Launching satellites into orbit is never as simple as merely pushing the "ignition button" in the mission control room and letting computers do all the work. Inevitably, there are obstacles to overcomesome anticipated and some surprisingand valuable lessons to learn along the way. The launch and deployment of Terra was no exception. Mission managers had to work through several delays in the launch date as well as some "exciting" episodes once the spacecraft reached orbit, but, thanks to a well-prepared launch support team, the spacecraft is working beautifully and now gathering important Earth system data. Terra Project Manager Kevin Grady, at NASAs Goddard Space Flight Center, shares some of the lessons he and his team learned along the way.
"It all begins with a launch support team that is resourceful and adept at identifying and solving problems quickly," says Grady. "But to launch a successful mission of this scope, you also need a robust spacecraft and a well-prepared flight operations team. Fortunately for Terra, we had both."
Although the launch team consisted of seasoned personnel, some with decades of experience, Terras flight operations system was brand new and had never before been used in a NASA launch. This explains one of the launch slips. Terra was supposed to launch in December 1998, but mid-way through that year the mission managers realized that the flight operations software they had been developing just wasnt going to get the job done. The EOS Science Data and Information System (ESDIS) Project decided to abandon that system and instead adopted another system that Raytheon Corporation was using for commercial launches. Of course, the new system had to be heavily modified to work in NASAs Earth Observing System (EOS) mission operations environment. So, the launch was pushed back to July 1999 to allow time for the modifications.
Then, as the summer of 99 approached, a launch failure occurred on another vendors rocket, which used an engine nearly identical to the Atlas IIas rocket that would launch Terra. The engine was grounded until, after months of detailed examinations, Lockheed-Martin reported to NASA that Terras launch vehicle was cleared for flight. A new launch date was set for December 16, 1999.
Those who work with him will tell you that Grady is the type of manager who tends to "see the glass as half full." Consider his advice to his team in the days leading up to Terras launch: "Spacecraft operations and activation will be full of opportunities to excel." His words proved prophetic. On the morning of December 16, Grady recalls, "I was a little anxious to get on with the launch. I wasnt really nervous because I was pretty confident that we were well prepared."
He didnt know it yet but Terra would stand ready on the launch pad another 48 hours before lift off.
To Launch or Not to Launch
After a long series of delays, Terra was ready for launch by December 16, 1999. However, a software glitch aborted the first launch attempt. (Photograph by NASA)
On December 16 at 10:47 a.m. PST, the countdown came within 40 seconds of ignition of the Atlas rockets first stage engines when one of the "housekeeping" computers issued an "abort" command. Up to the time of launch, according to Grady, computers are cycling through hundreds of rocket status checks to continually monitor the health and readiness of the rocket to launch. An "out of limits condition" was reported by one of the computers at the T-40 second point, and the launch was scrubbed. The limit violation proved to be erroneous, and after a thorough examination by the Lockheed ATLAS engineers, the launch was rescheduled for Saturday.
Grady says there was never any problem with the Terra spacecraft during this time. It remained powered up and perfectly responsive during the entire 48-hour delay.
Terra only had a 30-minute launch window. Due to the fact that Terra has strict requirements for the timing of its orbit (to descend across the equator at 10:30 a.m. local time), the launch had to occur sometime between 10:30 and 11 a.m. PST. If mission managers missed this window for any reason, then the launch would have to be postponed until the next day. On the morning of December 18, as the launch window approached, so too did a high-pressure front from out over the Pacific Ocean, bringing high winds aloft. Atlas mission managers always monitor wind intensity on the day of launch by launching a series of balloons with gauges onboard to measure wind speed and direction at various altitudes up through the lower atmosphere. As Terra slipped later into its launch window, the Atlas engineers were carefully examining the winds aloft to determine whether the wind velocity exceeded the Atlas design limits. Just before the close of the window, the Atlas engineers gave a "go to launch."
Dick Quinn, deputy program manager at Lockheed-Martin in charge of Terra launch operations, recalls feeling skeptical that the mission would launch that day. Quinn started working on the Terra Project in 1992. Not that he didnt share Gradys confidence. Quinn first began supporting NASA programs back during the Apollo missions. In his career, he supported the development of 21 Apollo lunar modules as well as a number of satellite missions. He has seen an evolution in the process of clearing a mission for launch.
"After the Challenger accident, the way rockets go up nowadays is much more deliberate and slow-paced," he observes. "This is still the business of exploration. Terra is one of a kind, built from the ground floor up. Thats a lot different than launching the fifth or the fiftieth version of the same thing.
"Even so," he states, "there was no way it wasnt going to
work. We were ready."
