Evaluation of SSU performance John Nash


Evaluation of SSU performance

John Nash


Factors affecting compatibility of measurements from different SSU

Radiometric Stability Leaks of pressure modulated cellsChanges in local time of observation [instability in orbits ]


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.


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.


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.


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.


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.


A leak of water vapour into the PMC

would have the effect of raising the weighting

function by about0.4 of that expected for a pressure leak

of carbon dioxide


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


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 25 0.1 r.u. : ch 27 0.3 r.u most latitudes ch 25 0.03 r.u polar winter ch2 7 0.6 r.u high latitudes


Example from NOAA-7

PMC frequency change at the end of 1984 for ch 27 was -0.83 Hz from a frequency after launch of 35.65 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 -0.45 Hz from Jul 85 to Jun 88

PMC frequency change at the middle of 1984 for ch 26 was -1.6 Hz given a frequency at launch of 40.49 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 -0.55 Hz from Jul 85 to Jun 88


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


Local time of day [h]

Example of temperature tide observed by channel 27


Example of temperature tide observed by channel 27


-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70

1

2

3

4

5

6

7

8

9

10

11

12

Latitude [degrees]

Mo

nth

Difference between zonally averaged radiances, channel 27 from local times 19.00-07.00 [approx], NOAA-6 1980-1982

3.5-3.73.3-3.53.1-3.32.9-3.12.7-2.92.5-2.72.3-2.52.1-2.31.9-2.11.7-1.91.5-1.71.3-1.51.1-1.30.9-1.10.7-0.90.5-0.70.3-0.50.1-0.3-0.1-0.1


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.


Variation of differences between spacecraft with time, SSU Channel 25

-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60

15

45

75

105

135

165

195

225

255

285

315

345

Latitude [degrees]

Day o

f the y

ear

Difference between zonal mean radiance as a function of time (TIROS-N/NOAA-7) - NOAA-6 [mw/(m2.sr.cm-1]

0.5-0.55

0.45-0.5

0.4-0.45

0.35-0.4

0.3-0.35

0.25-0.3

0.2-0.25

0.15-0.2

0.1-0.15

0.05-0.1

0-0.05

-0.05-0

-0.1--0.05

-0.15--0.1

-0.2--0.15


Difference NOAA-14-NOAA-11 1997/8

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

-80 -60 -40 -20 0 20 40 60 80

Latitude [degrees]

Te

mp

eratu

re d

iffe

ren

ce [K

]

ch25 ch26

ch27


Radiance differences between [TIROS-N or NOAA-7] and NOAA-6ascending plus descending orbits caused mainly by semidiurnal tides,

Channel 27

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

-50 -40 -30 -20 -10 0 10 20 30 40 50

S Latitude [degrees] N

Rad

ian

ce d

iffe

ren

ce [

R.U

.]djf mam jja son


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.


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.


Radiance differences between zonally averaged SSU radiance measurements from different spacecraft, channel 26X [3]

-5.00

-4.00

-3.00

-2.00

-1.00

0.00

1.00

2.00

3.00

4.00

5.00

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70

Latitude [degrees]

Rad

ianc

e di

ffere

nce

[r.u

.]

N7-N6,02/1982

N7-N6,07/1982

N7-N6,10/1982

N7-N6,02/1983

N9-N6,07/1985

N9-N6,07/1986

N9-N6,10/1986

average,07

average,10

average, well matched??


Radiance differences between zonally averaged SSU radiance measurements from different spacecraft, channel 35X [3]

-5.00

-4.00

-3.00

-2.00

-1.00

0.00

1.00

2.00

3.00

4.00

5.00

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70

Latitude [degrees]

Rad

ianc

e di

ffere

nce

[r.u

.]

N7-N6,02/1982

N7-N6,07/1982

N7-N6,10/1982

N9-N6,07/1985

N9-N6,07/1986

N9-N6,10/1986

average,07

average,10

average, well matched??


HIRS 2

190195200205210215

220225230235240

1 2 3 4 5 6 7 8 9 10 11 12 13

Month

Tem

per

atu

re [

K]

-70

-60

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

70

SSU 15X

190

195

200

205

210

215

220

225

230

235

240

1 2 3 4 5 6 7 8 9 10 11 12 13

Month

Tem

per

atu

re [

K]

-70

-60

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

70

MSU 4

195200205210215220

225230235240245

1 2 3 4 5 6 7 8 9 10 11 12 13

Month

Tem

per

atu

re [

K]

-70

-60

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

70

50 hPa

90 hPa


SSU 26X

195200205210215220

225230235240245

1 2 3 4 5 6 7 8 9 10 11 12 13

Month

Tem

per

atu

re [

K]

-70

-60

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

70

20 hPa HIRS\SSU 35X

215220225

230235240245250255

260265270

1 2 3 4 5 6 7 8 9 10 11 12

Month

Tem

pera

ture

[K

]

-70

-60

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

70

6 hPa

SSU 36X

235240245250255260

265270275280285

1 2 3 4 5 6 7 8 9 10 11 12 13

Month

Tem

per

atu

re [

K]

-70

-60

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

70

1.5 hPa SSU 47X

235

240

245

250

255

260

265

270

275

1 2 3 4 5 6 7 8 9 10 11 12 13

Month

Tem

per

atu

re [

K]

-70

-60

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

70

0.5 hPa

Evaluation of SSU performance John Nash

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