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From: rbk@hpctdke.HP.COM (Richard Katz)
Newsgroups: sci.space.shuttle
Subject: re: Magellan Data Volume
Message-ID: <4260014@hpctdke.HP.COM>
Date: 5 May 89 22:35:42 GMT
Organization: Hewlett-Packard CTD, Colo. Spgs.
Lines: 85

-Given that the Magellan is suppose to have "resolution down to football
-sizes" and all of Venus is dry, how many pixels would we be talking
-about here (assume pixel tone is function of height, so only one
-pixel per resolution cell).  How much data are we talking about here, 
-beside "a bunch"?

-I heard "500,000 books" on the shuttle launch TV program today.  Sounds
-like somebody divided the number of data bytes by the number of bytes
-in an average book.

I think 'a bunch' is a good approximation :-), but here's another one
using numbers.

First it takes 243 earth days to complete the primary mapping mission.
During this time, the radar is on for approximately 37 minutes out of
every 3.1 hours.

The radar sends 'data' to the spacecraft at the rate of 823 8-bit
bytes, every 8.333 mS.

So, the amount of data sent in the primary mission, assuming 100%
utilization of the radar to spacecraft link (it is VERY high so this
is a fair approximation) is, in bytes:

823 bytes   1000 mS   60 S    60 min   24 hr   243 days    37 min   1 hr
--------- x ------- x ----- x ------ x ----- x --------- x ------ x ------
8.33 mS        1  S   1 min    1 hr    1 day   1 mission   3.1 hr   60 min

                                                             ^
                                                             |
                                                           duty cycle
                 11
        4.12 x 10  bytes
   =    ----------------  =  a bunch.
                 mission



Using the 500,000 books, I assumed 60 characters per line, 60 lines
per page, and 250 pages per book, just for excercise.  This comes out
to: 

      60 char   60 lines   250 pages   500,000 books
      ------- x -------- x --------- x -------------  
         line       page        book       "mission"


                 11
         4.5 x 10  chars
   =     ---------------
               "mission"


So my above calculations seem consistent, although done from memory.


Above I used quotes around data.  The "data" that is sent to the
spacecraft for transmittal to earth is "reduced" on board.  Their are
several components to each block or burst of data.  First is the overhead
information which consists of sync info, time stamp, radiometer data, 
radar parameters and  thresholds, which are needed to "uncompress" the
radar info.   This stuff, along with it checkbits is small, only 108
bytes.   Next comes the radar data, compressed by 4:1 ratio, which can
range up to about 85 kbytes in length (this is variable, it can also
go down to about a couple of kbytes), and altimeter info, which is
always 2266 bytes.

                                                             12
So, I would approximate the amount of data as being 1.65 x 10   bytes
of data for a mission.  A 'bunch' of bytes if you ask me.

Yeah, I didn't calculate in the times that Magellan can't send data
back to earth.


rich katz
hewlett packard
p o box 7050
colorado springs, co
80933-7050

email: rbk@hpctdlb.hp.com


Disclaimer: I can't remember any good disclaimers and all of the above
            data is from memory.