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Satellite elevation question.

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Nov 3rd, 2007 at 5:12pm  
I am looking to get dedicated linkstar service at 1:1 with a 1.8m dish and a 4 Watt BUC. The satellite, Express A4 is only a dish elevation of 18 degrees at my location in Iraq. Will that low of an elevation severly effect my systems performance?

Obviously the more overhead the satellite the better, but I am wondering if it makes really that much difference?
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Reply #1 - Nov 3rd, 2007 at 7:47pm  
Yes, expect performance to be affected by ground noise. In my experience, 15 degrees is the bottom end. Below that is a waste of money. At 18 degrees, you have to weigh the cost of the connection with the actual need. I'd look for service with a more advantageous elevation angle. But if there's none available, it's a matter of weighing whether or not you're willing to pay for performance that you know up front you'll probably seldom see.

//greg//
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Eric Johnston
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Reply #2 - Nov 3rd, 2007 at 11:35pm  
The receive performance of your dish is degraded at lower elevation angles because more of the sidelobes are listening to the warm ground.  This page antenna noise temperature and elevation angle shows some examples of antenna noise temperature versus elevation angle.  

The antenna noise temperature needs to be added to the LNA/LNB noise temperature to get your system noise temperature, which is what goes to measure your antenna goodness (G/T = Gain(in dBi) - 10 log(system noise temp) dB.

When the first sidelobes and the main beam start intersecting with the ground the effect of ground noise is goes up rapidly.   The increase is most noticable if you have a very low noise temperature LNA/LNB, which can be tested simply by pointing it at the warm ground, versus the cold sky - i.e. straight up to a clear sky.

The lower the elevation angle the longer the slant path length through rain, so more rain outages at low elevation.  Rain causes attenuation of the wanted signal but also adds to the system noise temperature, since the rain is warm (up to ~290 degK).

At low angles under 15 deg you will experience scintillation, like the twinkling of a star near the horizon, the level goes both up and down, particulary in warm humid weather conditions and for many hours.

I  recommend a minimum of 5 deg at C band and 10 deg at Ku band.  Higher angles, particularly for Ku band operation in high rainfall areas are much preferred.

In high latitudes, low elevation operation cannot be avoided so you must put up with outages or use special techniques such as much larger dishes or diversity dishes with 2 antennas several miles apart, connected by cable.

At zero elevation signal levels vary wildly and are occasionally detectable when the satellite is below the horizon.  It depends on the beam bending in the troposphere.

Best regards, Eric.
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Reply #3 - Nov 5th, 2007 at 1:51pm  
Thank you very much for this valuable information .
i am facing big problems with Express A4 that made me to lost most of my terminals working on this sat .
When the service start it was the best service i was in my life , then day by day until i lost most of them .
no one will pay extra money for larfer dish or 4 watt BUC while he could get service from other sat without this problems .
but the question here , i offer service for iDirect modems over NSS7  22dgree west which it under Express A4 and we do not have any problem and the service working very smoothly . we are  using a 1.2 meter Andrew type dish and 3 watt BUC .
Mr. Eric your feedback is required .
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Reply #4 - Nov 5th, 2007 at 5:02pm  
Express A4, which I believe was launched in 2002 is approaching the end of it's life.  It had a 7 year anticipated lifespan and it is almost due for retirement.  They are using the "test engine" to maintain position. It appears that it was moved from it's original position to a new location which may have contributed to it not having enough fuel to stay in position. 

When satellites get old, they run out of the fuel required to keep them in their orbital slots.  Their orbits degrade and they go into what's called "inclined orbit."  In other words the satellite isn't exactly where it is supposed to be and although it's only moved a very small distance from where it is supposed to be, that's enough to mess up the satellite link unless you want to continually re-align dishes - particularly Ku . 

I understand that you can get bandwidth cheap on A4, but as with most things in life you get what you pay for.  You probably need an expensive tracking dish to use A4 successfully. 

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Reply #5 - Nov 5th, 2007 at 5:09pm  
it is so funny that you spend one hour using the internet per day and spend the rest to readjust the Dish .
Thanks for this info.
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Reply #6 - Jan 9th, 2008 at 9:24pm  
Its not due to noise temp. at all , its what pgannon said , he is absolutly right , we were getting a dedicated service , since Oct. 2007 , it was the best , i have to say , but in Dec. 2007 the service began to decrease in quality , and still untill now , the problem exist only at NIGHT !!!!!  I dont know why only at night the sat power decrease gradualy to zero !!!! then it return back at morning !!!!!!
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Reply #7 - Jan 9th, 2008 at 9:50pm  
Quote:
Its not due to noise temp. at all , its what pgannon said , he is absolutly right , we were getting a dedicated service , since Oct. 2007 , it was the best , i have to say , but in Dec. 2007 the service began to decrease in quality , and still untill now , the problem exist only at NIGHT !!!!!  I dont know why only at night the sat power decrease gradualy to zero !!!! then it return back at morning !!!!!!
I'd be careful of making declarations like that without a full grasp of the subject matter. Noise temperature indeed factors into an 18 degree look angle. The fact that the satellite has developed a station-keeping problem only serves to aggravate the issue.

