time the other station transmits ... but this assumes the other station knows how to compensate for Doppler Shift. Both of these techniques lead to QSO's merging into each other and causing QRM.

There is a third convention called ... "Doppler Shift, the one true rule" ... (this article by KB5MU is recommended reading and can be found on the AMSAT-NA website).      This "rule" suggests that all operators tune both the transmitter and the receiver ... to keep the frequency constant at the satellite ... in theory, this will prevent stations "wandering" into each other's QSO's.   Unfortunately, to achieve this requires a completely automated tracking station with auto-Doppler Shift tuning facilities ... in other words, a computer controlled rig and antenna system. As is well known ... many hundreds of amateur satellite operators simply don't have such equipment at their disposal.

So ... how do these stations combat Doppler Shift? ...

Well, first of all they ensure their EIRP (effective isotropic radiated power) is NEVER stronger than the satellite's beacon. (Using only the amount of power necessary to maintain reliable communications not only limits the amount of QRM it also preserves the batteries). If you can't hear the beacon at a reasonable level you should improve your receiving system ... either by a better antenna or by improving the quality of your coax. Better quality coax always means less signal loss. ALWAYS install the best possible coax you can afford ... it will save you a lot of time, money and effort later. Pay attention to connectors also. The cheap and common SO-239 and PL-259 types are more ‘lossy’ than the more expensive N types. 

Most operators without any form of automatic Doppler Shift correction tend to use the previously mentioned convention of tuning the higher of the two frequencies whether they happen to be the transmit or receive frequencies ... (depending on the satellite one is communicating through). After each over they "tweak" the receiver, tuning in to the transmitting station's signal until it sounds clear on SSB ... assuming one is operating via a linear transponder.

Remember, when the satellite is at AOS the Doppler Shift will be hardly noticeable but as it approaches TCA it will become more severe, depending on the frequency. Please see the tables and diagrams.

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