Keeping Us On Track

From: John Sonntag, ATM Senior Scientist and IceBridge Instrument Team Lead

Thule, Greenland — One of the many unusual aspects of flying with NASA’s Operation IceBridge (OIB) is that we fly very, very precisely. Getting the airplane where we want it to be, when we want it to be there is important for a variety of reasons. At its most basic, however, precise flying is necessary to match the limited footprint of our remote-sensing instruments with the target they are measuring. This target might be a spacecraft ground track, the centerline of the fastest-moving channel of a steep, sinuous glacier, or even a research camp established on a drifting ice floe.

Our pilots are outstanding professionals and the precise flying is primarily their doing. But to help them do it, OIB leverages NASA’s multi-decade investment in operations of the Airborne Topographic Mapper (ATM), a scanning lidar and key instrument of OIB. The ATM project, which is my own home team and where I learned most of what I know about airborne science, long ago addressed the need to fly a remote-sensing aircraft very accurately over regions with no fixed landmarks. The key, not surprisingly, is the Global Positioning System, or GPS. GPS, of course, gives us nearly continuous updates of our position with an accuracy of just a few meters. But using this knowledge effectively, especially at 250 knots, is a little tricky.

So the ATM team developed, and continues to modernize, a suite of software and hardware tools to effectively use GPS to help pilots steer an airplane very accurately, and even to steer the airplane directly. In their current form these tools are called “SOXMap and SOXCDI”.

SOXMap is really just a “moving map” system, at heart not unlike the ones in consumer GPS units used in cars or for hiking, but simpler and highly specialized for remote sensing applications. SOXMap displays the aircraft’s current position and orientation relative to a science target, such as the sinuous centerline of a glacier (see the SOXMap illustration, left). It also displays the measurement swath being collected by a science instrument such as the ATM, drawn to scale. This display is piped to the pilots up front, using little tablet computers with very sharp, bright displays, which are mounted right on the control yoke. Our pilots use this information, which is continuously updated, to steer the aircraft and its sensor swath right where we want it. Some of our OIB pilots refer to SOXMap as “the Pac Man display”, in reference to the old video game where the player guides a little mouth around a screen chewing up dots. The beauty of SOXMap is its versatility. Our bread-and-butter use for it with OIB is to follow sinuous glacier centerlines with any degree of curvature, but SOXMap is also useful for area-mapping applications, or really any targeted flying.

Even cooler than SOXMap is its sibling, “SOXCDI”. SOXCDI (image right) incorporates a moving map display similar to the one in SOXMap, but unlike SOXMap, SOXCDI can actually steer the airplane automatically! It is designed for cases where we fly long straight lines, such as grid lines or satellite ground tracks. We can do the same thing successfully using only SOXMap, but for long straight lines this demands lengthy periods of concentration from our pilots and can be extremely tedious for them. So SOXCDI continuously compares the current position of the aircraft to a desired straight-line track connecting a pair of waypoints (for any navigation geeks who might be reading along, the straight line is actually a great circle, and I plan to add a selectable rhumb line option as well). Doing a little math with that comparison, we always know how far we are from the desired track and whether we are right or left of it.

But how do we use this information to steer the aircraft automatically? Here is where things get extremely cool. It turns out that most large airplanes have what is called an “instrument landing system”, or ILS. The ILS is really just a radio receiver that is designed to listen to a simple pair of audible tones being broadcast from an airport runway, each tone angled slightly away from each side of the centerline. This allows them to land in bad weather when the runway isn’t immediately visible. If the ILS radio “hears” more of one of the tones than the other, it directs the pilot, actually the autopilot in our case, toward the “quieter” tone until the two tones are equal. When they are equal, the airplane is centered on the runway, right where it should be.

Can you guess where we’re going here? SOXCDI simply mimics an ILS signal, which we pipe into the aircraft’s autopilot. And it’s pretty simple, really. If we’re left of track, we generate more of the “left” tone than the “right” tone, and vice versa. The airplane’s autopilot does the rest. The tricky part comes in calculating how much of each tone we generate given how far right or left of track we are, but those are just details. The point is, the system is simple because we just piggyback on existing technology – the ILS. And though our software certainly has some complexity, the hardware is almost breathtakingly simple. We run the SOXCDI (and SOXMap) programs on very basic, run-of-the-mill computers – that 8-year-old laptop you tossed in the closet during the previous presidential administration would probably work fine once we installed our software on it. The tones are generated by the sound card built in to every modern computer has built in. We literally just plug a cable into the headphone jack of the computer, and plug the other end into a standard piece of lab equipment called a “function generator.” The function generator just takes the two tones and turns them into a radio signal, which we then pipe via another cable into the airplane’s ILS. And that’s it. You might not be familiar with a function generator, but to give you an idea what a basic piece of lab equipment it is, we recently bought one for just about $1,000, about what a basic laptop costs.

