# Presentation: 114214

## Ten Years of TRMM/VIRS On-Orbit Calibrations and Multi-Year Comparisons of VIRS and MODIS

**Ten Years of TRMM/VIRS On-Orbit Calibrations and Multi-Year Comparisons of VIRS and MODIS**
**Cheng-H. Joseph Lyu, William L. Barnes**†
**Science Systems & Applications Inc., Greenbelt, MD 20706, lyu@neptune.gsfc.nasa.gov**
- † University of Maryland, Baltimore County

**TRMM/VIRS**
- The TRMM/VIRS was in a non-sun-synchronous 350 km (402 km after maneuver, August 22, 2001) orbit, at 35o inclination, launched on Nov. 28, 1997.
**Terra, Aqua/MODIS**
- The Terra/MODIS and Aqua/MODIS, launched on 18 Dec. 1999 & 4 May 2002, were polar orbiting satellites in sun-synchronous orbits, at 705 km, inclined at 98.2o.

**Examples - ****Coincident Measurements, Comparisons**

- Case 1 – Mostly clear – VIRS ver.5/MODIS ver.2

- Case 2 – Mostly cloudy

## Updated Calibration Algorithms for the VIRS Solar Bands

**The ****spectral radiances**** for the two solar bands (i = 1, 2 = 0.62, 1.61 μm) from the earth view (EV) are determined as follows:**

- Where Δ(DNiSD - DNiSV ) is a correction term required because the baseline digital counts of SD (dark) and SV differ;
- Hisun(t) is the spectral solar irradiance at the time of calibration (H1sun(t0) = 168.16 and H2sun(t0) = 24.873, mW cm−2 μm−1, where t0 is the time when the sun is at 1 A.U.);

- 0.1286 + 0.8 × cos(θSD), from the measurements of BRDF of the diffuser by SBRS, represents the fraction of the reflected solar irradiance received by the detector for a given θSD;
- ρi(θDV ) is the spectral reflectance of the scan mirror viewed from the diffuser at θDV, where ρ1(θDV ) = 0.8976 and ρ2(θDV ) = 1.005 (Barnes et al., 2000; Lyu and Barnes, 2003).
- Note that the original adopted ρ2(θDV ) was 0.9231 in the VIRS 1B01 version 6 (Level 1B) algorithm was found to be in error.  The original ρ1(θDV ) is acceptable for the upcoming Level 1B (version 7) algorithm.
- Using the above corrected ρi(θDV ) terms, we find the initial (measured at launch) responsivities should be 67.34 and 388.14 for the 0.62 and 1.61 μm bands, respectively. The original initial responsivities (Lyu and Barnes, 2003) were 69.30 and 422.58, respectively.
- The adjustment of the responsivities *R**i*, based on a degradation rate of 0.05%/yr versus 1.15%/yr and the trending results for the 0.62 μm band minimize the radiance differences between MODIS and VIRS, especially for data after 2001.

**Fig. 1 – Nine years of ****on-orbit responsivities**** for the VIRS (a) 0.62 μm; (b) 1.61 μm bands**

**Fig. 2 – Disk-integrated ****spectral intensity**** (counts) of the VIRS 0.62 and 1.61 μm data vs.lunar phase angle**

**Diffuser measurements occur about twice a month, and the exposure of the diffuser to the Sun during each measurement lasts about two minutes. From these measurements, we calculate the responsivity R****i**** using the reflectance of the diffuser, the digital counts, and the irradiance from the Sun (Eq. (2)). **
**The responsivities (or gains) of the VIRS 0.62 μm and 1.61 μm bands from ****210 on-orbit solar calibrations**** are shown in Fig. 1. **
**VIRS lunar measurements have been made near monthly for over nine years. This nine year data set, as mentioned previously, was collected to enable separation of any solar diffuser degradation from that of the sensor. **
**We have compiled ****713 lunar measurements**** during 3400 observing days (Fig. 2). These data occur over a wide range of lunar phase angles, ranging from ****0.94° to 121.8°.**** **
**After sorting out the time series of the VIRS lunar calibrations and normalizing the integrated values with the best-fit phase curve of Fig. 2, the quadratic best fit through the VIRS lunar spectral intensity data indicates that the VIRS sensor has been degrading approximately 0.05%/yr (see Fig. 3).**

**Fig. 3 – ****Sensor performance trending**** of TRMM/VIRS, Terra/MODIS, Aqua/MODIS versus days since 1998. The dates of first lunar event for Terra and Aqua are March 24, 2000 and June 20, 2002, respectively. One-sigma error bar for VIRS is noted in the figure**

