Center of the box AMC 21

[chapelrun]

AMC 21 - - - which many of use for PBS - - - has a Center of the box of 124.9

Seems like a small thing but made a slight improvement for me on 125W KU.

http://www.ses.com/4405429/tool/centerofbox?download=y&sat=AMC-21

 

[Lone Gunman]

AMC 21 - - - which many of use for PBS - - - has a Center of the box of 124.9

Seems like a small thing but made a slight improvement for me on 125W KU.

http://www.ses.com/4405429/tool/centerofbox?download=y&sat=AMC-21

You'll have to forgive my ignorance here but this means..........................................?

 

[SatelliteAV]

Geosynchronous satellites are assigned a specific location in the Clarke belt. Gravitational forces, friction and other factors must be countered to keep the satellite in place. The satellite is never in a precise position, but rather contained within an area referred to as the box.

Station keeping is the procedure to use thrusters to correct the satellite's position and keep it within the box. The satellite's life is usually determined by the management of the fuel used during orbital placement and station keeping. Each correction reduces the amount of fuel and effectively shortens the life of the satellite.

The larger the dish, the narrower the beam width. Satellites will move in and out of the optimal signal location for both uplink and downlink dishes. Uplink dishes do not normally track the satellite's movement within the box, so as the satellite wanders near the edge of its assigned position, the satellite will not be receiving as much signal from the ground station. Automatic gain control amplifiers on the satellite and often also on the uplink transmitter increase or decrease the signal amplification to maintain the link budget signal strength and provide a constant signal level for the receive (downlink) sites.

A larger downlink dish will often reflect these signal increases and decreases as the satellite moves around in the station keeping box. The dish may be optimized for the signal in the morning, but that evening the aiming might be optimized by bumping up/down/east/west. A smaller dish with wider beam pick-up pattern may not notice these signal variations as the satellite movement is staying within the beam width of the reflector.

 

[Anole]

And at what size Ku dish would this likely become a serious concern?
- 4 foot molded, eg: ChannelMaster/Andrew/Prodelin offset
- 6 foot molded, eg: Prodelin offset
- 8 foot molded, eg: ChannelMaster/Andrew
- 8.5 foot spun aluminum, eg: Birdview
- 10 foot molded, eg: ChannelMaster/Andrew

Most of us don't have 16' commercial dishes such as Scientific Atlanta, or spun aluminum DH Satellite.
Those are more likely what the broadcasters use.

I doubt 10' and 12' mesh dishes have the surface accuracy at Ku to matter.
But it bears discussion.

What about large perforated dishes, such as the 10' Winegard Pinnacle?

.

 

[Lone Gunman]

Thanks Mike. I already knew how they kept the satellites in their proper place in the Clark Belt but just wasn't familiar with the terminology used to describe it. So now I know.

 

[SatelliteAV]

And at what size Ku dish would this likely become a serious concern?

Any reflector that resolves to <1 degree on the target band would certainly read movement within the box. Even standard size reflectors will also have noticeable BER changes when on a transponder that is affected by interference such as terrestrial or adjacent satellite. A thread 3 or 4 years ago documents the search for answers after the 11842 TP on G19 was newly launched. The satellite was not properly configured with the AGC (automatic gain control) turned ON for the new transponder. As the satellite moved in and out of the tight uplink beam, the AGC was not adjusting the signal levels and the Signal Quality reading dromed to threshold and was not adequate to maintain a lock on many receivers even using the link budget recommended 90cm. After collecting signal reports from many SatelliteGuy members, we established a signal quality graph that matched the movement of the satellite through the center of the box to the edges. The uplinker's equipment was verified to be aimed and operating correctly, so then the satellite owner was contacted to resolve the problem. A few hours later the satellite transponder parameters was corrected and the issue was solved.

Movement within the box can certainly affect any of us!

Brian