1

Differences between SSU channels

John Nash

Manager

Upper Air Technology centre , Met Office

2

Harmonising SSU measurements from different spacecraft(1)

SSU radiometers were checked very carefully in the laboratory oseveral time before launch. Each field of view was checked individually in laboratory calibration tests and a radiometric correction derived.

The radiometric accuracy should be better than 0.2 r.u., given the software correction of the space view counts is applied correctly. [J. Nash and J.L.Brownscombe, Adv. Space Res., Vol.2, No.6,pp59-62,1983].

Laboratory measurements of space view corrections over several years for a given instrument showed a variation in value of about 3 counts, corresponding to about 0.2 r.u. The resultant radiometric uncertainty in the earth views would normally be closer to 0.1 r.u. [note: 1 r.u. equivalent to about 0.1K in brightness temperature in channel25 and down to 0.07K when earth view radiance is high [.100 r.u.] in channel 27]

3

Harmonising SSU measurements from different spacecraft(2)

The spectroscopic performance of the SSU was mainly determined by CO2 gas in the pressure modulated cells (PMC’s) . D.R.Pick and J.L.Brownscombe, Adv. Space Res. Vol.1,pp. 247-260,1981.

The nominal fill pressures for carbon dioxide in the PMCs were , channel 25 106 hPa:

channel 26 37.1 hPachannel 27 11 hPa

Some air (mostly water vapour) leaked into the PMC’s during ground storage and modified the spectroscopic performance. Early SSUs were launched with additional PMC pressure between 4 hPa for NOAA-6 and 9 hPa for TIROS-N. Leak rates during storage in air were found to be close to 2 hPa/year.

4

Harmonising SSU measurements from different spacecraft(3)

After 1980, SSU storage was in dry nitrogen. Subsequently PMC frequencies were stable in storage, confirming that water vapour was the main origin of the problem.

The water vapour gas leaked out in space. Immediately after launch pressures increased slightly [probably as some water vapour outgassed from the internal fittings of the PMC], but then decreased rapidly. This leakage out of the PMC usually decreased with time , as indicated by the frequencies of the PMC systems. Changes in spectroscopic performance after a year in space were then in most cases small [see next slide].

Weighting functions of PMCs containing leak gas, as measured in the laboratory, showed a shift in weighting function for channel 26 and 27 of just under half that expected from the same pressure increase of carbon dioxide.

5

Examples of PMC frequency changes with time from early NOAA spacecraft,SSU P/F on TIROS-N, SSU F2 on NOAA-6 and SSU F3 on NOAA-7

NOAA-7 ch 26

NOAA-7 ch 27

NOAA-7 ch 25

6

TOVS satellites are in sun synchronous orbits. At a given latitude each satellite observes at only two local times during the day Solar diurnal and semidiurnal tides in the stratosphere are significant.Thus, the average zonal average radiance for a given latitude band depends on the local time of the sample. [Brownscombe,J.L., Nash,J., Vaughan,G. and Rogers,C.F., Q.J.R.Meteorol.Soc., 111,677-689,1985]

Thus, even if the SSUs on two NOAA spacecraft were perfectly matched the zonal averages for a given latitude band would not be identical because of the temperature tides. The two need to be adjusted for the tides to obtain equivalent results. The errors in the adjustment procedure will be proportional to the differences in the tidal fields from year to year. The variation from year to year is likely to be largest in the upper channels.

Harmonising SSU measurements from different spacecraft(4)

7

Synthesised SSU channels

The range of available SSU radiance observations was extended by using the measured radiance difference between near nadir and 35° off-nadir, see J. Nash , Q.J. R. Meteorol. Soc.(1988),114,pp 1153-1171.

The field of view difference measured by channel 25, could be subtracted from channel 26 radiance to give a channel 15X centred near 50 hPa, and also added to HIRS2 channel 2 radiance to give a channel at about 6 hPa , designated 35X.

As channel 25 was relatively stable compared to channel 26, channel 35X provided a reference against which the spectroscopic stability of channel 26 could be checked.

8

Synthesised SSU channels

The field of view differences measured by channel26, were subtracted from channel 27 radiance to give a channel 26X centred near 20 hPa, and added to SSU channel 25 radiance to give a channel at about 2 hPa , designated 36X.

