A Successful Strategy for Satellite Development and Testing - Inpe

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ment; convection enables relatively rapidcycling between hot and cold levels. Thermalvacuum testing does the same thing, but in avacuum chamber; cycles are slower, but themethod provides the most realistic simulationof flight conditions. In thermal balancetesting, also conducted in vacuum, dedicatedtest phases that simulate flight conditions areused to obtain steady-state temperature datathat are then compared to model predictions.This allows verification of the thermal controlsubsystem and gathering of data for correlationwith thermal analytic models. Burnintests are typically part of thermal cycletests; additional test time is allotted, and theitem is made to operate while the temperatureis cycled or held at an elevated level.For electronic units, the test temperaturerange and the number of test cycles have thegreatest impact on test effectiveness. Otherimportant parameters include dwell time atextreme temperatures, whether the unit isoperational, and the rate of change betweenhot and cold plateaus. For mechanical assemblies,these same parameters are important,along with simulation of thermal spatialgradients and transient thermal conditions.Thermal test specifications are based primarilyon test objectives. At the unit level,the emphasis is on part screening, whichis best achieved through thermal cycle andburn-in testing. Temperature ranges aremore severe than would be encountered inflight, which allows problems to be isolatedquickly. Also, individual components areeasier to fix than finished assemblies.At the payload, subsystem, and spacevehicle levels, the emphasis shifts towardperformance verification. At higher levels ofShock response spectra10,00010001001010.1100 1000Frequency (hertz)10,000Maximum predicted shock level for fairing separationMaximum predicted shock level for stage separationMaximum predicted shock level for spacecraft separationTypical shock test level is a smooth envelope plus 6 dB marginassembly in flight-like conditions, end-toendperformance capabilities can be demonstrated,subsystems and their interfacescan be verified, and flightworthiness requirementscan be met. On the other hand, at thehigher levels of assembly, it is difficult (if notimpossible) to achieve wide test temperatureranges, so part screening is less effective.At the unit, subsystem, and vehicle levels,Aerospace thermal engineers work with thecontractor in developing test plans that provethe design, workmanship, and flightworthinessof the test article. Temperature rangesare selected that will adequately screen or accuratelysimulate mission conditions, and theproper number of hot and cold test plateausare specified to adequately cycle the testequipment. Aerospace will provide expertiseduring the test to protect the space hardwarein the test environment, resolve test issuesand concerns, and investigate test articlediscrepancies. The reason, of course, is thatTypical test level usedto simulate the shockenvironment. Qualificationmargins at theunit level are typically6 decibels.identifying and correcting problems in thermaltesting significantly increases confidencein mission success.ConclusionSince the first satellite launch in 1957, morethan 600 space vehicles have been launchedthrough severe and sometimes unknown environments.Even with extensive experienceand a wealth of historical data to consult,mission planners face a difficult task in ensuringthat critical hardware reaches spacesafely. Every new component, new process,and new technology introduces uncertaintiesthat can only be resolved through rigorousand methodical testing. As an independentobserver of the testing process, Aerospacehelps instill confidence that environmentalrequirements have been adequately definedand the corresponding tests have been properlyplanned and executed to generate usefuland reliable results.Left: A spacecraft is placed in the acoustic chamber and is ready for testing. Air horns at the corners of thechamber generate a prescribed sound pressure into the confined space and onto the spacecraft. Microphoneslocated around the spacecraft are used to monitor and control the pressure levels. Middle: The sudden separationof the payload fairing is used to expose spacecraft components to the shock environment expected inflight. Right: Space instrument placed on an electrodynamically controlled slip table for vibration testing. Thecontrol accelerometers are mounted at the base of the test fixture at a location that represents the interface tothe launch vehicle adapter. Accelerometers mounted on the test specimen measure the dynamic responses.Crosslink Fall 2005 • 15
Software Testingin Space ProgramsAs space-system software grows in size and complexity, adequate testingbecomes more difficult—and more critical.Myron Hecht and Douglas BuettnerPhoto courtesy of European Space Agency; Ada code from http://www-aix.gsi.de/~giese/swr/ariane5.html...declarevertical_veloc_sensor: float;horizontal_veloc_sensor: float;vertical_veloc_bias: integer;horizontal_veloc_bias: integer;...begindeclarepragma suppress(numeric_error, horizontal_veloc_bias);beginsensor_get(vertical_veloc_sensor);sensor_get(horizontal_veloc_sensor);vertical_veloc_bias := integer(vertical_veloc_sensor);horizontal_veloc_bias := integer(horizontal_veloc_sensor);...exceptionwhen numeric_error => calculate_vertical_veloc();when others => use_irs1();end;end irs2;The Ariane 5 launch vehicle failed on its maiden flight in June1996. About 40 seconds after liftoff, a software bug in theflight controller made the rocket veer off course, leading to itsdestruction via ground command. Ariane 5 reused softwarefrom Ariane 4 without proper testing. Contributing to themishap, run-time range checking had been turned off becauseof processor limitations. Also, the backup channel had failedmilliseconds earlier because of the same coding defect.Failures attributed to software defects are becoming increasinglyvisible in space systems. Recent newsworthy examples include thefailure of the Mars rover Spirit to execute any task that requestedmemory from the flight computer; the unanticipated descent of the MarsClimate Orbiter into the Martian atmosphere, ultimately traced to a unitconversion defect in a navigation system; and the crash of the Mars PolarLander onto the Martian surface after a premature shutdown of its descentengines. In 1996, the first launch of the Ariane 5 booster ended with aspectacular crash off the coast of French Guiana. The cause was traced toa variable overflow that affected software running in both channels of itsdual redundant inertial reference system. Earlier this year, the EuropeanSpace Agency’s Huygens probe successfully beamed back only half ofits image data. The other half was lost because of a single missing line ofcode.In the period from 1998 to 2000, nearly half of all observed spacecraftanomalies were related to software. Anomalies, less severe than failures,have been occurring with increasing frequency on U.S. national securityspace vehicles. One reason is that space-system software has been growingmore complex to meet greater functional demands. Another reason isthat software quality is inherently difficult to determine. The challenge indeveloping the next generation of national security space vehicles will beto ensure reliability despite increasing software size and complexity. Softwaretesting is an important factor in meeting this challenge.Types of Software TestingSoftware testing methods generally fall into two categories: “black box”and “white box” (while some authors also identify a third category, the“ticking box,” which involves not doing any testing).Black-box methods disregard the software’s internal structure andimplementation. The test data, completion criteria, and procedures aredeveloped solely to test whether the system meets requirements, withoutconsideration of how the software is coded. Black-box testing is used at alllevels of testing and is particularly applicable at higher levels of integration,where the underlying components are no longer visible.White-box testing, on the other hand, does account for the internalsoftware structure in the formulation of test cases and completion criteria.The most common types of white-box testing include branch testing,
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