Last week I had a meeting with the vendor that does some of our aerial photography. Someone from our side asked about their involvement with the Department of Homeland Security. They said they do provide them with services. That piqued the interest of the questioner on my side and he pressed further. He asked if they are regulated or monitored in some way by the Feds. He was a little evasive but ended up answering yes. He pressed a little further into how they monitor them or how often. He again was evasive, but ended up answering that when the plane lands on a DHS flight that there are people waiting for them on the tarmac and they remove all of the computers and hard drives that were used during the flight. Cool!

They also said that the huge high resolution produce so much data so fast that the planes are outfitted with RAID arrays! Not so much for the data capacity, but for the ability to write the data to disk very fast.

RAID stands for Redundant Array of Inexpensive Disks. A bunch of hard drives are strung together so that to the computer they look like one big hard drive. The reason it makes it faster is because data is written across all of the disks at the same time rather than to just one disk at a time.

Normally the computer will send a file to the hard drive and the drive controller will say "OK, send this little pack of data to this area on the disk." Then it has to wait while the drive head moves to that area, and then wait until the spinning disk moves into the correct position, and then actually write the data to the disk, and then it does it again. Nothing gets written to the disk while the head is moving to the correct position.

With a RAID array there is a RAID controller. It sends a little packet of data to the first drive and then immediately moves onto the next drive and sends it a little packet of data to write, and so on. It doesn’t have wait until that data is written to disk before it can dump the next bit of data out of the controller's buffer. By the time it has sent a little bit of data to every drive and makes its way back to the first drive, the first drive should have been able to write its piece of data to the disk and is waiting for the next bit of data.

So with a RAID array on the plane where they might be snapping almost 1 gigabyte images every few seconds you cannot afford to wait for the data to get dumped from the camera's buffers.

He made a comment during the meeting along the lines of "Well what do you expect from a $3 million camera?" I don't know if he was referring to the whole specially equipped plane, to just the camera and associated electronics, or to just the camera, but in any case it is pretty impressive.

The camera has to be mounted to the plane in a hole cut through the bottom of the plane. There are several gyroscopes that record the exact pitch and yaw of the plane for every shot, and of course there is a survey-quality GPS receiver interfaced with the camera and the exact position is recorded along with every image. The camera itself is also gimbled so that can remain as close to horizontal as possible.

Before each flight they get the best results if they can place targets on the ground over precisely surveyed geodetic monuments. These targets are basically big white canvas X's that can be seen from the air. When the photogrammetrists are orthographically correcting each image they use these known positions as reference points.