SatSig topic:

Satellite Orbit Slot

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Mar 27^th, 2007 at 7:23pm

I am new here. Can any1 explain what satellite slot is and its relation to VSAT installation. Thanks

Mar 28^th, 2007 at 12:13am

The currently accepted vernacular defines "slot" as a number and letter representing the subsatellite point of an equatorial geosynchronous satellite (GEO). A GEO satellite is one that orbits the the earth's equator at a distance of 22,300 miles. That speed is calculated to match the rotation of the earth, and the satellite is orbiting in the same direction as the earth is rotating.

This combination makes a GEO satellite appear to remain in approximately the same spot in the sky at all times. Since it orbits around the equator, it then seems to be above the same spot on the equator at all times. As this spot on the equator has a longitude, that number is part of the slot designator; let's say 110 degrees. From there it's further identified as east longitude or west longitude. So a satellite in an orbital position over an equatorial longitude of 110 degrees east is said to have the 110E slot.

//greg//

Mar 30^th, 2007 at 8:27am

Thanks for your reply. Let me further ask you this, does a transponder cover a given slot or can it cover all the slots in a satellite? Why I am asking this is that someone said that there is an amount (cost) for each satellite slot and it made me feel a slot is the same as a range of Rx or Tx frequency.

Mar 30^th, 2007 at 9:34am

The term "slot" is used to describe the satellite's location, e.g. 22 deg West longitude.

If you are planning a satellite you need to select a suitable proposed orbit location and then negiotiate with the nearby satellite owners what interference you will accept from them and what interference you will cause to them. Regarding C, Ku and Ka band frequencies you will reach different agreements with other orbit users. This may take a year or more of intersystem negotiations and time and effort spent on this is costly. Once you have agreement then the registration of your proposed orbit slot is successful and you go ahead with your satellite. If you then decide not to launch then you can sell the slot to someone else who has a satellite with similar frequencies, beams patterns, sensitivity, powers etc. Countries that register spurious "paper" satellites, to try and grab orbit slots, are punished by having their slots cancelled if they have not used them by a given date.

Each satellite has many transponders, each transponder covering a particular narrow frequency range and having an associated uplink beam and downlink beam. When a transponder is operated in multicarrier mode you may lease capacity in terms of bandwidth (MHz) or power (dBW), whichever is the greater as a proportion of the resource available. A transponder may have a bandwidth of 40 MHz. 1 MHz of bandwidth costs about US$5000 per month. Long leases of several years for whole transponders are lower cost per MHz per month.

You can get a variety of different (say 400k to 3M) bit/s out of 1 MHz of bandwidth depending on dish size and modulation method. Using QPSK 3/4 FEC, 1.25 Mbit/s out of 1 MHz is normal. Using 16-QAM 7/8 FEC, 2.9 Mbit/s out of 1 MHz difficult - you spend more initially on advanced modems and extra large dishes, but save on the monthly costs.

Best regards, Eric.

Mar 30^th, 2007 at 1:39pm

Thank you Eric and Greg for your contributions. You have been very helpful.