On-orbit Cross-calibration of AM Satellite Remote Sensing Instruments Using the Moon

International Workshop on Radiometric & Geometric Calibration

Grand Casino Gulfport HotelGulfport, MS 39501December 4, 2003

Jim ButlerNASA’s GSFC

Code 924 Laser Remote Sensing Branch

Greenbelt, MD 20771Phone: 301-614-5942

Fax: 301-614-6744E-mail: James.J.Butler@nasa.gov

• Acknowlegements– Tom Stone-USGS Flagstaff– Hugh Kieffer-USGS Flagstaff (emeritus)– Bob Barnes-SAIC– Bob Kozon-NASA’s GSFC Mission Ops and his entire

team– Carol Johnson, Steve Brown, Ted Early-NIST– Stu Biggar, Kurt Thome-U of Arizona

The Moon as an On-orbit Calibration Target

The Moon has advantages (+) and disadvantages (-) when used as a satellite instrument calibration target.

• Advantages:

+Appropriate radiance range for Earth-viewing instruments

+Photometric properties are virtually invariant (<10-8/yr)

+Spectrally bland (from returned Apollo samples)

+Accessible to spacecraft regardless of orbit

+Useful as a common transfer source between spacecraft

• Disadvantages

-Non-uniform reflectance and complex photometric behavior

-Satellite instrument lunar views may require a spacecraft attitude maneuver

•MISR, ASTER, ALI, Hyperion, SeaWiFS, &

MODIS (Earth view port) on 4/14/03

•MODIS (Space view port) 8 to 12 times a year w/ no maneuver

The Moon is equivalent to a 6 km target from a 705 km orbit

On-orbit Lunar Viewing by AM Constellation Instruments on April 14, 2003

AM Constellation Spacecraft & Instruments

Satellite Optical Instruments

Altitude (km) Orbital Inclination (deg)

Equatorial Crossing Time

(LST)

EO-1 ALI

Hyperion

Atm. Corrector

705 98.2 10:01

Landsat-7 ETM+ 705 98.2 10:05

SAC-C MMRS

HRTC

HCS

702 98.2 10:15

Terra MODIS

MISR

ASTER

MOPITT

CERES

705 98.1 10:30

SeaStar SeaWiFS 705 98 12:00

ab

c

d

e

f

g

h

i

Terra Event Elapsed time (min)

a. Maneuver start 0

b. Zero pitch accel. 3.03

c. Nadir at Earth’s limb 7.37

d. Nadir at Moon 11.90

e. Midpoint of maneuver 17.95

f. Nadir at Earth’s limb 28.53

g. Maneuver end 35.54

h. Terra enters sunlight 41.6

On-orbit Lunar Viewing by AM Constellation Instruments on April 14, 2003

• Instrument Lunar Viewing Times (UT) 1. ALI MS/Pan 1 21:39:47

2. ALI MS/Pan 2 21:44:34

3. ALI MS/Pan 3 21:49:19

4. Hyperion 21:54:31

5. MISR Df camera 22:01:38

6. MISR Cf camera 22:02:33

7. MISR Bf camera 22:04:05

8. MISR Af camera 22:06:23

9. MODIS 22:09:24

10. MISR An camera 22:09:34

11. ASTER SWIR 22:09:34

12. ASTER VNIR-nadir 22:09:34

13. MISR Aa camera 22:12:46

14. ASTER VNIR-aft 22:13:22

15. MISR Ba camera 22:15:04

16. MISR Ca camera 22:16:37

17. MISR Da camera 22:17:33

18. SeaWiFS 22:34:14

EO-1 Lunar Maneuver

MS/PAN1MS/PAN2MS/PAN3

Start

Yspacecraft

1.26°

0.75°

41.5°

Pitch

Roll

End

Hyperion 1

X spacecraft

ASTER 560nm Band

MISR 672nm Band

MODIS 645.5nm Band

Robotic Lunar Observatory (ROLO) and the Lunar Irradiance Model

• Goal: provide on-orbit stable and accurate radiometric calibration at solar reflectance wavelengths using the Moon

• Two 20 cm, 1m focal length Richey-Chretien telescope systems

– VNIR: • 512 square Si-CCD• 23 filters

– SWIR: • 256 sq. HgCdTe• Identical to NICMOS on Hubble• 9 filters

• Calibration (absolute accuracy is TBD)

– 12” sq Spectralon™ panel + 1000W irradiance standard lamp

– Irradiance of spectrophotometric standard star Vega

– External 0.4 m dia. collimator + light source calibrated for radiance

Robotic Lunar Observatory (ROLO) Irradiance Model

• The ROLO Irradiance Model is used to compare on-orbit lunar measurements made by satellite instruments over a wide range of lunar phase and libration.

– Empirical model based on 5+ years of lunar observations

• ~85,000 lunar images; >106 star images

• 32 bands: 23 VNIR, 9 SWIR• ±90º lunar phase coverage14

coefficients for each band, 4 are wavelength coupled.

