In NASA's salt-seeking probe, new and old come together to show us the future
The Fifth Dimension advised us in its massive chart-topper of 1969 that "This is the dawning of the age of Aquarius." A time when "Peace will guide the planets and love will steer the stars," an era that brings about "the mind's true liberation." And, looking back, we can say with some confidence that many minds certainly experienced at least momentary liberation in the wild and experimental days of the late 1960s.
Today, however, there is an all-new age of Aquarius, one that began on June 10, 2011. It was on that day that NASA, in conjunction with Argentina's Comisión Nacional de Actividades Espaciales (CONAE), launched a Delta II rocket from Southern California's Vandenberg Air Force base.
Atop that rocket sat a spacecraft known as "SAC-D" (Satellite for Scientific Applications-D), a vehicle designed to orbit the earth for many years hence. A vehicle veritably festooned with scientific devices—known as "instruments" in the outer space fraternity—each developed to monitor various activities on or above the earth. A vehicle and payload that will ultimately help scientists better understand our world and what we're doing to it, and that many consider a critical step in the gauging of our future.
That's all well and good, you say, but where's the Aquarius connection? Well, nestled amongst all those instruments - and the high-tech awesomeness they represent - is perhaps the slickest and most important instrument of all: a device that will observe the surface salinity of our oceans and hopefully pave the way to a firmer grasp of climate change.
Dubbed "Aquarius" in honor of the water-bearer constellation, it quickly became the focal point of the mission for many. Inevitably, it was only a matter of time before Aquarius the instrument had also become the erroneous yet popular name of the spacecraft itself.
And that, boys and girls, is how SAC-D became known as Aquarius. Until we put a Millennium Falcon up there, it's one of the spiffiest pseudonyms to ever orbit the earth.
Convoluted naming conventions aside, the SAC-D spacecraft and its cargo are doing a lot of interesting stuff right now, 408 miles above our heads. We recently had a chance to speak with several men intimately familiar with the Aquarius instrument in particular—Principal Investigator Gary Lagerloef, Aquarius Instrument Manager Simon Collins, and Technical Lead for Flight Software Alex Murray—to get a better understanding of the tech behind the headlines. We can confidently say that if you've ever wondered what makes a mission like this tick, you've come to the right place.
We Have Liftoff
Though launched just a few months ago as part of the SAC-D mission, NASA's Aquarius instrument is no newcomer. Not by a long shot. Indeed, says Lagerloef, "We started in the mid-1990s thinking about this stuff. Aquarius was eventually one of two missions (from an original roster of 18 candidates) that were selected in a 2002 call for proposals."
From the very start, the mission has been linked with the Argentine space agency. For SAC-D, CONAE would provide the satellite—and several additional instruments—while NASA would deliver the primary instrument (Aquarius) and the rocket launch. Like many rocket flights, SAC-D was delayed more than once as scientists and designers put together the final pieces. But in 2009, when NASA shipped the Aquarius to Argentina in June for mounting on the assembled satellite, the proverbial ball really began to roll. The finished product was then shipped back in March of 2011, and in June it was space-bound.
As we alluded earlier, Aquarius is but one of several instruments aboard what is very much an international satellite. Other prominent instruments include ROSA (Radio Occultation Sounded for Atmosphere) from the Italian space agency ASI (Agenzia Spaziale Italiana). Essentially a highly sophisticated GPS receiver dedicated to the analysis of climate change, ROSA detects modifications to its signal as it passes through the earth's atmosphere and with that creates atmospheric profiles.
From France's CNES (Centre National d'Études Spatiales) comes two instruments—ICARE and SODAD. The former studies the effect of cosmic radiation on electronics, and the latter observes orbital debris and micrometeorites. Five CONAE instruments complete the picture, including radiometry, imaging, and infrared data collection devices.
Imagine all of these devices working harmoniously together for years in the blackness of space, controlled remotely and without hands-on input, while the craft they inhabit orbits at constant elevation of 408 miles and completes an orbit every 98 minutes (!), and you begin to see the inherent fragility of such a mission.
Dig a little deeper though and the whole thing seems only more incredible.
For one, Alex Murray tells us the core SAC-D spacecraft is…well, not particularly large, at approximately eight feet tall and eight feet wide. Apart from the size—or lack thereof—and all the goodies tagging along for the ride, Murray also points to the cabling as one of the biggest headaches, likening it to the interior of a desktop computer but far, far more complex.
Alex, we PC nerds feel your pain.
Just consider what it takes down here on Earth simply to watch over the spacecraft and its primary instrument. Mission operations in Cordoba, Argentina, handles observatory operations and control, service platform processing and storage, telemetry and stored data processing, orbit maneuvering, and much more. Tracking stations—a total of six—in such far flung locales as Poker Flats, Alaska; McMurdo, Antarctica; Svalbard, Norway; Wallops, Virginia; Matera, Italy; and Malindi, Kenya ensure SAC-D never disappears from view. Ground stations help keep everything together. NASA's Goddard Space Flight Center processes the data it receives from CONAE, and NASA's Physical Oceanography Distributed Active Archive Center disseminates that data.
