Mars Reconnaissance Orbiter (MRO) uses embedded technology ...

Special feature: COTS over MarsMars Reconnaissance Orbiter (MRO)uses embedded technology formartian imagesBy John CarboneImage courtesy of NASA/JPL/Arizona State UniversityThe High Resolution Imaging Science Experiment (HiRISE), now flying on the MRO, provides the highest-resolution imagestaken of deep space. From 190 miles away, HiRISE can identify objects only a meter across, providing a better understandingof the geologic and climate processes on Mars and making it possible to scout out possible landing sites. It will play a keyrole in the MRO’s mission of characterizing the surface, subsurface, and atmosphere of Mars and identifying landing sitesfor future missions.When they wrote the embedded software that controls HiRISE,a team of Ball Aerospace engineers led by Steve Tarr knewthey only had one chance to get it right. If there was a seriousflaw in the program, the $720 million spacecraft would haveno more value than a digital camera dropped in a bathtub. Tarrand his team wrote 20,000 lines of code running on an AtmelSPARC TSC695F processor, using Express Logic’s ThreadXRTOS. The code sets up and operates the camera and sends theresulting pictures to the spacecraft software. The spacecraftsoftware transmits the pictures to the ground. The softwarehas worked flawlessly, resulting in history-making photographssuch as one that shows the Opportunity rover movingacross the surface of Mars (Figure 1, courtesy of NASA).The MRO has four main goals: determine whether life everexisted on Mars, characterize the climate of Mars, characterizethe geology of Mars, and identify promising locations forscientific study. The HiRISE camera plays a key role in eachof the objectives. HiRISE can zero in on water-related surfacefeatures such as outflow channels from ancient floods. HiRISEwill look particularly for geologic settings that indicate thepresence of liquid water on the surface at some point in theplanet’s history. Hundreds of locales will be examined inunprecedented detail to reveal water-related mineralogy andwater’s role in shaping the terrain. The camera can even honein on rocks as small as 3 or 4 feet to help evaluate the safety ofpotential landing sites.HiRISE provides images of the surface of Mars to a much finerresolution and higher level of contrast than ever before. Lightenters the front of the camera, is gathered by a 50-centimeterdiameter primary mirror, and is then sent by a series of othermirrors to be focused on the detectors. HiRISE doesn’t take asingle image of a scene all at once but rather grabs one row ofabout 20,000 pixels at a time, imaging a 6 km swath, while thespacecraft sweeps over the surface of Mars.Figure 1

