8g.1 Recall that satellites orbit the Earth at heights above 150 km,
and understand that amateur satellites are moving in relation to the
Earth and will only be above the horizon at certain times.
Amateur
satellites are put into a low orbit round the Earth at a height of
about 150km. They are all in Polar Orbits which means that they go over
the North Pole, over the equator, over the South Pole. It takes about
90minutes for one orbit. During this time the Earth has moved, so on
their next orbit they pass over a different part of the Earth. After so
many orbits they go back over the same part of the Earth.
From the Earth observer's point of view they come over the horizon and
after about 15minutes disappear over the horizon. In the UK they can make 5
passes a day, although some will not be in range for long.
8g.2 Recall that the up-link and down-link frequencies are generally in
different amateur bands and that details are published by amateur
organisations. Recall that the transmitting station must be able to
receive both the up-link and down-link signals.
To
use a satellite for a QSO you usually need to be able to transmit on a
section of one band and receive on a section of another band. Every
satellite has a set frequency or band of frequencies for the uplink and
a different set for the downlink. You can find these frequencies on the
AMSAT website (http://www.amsat.org/amsat-new/satellites/status.php)
Although the transmitting
station transmits and receives on different frequencies it must be
capable of receiving on the up-link so that they are sure that they are
not causing interference to another station.
The table opposite is for Oscar 52. You do not need to learn this, but look at Mode U/V Dutch Transponder.
The uplink is on a section of the 70cm band and the downlink is on a section of the 2m band.
You can also see that you can use SSB and CW.
Frequency Information for Oscar 52 Mode V Beacon (Indian Beacon): Non-Operational Downlink - 145.9360 MHz Carrier
Mode V Beacon (Dutch Beacon): Operational
Simplex:145.8600 MHz CW
Mode U/V (B) Linear Transponder (Inverting) (Indian Transponder): Non-Operational
Uplink:435.2200 - 435.2800 MHz SSB/CW
Downlink145.9300 - 145.8700 MHz SSB/CW
Mode U/V (B) Linear Transponder (Inverting) (Dutch Transponder): Operational
8g.3 Understand that amateur satellites can only be used when they are
above the horizon at both the sending and receiving stations, and that
the movement of the satellite will cause frequency variation (Doppler
shift) on the received signal, which must be allowed for when selecting
operating frequencies.
To
contact another station both stations have to have the satellite above
the horizon at the same time. Most computing plotting programs will
place a circle around the piece of earth below the satellites showing
its coverage.
If the satellite is moving towards you the received signal will appear
to get higher in frequency and if it is moving away from you the
signals will appear to go lower in frequency. This is called the
Doppler shift and must be taken into account when selecting operating
frequencies.
NB some computer programs that can control the frequency of your transceiver will do this automatically.
This drawing shows the position of the International Space Station
above West Africa. The path of the satellite is shown. Stations withing
the acquisition circle should be able to communicate with the ISS. I
would not be able to work through the ISS at this moment in time
because I am outside of the acquisition circle. the satellite is below
my horizon.
8g.4 Understand that satellites have a very limited power supply,
derived from solar panels, and that excessive up-link power may result
in wasteful and unfair use of the satellite’s power.
Amateur
satellites derive their power from solar panels which top up
rechargeable batteries. the batteries have to hold sufficient charge to
power the radio when the satellite is not in sunlight. For this reason
their power output is quite
low.
In the case of most satellites, the more power is used in the uplink,
the more is used in the downlink. For this reason it is important to
reduce your transmit power to the minimum required for successful
access. Using too much power is wasteful and unfair to users who limit their power.