**Fig. 4 – ****Irradiance vs phase****. The irradiance of the Moon is represented by the best fits to the VIRS 0.62 and 1.61 μm data, normalized at 0° phase. The dotted line corresponds to the irradiance expected from a Lambertian surface. (a) Terra MODIS normalized irradiance vs VIRS; (b) Aqua MODIS normalized irradiance vs VIRS. One-sigma error bars for the VIRS 0.62 and 1.61 μm data are noted in the figures (a) & (b)**

- Because there are some bad detectors in the MODIS 1.64 μm band, together with significant (optical) signal leaks and cross-talk issues, we could not apply the lunar data for this band properly. Thus, we did not perform comparisons of sensor performance trending between the MODIS 1.64 μm band and the VIRS 1.61 μm band.
- Please refer to Lyu et al. (2006) for a detailed description on how we adopt the scaling factors (m 0 and m 1 for MODIS scan mirror sides 0 and 1, respectively) to determine the MODIS sensor performance changes.
- Note also that for trending studies, both for the VIRS degradation data shown in Fig. 3 and the irradiance curves shown in Fig. 4, the VIRS data are derived by applying best fits to 9 years of lunar measurements, whereas the MODIS results are based on individual lunar measurements.
- We find that the derived lunar irradiances from both Terra/MODIS and Aqua/MODIS are comparable to, but slightly higher than VIRS.
- Since different instruments view the Moon at somewhat different angles (e.g., the Moon appears in  waxing or waning phases), improved lunar models for TRMM/VIRS may have a significant impact on the derived irradiances.

**(1)**

**(2)**

## VIRS Calibration Uncertainties 

**Fig. 5 – Radiometric comparisons of TRMM/VIRS ****version 6****, Terra/MODIS ****version 5**** data observed in Mar. 2001, Jun. 2002, Sep. 2003, Dec. 2004, Mar. 2006 and Apr 2007**

- In Fig. 5, we present comparisons of the reflectance data of similar solar bands obtained from VIRS version 6 and MODIS version 5 in the years between 2001 and 2007.
- For the VIRS 0.62 μm band, we adopted a radiance degradation rate of 1.15%/year in the version 6 data products.
- For the VIRS 1.61 μm band, as indicated in earlier studies, the derived reflectance is consistently lower than the MODIS 1.64 μm band by about 14%.

**Fig. 6 – Brightness temperature comparisons of TRMM/VIRS ****version 6****, Terra/MODIS ****version 5**** data observed in Mar. 2001, Jun. 2002, Sep. 2003, Dec. 2004, Mar. 2006 and Apr 2007**

**VIRS Calibration ****Uncertainties**** **

**The radiometric accuracies calculated pre-launch for the in-flight calibration of the VIRS solar bands (0.62 and 1.61 μm) are about ****5.6%**** (TRMM, VIRS Final Technical Description Document, 1996).**
- The important uncertainty contributors include calibration sources (either from the solar diffuser or the lunar measurements), scan mirror reflection angle, stray light, out-of-band spectral effects (imperfect spectral filters), and out of field (near field) response.
- The most significant error source comes from the calibration source (4.0%). The next one is from out of field effects (3.0%), and then the out-of-band spectral effects (2.0%).
- The uncertainty contribution from the mirror reflectance is small, since the reflectance corrections for these bands are less than 1%. Other instrumental uncertainties are also small.

**The radiometric accuracies calculated pre-launch for the in-flight calibration of the thermal bands (3.78, 10.83, and 12.03 μm) are about ****2.7%**** (TRMM, VIRS Final Technical Description Document, 1996).**
- The primary uncertainty contributors include scan mirror reflection angle and in-band and out-of-band spectral effects.
- The uncertainty in the radiance from the calibration source is less than 0.9% for the 3.78 μm band and is less than 0.5% for both the 10.83, and 12.03 μm bands. In terms of temperature for an Earth scene at 300 K, the uncertainties are about 2 K.
- From measurements on-orbit by Lyu et al. (2000), the reflectances for the 10.83, and 12.03 μm bands differ by up to 2%, but the prelaunch reflectances adopted from MODIS measurements for these bands are the same (Barnes et al. 2000).
- The uncertainty of the on-orbit reflectances would be additional to the primary sources of uncertainty for the VIRS thermal radiances.

- In Fig. 6, we find that all three VIRS thermal bands (3.78 μm, 10.83 μm and 12.03 μm) are comparable to the MODIS thermal bands at 3.75 μm, 11.03 μm and 12.02 μm, respectively.
**Calibration algorithms**** for the VIRS ****thermal Bands **can be found in Lyu and Barnes (2008, submitted to JAOT).

