From: gat@forsight2.jpl.nasa.gov (Erann Gat)
Newsgroups: comp.robotics
Subject: Re: How to Explore Mars
Date: 11 Jan 1993 22:04:38 GMT
Organization: Jet Propulsion Laboratory
NNTP-Posting-Host: forsight2.jpl.nasa.gov

In article <GERRY.93Jan8231255@onion.cmu.edu> gerry@cmu.edu (Gerry Roston) writes:
>By today's standards, the Viking landers are large craft.  The
>question of using small robots versus large ones is almost a religious
>question.  The greatest failing of those proposing small robots is the
>naive assumption that using multiple, small robots increases overall
>system reliability.  This is not true because one can not assume a
>priori that the failure modes are independent.  That is, if the robots
>are identical, and one of them fails in a particulr manner, this
>implies that the others will also be prone to that particular failure
>mode.

You know Jerry, I'm really happy when you shoot your mouth off like this
because it makes me look diplomatic by comparison.

That using multiple small robots increases realiability is not a "naive
assumption", it is a theoretically and empirically verifiable fact.  It
does not matter whether failure modes are independent.  Unless there is
100% correlation among failures in multiple units (which is never the case)
having more units will increase the overall system reliability.

>From a technical viewpoint, the major drawback to small robots in
>telemetry.  To transmit a signal from Mars, you need a moderately
>large antenna and a bunch of power, if you want reasonable data rates.

What is a "reasonable" data rate?

>I do not have my references in front of me for doing link
>calculations, but from the moon (which is right next door), you need a
>.5 m dish on the moon, putting out 20 W of power to a 10 m dish on the
>earth to get data rates of between 10k-100k bits per second.  The
>problem from mars is much more difficult.  In addition, mars has winds
>which will make antenna pointing difficult, thus degrading data
>transmission.  To overcome these problems, a larger vehicle is needed.
>By larger, I am thinking of a vehicle with a mass of 100-200 kg, as
>opposed to a number of micro rover concepts that have a mass of < 10
>kg.  These micro rovers would be incapable of meaningful scientific
>exploration. 

There you have it, folks: Jerry Roston cannot think of a solution to
the telemetry problem, ergo it is impossible to solve.

First, a lander and orbiter relay system could easily provide high-
bandwidth communications with a small rover.  All of the components
for such a system have been implemented at JPL and tested in field
conditions.

Second, meaningful scientific exploration can be done with *extremely*
low data rates.  A question of current scientific interest is whether
there remain trace amounts of water on Mars.  A flock of microrovers
equipped with water detectors could answer this question by transmitting
back only a single bit of information over the lifetime of the mission.
(There are designs for systems which would not require any transmitting
power at all, but instead rely on corner reflectors which could be
detected by laser beams transmitted from Earth.  This is speculative
technology, but the potential feasability of such a scheme was recently
demonstrated on Galileo.)

Erann Gat
gat@robotics.jpl.nasa.gov

Newsgroups: comp.robotics