The Code 916 Frequently Asked Questions (FAQ) List

But there are other reasons as well. It has proven beneficial over the years for NASA to have in-house expertise in atmospheric science. For example, it helps, when your organization flings ozone-measuring instruments into orbit, to have people around who know how to turn raw instrument counts into physical quantities, and who are intimately familiar with the subtle problems that can creep in to corrupt those measurements.

The earth's ozone layer is far from uniform. There are regions of high ozone and regions of low ozone, and these regions are constantly moved around by the winds. As a result, the global ozone field changes from day to day.

Data from a ground-based instrument is taken from a station at a particular location. One can observe the daily fluctuations in the ozone overhead with such data, but it says little about what is happening on a larger scale. There is a network of ground-based instruments to measure ozone, and that helps, but these instruments are expensive to maintain and operate. After all, great care must be taken to keep them in calibration, since incorrect data are worse than no data at all. Thus, there are only a couple of hundred sites in the network, and these are restricted to locations that can support them (omitting the three-quarters of the earth's surface covered by oceans, for example). In addition, one has to deal with small, subtle differences between instruments and separate them out from actual differences in the ozone layer over the instruments' sites.

With space-based instruments, the sparseness and uneven distribution of measurement points is very much reduced. The Total Ozone Mapping Spectrometer (TOMS) instrument, which is managed by our branch, makes measurements of the total ozone in a column of air (from ground to space). By making 14 polar orbits each day, this single instrument provides daily maps of ozone covering most of the globe. With just the one instrument circling around and around, the problems of calibration differences between sites are very much reduced. And with the abundance of global data, one can calculate statistics on ozone changes that are much more reliable and robust.

Of course, there are problems associated with space-based remote sensing, too. Calibration, for instance, requires great ingenuity when you cannot take your instrument back into the lab periodically.

To achieve with ground-based instruments alone the set of measurements one needs to evaluate long-term changes in the earth's ozone layer, one would need many, many more such instruments distributed across the planet's surface in very inconvenient locations (e.g., the middle of oceans), maintained and continuously recalibrated at enormous expense. Satellites, in this purpose, are just better suited for the task. The ground-based network that does exist today is still crucial, of course, since without it we would have nothing against which to compare and verify the satellite data.

One TOMS instrument, which measures ozone, is currently functioning in orbit. Earth Probe TOMS was launched on July 2, 1996, and it continues to return global maps of total ozone.

Designed to last about two years, the first TOMS instrument, which flew on board the Nimbus 7 spacecraft (at an altitude of about 950 km), began operations in November 1978 and lasted until May 1993. The second TOMS instrument was launched in August 1991 aboard the Russian Meteor-3 spacecraft (at about 1205 km altitude); it failed in December 1994. ADEOS TOMS was successfully launched by the Japanese on August 16, 1996, and returned data until June 1997.

No, not exactly. The Total Ozone Mapping Spectrometer (TOMS) instrument on board the Nimbus 7 spacecraft had been gathering ozone data since its launch in November 1978. Ozone first dropped below 180 DU, the lowest amount that had ever been reliably reported previously, in October of 1983. When scientists here in Code 916 processed this data in July of 1984 (computer systems were slower back then) they noticed the unusual number of "flagged" low ozone values. The computers did not, however, automatically reject these data!

The question the scientists had to ask, though, was: are these low readings real, or is there some subtle, unforeseen problem with the instrument? The TOMS instrument produces values of the total ozone in a column of air (from ground to space) by measuring certain frequencies of sunlight scattered back into space by the earth's atmosphere. In the springtime over Antarctica, the sun is very low in the sky, and people were not sure how that might affect the readings. But more importantly, data from the South Pole station said that ozone was 300 when TOMS was measuring 180. Several months were spent analyzing the data before we convinced ourselves that our data were correct and the South Pole data were wrong.

Meanwhile, down at Halley Bay in Antarctica, Dr. Joe Farman and his colleagues were measuring ozone using ground-based instruments. These measurements were easier to verify, since the instruments could be hauled back into the lab for a calibration check. In 1985, Dr. Farman published his findings in a paper in Nature which showed large annual decreases in total ozone around October. (see Farman et al., Nature, Vol. 315, pp. 207-210, May 1985.). This was followed by our paper on the TOMS observations, which showed that the phenomenon was truly a "hole" covering almost all of Antarctica, and not something local to Halley Bay. (see Stolarski et al., Nature, Vol. 322, pp. 808-811, August 1986.)