Cluster (spacecraft)

This article is about the original, failed mission. For the successful replacement mission, see Cluster II (spacecraft).

Cluster

Operator	ESA
Mission type	Magnetospheric
Launch date	4 June 1996 12:34:06 UTC
Carrier rocket	Ariane 5G V88/501
Launch site	Kourou ELA-3
Mass	1,200 kilograms (2,600 lb)
Orbital elements
Regime	Highly eliptical (planned) Failed to orbit

Cluster was a constellation of four European Space Agency spacecraft which were launched on the maiden flight of the Ariane 5 rocket, Flight 501, and subsequently lost when that rocket failed to achieve orbit. The launch, which took place on Tuesday, June 4, 1996, ended in failure due to an error in the software design caused by inadequate protection from integer overflow. This resulted in the rocket veering off its flight path 37 seconds after launch, beginning to disintegrate under high aerodynamic forces, and finally self-destructing by its automated flight termination system. The failure has become known as one of the most infamous software bugs in history.^{[citation needed]} The failure resulted in a loss of more than US$370 million.

Spacecraft

Cluster consisted of four 1,200 kilograms (2,600 lb) cylindrical, spin-stabilised spacecraft, powered by 224 watt solar cells. The spacecraft were to have flown in a tetrahedral formation, and were intended to conduct research into the Earth's magnetosphere. The satellites would have been placed into highly eliptical orbits; 17,200 by 120,600 kilometres (10,700 by 74,900 mi), inclined at 90 degrees to the equator.

Launch failure

The Ariane 5 software reused the specifications from the Ariane 4, but the Ariane 5's flight path was considerably different and beyond the range for which the reused computer program had been designed. Specifically, the Ariane 5's greater acceleration caused the back-up and primary inertial guidance computers to crash, after which the launcher's nozzles were directed by spurious data. Pre-flight tests had never been performed on the re-alignment code under simulated Ariane 5 flight conditions, so the error was not discovered before launch.

Because of the different flight path, a data conversion from a 64-bit floating point to 16-bit signed integer value caused a hardware exception (more specifically, an arithmetic overflow, as the floating point number had a value too large to be represented by a 16-bit signed integer). Efficiency considerations had led to the disabling of the software handler (in Ada code) for this error trap, although other conversions of comparable variables in the code remained protected. This caused a cascade of problems, culminating in destruction of the entire flight.

Although the report identified a software bug as the direct cause, other investigators see the causes as system design failures and management issues:

h) On the basis of those calculations the main computer commanded the booster nozzles, and somewhat later the main engine nozzle also, to make a large correction for an attitude deviation that had not occurred.

i) A rapid change of altitude occurred, which caused the launcher to disintegrate at 39 seconds after H0 due to aerodynamic forces.

m) Ariane 5's inertial reference system is essentially the same as a system presently flying on Ariane 4. The part of the software that caused the interruption in the inertial system computers is used before launch to align the inertial reference system and, in Ariane 4, also to enable a rapid realignment of the system in case of a late hold in the countdown. This realignment function, which does not serve any purpose on Ariane 5, was nevertheless retained for commonality reasons and allowed, as in Ariane 4, to operate for approx. 40 seconds after lift-off.

n) During design of the software of the inertial reference system used for Ariane 4 and Ariane 5, a decision was taken that it was not necessary to protect the inertial system computer from being made inoperative by an excessive value of the variable related to the horizontal velocity, a protection provided for several other variables of the alignment software. When taking this design decision, it was not analysed or fully understood which values this particular variable might assume when the alignment software was allowed to operate after lift-off.

o) In Ariane 4 flights using the same type of inertial reference system there has been no such failure because the trajectory during the first 40 seconds of flight is such that the particular variable related to horizontal velocity cannot reach, with an adequate operational margin, a value beyond the limit present in the software.

p) Ariane 5 has a high initial acceleration and a trajectory, which leads to a build-up of horizontal velocity five times more rapid than for Ariane 4. The higher horizontal velocity of Ariane 5 generated, within the 40-second timeframe, the excessive value that caused the inertial system computers to cease operation.

