Onboard Blackbody Calibration Algorithm of Chinese FY-2 Geostationary Imager Based on GSICS Reference Radiances

Transcription

1 Onboard Blackbody Calibration Algorithm of Chinese FY-2 Geostationary Imager Based on GSICS Reference Radiances Xiuqing (Scott) Hu 1,4, Likun Wang 2, Fuzhong Weng 3, and Na Xu 4 1. ERT@NOAA/NESDIS/STAR, Camp Spring, MD, USA; Xiuqing.Hu@noaa.gov 2.UMD/ESSIC/CICS, College Park, MD, USA 3.NOAA/NESDIS/STAR, Camp Spring, MD, USA 4.CMA/NSMC, Beijing, China 1

2 Outlines Motivation: Demonstration of inter-calibration using AIRS, IASI, and CrIS can help for improving calibration of Chinese meteorological operational satellites under the Global Spacebased Inter-Calibration System (GSICS). Content Calibration challenge for Chinese FY-2 geostationary imager Inter-calibration under GSICS framework Onboard Calibration Algorithm using GSICS calibration Conclusion 2

3 On-Orbit Satellites FY-3A FY-1D FY-2F 112 FY-3B FY-2D 86.5 FY-2E 105 FY-2C

4 Instrument Specification of S-VISSR of FY-2 series Specification/spacecraft FY-2(01)-FY-2A/2B FY-2(02)-FY-2C/2D/2E FY-2(03)-FY-2F/2G/2H Bands( m) IFGOV (µrad) Spatial Resolutio n (km) Dynamic Range Data Quantizat ion Visible IR thermal IR11: IR12: IR11: IR12: Midium-IR NO IR Water vapor Visible IR (all 4 bands) visible IR(all 4 bands) Visible (Reflectance) 0-98% 0-98% 0-98% IR (TB) K 180K-330K 180K-330K Midium-IR(TB) NO 180K-340K 180K-340K Water vapor (TB) K 190K-300K 190K-300K Visible 6bits 6bits 6bits IR (all 4 bands) 8bits 10bits 10bits

5 Instrument Improvement of FY-2F/2G/2H -- Stray light and SRF

6 Challenges for FY-2 Imager calibration Just like the first generation of spin-scan radiometer. There are two optical paths in FY-2 satellites: HgCdTe detectors Filters Second relay lens Primary relay lens 1. earth and space view signal go through the fore- and aftoptics to reach the detectors; 2. Signals from onboard blackbody go through the caland aft-optics to reach the detectors. So there is no full-optics onboard blackbody calibration Visible calibrator space Secondary mirror earth aft-optics Calibration plane mirror Separate lens fore-optics cal-optics Calibration lens Blackbody Silicon optical detectors Transition mirror O Primary mirror blackbody

7 Calibration challenge? Calibration line CT_BB Blackbody counts Space clamp point? R(Tb) Blackbody radiances 7

8 Why so difficult? Deep Space CT_sp = (R_fore + R_aft + R_space) *Gain Earth if treated as R(T) CT_e= (R_fore + R_aft + R_e) *Gain Blackbody CT_bb = (R_aft + R_bb ) * Gain When Space clamp CT_sp=1023 CT_e - CT_sp = R_e * Gain CT_bb CT_sp =(R_bb R_fore) *Gain Note that: R_bb=e*R(T) + (1-e)*R_Cal Courtesy of Image from EUMETSAT report EUM/OPS-MSG/TEN/03/0064 8

9 FY-2 IR Calibration Milestones Before 2005: Earlier FY-2A/2B IR calibration based on Vicarious Calibration using Qinghai Lake and had no real time or near real time operational cal. Early sensor worked unstably and has heavy stray light and didn t have good geolocation. After 2005: FY-2C can work stably and VISSR IR operational calibration based on intercalibration using AVHRR and HIRS since FY-2D/2E also continues this kind of operational model since they begun to work operationally. After 2009: FY-2C/2D GSICS IR calibration experiment using the AIRS and IASI since Oct., After 2011: FY-2D/2E operational calibration monitoring test using GSICS IR calibration since May, FY-2E GSICS GEO-LEO IASI results has become the operational input of L1 calibration LUT to replace AVHRR/HIRS since Jan 12, After 2012: FY-2F/2G/2H completely operational calibration based on GSICS calibration using AIRS/IASI or CrIS and diurnally adjustment using half-optical BB calibration model. On-going work: Development of onboard half-optical BB calibration model based on GSICS reference standard. Retrospectively: Recalibration for FY-2C/2D/2E is conducted based inter-calibration using AIRS and IASI.

10 GSICS calibration for FY-2D/2E The slope derived from GSICS inter-calibration shows strong periodical cycle. So what controls the instrument slope?

11 What control slope (gain)? Self-emission An infrared detector detects and converts the thermal energy into an electrical signal Target Detector temperature Determines the efficiency of the detector in response to coming radiation This is why the detector temperature must be well controlled for IR sensors From Cao et al Instrument self-emission For the HgCdTe detector that has a nonlinear response, the significant increase in the radiation reaching the detector causes the response to shift to a different portion on the nonlinear response curve, which has a different gain (becomes less responsive at higher temperatures) 11

12 Self-emission push the slope Self-emission Target Calibration relationship Slope time series Cao et al. 2007; Mittaz and Harris,

13 Prelaunch test (FY-E IR Ch2) The pre-launch tests have been performed by changing the temperature of optical parts to check how the calibration slope changes 13

14 Slope vs. Temperature of optical parts Detector temp: 93.6 K Detector temp: K Detector temperature control the slope 14

15 Slope vs. Temperature of optical parts Cal. & Relay Primary 2 nd The temperature of aft-optics control the slope! Not the fore optics! 15

16 FY-2E Telemetry data (1) HgCdTe detectors Detector Temp. Filters Second relay lens Primary relay lens Calibration lens Blackbody Blackbody temperature Calibration plane mirror Separate lens Silicon optical detectors Visible calibrator Transition mirror Secondary mirror O Primary mirror 1. Detector and BB temperatures were well controlled. 2. Aft-optics temperature varied periodically 3. Temperature of relay and calibration lens are highly correlated. 16

17 GSICS derived Calibration Coeff. Relay Temp. Slope Intercept Note that the ct =1023-ct The slope are highly correlated with temperature of aft-optics 17

18 Proposed on-board calibration algorithms AIRS IASI FY Ob. CrIS GSICS X-Cal. Cal. Coeff. Self-emission Real-time Telemetry Data OBC counts Self. Emission Model Real-time Calibration Coeff. 18

19 Relationship of Slope and Self-emission Day time data Night time data 19

20 Calculated Slopes Trend Seasonal Cycle Diurnal Cycle 20

21 Conclusion Hyperspectral radiances with good radiometric and spectral calibration is very helpful to improve operational calibration of Chinese satellites. Using GSICS inter-calibration results as a reference combined with diurnally adjustment using half-optical BB calibration model further improve the data quality of Chinese satellites 21