The satellite is NOT decreasing it's power. It's apparently just wandering a bit, which magnifies the otherwise normal Center of Box phenomenon. All satellites - no matter how stable - will APPEAR to move around a little as perceived from the ground. The actual fact of the matter though, is that it's the earth that's wobbling on its axis. Since the earth revolves once every 24 hours, the satellite appears to scribe a pattern in the sky that's approximated every 24 hours.

Imagine that pattern being scribed inside a box. When you point your dish, you ideally want to be pointing dead center on that box. That way the apparent movement will always be within the capture range of your otherwise fixed dish. But when a satellite becomes unstable - and/or when you're pointing toward one side of the box rather than the center - the satellite will cyclically appear to wander too far away for your dish to capture a usable signal. A few hours later, it wanders back into range.

//greg//

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Eric Johnston
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Reply #8 - Jan 9th, 2008 at 11:15pm  
They are probably using the small amount of remaining fuel for east-west station keeping only, and allowing the north-south inclination to gradually increase.

As Greg says you will see the satellite moving about.  It will go above (north) and below (south) of the geostationary line with a daily cycle.
If you are clever, you can point a fixed dish around the middle and you will have two maximums per day and you will have two minimums when the satellite is at its extreme north or south excursion.  The inclination increases at about 0.8 deg per year so you soon start losing it completely for some of the time.  The transmit fails first as the beam is narrower.

If you can organise your antenna axes so that one is at right angles to the orbit line you can use a single axis actuator and tracking controller to track the satellite.  An az/el mount is fine if the satellite is due south of your site.  For all other locations you need a polar mount or a suitably tipped az-el mount.

Read more here about satellite dish tracking controllers:

http://www.researchconcepts.com/Files/track_wp.pdf

http://www.neterra.net/en/content.html?link=show_highlight.html&acc_id=2

http://www.satsyscorp.com/products/atx3000.html

Before spending any money check out the far end.  The teleport hub will have similar tracking problems and there is little point in you upgrading if the hub does not have tracking.  Ask how long will it be before the satellite is turned off ?

Best regards, Eric.
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Reply #9 - Jan 10th, 2008 at 7:21am  
I just wrote what is hapenning with my service , and what I see , eb/no decrease from 6 to 0 within 2 hrs at night still 0 for 8  or 9 hrs then it return back , so what i see is decreasing in power , i dont care where the hell is the satellite going to , the satellite is dying , and this is the truth ..
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Eric Johnston
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Reply #10 - Jan 10th, 2008 at 10:05am  
Call the other end and ask if they have their tracking working and if they are able to see the carrier intended for reception by you and if it stable all day.

If they can see that your downlink carrier is stable all day then I suggest you try repointing your dish at about the time the signal is about to disappear.  Does repointing fix the problem ?

If repointing makes the signal good again this tends to confirm the inclined orbit satellite explanation.  You will probably find the signal drops again after a few hours and will need another re-peaking.  This is what a single axis inclined orbit tracking controller does.

Think carefully about how the satellite moves.  It will be in a north/south line at right angles to the geostationary orbit.  It will be aligned with polarisation planes.

If the satellite is to the south east and your beam pointing rather low you might see something like this:
...
In this example you lose the signal when the satellite is to the upper left.  Move the dish up and to the left slightly.

It may take several days of observations and dish adjustments for you to find the centre of the box, where the satellite crosses the equator.


...
This image above shows a normal satellite, which has its inclination controlled to within 0.15 deg north-south.  The earth station dish is mispointed.  Note that the rotational orientation of this image corresponds to a due south orbit position at the top of the orbit.  For other orbit locations the entire image needs tilting accordingly.

The apparant movement involves both north south and small east west daily cycles.  It may describe a figure of 8, a circle or any tilted ellipse, plus slight and steady east or west drift.

Consider your antenna beam width using this antenna beamwidth calculator.

Best regards, Eric.
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« Last Edit: Jan 10th, 2008 at 3:59pm by Eric Johnston »  
 
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Reply #11 - Jan 12th, 2008 at 11:57pm  
Sigh.

If only network operators could honestly tell what they're selling. We've lost a few of our customers because they went to A4, attracted by cheap pricing. How many people I told that A4 is probably an option if you need a service occasionally for a month or two and it is not mission-critical, but not really much more. Even RSCC tells on their website that they don't guarantee anything on that particular bird. I strongly support the idea it is a centerbox problem.
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