## References

- Barnes, R. A., Barnes, W. L., Lyu, C.-H. Gales, J. 2000: An Overview of the VIRS Radiometric Calibration Algorithm. J. Atmos. Oceanic Technol., **17**, 395-405
- Lyu, C.-H., Barnes, W. L. and Barnes, R. A. 2000: First Results from the On-Orbit Calibrations of the VIRS for the TRMM, J. Atmos. Oceanic Technol., **17**, 385-394
- Lyu, C.-H., and Barnes, W. L. 2003: Four Years of TRMM/VIRS On-Orbit Calibrations and Characterization Using Lunar Models and Data from Terra/MODIS, J. Atmos. Oceanic Technol., **20**, 333-347
- Lyu, C.-H., Sun, J., Xiong, X. and Barnes, W. L. 2006: Multi-Year Lunar Observations from TRMM/VIRS, Terra/MODIS and Aqua/MODIS. SPIE, **6298**, 629811-9
- TRMM, VIRS, CDRL no. 507, Final Technical Description Document, Contract No. NAS5-32044, prepared by SBRS Hughes, June 1996

**VIRS level 1B ****ver**** ****6**** vs MODIS ver 5**

**VIRS level 1B ****ver**** ****7**** vs MODIS ver 5**

**Fig. 7 – Radiometric comparisons of TRMM/VIRS ****version**** ****6**** & ****7**** and Terra/MODIS ****version 5**** data observed in Apr. 2007**

**Concluding Remarks**** **

- The measured reflectances of the VIRS 0.62 μm band deviate from those of the MODIS 0.65 μm band and increase with time after the launch of TRMM/VIRS. This is due to the use of the degradation rate of 1.15%/yr resulting from the SD on-orbit measurements.
**The lunar data should enable separation of any solar diffuser degradation from that of the VIRS sensor**.
- However, in our last report by Lyu and Barnes (2003), we did not have sufficient lunar measurements to determine precise degradation rates. We assumed that the degradation in the response of the 0.62 μm band was due solely to the sensor.

**A study of the trend in lunar reflectance measurements over 9 years indicates that the VIRS sensor is degrading ~ 0.05%/yr for the 0.62 μm band**.
- With this revised degradation rate (in the upcoming level 1B version 7 algorithm), the radiometric measurements of both **the VIRS 0.62 μm**** and the MODIS 0.65 μm band are comparable** (Fig. 7).
- Comparison of the reflectances from the VIRS 1.61 μm band and the MODIS 1.64 μm band indicate that the VIRS 1.61 μm band is lower by about 14%. We examined the calibration algorithm, but we did not find any clue to the problems until we reviewed the calibration documents.
- We found that the BRDF of the SD and the reflectance of the scan mirror were not clearly defined. After determining the correct values of the scan mirror reflectance, we find the reflectance measurements of **the VIRS 1.61 μm band**** are comparable to those of the MODIS 1.64 μm band** (Fig. 7).
- The remaining 5% difference is mostly due to differences in the sensors in terms of band widths, wavelength centers, detection or saturation limits, and the intrinsic uncertainty in the measurements.

- The RObotic Lunar Observatory (ROLO) model is being developed (Tom Stone, private communications) to establish a spectral irradiance model of the Moon to account for variations with lunar phase, lunar librations, and the location of Earth-orbiting spacecraft. This program is capable of characterizing the brightness of the Moon for the on-orbit calibration of Earth sensing imaging radiometers.
- As an alternative to the approach presented here, the ROLO model should be able to determine integrated lunar irradiances for each of the VIRS lunar measurements. This will enable us to improve our on-orbit radiometric calibrations and to more directly track changes in sensor response.
**The TRMM/VIRS thermal bands**** are comparable to similar MODIS bands**. The current VIRS version 6 thermal band calibration algorithm will be retained for the upcoming version 7 data products.
- We expect, with the version 7 algorithms, VIRS should provide significant improvements for inter-calibrating multiple Earth-observing sensors.

**Acknowledgments**
- We would like to express our thanks for technical support from the TSDIS (TRMM Science Data and Information System), especially, Yong Zhang, who provided VIRS level 1B operational codes and SYBASE database tool kits which helped us greatly in checking modified level 1B data products.
- The authors thank R. Barnes for many useful discussions regarding the current calibration algorithm, and also wish to thank Jack Xiong and J. Sun for technical support from the MODIS Characterization Support Team (MCST).