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Re: AMSAT has a SECRET!



Phil Karn wrote:

> >I stumbled across the web pages for the soon-to-be-launched HESSI satellite.
> >This satellite will be in a 600 km high orbit, and will have a 5-watt
> >transmitter on 2215 MHz, with a 5 MBPS data rate.  To receive this, their
> >ground station needs a 11-meter dish!  Check out:
> >http://hessi.ssl.berkeley.edu/ground_systems/Gif/antenna_install_79.jpg
> >This antenna is speced to point with 0.05 degrees of accuracy,
> >while slewing fast enough to track a LEO satellite!
>
> Their link seems a little overdesigned.
>
> For a slant range of 2000 km, the path loss on 2215 MHz would be 22 +
> 20log10(d/lambda) = 165.4 dB.  The spacecraft transmit antennas are
> omni, so that's an EIRP of +7dBW.  Their 5 (*not* 11) meter dish has a
> specified G/T of +16.2 dB/K. So the received C/No would be
>
> +7dBw - 165.4dB + 16.2 dB/K - -228.6 dbJ/K = 86.4 dB-Hz.
>
> At a data rate of 3.5 (not 5) Mb/s, or 65.4 dB-bps, that's an Eb/No of
>
> 86.4 - 65.4 = 21 dB.
>
> That's 18 dB above the 3dB or so needed for the FEC code they say
> they'll use. I don't know where their 2.6 dB margin figure comes from;
> they don't provide a complete link budget.
>
> It's not uncommon to see highly overdesigned links on scientific
> spacecraft. The ACE spacecraft at the L1 point between the earth and
> the sun is generally received with 11 meter dishes that give 25 dB
> Eb/No most of the time. Because ACE is never very far from the sun as
> seen from earth, the extra margin is provided for when the sun is
> unusually active -- which is exactly when the data is needed most.
> HESSI's 5 meter ground antenna is probably only a small fraction of
> their total mission cost, so it provides relatively cheap insurance.
>
> Satellites whose primary mission is communications cannot be so
> extravagant.  There are many ground stations, so their total cost is
> significant. There's always a system-level tradeoff between the cost
> of the spacecraft and the cost of the ground station. At the very
> least, this implies the use of directional antennas on the spacecraft
> that would greatly reduce the size of the ground station antennas.
>
> Phil
> ----
> Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
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If you want to see the full build and install of the ant mentioned above just use

http://hessi.ssl.berkeley.edu/ground_systems/Gif/ and it will give you a list of
GIF's the ones with an "s" on the end of the name are small thumbnails of the
bigger file. Pretty neat to see all the gears and stuff inside these big
antennas.


--
Bill
WA2TQI


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