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Atmospheric Science
January 2010 Vol.55 No.1: 84−89 doi: 10.1007/s11434-009-0335-8

SPECIAL TOPICS:

FY-3 satellite Ultraviolet Total Ozone Unit
WANG YongMei1*, WANG YingJian1, WANG WeiHe2, ZHANG ZhongMou1,2, LÜ JianGong1, FU LiPing1, JIANG Fang1, CHEN Ji1, WANG JiHong1, GUAN FengJun1, HUANG FuXiang1 & ZHANG XingYing2
1 2

Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing 100190, China; National Satellite Meteorological Center, Beijing 100081, China

Received January 5, 2009; accepted April 2, 2009; published online November 3, 2009

FY-3 satellites are Chinese second-generation polar orbit meteorological satellite series. Ultraviolet Total Ozone Unit (TOU) is one of the main payloads on FY-3 satellite and the first instrument for daily global coverage of total ozone monitoring in China. The main purpose of TOU is to measure the Earth backscatter ultraviolet radiation for retrieving daily global map of atmospheric ozone. TOU will provide the important parameters for environmental monitoring, climate forecasting and global climate changing research. At present, the in-orbit testing of TOU has accomplished and won a consummation, and it will be delivered to National Satellite Meteorological Center for the operational phase of the project. In this paper we introduce the instrument of TOU and its measuring principle, in general. We also analyze the recent working status of the instrument, including the sensitivity, measuring precision of solar irradiance, diffuser degradation and wavelength drift. The inversion results show that TOU can provide good global ozone maps, and a comparison with the OMI total ozone product shows that their RMS deviation is about 5%. It is indicated that the satisfied global distribution of total ozone can be obtained through TOU observational data and self-developed inversion method. Ultraviolet Total Ozone Unit, total ozone, atmospheric composition, satellite remote sensing, FY-3
Citation: Wang Y M, Wang Y J, Wang W H, et al. FY-3 satellite Ultraviolet Total Ozone Unit. Chinese Sci Bull, 2010, 55: 84−89, doi: 10.1007/s11434-009-0335-8

Atmospheric ozone serves as a natural protective barrier for all life on the Earth from excessive solar UV radiance, which is very important to human survival and health, the ecosystem, etc. However, the destructive effects on atmospheric ozone due to human activities have reached a considerable level. The discovery of the Antarctic ozone hole attracted the world’s attention. Today, the ozone depletion has become a significant environmental problem of the humanity faces. Monitoring the temporal and spatial variation of atmospheric ozone has become a hot spot in environmental remote sensing. The Total Ozone Mapping Spectrometer launched aboard Nimbus 7 Satellite (N7/TOMS) in 1978 started the NASA’s long-term observations of the global distribution of total ozone. So far, N7/TOMS and its successors have obtained
*Corresponding author (email: [email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2009

nearly three decades of global total ozone data [1−5]. FY-3 satellites are Chinese second-generation polar-orbiting meteorological satellite series to offer global, all day, threedimensional, quantitative, multispectral remote sensing study. On May 27, 2008, the FY-3A satellite was successfully launched which is its first one with an 831 km circular, 98° inclination and sun-synchronous orbit [6]. Ultraviolet Total Ozone Unit (TOU) onboard FY-3A satellite is the first instrument for global coverage of total ozone monitoring in China. Its main purpose is to measure the Earth backscatter ultraviolet radiation for retrieving daily global map of total ozone. TOU will provide the important parameters for environmental monitoring, climate forecasting and global climate changing research. Based on more than 4 months of in-orbit testing, the results indicate that TOU’s functionality and performance are stable, and it will be delivered to National Satellite Meteorological Center for the operational
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phase of the project. In this paper we introduce the instrument of TOU, its measuring principle, and the main in-orbit testing results first, and then we give total ozone product retrieved from TOU.

