Comments on locations and methods

Uccle (Belgium)	50°48' North 4°21' East altitude = 120m a.s.l.	Brewer spectrophotometer automatic operation models: MKII and MKIII	zenith measurements or direct sun measurements
Diekirch (Luxembourg)	49°52' North 6°10' East, altitude = 218m a.s.l.	Microtops II sunphotometer manual operation	only direct sun measurements

The Microtops measurements are manual measurements, done by the operator when weather conditions allow, e.g. when bright sun-shine is available. The instrument is always directed to the sun; the readings correspond to the 'best' mean computed from 40 fast measurements done during approx. 10 seconds; usually 3 readings are made, and the mean is recorded.
Several other parameters are simultaneously measured: barometric pressure, precipitable water column, aerosol optical thickness. Location, altitude and time are programmed into the device according to the measurement place.

Both instruments are located on satellites moving on polar orbits. Toms passes over Uccle every day between approx. 09:30 and 11:05 UTC; there is one recording per day. Toms data file for the Uccle location can be found at ftp://jwocky.gsfc.nasa.gov/pub/eptoms/data/overpass/OVP053_ept.txt. It is assumed, that the total ozone column over Uccle is comparable and close to that over Diekirch. Nasa warns that the Toms data should not be used for long term trend analysis, due to instrument degradation.

The newer Sciamachy sensor on the Envisat satellite (sun synchronous polar orbit, repeat cycle = 35 days, altitude = 800km) makes one downward measurement every 5 seconds, covering a surface whose corner and center coordinates are known. The public available Sciamachy data can be found at http://www.temis.nl/protocols/o3col/o3month2005.html . The monthly files are very big 80 MB; global ; the author has written a Perl script to extract the data corresponding to a rectangle whose center coordinates are close to Diekirch (between [48 - 52] North latitude and [5.8 - 6.2] East longitude); the mean of these (usually 1 to 4 per pass) data is kept for the comparison. The satellite passes through this location between 09:30 and 10:30 for approx. 5 to 12 days a month; so the datafile with the Sciamachy readings is very spotty compared to the Toms file.

A daily map of total ozone column over Europe measured by NOAA satellite no.14 can be found here.