Terras flight controllers were responsible for seeing the spacecraft safely into orbit. They also successfully guided Terra through a a series of surprises during activation of the spacecraft for science operations. (Photograph courtesy Terra Project)
With only 10 seconds left in the launch window, the Atlas IIas rocket lifted off from the Vandenberg Air Force Base launch pad and began a picture-perfect ascent through the atmosphere, (view video [2.8MB]) carrying Terra--flagship of NASAs Earth Observing System program--in its nose-cone (called a "fairing"). Three minutes later, the fairing separated from the rocket and carried its precious cargo into a low-Earth orbit, flying elliptically 655 km by 695 km above our planet. Four minutes after that, Terras transponders turned on and began sending data to mission control via NASAs Tracking and Data Relay Satellite System (TDRSS).
"Everything was happening pretty much as the ascent timeline called for," recalls Grady, "which gave us a good feeling. Then, when we received housekeeping data from TDRSS right on schedule, that was the confirmation that everything was proceeding on the mark. That was when we breathed our first sigh of relief."
Fourteen minutes after launch, the Centaur (second stage) engine shut down and the spacecraft separated from the booster. Immediately, Terras Guidance Navigation and Control system oriented itself with respect to Earth and performed very well. Next came deployment of the solar array, without which the mission would have an extremely short lifetime. According to Grady, Terras was the most complicated solar array deployment in NASAs historyall controlled by software. The solar array unfurled facing the Earth almost directly over Antarctica and worked so efficiently that it almost immediately gleaned about a kilowatt of power from the sunlight reflected off the bright, ice-covered surface.
"At that point I was extremely happy," says Grady, "because we were charging the batteries and I knew the power system was working. All signs were pointing to a successful mission."
Of course, Terra still had to be raised to its final orbital altitude of 705 km before science operations could begin.
Ten seconds before Terra's launch window closed, it soared into the clear sky over Vandenberg Air Force Base. Fourteen minutes after that, Terra was in orbit. (Photograph and video by NASA [2.8MB])
On Sunday evening, December 19, the Terra flight operations team was surprised when the satellites High Gain Antenna spontaneously "safed itself." This antenna is the one that is used for routine communications with TDRSS satellites, including the downlink of Terras science data. The flight operations team quickly initiated a series of diagnostic tests to find out what went wrong.
Quinn explains that there are built-in electronics in the High Gain Antenna that constantly monitor the electric current being drawn by the motor drive assembly, which controls the pointing direction of the antenna. If the electric current either exceeds or falls below a certain limit, then the antenna is preprogrammed to flag the problem and can decide to park itself until the flight operations team decides to restart it. In this case, telemetry data from the spacecraft indicated an anomalously high current passed through the motor drive assembly.
"At first, only one axis on the antennas gimbal was affected and the other axis kept moving," Quinn recalls. "Then an additional fault detection shut down the other axis. We knew right away that this wasnt due to a failure of the High Gain Antenna. So we began a brainstorming session in which we listed all the possible reasons why this could have happened."
"It turns out, there is a semi-conducting opto-coupler in the High Gain
Antenna electronics that is susceptible to radiation transient single event
upsets," Grady explains. Thats a technical way of saying that the
satellite was exposed to a high dose of proton radiation in the Earths
magnetic field. Because electronic devices are getting smaller and smaller, they
are increasingly susceptible to interference or damage from radiation.
Terra's high gain antennathe parabolic dish shown here above the spacecrafttransmits science data from the five instruments to scientists on the ground through a relay satellite. Without it, data from Terra would slow to a trickle. (Image and video by Reto Stöckli, Terra Project Science Office [4.7MB])
There are three regions over the Earth where scientists typically observe high levels of radiationover the North and South Poles, and another region centered partly over Brazil and extending out over the Atlantic Ocean that scientists call the "South Atlantic Anomaly." In this region, scientists observe very high levels of proton radiation. The Terra flight operations team nicknamed the region "The Duck" because when you draw a contour map of the central part of the South Atlantic Anomaly where the radiation levels are highest, it resembles the profile of a duck.
According to Grady, exposure to radiation in The Duck essentially fooled Terras High Gain Antenna into thinking that a fault condition existed in its Motor Drive Assembly. The antenna responded by turning itself off in order to prevent any possible damage from occurring.
Interestingly, the Multi-angle Imaging SpectroRadiometer, or MISR, instrument
aboard Terra confirmed that the spacecraft was being exposed to high levels of
radiation over the South Atlantic Anomaly. By February, the instrument had
already been turned on and, although its aperture doors were still closed, MISR
was fully ready for operation. Every time Terra flew through The Duck, MISR
measured very high numbers of protons striking its highly sensitive detectors.
Intrigued, the MISR Team began counting these "hits" and produced the
first MISR image before they even opened its doors!