1 Measuring principle
The TOU is an instrument used to measure the incident solar radiation and backscattered ultraviolet radiance for retrieving total ozone. Considering the characteristics of atmospheric ozone profile, in order to ensure the accuracy of measuring total ozone from the satellite, the measuring wavelength should be selected that its effective scattering layer is situated in the troposphere. Figure 1 gives the simulation results for 1976 U.S. standard atmosphere. It shows that the effective scattering layer for 302.5 nm is situated above the height of ozone maximum and that for the 307.5 nm has two effective scattering layers. Therefore, the shortest wavelength for measuring total ozone should be longer than 308 nm. According to the Beer law, the atmospheric backscatter ultraviolet radiation intensity I received by satellite can be written as

molecular scattering and aerosol scattering. In addition, different underlying surfaces also have an obvious influence on the backscattering ultraviolet intensity. So using the single-wavelength method is difficult to obtain total ozone precisely. Therefore, a wavelength pair method is used to eliminate the effects of atmospheric molecular scattering and aerosol scattering. In this method, a non-ozone-absorbing wavelength is used to obtain the reflectivity of underlying surfaces and the heights of cloud top. At higher latitudes, due to the enhancement of effective reflection layer, it will not be able to obtain accurate total ozone using the wavelength pairs at the lower and middle latitudes, so it needs using other pairs of wavelengths.

2 Instrument general
To achieve mapping of daily global coverage of total ozone, TOU should have the following basic characteristics: (1) It should have the multi-wavelengths detectability to ensure the inversion accuracy of total ozone. TOU has six measuring channels (Table 1). The channel at 360 nm is to measure the reflectivity of underlying surfaces, and the 308 nm channel is to monitor atmospheric SO2. (2) It should have two-dimensional scanning. One-dimensional scanning depends on the satellite orbit tracking, and another scanning is completed by a scanning mirror. (3) It should have the capability of quick wavelength scanning to ensure the spatial resolution of 50 km at nadir. A fixed grating and slit-array Ebert-Fastie grating spectrograph system is used and the measuring wavelengths are individually selected using a chopper wheel. (4) The chopper wheel and detected signal should be synchronized precisely, just for the synchronization problem causes N7/TOMS’s complete failure [7]. (5) The measured backscatter signal should be modulated to reduce the effect of the South Atlantic Anomaly of radiation belts on the zero drift of photomultiplier tube (PMT). (6) To reduce effects of the degradation of solar diffusers and wavelength drift on the inversion precision of total ozone, TOU uses a three-diffuser system to analyze the degradation of solar diffuser in space and monitors the relative variation of 296.8 nm Hg line intensity to analyze TOU’s wavelength drift. TOU is composed of the optical assembly and the electronics assembly. The optical assembly is a grating spectrometer with responsible for the selection of measuring channels, the measurement of their backscatter ultraviolet radiation and solar irradiance signals and onboard calibration. The electronics assembly is responsible for the power supply, operation control, data acquisition and communication with satellite. Key instrument technical specifications are shown in Table 1. TOU has three work modes, i.e., scanning mode, radiometric calibration mode and wavelength monitoring mode. The

I ∝ exp(−nσ ) ,

(1)

where n is the atmospheric ozone content and σ is the ozone absorption cross section. In this way, as long as extraterrestrial solar radiation intensity and atmospheric backscatter ultraviolet radiation can be measured, the total ozone can be given in principle using the ozone absorption cross section obtained from the laboratory. The main difficulty in using the single-wavelength method for measuring total ozone is that the attenuation of backscattering ultraviolet intensity is not only caused by atmospheric ozone absorption, but also by the atmospheric

Figure 1 radiation.