As the field of view differences in channel 26 were relatively stable for normal contamination leaks 26, channel 36X provided a reference against which the spectroscopic stability of channel 27 could be checked.

Field of view differences measured by channel 27 were added to SSU channel 26 radiance to give a channel 47X in the lower mesosphere.Field of view differences in channel 27 were very sensitive to its spectroscopic performance, and it is not possible to compensate the differences between spacecraft in channel 47X to the same accuracy as in the lower channels.

9

If the times of observation gradually drift from year to year, as was the case on most NOAA spacecraft, then the sampling of the tides changes. This introduces changes in average radiances, even if the average state of the atmosphere were unchanged.

The SSUs observe emission from carbon dioxide so the long term increase in atmospheric carbon dioxide with time also changes the spectroscopic sampling of the atmosphere. The SSU’s will observe slightly higher in the atmosphere as the carbon dioxide increases. This produces a small increase in radiance in most SSU channels apart from SSU47X where the radiance would decrease.A 2 per cent increase in carbon dioxide would increase the measured radiance by about 0.2 r.u. in channels 25 and 26 and much smaller than this in channel27[Barnett, personal communication]. Changes in ozone may cause a compensating change in radiance.

Other influences on trends in SSU radiances

10

How were the SSU measurements adjusted to be consistent in spectroscopic sampling?

Simultaneous SSU measurements from the early NOAA spacecraft were compared , see the following slides, and decisions made on the magnitude of radiometric errors, spectroscopic differences and tidal differences. The stability of the PMC frequencies was used to check for the magnitude of possible gas leaks from the PMC’s

The spectroscopic corrections applied to channel 26 and 27 were checked by comparison of global radiance trends with those of the synthesised channels 35X and 36X for all spacecraft

In addition the effects of the corrections on channel 27 were checked by showing that channel 26 X had similar global trends to SSU channel 25.

Finally it was also checked that SSU 15 X had similar long term trends to MSU channel 4.

11

Comments on following comparison plots In the following plots data sets are identified by the month

at the centre of the data sample. Here month means a 30-day period, with the last five days of the year orphaned.

Usually the values plotted are averages over several months.

At all levels apart from SSU 47X the SSU radiance observed in the atmosphere would normally increase with a decrease in PMC pressure (PMC frequency).

The influence of the temperature tides is similar between May and August, and also between November and January, with the diurnal tidal structure more symmetric about the equator in months near the equinoxes. This can be seen in the “day-night” differences for NOAA-6 channel 27 in the next slide.

12

Differences shown are mainly caused by solar diurnal temperature tides

-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 from SSU channel 27 from local times 19.00-07.00 [approx], NOAA-6/8 1980-1983

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

13

SSU channel 25 (channel 1 or channel15?) centred at about 15 hPa

Differences between zonal averages of SSU channel 25 radiance,simultaneous measurements from different spacecraft

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

-70 -50 -30 -10 10 30 50 70

Latitude [degrees]

Ra

dia

nc

e d

iffe

ren

ce

[r.

u.]

TN-N6

N7-N6,02/1982

N7-N6,07/1982

N7-N6,11/1982

N7-N8,08/1983

N7-N8,02/1984

N9-N6,1985/86

average ofnormal

14

Conclusions about channel 25 All the SSUs from NOAA-6 to NOAA-8 were judged to match

with a radiometric accuracy of ± 0.1 r.u. The larger average differences found between the SSUs are consistent with the differences expected from the semidiurnal solar temperature tide. Large discrepancies at high latitude were the result of inadequate data coverage in some months.

Channel 25 on NOAA-9 clearly did not measure the same radiances as the other spacecraft. The radiances appear too large by between 0.6 and 0.7 r.u.. This is most likely to be radiometric error, caused by an error (wrong sign?) in correcting the space-view offset problem. There were no large anomalies in the gas pressure within NOAA-9 relative to the other spacecraft, when checked in laboratory transmission measurements, so a spectroscopic error is unlikely.