The region of the South Atlantic Anomaly was dubbed "The Duck" by the Terra flight operations team because when you draw a contour map of the central part of the anomaly, it resembles the profile of a duck. (Image by Mark Woodard, NASA Goddard Space Flight Center)
|Even before the cover opened, the Multi-angle Imaging SpectroRadiometer (MISR) instrument aboard NASA's Terra spacecraft began making scientific measurements. The MISR cameras, designed to detect visible light, are also sensitive to energetic protons in Earth's upper atmosphere. With the cover closed, background levels of protons stand out. This map was created by specially processing MISR "dark" data taken between February 316, 2000, while the cover was still closed. Each orange picture element on the map shows one or more proton hits. (Image courtesy MISR Science team)|
Of course, Gradys team spent a few days brainstorming and running diagnostic tests to be sure the South Atlantic Anomaly was the cause of the problem. Once convinced, and after making sure there was no danger to the antenna, they simply modified the computer program to restart the antenna gimbal subsequent to a radiation event. Quinn says the antenna still turns off over this region about once every 30 orbits, but then turns itself on again with no impact to mission operations.
"There was quite a bit of effort expended to make sure we didnt have these kinds of parts [overly sensitive to radiation anomalies] in the electronics," Grady states. "Of the tens of thousands of parts that we examined before launch, this one somehow got by us. This is where a robust spacecraft design is critical, as it allows the flight controllers to implement an operational modification with no resulting impact on the missions science. The antenna continues to work fine."
With the antenna problem solved, Gradys team began preparations for raising Terra to its final orbital altitude for mission activation. They didnt know it yet, but the satellite had another surprise in store for them.
The Unplanned Roll
Throughout the course of its (at minimum) 6-year lifetime, there is a series of maneuvers planned for Terra. These maneuvers fall into two categories: (1) attitude maneuvers that involve changing the orientation of the spacecraft in any of three possible axes (called roll, pitch, and yaw); and (2) orbital maneuvers that involve changing either the orbital altitude or the orbital plane of the satellite in order to adjust the local time at which Terra crosses the equator.
There are "thrusters" positioned strategically at locations on the
spacecraft that allow the flight operations team to adjust the orbital position
of Terrato fly it, essentially. Onboard tanks contain hydrazine, a
reactive gas commonly used on spacecraft. In firing the thrusters, the ground
operations team is actually releasing some hydrazine from the tank and routing it
over a heated catalyst that breaks down the gas into products that are forced out
through small rocket engines, thereby providing thrust for the spacecraft.
|Throughout its lifetime, Terra will periodically perform a variety of attitude maneuvers for calibration purposes. In these maneuvers, Terra changes its orientation with respect to the Earth. The spacecraft will pivot on one or more of three possible axes to enable its sensors to view the blackness of deep space or the moon. Spinning on its X-axis is called a roll; spinning on its Y-axis is a pitch; and spinning on its Z-axis is a yaw maneuver. (Image by Reto Stockli)|
The first major maneuver planned for Terra, on January 11, was to raise it from its lower elliptical orbit to a higher, circular orbit of 705 km above the Earth. The flight operations team had to meet the requirement of flying in formation with Landsat 7 so that Terra descends across the equator at roughly 10:30 a.m. local time, about 15 minutes behind Landsat 7 on the same flight path. To do this orbital maneuver, the Terra team had to fire Terras onboard thrusters to both raise its orbit as well as keep the satellite properly oriented with respect to the Earth. At 6:19 p.m. EST, the team began firing the thrusters. Moments later Terras flight computer aborted the maneuver when it correctly diagnosed that Terra was beginning to roll more than expected.
"I was in the mission control room and was growing very concerned about the increasing roll rate," Quinn recounts. "At the time, I didnt have an explanation of why Terra was rolling." Quinns mind raced as he watched Terra enter "safe mode." There are two separate protective programs built into the spacecraft to limit how much error is allowed in its orientation. If any error limits are exceeded, then the thrusters stop firing automatically and the protective programs take over to re-establish Terra into an Earth-pointing orientation. In this way, if the ground controls fail for any reason, the sensors will by default be pointed at the Earth and the missions science objectives can still continue safely.
Again, Grady scrambled his flight operations team into a brainstorming session
that lasted, off and on, for a couple of weeks. Why did Terra begin an unplanned
roll when they started boosting its orbit? After many days filled with long
hours of exhaustive work examining all the possibilities, and creating and
modifying computer models of the spacecrafts behavior, the team narrowed
the list of possibilities to a few key suspects.
Gradys team determined that the roll was caused by a combination of two things: (1) a misalignment in the satellites center of gravity with respect to the thruster settings, and (2) plumes from two of the thrusters impinging on the solar array. "There is a thruster pairing matrix that resides in the software system aboard the spacecraft that describes how much torque you get from each thruster," Grady elaborates. "When the control system needs a certain amount of torque, it uses that matrix to decide which thrusters to fire to get the desired thrust. We discovered that these thruster pairing matrices werent exactly correct for the center of gravity of the spacecraft, but we knew wed have to fine-tune the matrices on orbit."