The Time Delay and Integration (TDI)method improves image quality as thespacecraft covers the ground at a speedof approximately 3,200 meters per second.The TDI continuously collects andreads out the accumulated signal from theCharge Coupled Device (CCD) one rowat a time to match the ground. As a rowis read out, the charges in the remainingrows are shifted down by one row. Thishas the same effect as increasing theexposure time in a conventional camera.While HiRISE delivers a high-resolutionimage of Mars surfaces, the Context Camera(CTX) imager provides a wider-angleview of the same region and the CompactReconnaissance Imaging Spectrometerfor Mars (CRISM) multispectral imagerobtains near-infrared spectral images toidentify surface composition.Functionality of embeddedsoftwareThe HiRISE embedded software writtenby Tarr and his group performs a numberof different functions and is integratedinto an overall system of programs. Forexample, the camera is aimed by thespacecraft software, a separate applicationwritten by Jet Propulsion Laboratoryengineers, which controls thrusters thatposition and orient the spacecraft so thatthe camera is pointing at the area that isto be photographed. When positioning iscorrect, the spacecraft software directsthe HiRISE software to take the picture.The HiRISE software configures thecamera by setting the TDI pixel line timeto match the ground velocity and maintainalignment with the spacecraft’s motionover Mars. The software then turns on thecamera and begins processing the imagesgenerated by the CCDs by adding headerinformation and sending them to thespacecraft image storage system, which inturn sends them to a ground station. In thebackground, the HiRISE software capturesengineering information from sensorslocated throughout the camera in order tohelp identify and diagnose problems. Thesensor readings provide information suchas the temperature of the camera’s keycomponents. HiRISE evaluates these readingsand issues a warning if any parametermoves outside the normal range.Tarr and Ball Aerospace addressed asimilar challenge when they developedsoftware for NASA’s Deep Impact missionin 2003. This mission involved crashinga spacecraft into a comet while a secondspacecraft photographed the collisionand recorded data about the dust and gasreleased by the explosion. Tarr’s team atBall wrote software to control the threeimaging instruments that generated imagesof celestial bodies with known positionsso that the spacecraft could determineits position and compute its course to itsdestination. The instruments also photographedthe exploding comet, as shown inFigure 2 (courtesy of NASA).Figure 2Just as with the Deep Impact mission, Tarrand his team selected the ThreadX Real-Time Operating System (RTOS) fromExpress Logic, San Diego, California tomanage the scheduling of the applicationthreads and service interrupts, and to passthe messages needed for the cameras tooperate correctly. On July 4, 2005 theDeep Impact spacecraft kept its rendezvouswith the Tempel 1 comet, plungedits impactor into the comet’s surface, andtransmitted more than 4,500 photographsof the event back to Earth.Reusing code from earlier projectThe success of the Deep Impact applicationmade it natural to reuse as much ofits technology and code on the HiRISEsoftware as possible. Yet there weresubstantial differences in the HiRISEapplication that required a substantialdevelopment project.First, the Deep Impact camera used ashutter to take a single image, whileHiRISE uses a more complex push-broomapproach to sweep a continuous image.This, along with TDI use, increasedthe complexity of the CCD interfacingtask. The application captures the bitsfrom the CCDs, then, when required bythe TDI configuration, sends the data tocharge the next CDI. Another differenceis that HiRISE uses a heater to maintainthe camera at a constant temperature toimprove image quality, and the new softwareapplication must control the heater.When the image is fully formed, theembedded code adds header informationand sends the data to the navigationsoftware, which communicates with theground tracking system.Multitasking critical to applicationMultitasking is a critical HiRISE softwarerequirement, such as checkingsensor readings at the same time it processesthe images from the CCDs. Tarrand his team provided this capability bytaking advantage of the RTOS’s InterruptService Routines (ISRs). The ISRs thatpoll sensors, for example, are triggered inresponse to periodic timers.Since application code execution is preemptedduring the execution of an ISR,the Ball Aerospace team’s applicationminimizes the amount of code in the ISRand relies instead on a different applicationthread to complete the processing.This approach allows the highest-priorityapplication code to be executed as quicklyas possible.Camera has worked flawlesslyThe new MRO software and the HiRISEcamera have worked flawlessly sincethe camera took the first high-resolutionpictures of Mars on September 29, 2006.HiRISE reveals unprecedented detail inits images taken of Mars, showing seeminglyendless fields of sand dunes, somecarved by gullies that possibly formedwhen sunlight heated carbon dioxide orwater frost in the dunes, triggering avalanchesof flowing sand.Other HiRISE images show layered aridterrains that resemble landscapes pro­

Special feature: COTS over Marstected as national parks on our own planet, along with a fossildelta inside a crater that once held a lake. HiRISE images resolvemeter-sized blocks within the delta channel that may be blocks ofsand and gravel carried along as the channels eroded.HiRISE images also capture numerous impact craters, includingthe Endurance crater that NASA’s Opportunity rover explored for10 months of its now nearly four-year mission. Details visible inthe HiRISE image of Opportunity’s landing site show the parachutelying on the martian surface, Opportunity’s heat shield ata different location, and the lander itself on the floor of the smallimpact crater where the airbag came to a stop.Figure 3, courtesy of NASA, shows gullies in an unnamed TerraSirenum crater on Mars. This region receives very little sunlightin the southern winter, and some areas consist of frost. At thelatitude of this image, frost is most likely composed of waterbecause the temperature is not low enough for carbon dioxidecondensation.Other images show layered polar terrains that likely record martianclimate changes, and also polygon-patterned northern plainsregions that are among candidate landing sites for the PhoenixLander in 2008.HiRISE: Higher resolutionThe HiRISE camera is generating images of Mars at a higherresolution than ever before on a daily basis. Key to the successof this camera has been the embedded software that sets up,operates, and monitors it. This software flawlessly and efficientlyhandles the large number of simultaneous tasks required to keepthe camera working. The result is a dramatic increase in ourknowledge of the red planet.John Carbone, vice president of marketing forExpress Logic, has 35 years of experience inreal-time computer systems and software,ranging from embedded system developer andFAE to vice president of sales and marketing.Prior to joining Express Logic, he was vicepresident of marketing for Green Hills Software. John has a BSin mathematics from Boston College. He can be contactedat sjcarbone@expresslogic.com.Express Logic858-613-6640www.expresslogic.comSee www.mil-embedded.com/articles/authors/lamie for moreinformation on the Deep Impact mission.Figure 3© 2008 OpenSystems Publishing. Not Licensed for distribution. Visit opensystems-publishing.com/reprints for copyright permissions.