Willy was ready to fly today!
I spent the day with James Jacobson and the Digital Mapping System or DMS. DMS uses a Canon 5D EOS Mark II digital camera to take photos or ‘frames’ of the ground (ice) every 1.2 seconds. The rate at which frames are taken can be varied; higher rate when flying lower or faster, and lower when flying higher or slower. Confused already? Don’t worry; with a lot of help from James, let’s get into it.
This was the mission flown today 4-1-13. The 'mowing the lawn' pattern is done where Operation IceBridge and others want to collect the most data and the Jakobshavn glacier is one of the fastest moving glaciers in the world. The long straight tracks are IceSat I passes, so IceBridge reapeats those tracks to continue gathering logitudinal data. Image courtesy of NASA/Operation IceBridge.
James grew up in Sunnyvale, California outside San Francisco. Sunnyvale, part of ‘Silicon Valley,’ is home to companies like Lockheed, Yahoo, and AMD, so maybe it’s no surprise that he got into science and engineering. James has undergraduate degrees in geology and earth science from UC – Santa Barbara and a graduate degree in remote sensing from UC – Santa Cruz. He’s done all kinds of neat stuff including remote sensing of rain forests, but you’ll have to watch the interview for more about James. He’s been working with Opertation IceBridge’s DMS for a year and a half.
James at work in front of the two DMS racks.
The Canon camera has 29,000,000 light sensing elements or 29 Megapixels. That means, crammed onto a piece of semiconductor material, a little more than one square inch, are 29 million light sensing pixels. Thanks to the photoelectric effect, which Einstein won the Nobel prize in 1921 for explaining how it works, the light is converted in to electricity which is then sent to a computer for processing. The computer processesses those 29 million inputs, stores them in its memory, and gets ready to take another frame. All within 1.2 seconds. It actually happens much faster than that and quite a bit more information is actually stored, but we’ll stick to the basics.
This is what the DMS images look like coming in real-time. In better focus than my picture, or course.
At the rate of one frame every 1.2 seconds that equates to about 15,000 frames per mission. Multiply by 29,000,000 and you get … well, a really big number. Each day the DMS stores around 120 GB of data. They’ll fly close to 30 missions during this campaign, and they fly 2-3 campaigns per year. When James isn’t flying; he’s often processing data. A lot of data.
If you are a camera buff this next part is for you. The camera is mounted in the belly of the plane where it’s not easy to get to from the cabin. Therefore the camera is completely computer-controlled. The focus is set to infinity. My math teacher friend, Rick Brenner, is going to object to calling 1,500 feet infinite, but the curve gets pretty flat, pretty quickly so we’ll round off to infinity.
Physics and math students, please click the video link below. All others may skip.
Did you skip the video? If you answered yes, then GO BACK – it’s great stuff! If you answered no, then pat yourself on the back and read on.
One of two DMS racks.
The ISO is set between 200 and 1600 depending on light conditions, the aperture is set to f11 and the exposure time is 1/2000th of a second. All those settings can be adjusted during the mission as light conditions change from sunny to cloudy and from dawn to dusk. DMS is a passive system, compared to the ATM and radar systems which are active systems – they transmit and receive energy. DMS only receives energy from sunlight reflected off the ground. James gets one of the best seats on the plane; right next to a bubble window, so he can monitor outside light conditions and adjust camera settings if necessary.
I spent a lot of time getting to know James, learning about the system he operates, and looking over his shoulder.
DMS doesn’t just take high resolution photographs – it correlates each pixel to a physical dimension on the ground. Flying at 1,500 feet each pixel corresponds to roughly a 10 cm x 10 cm square on the ground. Here’s a short video diagramming that process.
The DMS takes in data from the GPS system to locate the plane in the air. With the GPS data, the DMS now places that 10 cm x 10 cm square in the correct location on the surface of the earth, thereby creating a topographic map.
A close up of one of the DMS monitors.
But, and you knew this was coming, there’s more to it. The GPS locates the plane in three dimensions, but the plane also wobbles around while flying. The nose can go up and down (pitch), the wings can tilt (roll), and the whole plane can slide sideways (yaw). All these motions are amplified when looking at something on the ground, 1500 feet away.
A example of a DMS image. The resolution is in the 10 cm range. Image courtesy of NASA/Operation IceBridge.
It’s like when you shine a flashlight on the wall some distance away. You move the flashlight in your hand a little bit, and the spot on the wall moves a lot. The technical term for this is parallax. And parallax distorts the image. Think about a square piece of cardboard placed under a lamp directly overhead … nice square shadow in the ground. Now move the lamp off to the side but keep the cardboard in the same spot. The shadow distorts – it’s no longer a square. That’s parallax! Try it!
Even though the bricks are really the same size, because of the perspective, the bricks are not all the same size in the photograph. That's parallax.
By now, I hope it doesn’t surprise you that … these NASA folks can correct for parallax. DMS also takes in data from the inertial navigation system. What’s an inertial navigation system? Better look here for the details. Basically, there are three gyroscopes mounted in three mutually perpendicular planes and and they can tell you exactly how the plane is oriented in terms of pitch, roll, and yaw. The DMS system uses that information to correct the frames.
James is making adjustments to the DMS.
But, wait, there’s more! The frame rate is high enough that around 60% of two successive frames overlap. Do you know what you get when you view the same object from two different angles … 3D! The DMS can be used stereoscopically (that’s the technical term) to create 3D images of the surface. It’s incredible what these really bright people can do with their data. And don’t forget that all their data, all of it, is publically available. You could get DMS images and analyze them yourself. I know you want to.
DMS works on land, but so does the ATM – see the journal from 4-8-13. Maybe you remember that when things are smooth, or specular, the ATM sometimes misses the return reflection. Where do you find a lot of specular surfaces? Over the ocean. Besides glaciers and ice sheets, Operation IceBridge also maps a lot of sea ice. DMS works great over sea ice, especially when there is open water, because it can easily tell the difference between bright snow or ice and dark water. A calculation of how much sea ice there is can then be performed.
The two ATM elevation passes from April 10, 2013, plotted with the pass from April 4, 2013. One can see that approximately 200 meters of Jakobshavn glacier has flowed into the fjord in the six days and is 'missing' from today's plots. Image and caption coutesy of NASA/Operation IceBridge.
You can hear more about James and how he came to be a part of NASA and Operation IceBridge in the following interview.