[-empyre-] glitch device/divide

Julian Oliver julian at julianoliver.com
Fri Dec 9 06:54:01 EST 2011

Some point earlier, I wrote:

> In 1996 the European Space Agency's Ariane 5 Flight 501 self destructed due to a
> glitch in the guidance software.

> Again, a glitch itself doesn't have to be seen for it to have been highly
> significant. No one saw the glitch at work in Flight 501, buried deep in the
> guidance software subsystem. They saw the craft explode as a consequence of the
> glitch.  Perhaps no one even saw the explosion, but a statistical representation
> of it a second after all their comms went down.

In fact they did see the explosion alright.. ouch:


Here's the anatomy of the glitch that caused that explosion. To summarise, it
came down to an Operand Error resulting from an unexpected high value from an
on-board alignment sensor:

The launcher started to disintegrate at about H0 + 39 seconds because of
high aerodynamic loads due to an angle of attack of more than 20 degrees that
led to separation of the boosters from the main stage, in turn triggering the
self-destruct system of the launcher.

This angle of attack was caused by full nozzle deflections of the solid boosters
and the Vulcain main engine. These nozzle deflections were commanded by the
On-Board Computer (OBC) software on the basis of data transmitted by the active
Inertial Reference System (SRI 2). Part of these data at that time did not
contain proper flight data, but showed a diagnostic bit pattern of the computer
of the SRI 2, which was interpreted as flight data.

The reason why the active SRI 2 did not send correct attitude data was that
the unit had declared a failure due to a software exception.

The OBC could not switch to the back-up SRI 1 because that unit had already
ceased to function during the previous data cycle (72 milliseconds period) for
the same reason as SRI 2.

The internal SRI software exception was caused during execution of a data
conversion from 64-bit floating point to 16-bit signed integer value. The
floating point number which was converted had a value greater than what could be
represented by a 16-bit signed integer. This resulted in an Operand Error. The
data conversion instructions (in Ada code) were not protected from causing an
Operand Error, although other conversions of comparable variables in the same
place in the code were protected.

The error occurred in a part of the software that only performs alignment of
the strap-down inertial platform. This software module computes meaningful
results only before lift-off. As soon as the launcher lifts off, this function
serves no purpose.

The alignment function is operative for 50 seconds after starting of the
Flight Mode of the SRIs which occurs at H0 - 3 seconds for Ariane 5.
Consequently, when lift-off occurs, the function continues for approx. 40
seconds of flight. This time sequence is based on a requirement of Ariane 4 and
is not required for Ariane 5.

The Operand Error occurred due to an unexpected high value of an internal
alignment function result called BH, Horizontal Bias, related to the horizontal
velocity sensed by the platform. This value is calculated as an indicator for
alignment precision over time.

The value of BH was much higher than expected because the early part of the
trajectory of Ariane 5 differs from that of Ariane 4 and results in considerably
higher horizontal velocity values.



We hope you enjoy your flight,


> Julian:
>> Indeed it is what is relevant, as it is with any cultural trope. It's here
>> however that software developers like myself find ourselves cynical  
>> about Glitch
>> Art precisely because we know that what we're often looking at/listening to is
>> not a glitch, rather an event designed to have the appearance of one.
>> A glitch-concert using Max MSP is not glitch, rather the application  
>> of digital
>> synthesis to mimic sounds that sound like what we understand to be glitch,
>> namely electrical sparks, servos breaking under load, etc.  Similarly, someone
>> playing with GTK or Quartz Composer to manipulate a desktop interface 
>> such that
>> it performs unexpectedly isn't glitch, it's UX/UI design.
>> This leads us to the question "Can you design a glitch?". Perhaps you can only
>> design /with/ glitches, not glitches themselves..
>> If glitches are political at all it's in because they represent a possible
>> entry-point within an otherwise closed system, a 'de-punctualisation' (from
>> Latour) of the Black Box. What many call glitches are in fact just the 
>> beginning
>> of what later becomes an exploit (whether that be jailbreaking a device or
>> injecting malicious code into a process running on a server). In this way
>> glitches signal the possibility of further action; an opening, they express
>> freedom of movement.
>> Purely aesthetic fetishising of glitch depreciates this potential, I think.
>> After all, some of the most potent and transformative glitches in  
>> technological
>> history are quite boring to behold. To most, they'd probably go unnoticed.
>> Cheers,
>> Julian
> _______________________________________________
> empyre forum
> empyre at lists.cofa.unsw.edu.au
> http://www.subtle.net/empyre

Julian Oliver
empyre forum
empyre at lists.cofa.unsw.edu.au

More information about the empyre mailing list