Nimbus satellite programme

A test bed for the sensors

NASA's Nimbus series, from Nimbus 1 on 28 August 1964 to Nimbus 7 on 24 October 1978, flew in near-polar, Sun-synchronous orbits that cross the same latitude at the same local solar time, a design that makes images from different days comparable. Nimbus 2 imaged the polar ice caps in 1966, and Nimbus 3 in 1969 retrieved atmospheric temperature profiles and flew the first SNAP-19 nuclear generator. Nimbus 4 carried an ultraviolet instrument that was a pioneer of satellite ozone measurement.

Each satellite carried a set of experimental instruments. The best of them were later copied into operational satellites.

Nimbus 7 and its long records

Nimbus 7 carried instruments including the Total Ozone Mapping Spectrometer (TOMS), the Solar Backscatter Ultraviolet instrument, the Coastal Zone Color Scanner, the first ocean colour sensor in orbit, and the Scanning Multichannel Microwave Radiometer, which gave the first continuous passive-microwave record of sea ice from 1978. It operated for sixteen years and was turned off in 1994.

Its ozone record is the source of a well-known cautionary tale. The data-processing software flagged very low ozone values over Antarctica as probable errors, so the satellite team did not report the depletion that ground observers at Halley had found in 1985. The data were reprocessed and confirmed the ozone hole.

Legacy

The sea-ice record from the passive-microwave radiometers, continued by later satellites, is the basis of the satellite record of the shrinking Arctic sea ice. The ozone instruments continued in the NOAA and NASA satellite fleets, and ocean-colour imaging returned with SeaWiFS and its successors. Nimbus established the idea that a mission is a source of a time series, with calibration to match.

A test bed with a cost

Nimbus satellites were research missions, expensive and complex, and the programme was criticised at times for cost and delay. Its legacy is the range of techniques it proved: sounding temperature profiles, mapping ozone, measuring ocean colour and mapping sea ice through cloud. Some of these continued without a break into operational use and some, such as ocean colour, had to wait many years for a successor. The gap in ocean-colour measurement between 1986 and 1997 is a reminder that a research mission does not guarantee continuity, and that a satellite record needs a plan for the long term.

Instruments and technologies

• Total Ozone Mapping Spectrometer A scanning spectrometer measuring backscattered ultraviolet light at several wavelengths to infer the ozone column over the whole globe each day.
• Coastal Zone Color Scanner A scanning radiometer observing colour in six bands to estimate chlorophyll in the surface ocean.
• Scanning Multichannel Microwave Radiometer A passive microwave radiometer receiving thermal emission from sea ice and ocean, which works through cloud and in the polar night.

Timeline

• 1964 Nimbus 1 launched on 28 August.
• 1966 Nimbus 2 images the polar ice caps.
• 1969 Nimbus 3 retrieves atmospheric temperature profiles and flies the first SNAP-19 generator.
• 1978 Nimbus 7 launched on 24 October.
• 1994 Nimbus 7 turned off after 16 years.

Key lessons for observation today

• Flag, never delete Ozone values were discarded as errors when they were real. Quality control must flag suspicious data and keep it for review.
• Design the orbit for comparability A Sun-synchronous orbit makes each pass at the same local time. Observing geometry is a design variable for any time series.
• A test bed feeds the operational fleet Instruments proved on Nimbus entered operational use. Experimental and operational systems need a deliberate path between them.

Sources

• NASA Science, The Legacy of Nimbus
• NASA NSSDCA, Nimbus 1
• NASA Science, Nimbus-4: A Satellite Pioneer For Ozone Research
• Wikipedia, Nimbus program