q) The purpose of the review process, which involves all major partners in the Ariane 5 programme, is to validate design decisions and to obtain flight qualification. In this process, the limitations of the alignment software were not fully analysed and the possible implications of allowing it to continue to function during flight were not realised.

r) The specification of the inertial reference system and the tests performed at equipment level did not specifically include the Ariane 5 trajectory data. Consequently the realignment function was not tested under simulated Ariane 5 flight conditions, and the design error was not discovered.

s) It would have been technically feasible to include almost the entire inertial reference system in the overall system simulations which were performed. For a number of reasons it was decided to use the simulated output of the inertial reference system, not the system itself or its detailed simulation. Had the system been included, the failure could have been detected.

t) Post-flight simulations have been carried out on a computer with software of the inertial reference system and with a simulated environment, including the actual trajectory data from the Ariane 501 flight. These simulations have faithfully reproduced the chain of events leading to the failure of the inertial reference systems

Arithmetic Overflow

According to a presentation by Jean-Jacques Levy (who was part of the team who searched for the source of the problem), the source code in Ada that caused the problem was as follows.

L_M_BV_32 := TBD.T_ENTIER_32S ((1.0/C_M_LSB_BV) * G_M_INFO_DERIVE(T_ALG.E_BV));


if L_M_BV_32 > 32767 then


P_M_DERIVE(T_ALG.E_BV) := 16#7FFF#;


elsif L_M_BV_32 < -32768 then


P_M_DERIVE(T_ALG.E_BV) := 16#8000#;


else


P_M_DERIVE(T_ALG.E_BV) := UC_16S_EN_16NS(TDB.T_ENTIER_16S(L_M_BV_32));


end if;


P_M_DERIVE(T_ALG.E_BH) :=

 UC_16S_EN_16NS (TDB.T_ENTIER_16S ((1.0/C_M_LSB_BH) * G_M_INFO_DERIVE(T_ALG.E_BH)));

The last line (shown here as two lines of text) caused the overflow, where the conversion from 64 bits to 16 bits unsigned is not protected. The correct code would have been:

L_M_BH_32 := TBD.T_ENTIER_32S ((1.0/C_M_LSB_BH) * G_M_INFO_DERIVE(T_ALG.E_BH));


if L_M_BH_32 > 32767 then


P_M_DERIVE(T_ALG.E_BH) := 16#7FFF#;


elsif L_M_BH_32 < -32768 then


P_M_DERIVE(T_ALG.E_BH) := 16#8000#;


else


P_M_DERIVE(T_ALG.E_BH) := UC_16S_EN_16NS(TDB.T_ENTIER_16S(L_M_BH_32));


end if;


Aftermath

Following the failure, four replacement Cluster II satellites were built. These were launched in pairs aboard Soyuz-U/Fregat rockets in 2000.

The launch failure brought the high risks associated with complex computing systems to the attention of the general public, politicians, and executives, resulting in increased support for research on ensuring the reliability of safety-critical systems. The subsequent automated analysis of the Ariane code was the first example of large-scale static code analysis by abstract interpretation.

The failure also harmed the excellent success record of the European Space Agency's rocket family, set by the high success rate of the Ariane 4 model. It was not until 2007 that Ariane 5 launches were recognised as reliable as those of the predecessor model.

See also

• Computer bug
• Ada
• Apollo guidance computer – PGNCS trouble, another case where a spacecraft guidance computer suffered from having a subsystem inappropriately left running
• Mars Climate Orbiter software that had been adapted from an earlier Mars Climate Orbiter was not adequately tested before launch

References

Thomas, L.D. (2007) Selected Systems Engineering Process Deficiencies and their Consequences. Acta Astronautica, 61, 406–415.

External links

• Jacques-Louis Lions et al., Ariane 501 Inquiry Board report (html version at the Wayback Machine (archived August 15, 2000))
• Spaceflight Now - Cluster II - Ariane 501 explodes — Footage of the final seconds of the rocket flight (216 kB QuickTime file).
• Wired - History's Worst Software Bugs — An article about the top 10 software bugs. The Ariane 5 Flight 501 software glitch is mentioned as one of the these bugs.
• (German) Ariane 5 - 501 (1-3) — A good article (in German) where the actual code in question is given