Happy New Year from Antarctica! This year is off to a great start. The STO-2 mission had a hang test late on January 1, and passed the flight readiness review January 3. That means we are ready for launch and are just awaiting good weather before we fill the balloon and let go!
If you remember from previous posts, the hang test is a test where the payload is picked up by the launch vehicle and hangs freely while the team performs various tests. First of all, the hang test serves as a practice run for getting the instrument off of its dolley in the hangar and connected to the launch vehicle (more on that later). The launch team practices attaching last-minute equipment such as crash pads and filling the ballast (dead-weight) used to maintain a steady altitude during flight. After the payload is flight-ready, the instrument team can take over and run tests from the ground.
One of the most important checks that occurs during the hang test is the communications tests. The gondola uses several antennas that communicate over three different satellite and ground-based network systems. For the first 24-48 hours after launch, the gondola communicates via line-of-sight antennas with the ground based station at the Long Duration Balloon facility. This is the most important form of communication because the data rate is the highest, so we want to try and do all of the commissioning activities while we have line-of-sight (LOS) communication. As the name implies, LOS communication will only work until the balloon mission disappears beneath the horizon, which occurs sometime after the first day of the flight, depending on the weather patterns and wind direction. Some of the activities that have to occur for commissioning is focus the telescope and align our star tracking cameras to the main telescope.
The other communication systems on the gondola are antennas that communicate with satellite networks. You can think of them as two systems, one that is high data rate but must be aligned very precisely and does not have 24 hour coverage, and a second system that has continual coverage but has a slower data rate. In general, we use the high data rate to send commands to the gondola to plan and execute our observing strategy. The slower system will be used primarily to return our (compressed!) data down so we can make sure that it looks like what we expect in real time. This is an important system, because otherwise the raw data is stored on the hard drives in the main computer on the gondola which MUST be recovered. This is a risky situation because there is a good chance we may not be able to recover the gondola when it lands. Some of the reasons a gondola may be unrecoverable are if the weather is too bad at the crash site to land a plane safely, if the gondola gets pushed out to sea or lands in a crevasse, or if we loose communication with the gondola entirely and don’t know where it landed.
The other main thing to check during the hang test is the pointing, tracking, and stabilization system of the gondola. The gondola must move freely during flight to allow it to point at what we want to study! We also want to keep the gondola steady, which is hard on a free-hanging object because the motion of the telescope causes vibration and pendulation that shake the gondola. To move the gondola we use two reaction wheels and a motorized telescope. The motor causes the telescope to tip vertically, so it swings out of the upright position that you see in all these pictures. The two reaction wheels are motorized and are controlled by gyroscopes that track the motion of the payload. The motors spin the opposite way that we want to move, and the gondola spins because, as Newton put it, ‘every action has an equal and opposite reaction’.
One of the problems of using a telescope is that they are designed to be very powerful at detecting light, but you want to have a fairly small field of view so that you can resolve fine details of whatever you are looking at. However, the narrow field of view makes it difficult to know what you are looking at. It would be like getting dropped off in a neighborhood that you know, but trying to figure out where you are but could only look through a straw. To solve this problem, we have installed to cameras on the sides of the telescope, each with a wider field of view. We look at the night sky with these cameras and observe a bunch of stars. A software package then triangulates the distances between the brightest stars in the image which provide a unique location of the night sky in the direction you are looking. This information is only helpful to you though if the telescope and the two star-tracking cameras are pointed in the same direction.
During the hang test, we use the star cameras to find specific targets and make sure the telescope moves to the correct location. We can also determine if the star cameras and telescope are properly aligned (though all of these things must be re-checked during the commissioning phase of our flight!!). The gyroscopes track the motion of the gondola, and make sure the gondola moves when it is supposed to. They also tell the reaction wheels if there is any jittery motion, and the reaction wheels correct for it.
Our hang test took place late at night, which was a first for the Columbia Scientific Ballooning Facility. We began our test at 7 PM and didn’t finish until 3 AM the next day. Since there is 24 hours of daylight in Antarctica this time of your, that wasn’t a problem for us, although it was really really cold! Most of the team got interviewed by a National Science Foundation representative. The interview is mostly for the reporter’s notes but the NSF also stores them in their archive.
After the hang test was declared a success (and we went home to get some sleep!) we had a flight readiness review. This was mostly a formality going through a checklist of things that still needed to be done before launch. Now that we have everything ready, we are awaiting good launch weather. There hasn’t been a calm enough day to do the launch yet, although we have gone to the hangar to make the preparations. The other balloon payload here, GRIPS, is also trying to launch, and we are behind them in the que. Word on the intranet is that a huge storm will move in on Monday, but there might be a big calm afterwards. Hopefully, the CSBF folks can get both of us launched back-to-back. The double launch has never been done before, but the team is excited to give it a try! Wish us luck!