• Typically 14 coefficients and >1000 data points per band.

• Smooth wavelength interpolation.

NOTE: The ROLO lunar image database is unique and extensive. The substantial spatial, phase angle, and wavelength coverage are far beyond any published prior work.

Applications of the ROLO Lunar Irradiance Model

– The irradiance model has been used to:• Determine relative differences in the radiometric scales of

satellite instruments ()• Determine long-term degradation in the radiometric

responsivity of satellite instruments ()• Determine band to band radiometric differences in satellite

instruments ()• Validate the absolute radiometric scales of satellite

instruments (accuracy is TBD)

Determining Relative Radiometric Differences Between Instruments Using the Moon

April 14, 2003 AM Instrument Lunar Comparison

-10

-5

0

5

10

15

20

25

300 600 900 1200 1500 1800 2100 2400 2700

Wavelength (nm)

% D

iffe

ren

ce f

rom

RO

LO

Mo

del

MODIS MISR (9 camera avg.) SeaWiFS Hyperion ALI

-Absolute accuracy of ROLO measurements is TBD-Relative differences between instruments due to 1. Use of different solar irradiance spectra 2. Different approaches in calculating integrated lunar radiances from instrument lunar images 3. Inherent differences/uncertainties in instrument calibrations

Determining Relative Radiometric Differences Between Instruments Using the Moon

-Comparison of MODIS instruments at three different lunarphases.

1. MODIS Terra: 1 through Earth view port + 25 through Space view port (2 angles of inc. on scan mirror).2. MODIS Aqua: 10 through space view port.

MODIS Terra/Aqua Lunar Comparisons

0

2

4

6

8

10

12

14

16

18

300 400 500 600 700 800 900 1000

Wavelength (nm)

% D

iffe

ren

ce f

rom

RO

LO

Mo

del

MODIS Terra 4/14/03 (-27.7 deg.) MODIS Terra (-55 deg.) MODIS Aqua (+55 deg.)

Determining Relative Radiometric Differences Between Instruments Using the Moon

AM Instrument Lunar Comparisons

-10-8-6-4-202468

101214161820

300 600 900 1200 1500 1800 2100 2400 2700

Wavelength (nm)

% D

iffe

ren

ce f

rom

RO

LO

Mo

del

MODIS Terra SeaWiFS MISR ALI Hyperion

-Average of all lunar views for each instrument: SeaWiFS (70); ALI (29); MODIS Terra (26); Hyperion (9); MISR (1)

Determining Long-term Degradation in the Radiometric Responsivity of Satellite Instruments

SeaWiFS

-Correcting for correlated time jitter (left), a clear asymptotic degradation trend is seen in Bands 7 and 8 (right).-Degradation information determined through repeated lunar views is used in the generation of SeaWiFS standard data products.

Determining Long-term Degradation in the Radiometric Responsivity of Satellite Instruments

ALI Radiometric Stability -Band 3

0.925

0.95

0.975

1

1.025

1.05

1.075

0 100 200 300 400 500 600 700

Mission Day Number

Re

lati

ve

Re

sp

on

se

Solar Calibration

Ground Truth

Lunar Calibration

Lamp

ALI

Determining Band-to-band Differences in Satellite Instruments

MISR An Camera Band-to-band Differences

-Results from 5 VC expts. + 1 lunarview (4/14/03)-Normalized VC and lunar results show identical band-to-band trends

MODIS Terra & Aqua Band-to-band Differences

MODIS Terra/Aqua Band-to-band Differences

0

2

4

6

8

10

12

14

16

18

300 400 500 600 700 800 900 1000

Wavelength (nm)

% D

iffer

ence

from

R

OLO

Mod

el

-6

-4

-2

0

2

4

6

8

10

12

% D

iffer

ence

from

U

of A

rizo

na

MODIS Aqua MODIS Terra MODIS Aqua (U of A) MODIS Terra (U of A)

-Lunar and U of A results show similarband-to-band trends-Absolute scale not established in lunarcase

Summary and Conclusions

• On-orbit satellite instrument calibration/characterization using the Moon complements current on-board and vicarious calibration approaches.

– Effective in long-term trending of degradation of instrument radiometric response– Useful in determining relative radiometric differences between instruments– Useful in determining band-to-band radiometric differences within instruments

• Absolute calibration of lunar system needs additional work– Analysis of calibration/characterization data obtained in Sept/Oct at Flagstaff– Detailed analysis of atmospheric correction codes

• The lunar irradiance model is currently running at GSFC

• The Moon (i.e. lunar views + model) is a viable prospect for meeting NPP/NPOESS global change/climate monitoring goals

“All you have to do is look at the Moon.” Hugh Kieffer

“Once you look at the Moon, you may see amazing things.” Hugh Kieffer