🚀 go-pugleaf

Many thanks to the person who posted the 1987 articles from comp.risks.
Thanks also to Fred Webb for explaining where the DO-loop version came
from -- that article has been forwarded *to* comp.risks.

Finally, here's one more comp.risks item giving further details; this
from issue 8.75, sometime this past summer.
----------------------------------------------------------------------

Date: Sat, 27 May 1989 15:34:33 PDT
From: Peter Neumann <neu...@csl.sri.com>
Subject: Mariner I -- no holds BARred

Paul Ceruzzi has written a truly outstanding book for the new show that opened
two weeks ago at the Smithsonian National Air and Space Museum.  The exhibit
and the book are both entitled "Beyond the Limits -- Flight Enters the Computer
Age".  Both are superb.  Go for it (them).

Paul has dug into several cases treated previously in RISKS and in issues of
the ACM Software Engineering Notes, and has been able to resolve several
mysteries.  In particular he considers the case of Mariner I, about which
various inaccurate stories have been told.  Intended to be the first US
spacecraft to visit another planet, it was destroyed by a range officer on 22
July 1962 when it behaved erratically four minutes after launch.  The alleged
missing `hyphen' was really a missing `bar'.  I quote from Paul's book, pp.
202-203:

  During the launch the Atlas booster rocket was guided with the help of two
  radar systems.  One, the Rate System, measured the velocity of the rocket as
  it ascended through the atmosphere.  The other, the Track Ssytem, measured
  its distance and angle from a tracking antenna near the launch site.  At the
  Cape a guidance computer processed these signals and sent control signals
  back to the tracking system, which in turn sent signals to the rocket.  Its
  primary function was to ensure a proper separation from the Atlas booster and
  ignition of the Agena upper stage, which was to carry the Mariner
  Spacecraft to Venus.

  Timing for the two radar systems was separated by a difference of forty-three
  milliseconds.  To compensate, the computer was instructed to add fourty-three
  milliseconds to the data from the Rate System during the launch.  This
  action, which set both systems to the same sampling time base, required
  smoothed, or averaged, track data, obtained by an earlier computation, not
  the raw velocity data relayed directly from the track radar.  The symbol for 
  this smoothed data was ... `R dot bar n' [R overstruck `.' and `_' and
  subscript n], where R stands for the radius, the dot for the first derivative
  (i.e., the velocity), the bar for smoothed data, and n for the increment.

  The bar was left out of the hand-written guidance equations.  [A footnote
  cites interviews with John Norton and General Jack Albert.]  Then during
  launch the on-board Rate System hardware failed.  That in itself should not
  have jeopardized the mission, as the Track System radar was working and could
  have handled the ascent.  But because of the missing bar in the guidance
  equations, the computer was processing the track data incorrectly.  [Paul's 
  EndNote amplifies: The Mariner I failure was thus a {\it combination} of a
  hardware failure and the software bug.  The same flawed program had been used
  in several earlier Ranger launches with no ill effects.]  The result was 
  erroneous information that velocity was fluctuating in an erratic and
  unpredictable manner, for which the computer tried to compensate by sending
  correction signals back to the rocket.  In fact the rocket was ascending 
  smoothly and needed no such correction.  The result was {\it genuine} instead
  of phantom erratic behavior, which led the range safety officer to destroy
  the missile, and with it the Mariner spacecraft.  Mariner I, its systems
  functioning normally, plunged into the Atlantic.

------------------------------  
Forwarded to alt.folklore.computers by
-- 
Mark Brader, SoftQuad Inc., Toronto, utzoo!sq!msb, m...@sq.com
	"I'm a little worried about the bug-eater," she said.  "We're embedded
	in bugs, have you noticed?"		-- Niven, "The Integral Trees"