Once it’s all set up, SOXCDI generally keeps the airplane within just a few meters of where we want it to be, and can do it literally all day long. It does this, in essence, by “singing” a two-note chord to the airplane, the relative volumes of the two notes determining the steering correction. And it works equally well with both of NASA’s workhorse OIB airplanes – the P-3 turboprop and the DC-8 jet. And that points to another strength of the SOXCDI design. Because it mimics standard ILS signals, it should work with any ILS-equipped airplane.

If I sound enthusiastic about our precise navigation systems, well, I am. I am the developer of SOXMap and the co-developer of SOXCDI with my colleague Rob Russell, and I’m very proud of their role as a key enabling technology for IceBridge. They are also just about the most fun projects I’ve ever taken on at work, a perfect task for an at-heart aerospace tinkerer like me. But Rob and I can’t claim all the credit, or even most of it. We built SOXCDI and SOXMap on ideas originally conceived and developed by our colleagues Wayne Wright (SOXCDI predecessors), and Richard Mitchell and Bob Swift (SOXMAP predecessors), respectively.

And finally, what about those names? Well, the original implementation of the moving map concept with the sensor swath drawn to scale, way back in the 90s, was called XMAP. So in homage to that, I originally called my implementation “Son Of XMAP”, which became SOXMap. Later when we developed SOXCDI, we essentially merged elements of SOXMap and and older system called CDI (for Course Deviation Indicator), yielding SOXCDI.

Weathering the Storm

Spring officially arrived on Sunday, but for IceBridge scientists in Thule, Greenland, spring was nowhere to be seen. Teams awoke Monday to a storm that continued through the day and by 5 p.m. local time, Thule Air Base had declared “Delta” status, prohibiting on-base travel and confining personnel to the buildings.

The storm on Monday in Thule intensified to “Delta” status. IceBridge scientists waited out the storm indoors. Credit: NASA/Jim Yungel

While snow blows outside, IceBridge project scientist Michael Studinger briefs the teams at the nightly meeting. A science flight the following morning would require clear skies. Credit: NASA/Jim Yungel

With no other options for food, some scientists turned to MREs (Meal, Ready-to-Eat), including this southwest style beef and beans with Mexican style rice, picante sauce, and chocolate disk cookie dessert. Credit: NASA/Jim Yungel

The ground-based GPS antennas did not fair as well in the storm, although both stations have since been reestablished and ready for a science flight. Credit: NASA/Kyle Krabill

By Tuesday morning the storm had passed and crews were quick to plow the runway. Credit: NASA/Jim Yungel

With a clear runway, the P-3 was rolled out for a long-awaited, high-priority sea ice flight to Fairbanks, Alaska. Credit: NASA/Jim Yungel

An Uncommon Routine

From: Michael Studinger, IceBridge project scientist, Goddard Earth Sciences and Technology Center at the University of Maryland, Baltimore County

Thule Air Base, Greenland — The IceBridge team arrived in Thule last week and the campaign is off to a good start. We flew four out of five days last week and accomplished three sea ice missions including an underflight of the European Space Agency’s CryoSat-2 satellite over the Arctic Ocean. After a week here in Thule, we are settled in and our operations have become routine.

Operation IceBridge accomplished three science missions during the first week, including an underflight of ESA’s CryoSat-2 over the Arctic Ocean just 120 miles from the North Pole. Credit: NASA

A typical IceBridge day in Thule Greenland starts at 5 a.m. when my alarm clock goes off. I start downloading satellite images to get an idea which missions may be possible to fly before I go to breakfast at 5:45 a.m. At 6:15 a.m., the pilot in command, mission manager, John Sonntag and myself meet at Base Ops to get a weather brief for the day.

The three meteorologists at Thule Air Base have known us for years and do an excellent job in providing us with a very detailed and specialized weather brief that we require for decision making. The demands for research flights are different from everyday air travel, and the polar environment poses great challenges in terms forecasting the weather. There is not a single weather station within hundreds of miles of our survey area that we could use to get a weather observation or that would provide observational data as input into a forecast model. Instead, we depend on satellite images that are several hours old. Visible images are dark for any area west of us. It requires experience and skill to interpret the forecast products for our purpose.

A few days ago, a model transect along our flight path showed dense cloud cover along the entire mission profile at 500 meters flight elevation calling for a no-fly day. We spent time with the meteorologists to understand the weather situation and decided to fly, despite the grim looking forecast. It was the right decision. The cloud layer depicted by the forecast model turned out to be a thin layer of haze that did not pose any difficulties for our laser and digital imagery sensors.

