DATA STABILITY AND CONTINUITY FOR 
GLOBAL ENVIRONMENTAL MONITORING: 
SOME BOUQUETS AND BRICKBATS 
F.J. Ahern 
Canada Centre for Remote Sensing 
ABSTRACT 
Absolute radiometric calibration, atmospheric correction, and georadiometric corrections are all essential for 
long term global monitoring, in order to enable researchers to distinguish real changes from artifacts caused 
by sensor drifts and mis-calibration, atmospheric changes, and changes caused by differences in illumination 
and viewing geometry. In addition, long-term data sets are required for global change studies. 
Much progress has been made in optical sensor calibration since the first Landsat MSS, which began its 
remarkable record of nearly continuous earth observation in 1972. The designers of the MSS, especially Jack 
Lansing and Virginia Norwood of the Hughes Santa Barbara Research Center, recognized the need for 
absolute radiometric calibration, and devised a method using an on-board calibration lamp as a reference. 
John Barker has done the world a tremendous service through the effort he has made in MSS and TM absolute 
calibration. He has set a high standard for all other optical sensors to try to match. Also, his enthusiasm has 
been infectious, and he has undoubtedly convinced many key people of the importance of accurate, precise 
radiometric calibration of earth-observation sensors. 
LANDSAT represents the best long-term global data source. It is reasonably well calibrated, data go back to 
1972, and the data flow been essentially continuous. Considering its value, it has been under-appreciated, by 
its parents, NASA, and even more by its step-parents, the National Oceanographic and Atmospheric 
Administration (NOAA). The wavering commitment by the US to a long term LANDSAT program has 
aroused worldwide concern. While many global environmental monitoring objectives could be met with serious 
commitment to continuity in the LANDSAT and NOAA series, this fact often seems lost in the glare of 
publicity for the massive, complex Eos. 
Data from the NOAA Advanced Very High Resolution Radiometer (AVHRR) sensor provide data which 
nicely complement Landsat MSS and TM data for global change studies. Jim Tucker of the NASA Goddard 
Space Flight Center deserves credit for recognizing, demonstrating, and supporting the use of AVHRR data 
for global environmental monitoring. NOAA satellites each have frequent temporal coverage (twice per day) 
of the entire Earth, at the expense of spatial resolution (1km). However, much more work needs to be done 
on AVHRR calibration to provide the confidence necessary for global environmental monitoring. One serious 
impediment to AVHRR calibration has been NOAA’s policy of stockpiling sensors years before they are 
needed. This has resulted in poor calibration and obsolescence. In addition, only recently have NOAA 
scientists, engineers, and management appreciated the calibration problem and how it affects long term global 
change studies. 
The commitment to both data continuity and calibration by the SPOT program has been very reassuring, but 
SPOT data are very expensive and the areal coverage is too limited for global change studies except through 
a sampling approach. 
Nonetheless, the present situation is far from gloomy. LANDSAT 5 is still working, LANDSAT 6 is under 
construction, and the US is beginning to make serious plans for LANDSAT 7. The continuity of the SPOT 
and NOAA series seems assured, and the Japanese are beginning to launch and maintain high-resolution 
optical-band satellites. 
Organizations proposing SAR satellites are taking calibration seriously. Although SAR satellites are twenty 
to twenty five years behind their optical counterparts in the development of a continuous earth-observation 
136