15

Conclusions about channel 25(2) Several channel 25 PMC’s have had significant PMC

frequency changes, both before and after launch, but the changes in gas pressure do not seem to have changed the measured radiance by more than 0.1 to 0.2 r.u.

SSU channel 25s on NOAA-11 and NOAA-14 are judged to be similar to NOAA-6 to NOAA-8

The results for TIROS-N shown here may not correspond exactly to those in the NOAA-data archive, since there was some confusion in the application of correct corrections for radiometric field of view errors and also for the first three or four months of NOAA-6 operations.

16

SSU channel 26 (channel 2 or channel16)Differences between zonal averages of SSU channel 26 radiance,

simultaneous measurements from different spacecraft plus the spacecraft differences* assumed for NOAA-11 and NOAA-14

-2

-1

0

1

2

3

4

-70 -50 -30 -10 10 30 50 70

Latitude [degrees]

rad

ian

ce

dif

fere

nc

e [

r.u

.]

TN-N6(1979/80)

N8-N6(1984)

N7 -N6(09/1981)

N7 -N6(03/1983)

N9-N6(1985/86)

N11-N6(02/1989)

N11-N6(02/1992)*

N14-N6(01/1996 )*

N14-N6(01/1998)*

average differencebetween orbits

N7 PMC frequency change of -1.3 Hz, CO2+ water vapour /air leaking continued

N9 PMC frequency change of -0.5 Hz, water vapour /air leak nearly stabilised +radiometric error

17

Conclusions about channel 26 The radiometric accuracy should be ± 0.1 r.u., as for channel

25, apart from channel 26 on NOAA-9. The radiances of NOAA-9 were probably too large by around 1 r.u., since the PMC fill pressure should have been similar to the earlier SSUs. The origin of this radiometric error was probably associated with the origin of the radiometric error in NOAA-9 channel 25.

Channel 26 on NOAA-7 was much more unstable than any other SSU 26. The spectroscopic performance changed by about +3.0 r.u. in 18 months, i .e. the SSU weighting function moved up by about one quarter of the usual separation between SSU channel 26 and channel 27. The black body view went off scale by mid-1983, so subsequent calibrations were not valid. The observation height increased so significant amounts of the weighting function were above the stratopause and the field of view differences became smaller than usual.

18

Conclusions about channel 26(2) It is judged that NOAA-7 was leaking both carbon dioxide

and air from a leak which was much larger after launch than before launch.The ratio of change of mean spectroscopic offset for a given PMC frequency change (-2.5 r.u./Hz) was high compared to the other SSUs leaks (about -1 r.u ./Hz)

Typical spectroscopic+radiometric offsets relative to NOAA-6 are estimated to be TIROS-N 0

NOAA-8 -0.5 NOAA-9 0.9

NOAA-11 -0.2 NOAA-14 -0.9 The increase in spectroscopic offset of these SSU’s during a spacecraft lifetime was in the range 0 to +0.7 r.u., as could be estimated from PMC frequency change

19

SSU channel 27 (channel 3 or channel17)

Differences between zonal averages of SSU channel 27 radiance, simultaneous measurements from different spacecraft

plus the spacecraft differences assumed for NOAA-11 and NOAA-14*

-4

-3

-2

-1

0

1

2

3

4

5

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

Latitude [degrees]

rad

ian

ce

dif

fere

nc

e [

r.u

.]

N7-N6,01/1982

N7-N6,07/1982

N7-N6,12/1982

N7-N6,07/1983

N7-N8,08/1983

N7-N8,04/1984

N7-N6,11/1984

N9-N6,07/1985

N9-N6,12/1985

N9-N6,07/1986

average,well matched?

n11-n6,2/1990*

n11-n6,2/1995*

n14-n6,8/1995*

n14-n6,10/1997*

20

Conclusions about channel 27 Spectroscopic and tidal differences dominate in this channel. TIROS-N was extremely noisy. Asymmetry in the response of

the signal channel to very high levels of noise produced large radiometric errors in the measurements of TIROS-N.

Channel 27 on NOAA-7 was known to have the largest leak during storage on the ground. The PMC frequency fell by about 0.9 Hz in space from 1991 until 1995. Gas leaked out very rapidly for a couple of months after launch and then slowed down to a relative trickle, see earlier slide .