The weight function of atmospheric backscatter ultraviolet

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Table 1 Key technical specifications

WANG YongMei, et al. Chinese Sci Bull January (2010) Vol.55 No.1

Term Central wavelengths (nm) Spectral bandwidth (nm) Sensitivity Dynamic range Spectral stray light Scan swatch Spatial resolution at nadir Wavelength accuracy Relative radiation calibration accuracy Goniometric accuracy of solar diffuser

Values 308.68±0.15 312.59±0.15 317.61±0.15 322.40±0.15 331.31±0.15 360.11±0.25 1.0+0.3, −0.0 ≤0.004 μW cm−2 sr−1nm−1 (S/N=1) 104 <10−3 ±54° ~50 km 0.03 nm 2% 3%

voltage power supply will provide an operating voltage for the PMT. The phase-locked steady velocity circuit drives the brushless DC motor turn steadily. The stepper motor driver circuits drive the scanning motor and radiometric calibration motor to achieve scene scan and the selection of diffusers respectively. The wavelength monitoring power supply provides an ignition voltage of mercury lamp. The signal acquisition-control interface circuit achieves signals and dark current collection, A/D conversion, data pre-procession and TOU operation control. The CPU-RTU realizes the collection and packaging of scientific data and engineering parameters and the communications with satellite through 1553B bus.

scanning mode is a main work mode, which will collect the scientific data at six wavelengths. The radiometric calibration mode is operated once per orbit and the wavelength monitoring mode is operated about once per two days when the satellite moves in the shadow area. 2.1 Scanning unit TOU’s scanning unit used a 45° scanning mirror in object space driven by a stepper motor. It scans perpendicular to the orbital plane across the track ±54° from the nadir in 3.6° steps for a total of 31 samples. 2.2 Optical system TOU’s optical system consists of foreoptics unit, monochromatic unit, wavelength selector and focusing unit, etc. The foreoptics unit is used to match TOU’s field of view with the f-number of monochromator, and to depolarize the entrance light. The monochromatic unit is a single EbertFastie monochromator with a fixed grating and an array of exit slits, which cooperates with the wavelength selector to achieve the selection of the detected wavelengths and the measurement of their signals and dark currents alternately. The focusing unit collects the ultraviolet radiation from the six exit slits on PMT photocathode and reduces the non-uniform influence of PMT photocathode sensitivity. 2.3 Electronics system TOU’s electronics system consists mainly of electrometer circuit, phase-locked steady velocity circuit, stepper motor driver circuits, high-voltage power supply, wavelength monitor power supply, signal acquisition-control interface circuit, secondary power supplies and CPU- RTU, etc. The detector is a dual-alkaline cathode PMT. The electrometer circuit amplifies the output of PMT, and supply A/D conversion. To achieve rapid signals response and required dynamic range, the electrometer circuit includes three amplifiers in parallel with different gains. The high

3 Primary results
3.1 Wavelength drift TOU wavelength drift is monitored by observing the relative change of the 296.8 nm Hg line intensity through two exit slits. Figure 2 shows TOU wavelength shifts during June 3 to November 18, 2008. Compared with the prelaunch data, the wavelength drift onboard is about −0.065 nm. In more than five months, the standard deviation of wavelength drifts is about 0.002 nm and the maximum deviation is less than 0.01 nm. Due to the very small changes of TOU’s temperature during this period, so no obvious temperature effect on wavelength drift is found. As compared with TOU’s, the wavelength shifts of M3/TOMS and ADEOS/TMOS are −0.11 nm and 0.15 nm respectively prior to launch, since launch the drifts are 0.25 nm and 0.02 nm [8,9] respectively. 3.2 Sensitivity In the space environment conditions, the PMT dark current will be severely affected by energetic particles, especially in the South Atlantic Anomaly zone and high latitudes [10]. Figure 3(a) demonstrates an example of the sensitivity distribution measured by TOU on July 18, 2008. It shows that the sensitivity is less than 0.004 μW cm−2 sr−1 nm−1 with the exception of the South Atlantic Anomaly zone and the polar regions, while in the South Atlantic Anomaly region

Figure 2

The measuring results of wavelength drift of TOU.

