Tuesday, 18 January 2011

Finding the right orientation for the ozone spectrophotometer

During the last week, we installed the Brewer ozone spectrophotometer on the roof of the station. As already explained in the last post, this instrument will serve to monitor very precisely the amount of total ozone in the atmosphere. It will not only help to monitor the evolution of the ozone layer, but it will also be valuable to validate satellite measurements of the ozone amount. The retrieval above highly reflective areas like the Antarctic is not always easy and therefore ground-based measurements are important.

In order to do its measurements, the ozone spectrophotometer turns with its optics always towards the sun. This is not that simple as it sounds. One essential parameter is the azimuth angle. This angle is defined as the angle between the North and clockwise going to the position of the sun. In the northern hemisphere this angle starts at 0 degree in the North and goes over 90 degree East, 180 degree South, 270 degree West back to 360/0 degree North. On the contrary, in the southern hemisphere the azimuth starts (as the definition stays the same, but the sun moves from East over North to West) with 180 degree in the South, goes back to 90 in the East, to 0/360 in the North, over 270 degree in the West back to 180 in the South. As the suntracker of the Brewer with its motor system is able to do one clockwise rotation of 360 degrees and then turning back to its starting position, the tricky thing with orienting the Brewer in the Antarctic arises with the transition point around local noon when the azimuth makes a jump from 0 to 360.

After some tests in order to find the correct positioning I found two possible solutions with the great help from Hugo in Brussels and Arjan in Delft. One possibility is to orient the tracker as in the northern hemisphere with its zero point to the North. At local noon, at the switch from 0 to 360 degree, the tracker makes a full 360 degree rotation. This setup has the disadvantage that there is some time lost due to the tracker’s rotation. And this just around the short precious time when one can get the best measurements in a place like Antarctica with its (mostly) low sun elevations. The other possibility is, to orient the tracker with its zero point to the South. However, as the software calculates the correct azimuths (starting at 180, not at 0), and the tracker counts its degrees clockwise, one has to set an internal offset of +180 degree. For the azimuths when +180 and the azimuth results in more than 360 degree (and, as the tracker’s rotation is limited to 360 degree), the tracker internally knows that it cannot move clockwise more than 360. So, it subtracts automatically 360 degrees. In this setup, it does not need to make a full rotation around local noon. This full rotation is then done around local midnight, when the sun is anyway too low for measurements.

Besides the instrument installation, there are many opportunities to make some nice photos of the clouds and the landscape here. Above, I put some new ones.

No comments:

Post a Comment