By Jennifer Petrosky, MathWorks and Frances Flynn, MathWorks
The ten minutes it takes for data sent from Mars to reach Earth only intensified the suspense for engineers and scientists at the Jet Propulsion Laboratory (JPL) as they waited in the California night for news of the landings of the Spirit and Opportunity Mars Rovers on January 3 and January 25, 2004. Landing the rovers safely, just three weeks apart, at predefined sites after a 320-million-mile-journey was equivalent to throwing a dart in London and hitting the bull’s-eye in Los Angeles.
The landings, and subsequent independent rover missions, were the culmination of three-and-a-half years’ work, and expenditure of more than $800 million. Their successes required exquisitely precise science, technology, and engineering.
JPL engineers are using MATLAB and Simulink for numerous phases of the Mars Exploration Rover (MER) mission, including navigation, data analysis, and entry, descent, and landing (EDL) system design.
Prior to the rovers’ seven-month journey, navigation engineers had to prove that the rovers would make it to the Martian Gusev Crater and Meridiani Planum landing sites. Using data from previous missions, including Viking and Pathfinder, and the latest data from both Mars Global Surveyor and Mars Odyssey, they built a simulation tool with MATLAB that took into account various attributes of the Martian surface to confidently predict that the chosen landing sites could be reached.
With the Rovers in full operation, MATLAB is being used to reconstruct the data to analyze the mission’s performance and understand the behavior of the various systems. Scientists are also using MATLAB to understand more about the geological data the rovers are collecting.
EDL is a three-stage operation that includes entry into the Martian atmosphere, slowing and controlling the descent through the atmosphere, and landing the craft safely and precisely. It is one of the most challenging stages of the mission and is the topic of this story.
JPL engineers called the heart-stopping suspense of EDL the "six minutes of terror." During this time, the spacecraft travels at speeds of up to 25 times the speed of sound relative to the Martian surface. After every critical event during EDL, approximately ten minutes were needed for signals recording the successful events from the landers’ computers to reach Earth, making real-time communications to correct for any anomalies impossible. The exultation that followed in the wake of the terrifying minutes was the payoff for three-and-a-half years of solid engineering.
With such a short time-frame in which to complete the program, NASA decided to build on existing technology from the Mars Pathfinder. One of EDL’s systems engineers was responsible for ensuring that the landing craft would withstand disturbances in the Martian atmosphere from two to three kilometers above the surface to the landing site, and determine how the onboard systems would react to those disturbances.
The terminal descent analyst, a long-time MATLAB user, developed a statistical program using MATLAB to propose, develop, test, and implement two new onboard EDL systems.
This, in turn, was based on Simulink multibody, six-degrees-offreedom simulations and higher-fidelity structural analysis simulations. He also worked hand in hand with the designer of the EDL flight software algorithms.He said, "It was two parallel development paths with a lot of cross-talk. Two guys, integrated and working to make sure we had the same algorithm implementations and parameter settings in both the analysis tools and the flight software."
The two new systems were manifested on the flight vehicles as TIRS and DIMES. TIRS, the Transverse Impulse Rocket System, was designed to add a last-second attitude correction to protect the landing craft's airbags from self-induced excess horizontal velocity resulting from multibody excitation due to wind shears and gusts. DIMES, the Descent Image Motion Estimation System, used pictures to detect prevailing wind and residual trajectory velocity effects on the system. The craft's onboard computer processed images to correlate features properly and aid the Inertial Measurement Units (IMU) with a horizontal velocity measurement.
Using MATLAB, they were able to test, tune, and implement the onboard descent systems, which told the rover which TIRS rockets to fire and when to fire them, based on images taken of the surface and IMU measurements acquired.
During the landing of Spirit, the three components of the landing craft—the lander that contained the rover, the parachute, and the heat shield—had to separate. As predicted, the parachute deployed to slow the spacecraft from a subsonic velocity (Mach 2) to about 250 miles per hour relative to the surface of Mars, at which point the heat shield disengaged. At Gusev crater, the lander responded as predicted to instabilities produced by turbulence in the atmosphere.
In the last few seconds, Spirit’s airbags inflated and the onboard computer activated the retro rockets to slow the lander from 150 miles per hour to almost zero at approximately 20 meters above the surface. As the bridle was cut to release the lander, the rover hit the ground at approximately 24 meters per second. It then bounced almost 30 times and finally rolled to a complete stop, completing the EDL phase of the mission.Without the addition of TIRS and DIMES, the total impact velocity would have been somewhere near the limits of the landing system capabilities.
NASA had to test components of the new system before they could implement them into the flight hardware.Many tests were conducted in the deserts of southern California to prove that these systems— both hardware and software—would perform as expected.MATLAB was used extensively for data reduction and analysis from these tests. In tuning TIRS/DIMES, the EDL team created four-dimensional trade space visualizations that enabled them to very quickly understand system strengths, weaknesses, and pinpoint areas where optimization was necessary. Those studies led to a modified implementation that added flexibility in timing of firing the TIRS motors, which became known to the MER team as "Dual-TIRS."
Opportunity had a similarly well-behaved landing in a less windy environment, which meant that the TIRS system did not activate, based on measurements from DIMES and the other terminal descent sensors. At a standstill, the MER lander is at EDL t = final, but only t = beginning for the next phase of the mission, Impact to Egress (ITE)—a week-long sequence to deploy the rover from its lander.
The landings of the Spirit on January 3, and the Opportunity on January 24, were flawlessly executed and happened precisely as the EDL team had predicted. "It was absolutely incredible—the most suspenseful moment of my life," said one of the team’s systems engineers. "Those 15 minutes (through EDL, and first-signal response) seemed to last an eternity. However, once we got that final signal, we all felt an incredible sense of elation seeing three-and-a-half years of our work succeed in delivering the fourth and fifth successful landers to the surface of Mars."
On Spirit’s 90th day on Mars, the MER team announced that the rover had completed its primary mission and that Opportunity was well on its way to completing its tasks. Both rovers have performed so well, and are in such good condition that NASA has extended the joint mission through September.
In 2009, NASA is planning to land the Mars Science Laboratory, a rover whose present design is as large as the size of a small car and powered by nuclear energy, instead of solar power. That mission will last longer and cover a considerably larger area than the current one and will require a completely different landing method. The success of the Spirit and Opportunity missions will provide an important knowledge base for that challenge.