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Re: Re: Asteroid echos on 2380 MHz



Dan,

Good researching, you got the numbers pinned down a bit closer.  

Your assumption that eme'rs are using 1kW is correct...for 2m and 70cm.  At
1296 the average eme'r is running 200w and at 2.3 GHz getting 100-150w is
quite an achievement. Since we are trying to use the typical eme station as
a bench-mark of the ham radio state of the art, I will stick with my
original 100w! 

My assumption of 40dB antenna gain differential was very crude.  A ten-foot
(3m) dish at 2380 MHz will have about 36 dBi gain, so Arecibo's 73 DBi dish
is only 37 dB more!  So we have 77 dB difference using this simple
comparative approach.  S/N = 77-82.3 = -5.3 dB.  So one would need a dish
almost twice the 10-foot example given to achieve S/N=0 dB*.

Of course we are assuming this is a CW signal so that using same narrow
bandwidth would apply.  In fact they are talking about using swept
frequency radar which may take MHz sized Bandwidths, so there is another
40+ dB needed, assuming the typical eme receiver BW of 100Hz (in fact it is
assumed by many eme'rs that one's hearing/brain adds filtering down to 50Hz
or less).

The only hope (for us) is that they operate in CW and you can search with a
2 Hz BW DSP program (thus giving 17 dB more S/N).  Doppler is another
unanswered question.  This object is in a highly elliptical solar orbit so
its traveling much faster than the Moon!  You may need a chirping DSP
program to keep the 2 Hz window centered on the signal.  Doppler will also
change rapidly as the object makes closes approach.

BTW similar discussion is going on this topic on the eme reflector as we
speak!

Its going to take some doing for hams to hear this one, I think.  {if you
have an AO-40 system tunable to 2380, with a very good LNA and 10-foot or
larger dish, this might be worth considering...if you are up for the
challenge...and in a common view of the Asteroid with Arecibo}

Ed, AL7EB
*Nice radar analysis Domenico!  Amazing how close our different approaches
come out!
============================================
At 05:39 PM 8/16/2002 -0400, Dan Schultz wrote:
>A good source of information on the asteroid flyby is
>http://skyandtelescope.com. The site includes finder charts.
>
>Sky and Telescope reports the diameter as 500 meters (1/3 mile) and a
>flyby distance of  527,000 kilometers (327,000 miles). Given that the
>moon is 3475 kilometers (2159 miles) diameter at an average distance of
>384,000 kilometers (239,000 miles), we can calculate as follows:
>
>Size difference:
>(500 meters) ^ 2 / (3475 km) ^ 2 = 48,302,500 times smaller projected
>area than the moon
>10*LOG (48,302,500) = 76.8 dB worse.
>
>Range difference:
>(527,000 kilometers) ^ 4 / (384,000 kilometers) ^ 4 = 3.55 times less
>signal
>10*LOG(3.5) = 5.5 dB worse.
>
>Total: 82.3 dB worse.
>
>Using your figures for Arecibo uplink power and antenna gain, the 80 dB
>gain will just about offset the 82.3 dB loss due to the smaller size and
>greater distance of the asteroid. It should be only 2.3 dB worse than a
>100 watt amateur EME station. However I think most EME stations run more
>than 100 watts, the asteroid reflection would be 12.3 dB worse than an
>amateur EME station running 1 KW. So let's give it a listen and see if
>you can hear it.
>
>Dan Schultz N8FGV
>
>
>>The asteroid is purportedly about 1/2 mile in diameter and passing at a
>
>>distance of 300,000 miles (just beyond the orbit of the moon). So how
>>strong an echo will this tiny rock produce compared to a moonbounce
>signal
>>(off a much bigger rock!)? Of course Arecibo will be running a signal
>9000
>>times as powerful as is used on 2304 MHz eme (avg 100w), and with 40 dB
>
>>more antenna gain! That's about 80 dB stronger! the moon is about a
>>quarter the size of the earth (guessing 2000 miles diameter) so that is
>
>>(very) roughly (Drock)^2/ (Dmoon)^2 = 1/16,000,000 weaker reflection!
>
>>So whatdya say? Is this possible?
>
>>Ed, AL7EB
>
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