[Sis-csi] Limit of Cislunar domain

Sat Oct 8 23:15:05 EDT 2005

Mike,

L2 being on the far side of the Moon is what makes it nice as a comm relay for assets that are also on the far side.  I'm not sure about L4 and L5, but I'm with you about not being confident in my knowledge of how these points could be used.  Off-hand if you're using relays to save power/mass on a lander, it would seem using L4 & L5 wouldn't help much.  However, L4 & L5 would give some coverage of the far side.  

Chris

-----Original Message-----
From: sis-csi-bounces at mailman.ccsds.org on behalf of Kearney, Mike
Sent: Sat 10/8/2005 10:10 PM
To: sis-csi at mailman.ccsds.org
Subject: RE: [Sis-csi] Limit of Cislunar domain

Keith (Hogie):  I was plugging your table below into section 2...

0000.1 sec - Interaction between rovers, landers, (e.g. local
environment)
0000.1 sec - Low-Earth orbit  ( a few hundred kilometers one-way)
0000.1 sec - Low-Lunar orbit ( a few hundred kilometers one-way)
0000.1 sec - Low-Mars orbit   ( a few hundred kilometers one-way)
0000.5 sec - Earth geosync orbit  (36,000 kilometers one-way)
0002.5 sec - Earth-to-Moon (384,000 kilometers one-way)
0010.0 sec - Earth to L1 or L2 (1,500,000 kilometers one-way)
------------------Limit of Cislunar domain
0366.0 sec - Earth to Mars (closest = 55.000.000 kilometers one-way, 6
minute, RTT)
2673.0 sec - Earth to Mars (farthest = 401,000,000 kilometers one-way,
45 minute RTT)

But I started wondering about the usage of L1 and L2.  L1 is closer to
earth than the moon, so it would really not be a factor in establishing
the boundary of cislunar missions in terms of time delay.  L2 is on the
far side of the moon and wouldn't have much value for comm relays.  Did
you have some other mission in mind?  

In terms of comm relays, I was wondering if L4 or L5 (preceding or
following the moon in earth orbit) have more value as comm relays.  And
I started wondering if anyone what Exploration was considering as
possible uses for those Lagrangian points.  I'm not confident in my
understanding, and I just want whatever text that goes into the Cislunar
GB to be credible.   

Anyone have any insight into that?  

I ask the question, because a better description of what might be done
at the Lagrangian points would help the "mission characteristics"
section.  

   -=- Mike

Mike Kearney

NASA MSFC EO-01

256-544-2029

________________________________

From: sis-csi-bounces at mailman.ccsds.org
[mailto:sis-csi-bounces at mailman.ccsds.org] On Behalf Of Keith Hogie
Sent: Thursday, September 08, 2005 12:55 AM
Cc: sis-csi at mailman.ccsds.org
Subject: Re: [Sis-csi] IP Header Compression

Adrian J. Hooke wrote: 

At 01:40 PM 9/6/2005, Keith Hogie wrote:

  I agree we need to consider issues with small packets and low rates,
but how low do we need to go.  In all of the missions I have seen (non
deep space), the lowest data rates are 125 bps.  This is over an order
of magnitude difference from your 10 bps.  

  For the Cislunar environment, we need to figure out what some of our
limits are.  Do we really want to burden the Cislunar design with issues
that only relate to Deep Space?

Aren't the Lunar missions supposed to be "training" for going to Mars?
For critical emergency commanding operations, shouldn't we be developing
a robust, unified, reliable, tested system that works wherever you go?

  For emergency commanding I don't see any difference between Cislunar
or Deep Space.  In both cases the solution is to send a string of bits
that gets decoded by hardware and do not need any protocol.  The
critical hardware commands are their own frame sync, authentication, and
command all packed into a highly unique string of bits.  Most hardware
decoders pick off the bits they are looking at directly from the
receiver and don't involve any flight software.  This means that there
is no complex packet processing and the hardware is just looking for
particular sequence of bits.  The length of this sequence is not a
function of any CCSDS or IP headers.  Getting the command to the
spacecraft just requires radiating the proper string of bits.  The
length of the hardware command is just a function of how many bits you
think you need to make sure your command doesn't occur in normal data
transfers.  

 My main concern is for all the other operational modes there is a very
large disconnect between things that will work in a Cislunar environment
and a long haul link to Mars.  If you consider the following round trip
times (RTT):

0000.1 sec - Interaction between rovers, landers, (e.g. local
environment)
0000.1 sec - Low-Earth orbit  ( a few hundred kilometers one-way)
0000.1 sec - Low-Lunar orbit ( a few hundred kilometers one-way)
0000.1 sec - Low-Mars orbit   ( a few hundred kilometers one-way)
0000.5 sec - Earth geosync orbit  (36,000 kilometers one-way)
0002.5 sec - Earth-to-Moon (384,000 kilometers one-way)
0010.0 sec - Earth to L1 or L2 (1,500,000 kilometers one-way)
------------------Limit of Cislunar domain
0366.0 sec - Earth to Mars (closest = 55.000.000 kilometers one-way, 6
minute, RTT)
2673.0 sec - Earth to Mars (farthest = 401,000,000 kilometers one-way,
45 minute RTT)

  When you look at distances like these there is a huge break between
Cislunar ones and Mars.  In the Cislunar  area it is actually possible
to do interactive things like interactive audio, video, and data access.
You can consider security protocols that negotiate security details.  At
L1 and L2 things get a bit uncomfortable at 10 seconds RTT but that is
still manageable.  At Lunar distances you can do most anything you do on
Earth.  A 2.5 second delay is a bit long for some interactive operations
but it is not really any longer than what happens when you surf the open
Internet and hit a bit of congestion.  The main point is that out to L1
and L2 you can actually do interactive operations  This also applies to
systems on Mars and orbiting around Mars.  

  However, when you move to the long haul link between Earth and Mars,
the RTT jumps up to over 100 or 1,000 times that of the Earth and Moon.
With a 6 to 45 minute RTT, you can't carry on an interactive voice or
video conversation and lots of interactive data access just doesn't
work.  On a Earth-to-Mars link you are forced to shift to an operations
concept of two one-way links.  Operations must shift into email-like
file store-and-forward or one-way streaming of data.  

   So I don't see any real problem with using the same hardware
commanding solution in Cislunar or Earth-to-Mars scenarios.  Some file
store-and-forward and one-way streaming operations will also work for
both environments.  Of course any acknowledgments on the file-store-and
forward will take lots longer.  

  My concern is that other there are lots of protocols and applications
that will work fine in an interactive Cislunar environment but just
don't work for Earth-to-Mars.  We don't want to limit our Cislunar
solutions to only those that will also work for Earth-to-Mars.  I think
we need to develop our Cislunar solutions and then see if any of them
will also work in a Earth-to-Mars scenario.  

----------------------------------------------------------------------
  Keith Hogie                   e-mail: Keith.Hogie at gsfc.nasa.gov
  Computer Sciences Corp.       office: 301-794-2999  fax: 301-794-9480
  7700 Hubble Dr.
  Lanham-Seabrook, MD 20706  USA        301-286-3203 @ NASA/Goddard
---------------------------------------------------------------------- 

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