What Grady and his team hadnt known is that the gas molecules escaping the thrusters on the rear of the spacecraft would spread out into a cone shape so that enough of these molecules would impact the solar panel with an effective force of approximately one-quarter of a pound. (View an animation [6.2MB] of exhaust gasses hitting Terra's solar panel) "At 30 feet long, what you basically have with the solar array is a very long moment arm, or lever, on the spacecraft," he explains. "Even though Terra weighs approximately 11,000 pounds, the solar panel gives you such a long lever that it doesnt take much force on the end of it to roll the spacecraft."
After days of analyzing telemetry data and conducting simulations that nearly identically matched Terras behavior, Grady and his team were convinced that they understood the problem well enough to resume the ascent maneuver. This time, they did two things to avoid another unplanned roll: (1) they fine-tuned the thruster pairing matrices to more precisely account for the satellites center of gravity and external disturbances generated by the thruster plumes, and (2) they placed the solar array into a position where it would minimize any impingement from thruster plumes. They began the ascent with a series of short thruster bursts and once they knew they had solved the problem, they did a series of longer thrusts.
Terra reached its target orbital altitude on February 23 and its sensors began opening their aperture doors the next day for science operations. Terras activation had begun.
During the first maneuver to raise Terra into its final orbit the spacecraft rolled excessively. Using computer simulations, the mission controllers discovered that exhaust from a rocket thruster was pushing on Terra's large solar panel. (Image and animation by Reto Stöckli [6.2MB])
"Thats what I like about this business most," notes Quinn. "If things just worked, it wouldnt be all that exciting. But building a team, getting the grey matter to work, attacking and solving problemsthats what I enjoy.
"We had people from all disciplines participating," he continues. "Seeing a team of professionals putting their heart into reaching a goal like that is very rewarding." Quinn points to a similar situation when he was part of the multi-disciplinary Apollo 13 team assembled to do quick and creative problem solving.
Quinn is optimistic that Terras troubles are behind it and the mission will be highly successful. "Hopefully, weve done everything right and Terra will outlive its 6-year lifetime estimate. I hope our baby will serve the world well."
Grady echoes Quinns sentiment. "Terras spacecraft subsystems are now operating almost flawlessly," he says. "From the very first day the instruments doors opened, we have been collecting spectacular images. After years of hard work, our efforts are now being rewarded with Terras initial science images.
"The flight operations team is to be congratulated for performing superbly," Grady concludes, "and I am grateful to everyone who hung in there. Their dedication and sacrifice have given the Earth science community the extraordinary opportunity to study the planet in a manner which promises to have profound impacts on mankinds understanding of our home."
|The Moderate Resolution Imaging Spectroradiometer (MODIS) can image half the United States in just five minutes, with 250 meter resolution. (Image by NASA GSFC and the MODIS Science Team)|
|This false color image is from ASTER, the Advanced Spaceborne Thermal Emission and Reflection Radiometer. It shows docks and artificial islands built in Tokyo Bay. (Image by ASTER Science Team)|
|The above image shows longwave radiation emitted by the Earth (left) and shortwave radiation reflected by the Earth (right) as seen by the Clouds and the Earths Radiant Energy System (CERES). The data are a monthly composite from March 2000. (Image by Robert Simmon, based on data from the CERES science team.)|
|During the first maneuver to raise Terra into its final orbit the spacecraft rolled excessively. The mission controllers discovered that exhaust from a rocket thruster was pushing on Terra's large solar panel by using computer simulations. (Animation by Reto Stöckli)|
|Shortly after reaching orbit Terra unfolded its solar array, which will provide power over the 6-year mission lifetime. Next, the high gain antenna was deployed, enabling data to be transmitted to scientists here on Earth. (Animation by Reto Stöckli)|
|10 seconds before Terra's launch window closed, it soared into the clear sky over Vandenberg Air Force Base. 14 minutes after that, Terra was in orbit. (Video by NASA)|
|The MISR image above is a composite of red band data taken by the MISR forward 45.6-degree, nadir, and aftward 45.6-degree cameras, displayed in blue, green, and red colors, respectively. Color variations in the left image highlight spectral (true-color) differences, whereas those in the right image highlight differences in angular reflectance properties. The purple areas in the right image are low cloud, and light blue at the edge of the bay is due to increased forward scattering by the fast (smooth) ice. The orange areas are rougher ice, which scatters more light in the backward direction. This example illustrates how multi-angle viewing can distinguish physical structures and textures. The image is about 400 km (250 miles) wide with a spatial resolution of about 275 meters (300 yards). North is toward the top. (Image by MISR Science Team/JPL)|
|The Measurements of Pollution in the Troposphere instrument detects two import trace gasescarbon monoxide and methane. The above image shows the concentration of carbon monoxide from March 57, 2000. (Image by MOPITT Science Team)|