The weather forecast is shown along a survey line for a P-3 science mission. The forecast predicts dense cloud cover at the flight elevation (500 m), but after carefully studying the weather situation, we decided to fly. Credit: NASA

Between 6:30-6:45 a.m. we make a go/no-go decision. If we fly, the aircraft gets pushed out of the hangar and the fuel truck arrives. We need to collect one hour of static GPS data on the ground to calculate high-precision trajectory data from our flights. At 7:30 a.m. the door of the aircraft closes and we taxi to the runway to be ready for an 8 a.m. takeoff as soon as the tower opens.

We typically transit to the survey area north of Thule and then descend to 1,500 feet were we start collecting data. It’s still early in the season, which means missions west of Thule are flown in near-constant twilight, with the sun following us as we go west. When we turn around the western end of the line and fly back east, it immediately start getting lighter with every minute of the flight.

During the flight the operators monitor their instruments and make sure we collect high-quality data. Occasionally, adjustments need to be made to ensure the instruments keep working.

In-flight adjustments are often necessary to keep the instruments working and collecting high-quality data. Adjustments often require work below the deck to access the instrument sensors in the belly of the P-3. Credit: NASA/Michael Studinger

At 3:45 p.m. we typically land to leave enough time for a 1-hour post-calibration with the aircraft outside. By 5 p.m. the aircraft is rolled back inside the hanger and doors close for the night.

John Sonntag and myself quickly stop by Base Ops for another weather brief to see what’s in the mix for the next day. At 5:30 p.m. we have a science meeting where we discuss plans for the next day and talk about issues that are worth sharing with others. After the meeting, most people go straight to dinner followed by a late evening spent backing up data and processing data.

At 5 a.m. the next morning we start again.

A lateral moraine can be seen at the margin of the Greenland Ice Sheet near Thule Air Base. Credit: NASA/Michael Studinger

View From the Hut

We previously wrote about the Met Huts in Kangerlussuaq and Thule, Greenland –- the ground-based GPS stations that help scientists to ensure that GPS information collected on the aircraft is as accurate as possible. Kyle Krabill is back in Thule for the Arctic 2011 campaign making sure hut operations run smoothly.

From: Kyle Krabill, ATM Instrument Team Engineer, NASA’s Wallops Flight Facility

“Took a couple pictures this morning [March 17] from my view here at the hut. Sunrise is getting earlier by 13 minutes or so each day. The guys are flying another mission again today. Had a little warm front come through last night and its up to a balmy -17 (if you’re out of the wind)”

The P-3 passes over the Met Hut on March 17, 2011. Credit: Kyle Krabill

The sun rises in Thule, Greenland, as seen from the Met Hut on March 17, 2011. Credit: Kyle Krabill

Teacher, Student Blog From the Arctic

A teacher and student from the U.S. Naval Academy in Annapolis, Md., joined IceBridge in the field for the Arctic 2011 campaign. Follow their Arctic adventures here.

LCDR John Woods is a Meteorology and Oceanography Officer (METOC) currently teaching in the Oceanography Department at the United States Naval Academy (USNA). He is part of the Sea Ice Thickness Observation team currently participating in NASA’s Operation Ice Bridge 2011.

Eric Brugler is First Class Midshipmen who is an honors Oceanography major at the United States Naval Academy. He is interested in the polar regions of Earth because he believes they play a very important role to the Earth’s climate system.

Welcome to the 2011 Arctic Campaign

From: Kathryn Hansen, NASA’s Earth Science News Team/Cryosphere Outreach Specialist

On March 14, NASA’s P-3B landed in Thule, Greenland, for the start of the Arctic 2011 campaign of Operation IceBridge. Credit: NASA/Jim Yungel

On March 14, NASA’s P-3B aircraft landed in Thule, Greenland, where it will be based for the first leg of the Arctic 2011 campaign of Operation IceBridge. It’s our third annual campaign over the frozen north, ensuring the continuity of ice elevation measurements that scientists use to monitor change. This year, IceBridge is bigger than ever before and delving into new areas of exploration. Read more about the Arctic 2011 campaign and watch the video here.

Follow this blog throughout the 10-week campaign to read about the mission from the perspective of the scientists and crew on the ground (and in the air) who make the mission possible. They will give a behind-the-scenes account of individual flights and daily life in the field. They will share pictures and video from the sky and ground. And they will discuss the science questions being probed by the array of instruments onboard the flying laboratories. Welcome aboard!

Mission participants chat inside the P-3B during transit on March 14 from NASA’s Wallops Flight Facility in Wallops Island, Va., to the mission’s base in Thule, Greenland. Credit: NASA/Jim Yungel

IceBridge teams arrive in cold and sunny Thule, Greenland. Credit: NASA/Jim Yungel