WANG YongMei, et al. Chinese Sci Bull January (2010) Vol.55 No.1

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and the Antarctic region, the sensitivity is about one order of magnitude lower. However, compared with the earlier instrument N4/BUV [11], TOU has an improvement of two orders of magnitude resulting from added shielding approached. Figure 3(b) gives the electron contour maps of energy > 3 MeV at 900 km during the quiet sun [12]. As it can be seen, the distribution map of TOU sensitivity and that of > 3 MeV electrons are very similar indicating that TOU’s shielding is good for < 3 MeV electrons. 3.3 Solar irradiance measurement Taking into account the possible effects of space environment on instrument, in order to ensure the measuring- accuracy and long-term stability, the onboard radiomet- ric calibration is needed. As the solar radiation in the ultraviolet wavelength range (300–360 nm) remains almost invariable, it can be used as the reference radiation source of in-orbit calibration. TOU’s onboard calibration is performed by measuring the solar radiation reflected from the diffuser. In order to monitor the degradation of diffuser in space, TOU is equipped with three diffusers: cover diffuser (A1), work diffuser (A2) and reference diffuser (A3). The cover diffuser is observed once per orbit, while the working and reference diffusers are observed once per week, and once per 15 weeks respectively. Figure 4 gives the comparison of solar irradiances measured by TOU (the Square point) and UARS/SOL-STICE. It shows that the results of TOU are in accordance with those of SOLSTICE at four shorter wavelength channels (308 nm,

312 nm, 318 nm and 322 nm). However, at two longer wavelength channels: (331 nm and 360 nm), the solar irradiation measured by TOU is larger than those of SOLSTICE, and the relative deviation is in 15%–20%. It could be caused by the stray light reflected from the coat of satellite, which has been confirmed partially by measuring the spectral reflectance of the coat and analyzing the field of view of the diffuser. Figure 5 gives the relative change of irradiance measured by the work diffuser since the TOU operated. As it can be seen, the RMS deviation is less than 0.5%, which indicates the TOU is stable. Similar to ADEOS/TOMS, the onboard radiation calibration of TOU would be operated over the Arctic, so the atomic oxygen will seriously affect the diffuser degradation [13]. Table 2 gives the comparison of the degradation between the TOU diffuser A1 and the cover diffuser of ADEOS/TOMS [3]. It can be found that the diffuser degradation of TOU is a little smaller than that of ADEOS/ TOMS, due to using some special methods such as restricting the view field of diffusers on TOU, but it is larger than other TOMSs, whose radiation calibration would be done over the Antarctic [14]. 3.4 Product The purpose of TOU is to obtain the global distribution of total ozone. Using the data of TOU, the global total ozone has been retrieved and compared with the products from the AURA/OMI (http://toms.gafc.nasa.gov/). As an example, Figure 6 gives a comparison of their global total ozone on

Figure 4 Comparison of solar irradiance measured by TOU and SOLSTICE.

Figure 3 (a) The measuring result of TOU sensitivity (μW cm−2 sr−1 nm−1); (b) the electron contour maps of energy > 3 Mev at 900 km during the quiet sun [12]. The number in (b) is the logarithm of electronic flux (cm−2 s−1).

Figure 5

Stability of measuring for TOU working diffuser (A2).

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WANG YongMei, et al. Chinese Sci Bull January (2010) Vol.55 No.1

Table 2 Comparison of diffuser degradation at 308 nm Run time 1 month 2 months 3 months 4 months FY-3/TOU-A1 diffuser ADEOS/TOMS-Cover diffuser 0.937 0.89 0.876 0.80 0.806 0.72 0.695 0.63

July 18, 2008. From the figure, it can be seen that at middle and low latitudes and some high latitudes, their maps of total ozone are almost identical including the details of the distribution. Figure 7 gives the histogram of their differences. It can be seen that the differences of about 80% data are less than 5%. The inversion results from July to October, 2008 show that the RMS deviation of total ozone of TOU and OMI is about 5%, only in some special place, such as the polar, the RMS deviation is more than 10%. It indicates

that the satisfactory product of the global distribution of total ozone can be obtained using TOU data and self-developed inversion method. There are two main factors that cause the difference of the inversion results of TOU and OMI. Firstly, the FY-3A’s orbit is different from the AURA’s, so the time for observing the object at same place by TOU and OMI has an interval about four hours. Taking into account the change of atmospheric conditions and the geometric differences of observational condition, the relative deviation is reasonable; secondly, in the Polar Regions, due to lower sun elevation angle, the signals become weaker, so the calculation error of radiation transfer will increase and influence the inversion accuracy of total ozone. The Antarctic ozone hole occurs in September–November each year. TOU is the first onboard instrument in

Figure 6

The maps of the global total ozone on July 18, 2008. (a) TOU; (b) OMI.