We could go on, but we think you get the picture.
Yet despite all the earth lifelines, most of what goes on in the SAC-D and in the Aquarius instrument itself is designed to be autonomous. More than that though, it's designed to be long-lasting. Really long lasting. You see, although the mission is pegged as a three-year stint, Gary Lagerloef tells us that as long as something doesn't break down (even spacecraft are privy to unforeseen mechanical foibles as the rigors of time take their toll), the mission will last a whole lot more. "It's a minimum of three years. We'll have achieved our key objectives in three years, but as has been proven in the past, earth sciences satellites can continue doing their job for ten years or more."
Lagerloef points to QuickScat, a satellite launched aboard a U.S. Air Force Titan II launch vehicle in June of 1999 to monitor winds over the world's oceans and in turn help forecast critical stuff like save heights, aviation weather, and last but certainly not least, the development of major storms. QuickScat was thought to have a useful life of two to three years, yet it continued to fully function and relay key data more than a decade later.
Ever when its antennae essentially stopped spinning in November of 2009, thus rendering its primary objective defunct, all was not lost. Indeed, QuickScat continues even today to play the role of orbiting Energizer Bunny, albeit in a reduced capacity, cross-calibrating data for other earth sciences spacecraft.
Lions and Tigers and Scatterometers (Oh my!)
So, just what is Aquarius, and why is this $287 million device so potentially important?
For starters, Simon Collins tells us Aquarius is no teeny-tiny lab instrument. Indeed, it's a somewhat hefty fellow clocking in at 705 pounds. Some of that weight can be attributed to the device's sunshade, which at 8.7 feet in diameter, effectively engulfs the base of the instrument and its reflector (antenna), which deploys from the opposite end. With reflector deployed, Aquarius measures 12 feet in length, taller in fact than the SAC-D spacecraft upon which it hitches a ride.
With a little imagination, one could liken the instrument to a clamshell. The deployed reflector/antenna makes up the top shell, the sunshade the bottom shell, and the primary structure resides in the middle, as the body of the "clam." Unlike a clam, however, Aquarius concerns itself not with the ocean bottom, but instead with its surface.
As we mentioned earlier, it exists for a single purpose—to measure the salinity (the concentration of salt) at the uppermost inches of the world's oceans. More accurately, as days turn to months and then years and Aquarius has surveyed each section of ocean multiple times, NASA scientists will use its data to determine the degree to which that salinity has changed.
It does this because ocean salinity is a key indicator of ocean circulation, and a more thorough knowledge of ocean circulation will help us better understand the planet's water cycle (essentially how precipitation, evaporation, ice melt, and river runoff move water around the planet). If we can come to grips with that, we'll not only improve predictions of future climate trends and short-term climate events such as El Niño and La Niña, but also gain important insight into climate change.
A salinity study of this magnitude has never before been attempted. Though satellites have and are being used to measure sea surface temperature, winds, rainfall, water vapor, and sea level, ocean surface salinity measurements have generally been limited to bits and pieces of data taken from ships, buoys, and a small number of in-atmosphere airborne science campaigns.
Aquarius, conversely, is an ocean-scanning mega-machine of such proportions that NASA claims it will collect as many sea surface salinity measurements as the entire 125-year historical record from ships and buoys. And that's just within its first few months of operation.
At the forefront of the instrument is something called a "radiometer." To be more specific, a passive microwave radiometer. The Aquarius has three of them in total, each measuring microwave emissions from the sea surface at 1.4GHz in the L-band portion of the electromagnetic spectrum. The more salinity that resides in a given bit of ocean, the greater its "emissivity" and the more powerfully the signal is radiated.
Yet there is a fly in the radiometer ointment. If the water in a given oceanic zone is choppy, radiometer readings tend to "scatter," thus rendering gathered data far less effective. But that's where a device called a "scatterometer," one of which has been installed on Aquarius, comes in.
By NASA definition, a scatterometer is "a microwave radar sensor used to measure the reflection or scattering effect produced while scanning the surface of the earth from an aircraft or a satellite." In the case of Aquarius, the on-board scatterometer takes constant reading of ocean surface roughness in parallel with the radiometers, and in effect, corrects measurements skewed by rogue waves. Pretty slick, huh?
If that doesn't impress you, consider this: The trio of Aquarius radiometers (and the scatterometer, which alternates operations with the radiometers so the sensors look at the same piece of ocean simultaneously) completes a full scan of the Earth's entire ocean surface in just seven days. Granted, that has a lot to do with that 98-minute orbit time we referenced earlier, but it's also a function of three highly cooperative radiometers. Operatingin a harmonious "pushbroom" configuration, together they cut a swatch across the world's oceans that's 242 miles in width, with what NASA claims is accuracy equivalent to a pinch of salt in a bucket of water. Some pushbroom.
One other thing: according to Lagerloef, the potential malfunction of one, or even two, of Aquarius' radiometers doesn't spell an end to the mission. In such an instance, the unaffected radiometer(s) would simply keep on trucking.
But Can It Play Crysis?
