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Re: AMSAT has a SECRET!



>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
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