The Nimbus 6 Electrically Scanning Microwave Radiometer: data rescue

The single-frequency, dual-polarization 37.0 GHz Nimbus 6 Electrically Scanning Microwave Radiometer (N6ESMR) was launched on 12 June 1975 onboard the NASA Nimbus 6 satellite. Besides ESMR on Nimbus 5 (N5ESMR), which carried a single-polarization 19 GHz frequency, it is the only other resource for high-resolution microwave data in the mid to late 1970s. The original digital data have been lost and only the image format used for visual inspection of the brightness temperatures () remains, saved on 70-mm photofacsimile film strips as both positives and negatives. These have recently been made available by NASA online as scanned TIF images. Here, a process has been developed and evaluated to transform N6ESMR
TIF images into fully digitized NetCDF files. The resulting files include header data as well as time and geolocation information derived from the images.

The uncertainty of the derived Tb data is found to be highly dependent on the value itself. Visual inspection indicates that the data are noisy compared to the digitally stored N5ESMR data. However, uncertainties have been assigned to each data point and each processing step has been discussed in detail. To demonstrate the value of the data, we show an example of sea ice concentrations derived for the first time from the Nimbus 6 ESMR.