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Page 2© Crown copyright 2004 Evaluation of SSU performance John Nash

Page 3© Crown copyright 2004 Factors affecting compatibility of measurements from different SSU  Radiometric Stability  Leaks of pressure modulated cells  Changes in local time of observation [instability in orbits ]

Page 4© Crown copyright 2004 Radiometric Stability  Self calibrating radiometer, observing in 8 positions at 5,15,25 and 35 ° from nadir to each side of the spacecraft.  Scan cycle repeated every 256 s with a space view and internal black body view lasting 16s each  Sensing circuit for platinum resistance thermometer on black body noisy so thermistors were used to define black body temperature  Switching to the reference resistance for the blackbody platinum sensor changed the electrical offset in the signal channel output, so space views needed a special correction (determined in ground testing but also through inhibiting the scan mirror in orbit).  This offset seemed stable during ground testing and in flight, apart from the instrument on NOAA-9 where some temperature dependence was noted during testing.

Page 5© Crown copyright 2004 Radiometric precision of SSU functioning correctly  Extensive Laboratory testing in vacuum chamber with calibrated black body targets showed  Accuracy of self-calibration ± 0.1 R.U.  Linearity of signal channel ± 0.05 R.U.  Monitoring calibration coefficients in flight  calibration coefficients were monitored in terms of channel sensitivity and channel offset. An electrical offset gave blackbody view about 600 and space view about 3000 counts.  The detector gain and the PMC pressure/ drive amplitude were the main factors affecting the sensitivity, once assembled  A temperature cycle within the PMC contributed to the offset, and the magnitude of this depended on the cell fill pressure.The offset was particularly sensitive to pressure changes in ch 27 and to a lesser extent in ch 26.

Page 6© Crown copyright 2004 Radiometric precision of SSU functioning correctly  Monitoring calibration coefficients in operation  showed that global zonally averaged radiances could be repeated with very high precision from day to day  Short term changes in calibration unrelated to orbital temperature cycles Ch. 25 and 26 ± 0.05 R.U. Ch 27 ± 0.15 R.U.  Temperatures change around orbit associated with variation in backscattering of solar energy from cloud. PMC Temperatures lower ( as deduced from operating frequency) when external instrument temperature highest  Overall radiometric precision better or equal to 0.2 R.U., as long as space view offset stable.

Page 7© Crown copyright 2004 Spectroscopic performance of SSU determined primarily by gas pressure in PMC’s  When filled to correct pressure of carbon dioxide gas, gas cells were best modelled by weighting functions peaking at around 15.4, 5.9 and 1.9 hPa  The sealing of the PMC’s proved problematical during manufacture, especially the ceramic to metal junction on the electrical lead- through to the PMC piston drive. This was resolved by covering the leaky areas with epoxy resin.  The resonant frequency of the piston in the PMC depended on the cell pressure. This was found to be increasing with time during storage in air before launch  It was then realised that water vapour could pass through the epoxy resin and hence into the PMC through the leak much more readily than nitrogen or oxygen.

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Page 9© Crown copyright 2004  When leak gas was present in the PMC, the gas pressure was found to increase much more rapidly as the PMC warmed up to its thermostat temperature than would be expected by the perfect gas law. This was interpreted as water vapour outgassing from the PMC magnets as the system warmed up.  On the basis of the anomalous rate of increase in pressure with temperature, it was concluded that most PMC leaks were dominated by water vapour, with relatively small amounts of nitrogen and oxygen.  When PMC’s with this type of leak were launched the water vapour usually migrated out of the PMC within 2 years. SSU’s with relatively long operational life were usually stable in spectroscopic performance in later years. A limited number of warm up tests in space showed a return towards the perfect gas law temperature dependence in the PMC  However, on NOAA-7 the PMC in channel 26 developed a leak after launch with carbon dioxide and the extra gas leaving the cell.