According to Alex Murray, the spacecraft itself is home to three computers, all of which share architecture they've inherited from earlier Argentine spacecraft. The first handles command and data handling, measures the health of the machine, and communicates with the instruments.
The second is dedicated to the attitude control system, essentially keeping the entire assembly pointed in the right direction and in a "sun-synchronous" orbit in which it passes over the same part of the Earth at the same local time each day and whereby the Aquarius sensors are consistently pointed away from the sun to avoid signal contamination from solar L-band energy flux. The third and final SAC-D computer moderates and distributes the power accumulated through the spacecraft's solar panels.
Similarly, the Aquarius instrument itself also features three computing devices—the two smallest and least powerful of which are dedicated to controlling temperature. And yes, temperature is that important.
During orbit, Murray tells us, Aquarius is regularly exposed to temps as cold as -70C - most definitely not a good thing for the delicate systems within it. Yet even more potentially harmful are temperature variations, which can play havoc with optimal operation of certain components and the proper calibration of those oh-so-critical radiometers. A ten- or fifteen-degree temperature swing, says Murray, would be "intolerable."
The obvious solution? Install a heater. Not the heater from your old Chevy, mind you, but a state of the art "thermal control system" that ensures everything important remains at a nice, even keel. This was not an easy task, as it turned out, considering Aquarius' size, scope, and proposed longevity, and it ultimately forced scientists at NASA's Jet Propulsion Lab to work overtime designing special computer models beforehand. The reward? An instrument that, like Goldilocks' ill-gotten porridge, is neither too hot nor too cold.
The third Aquarius computer is where all the cool stuff happens. This is where the data and the command information flows, and it is, in effect, the brains of the operation. The big surprise, however, to all us space-challenged, earth-dwelling tech-lovers, is that Aquarius' most powerful computer by a country mile…isn't. At least not when compared to even one of today's entry-level desktops.
Lagerloef tells us that in Aquarius, "computation demand is not high." Murray goes a few steps further, spelling out for us that Aquarius' chief computer is one of the spares from NASA's long running Mars Exploration Rover (MER) program, an IBM RAD6000—a computer that operates at the 20 MIPs (millions of instructions per second) range.
How fast is that? Well, MIP speed alone is not an accurate measure of computer power, though we can say that virtually every new desktop-level CPU released in the last decade rates considerably higher. Intel's Core i7 Extreme, for example, screams along at 150,000 MIPs-plus.
Aquarius' main computer also does not have a hard drive. Well, of course not, you say, but surely it must incorporate some high-end, high-capacity solid state drive that would better stand up to the rigors of space. Well…no. Instead it features a whopping 128MB of RAM. Here, the flight software, amongst all the other information that's gather, is stored.
Everything Old is New Again
Fortunately, the folks at NASA have ensure Aquarius doesn't need a tons of on-board storage. Four times each day (typically twice during the morning and twice in the evening), new data is dumped from the instrument to the Cordoba, Argentina ground station.
Still, some may scoff at the seeming antiquity of Aquarius' computing hardware. Yet the truth is that when you're monitoring the salinity of oceans from space, the speed and power of the computing component is secondary to both its reliability and its survivability. Murray puts it succinctly. "When in space, you can't have reboots. You can't have any BSODs." We couldn't agree more.
One threat comes from vibration. Like Bill Haley and his appropriately named Comets, spacecrafts shake, rattle, and roll as they make the transition to space—a terrible spot for typically earthboard electronics and computers. But the far more urgent threat when outside our atmosphere, apart from brutally low temperatures of course, is radiation. And make no mistake—the level of radiation 408 miles up is not only deadly to humans, it's also absolutely toxic to unprotected computing equipment.
Essentially, says Murray, high-energy particles such as cosmic rays, so prevalent in outer space, can hit a binary digit (bit) and flip its switch from a 1 to a 0 or from a 0 to a 1. The result? An error. A "glitch." And in a situation where precision is paramount, where errors can cause irreversible mission damage, where hundreds of millions of dollars are on the line, and of course, where hands-on technicians aren't exactly hovering behind the next asteroid, such errors simply cannot be permitted.
Thusly, the Aquarius mission, and indeed all NASA missions, rely exclusively on "radiation hardened" computer components. The exact recipe for radiation hardening generally includes extra transistors that take more energy to switch on and off and unique approaches to the manufacture and installation of insulating layers.
The downside? Radiation hardened components are power hungry, tremendously expensive, and slow. Their inherent lack of speed can be attributed, at least partly, to the radiation hardened environment in which the chips sit, but there's also something else at play here—trustworthiness.
Basically, if computational demands aren't extremely high—as is the case with Aquarius—and if you have a given computing array that's already survived previous missions, the need to upgrade to a faster, more powerful array is not a prime driving force. And that's one of the main reasons relatively old school single-board computers such as the aforementioned RAD6000 still find favor in the NASA community.
Here, the old adage "If it ain't broke, don't fix it," seems more than apropos.
And certainly the Aquarius mission thus far is anything but broken. In full operation as of August 25, the instrument, its radiometers, and its scatterometer have already produced complete global maps of ocean salinity. Moreover, says Alex Murray, the "precision of the fineness of the measurements is much better than anticipated."
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