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Figure 7 OMI.

The histogram of the deviation of total ozone between TOU and

satellite is the first instrument for global total ozone monitoring in China. TOU can give a daily global map of total ozone using the self-developed inversion method. The data analysis shows that TOU is in a good working status during in-orbit testing. Its main performances are at the same level as the similar apparatuses, whereas some performances of TOU are slightly better. In addition, total ozone product derived from TOU agrees with that of other in-orbit instruments, indicating that both instrument development and retrieved method in China have come to an initial success.
1 2 3 4 5 6 7 Heath D F, Krueger A J, Roeder H A, et al. The solar backscatter ultraviolet and total ozone mapping spectrometer (SBUV/TOMS) for NIMBUS G. Opt Eng, 1975, 14: 323–331 Jaross G, Krueger A J, Cebula C, et al. Calibration and postlaunch performance of the METEOR 3/TOMS instrument. J Geophys Res, 1995, 100: 2985–2995 Krueger A J, Jaross G. TOMS/ADEOS instrument characterization. IEEE Trans Geosci Remote Sensing, 1999, 37: 1543–1549 McPeters R, Bhartia P K, Krueger A J, et al. Earth Probe total ozone mapping spectrometer (TOMS) data products user’s guide. NASA TP-1998-206895 Levelt P F, van den Oord G H J, Dobber M R, et al. The ozone monitoring instrument. IEEE Trans Geos Remote Sens, 2006, 44: 1093–1101 Dong Y H, Sun Y Z, Wang J H, et al. FY-3A polar meteorological satellite (in Chinese). Aerospace Shanghai, 2008, 1–11 McPeters R D, Bhartia P K, Krueger A J, et al. Nimbus-7 total ozone mapping spectrometer (TOMS) data products user’s guide. NASA RP 1384, 1996 Herman J R, Bhartia P K, Krueger A J, et al. Meteor-3 total ozone mapping spectrometer (TOMS) data products user’s guide. NASA RP-96-1393 Krueger A J, Bhartia P K, McPeters R D, et al. ADEOS total ozone mapping spectrometer (TOMS) data products user’s guide. NASA TP-98-206857 Beatty M E, Debnam W J, Meredith B R. Effects of 1- and 2-MeV Electrons on Photomultiplier Tube. NASA TN D-8125 Heath D F, Mateer C L, Krueger A J. The backscatter ultraviolet (BUV) experiment. Pure Appl Geophys, 1973, 106-108: 1238– 1253 Du H, Ye Z H. Handbook of Space Environment at Low Spacecraft Orbit (in Chinese). Beijing: National Defense Industry Press, 1996. 501 Wang Y J. Effects of space environment on solar diffuser (in Chinese). Chin J Space Sci, 2002, 22: 52–57 Jaross G, Cebula C, DeLand, et al. Backscatter ultraviolet instrument solar diffuser degradation. Proc SPIE, 1998, 3427: 432–444

Figure 8 2008.

The Antarctic ozone hole observed by TOU on October 14,

8 9 10 11 12 13 14

ozone hole in China that can provide the continuous observarion of ozone hole. As a sample, Figure 8 gives the distribution of the Antarctic total ozone measuring by TOU on October 14, 2008. From it, the distribution and intensity of the Antarctic ozone hole can be shown and the comparison with other days indicates that at this time its intensity is almost the strongest.

4

Conclusions

Ultraviolet Total Ozone Unit (TOU) onboard the FY-3

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