Page 10© Crown copyright 2004 A leak of water vapour into the PMC would have the effect of raising the weighting function by about 0.4 of that expected for a pressure leak of carbon dioxide

Page 11© Crown copyright 2004 Impact of gas leaks on observed radiances (1)  Laboratory test were performed to check whether the SSU filters allowed a water vapour signal to be detected. There was no discernible difference in SSU radiance between measurements through a White cell full of water vapour and vacuum, with the main SSU signal blocked out by suitable filters.  Thus, it is expected that changes in SSU radiance measurements caused by the gas leaks were dominated by the effects the leak had on the weighting function of the instrument.  Modelling the effects of change in weighting function using the available rocketsonde climatology did not provide a sensible explanation of the differences between zonally averaged radiances measured by channel 26 and 27 of NOAA-6 and NOAA-7

Page 12© Crown copyright 2004 Impact of gas leaks on observed radiances (2)  Hence, sensitivity to changes in weighting function in the vertical was taken using differences between zonally averaged radiances from off nadir views (35°) and near nadir views (5°)  For much of the world this was:- for ch 25 and 26 about 2 R.U. apart from polar regions where values were often lower in winter  for channel 27, the values were nearer 1 R.U. over much of the globe apart from high latitudes, with sensitivity dropping rapidly if the gas pressure in the cells fell (making the weighting function peak moved closer to the stratopause level)  Consequently, the sensitivity of zonal radiance change to a change of 1 hPa in PMC cell pressure was about :  ch r.u. : ch r.u most latitudes  ch r.u polar winter ch r.u high latitudes

Page 13© Crown copyright 2004 Impact of gas leaks on observed radiances (2)  Hence, sensitivity to changes in weighting function in the vertical was taken using differences between zonally averaged radiances from off nadir views (35°) and near nadir views (5°)  For much of the world this was:- for ch 25 and 26 about 2 R.U. apart from polar regions where values were often lower in winter  for channel 27, the values were nearer 1 R.U. over much of the globe apart from high latitudes, with sensitivity dropping rapidly if the gas pressure in the cells fell (making the weighting function peak moved closer to the stratopause level)  Consequently, the sensitivity of zonal radiance change to a change of 1 hPa in PMC cell pressure was about :  ch r.u. : ch r.u most latitudes  ch r.u polar winter ch r.u high latitudes

Page 14© Crown copyright 2004 Example from NOAA-7  PMC frequency change at the end of 1984 for ch 27 was Hz from a frequency after launch of HZ and the associated radiance change was around 1 r.u. in mid-latitudes  Frequency change for NOAA-6 ch 27 was -0.2 Hz from Oct 79 until Nov 86  Frequency change for NOAA-9 ch 27 was Hz from Jul 85 to Jun 88  PMC frequency change at the middle of 1984 for ch 26 was Hz given a frequency at launch of HZ and the associated radiance change was around 6 r.u. in midlatitudes. This leak was different to most other s, and had a significant carbon dioxide content  NOAA-6 channel 26 was stable throughout operations  NOAA-9 channel 26 changed by Hz from Jul 85 to Jun 88

Page 15© Crown copyright 2004 Adjustments to compensate PMC frequency pressure change  The pressure leaks appeared to have small effects on channel 25 and no compensation has been applied. Changes may have been as large as 0.2 r.u. at most, but could not be resolved from changes that may have been caused by drift in satellite observation time. NOAA-9 ch.25 observations were high by at least 0.6 r.u..  Adjustments to channel 26 were usually less than 1 r.u. for most spacecraft, except NOAA-7 and NOAA-9 where observations were too high by about 1 r.u.  Adjustments to channel 27 were usually less than 1 r.u. in mid latitudes

Page 16© Crown copyright 2004 Local time of day [h] Example of temperature tide observed by channel 27

Page 17© Crown copyright 2004 Example of temperature tide observed by channel 27

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Page 19© Crown copyright 2004 Zonally averaged data  In the Nash archive, ascending and descending orbits were always averaged together to minimise the affect of solar tides. Fragments from ascending or descending orbits could be more than a degree different from the average.  Averages were taken over 30 day periods to minimise fluctuations associated with lunar semidiurnal tides.

Page 20© Crown copyright 2004 Variation of differences between spacecraft with time, SSU Channel 25

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Page 23© Crown copyright 2004 Instability in time of observation  The overpass time of satellites such as NOAA-7, NOAA-9, NOAA-11 changed significantly with time, [several hours over the lifetime of the spacecraft]  The sampling of the semidiurnal tide changed significantly, and in the Nash archive an attempt was made to compensate the changes to some extent.  The problem was largest in the upper stratosphere where the semidiurnal tide was largest.

Page 24© Crown copyright 2004 Variation in zonally averaged brightness temperatures with height  The channels synthesised from the basic SSU channels using the radiance difference between near nadir and off-nadir indicate characteristic annual variation for brightness temperatures centred at different heights in the stratosphere.

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Page 27© Crown copyright hPa 90 hPa

Page 28© Crown copyright hPa6 hPa 1.5 hPa0.5 hPa