Some carbon dioxide may have leaked out of the NOAA-7 PMC by the time the performance stabilised in space and it was observing much higher than the other SSU channel 27s. Observations were centred on or just above the stratopause. Channel 27 field of view radiance differences were smaller than those of the other SSUs.

21

Conclusions about channel 27(2) NOAA-9 was observing lower in the atmosphere than the

majority of channel 27s. NOAA-9 channel 27 field of view difference was larger than the other SSUs. The radiance differences relative to NOAA-6 for NOAA-9 and NOAA-7 are much larger in polar regions where vertical temperature structure around the stratopause differs from mid-latitudes and tropics in some seasons.

Typical spectroscopic + radiometric offsets in mid-latitudes and tropics relative to NOAA-6 are estimated to be

NOAA-7 +1.0 NOAA-8 -0.1NOAA-9 -0.9NOAA-11 -0.3NOAA-14 -0.1 but

with the adjustments at high latitudes much higher than this for some seasons.

22

SSU channel 36X (computed from channel 25 and the field of view differences of channel 26)

Differences between zonal averages of SSU radiance measurements from different spacecraft in simultaneous operation, SSU channel 36X

-3.00

-2.00

-1.00

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

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

Latitude [degrees]

rad

ian

ce

dif

fere

nc

e [

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

TN-N6,07/1980

TN-N6,02/1980

average, 07

average, 02

average,wellmatched?

23

Comments of channel 36X

If a synthesised channel is to agree within 2 r.u. with that of another spacecraft then the field of view differences of the two channel 26 must match to better than 0.1 r.u.. The measurements do match this well for most spacecraft apart from NOAA-9. This again suggests that the radiometric corrections applied to NOAA-9 may have

been in error. NOAA-7 channel 36X gradually drifts negative to NOAA-6

in midlaritudes and tropics , as the channel 26 weighting function moves upward in late 1982 to levels where the field of view differences start to drop as the weighting function moves upwards. Other differences between spacecraft do not appear to be related to spectroscopic differences and can be adjusted by a radiance offset.

24

SSU channel 47X (computed from channel 26 and the field of view differences of channel 27)

Radiance differences between zonally averaged SSU radiance measurements from different spacecraft, channel 47X

-12.00

-10.00

-8.00

-6.00

-4.00

-2.00

0.00

2.00

4.00

6.00

8.00

10.00

12.00

-70

-50

-30

-10 10 30 50 70

Latitude [degrees]

Ra

dia

nc

e d

iffe

ren

ce

[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

average,07

average,10

average, wellmatched??

25

Comments on channel 47X NOAA-9 channel 26 was similar between summer 1985 and

summer 1986 , so most of the change shown in channel 47X ( a reduction in spectroscopic offset of about 1.8.±0.2 r.u) is considered to be the result of a drop in PMC pressure with a PMC frequency decrease of 0.3 Hz. The variation of differences with latitude were used to estimate corrections for spectroscopic drift in later SSU channel 47X.

NOAA-7 channel 47Xradiance was much lower , but was relatively stable from early 1982 until early 1983. During this time NOAA-7 channel 26 spectroscopic offset increased by about 2 r.u. , but was compensated by a decrease in the field of view differences in channel 27 introduced by the pressure drop with a PMC frequency change of - 0.23 Hz.

Hence, SSU 47X appears to have a spectroscopic offset sensitivity of about 7 ± 1 r.u./Hz for a normal water vapour leak in space.

26

Distribution with height of temperature differences

between NOAA-7/9 orbits and NOAA-6/8 orbits -resulting mainly from semidiurnal tides

Estimated differences between well matched SSU's observing in orbits similar to NOAA-7/NOAA-9 and NOAA-6

-2

-1

0

1

2

3

4

5

6

7

8

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

Latitude [degrees]

Ra

dia

nc

e d

iffe

ren

ce

(r.

u.)

off

se

t w

ith

c

ha

nn

el h

eig

ht

53km ,SSU47X+6

44km ,SSU36X+4

44km ,SSU27+4

34km ,SSU35X+2

34 km ,SSU26+2

29km ,SSU25

27km ,SSU27X