Though both of the current Landsat instruments, Landsat-7 Enhanced Thematic Mapper+ (ETM+) and Landsat-5 Thematic Mapper (TM), include on-board calibration systems, since 2001, pseudo-invariant calibration sites (PICS) have been added to the suite of metrics to assess the instruments calibration. These sites do not provide absolute calibration data since there are no ground measurements of the sites, but in monitoring these PICS over time, the relative calibration can be tracked. The sites used by the Landsat instruments are primarily in the Saharan Desert. To date, the trending from the PICS sites has confirmed that most of the degradation seen in the ETM+ on-board calibration systems is likely not degradation of the instrument, but rather degradation of the calibration systems themselves. However, the PICS data show statistically significant degradation (at 2-sigma) in all the reflective spectral bands of up to -0.22%/year since July 2003. For the TM, the PICS were instrumental in updating the calibration in 2007 and now suggest two bands may require another update. The data show a statistically significant degradation (at 2-sigma) in Bands 1 and 3 of -0.27 and -0.15%/year, respectively, since March 1999. The data filtering and processing methods are currently being reviewed but these PICS results may lead to an update in the reflective band calibration of both Landsat-7 and Landsat-5

Barsi, Julia A.

Helder, Dennis L.

Markham, Brian L.

NASA Technical Reports Server

CONTINUED MONITORING OF LANDSAT REFLECTIVE BAND 
CALIBRATION USING PSEUDO-INVARIANT CALIBRATION SITES 
 
Julia A. Barsi, Brian L. Markham, Dennis L. Helder 
 
Science Systems and Applications, Inc., NASA/GSFC Code 618, Greenbelt MD 20771; NASA/GSFC 
Code 618, Greenbelt MD 20771, South Dakota State University, Brookings SD 57007 
 
 
ABSTRACT 
 
Though both of the current Landsat instruments, Landsat-7 
Enhanced Thematic Mapper+ (ETM+) and Landsat-5 
Thematic Mapper (TM), include on-board calibration 
systems, since 2001, pseudo-invariant calibration sites 
(PICS) have been added to the suite of metrics to assess the 
instruments’ calibration. These sites do not provide absolute 
calibration data since there are no ground measurements of 
the sites, but in monitoring these PICS over time, the 
relative calibration can be tracked. The sites used by the 
Landsat instruments are primarily in the Saharan Desert. 
 
To date, the trending from the PICS sites has confirmed that 
most of the degradation seen in the ETM+ on-board 
calibration systems is likely not degradation of the 
instrument, but rather degradation of the calibration systems 
themselves. However, the PICS data show statistically 
significant degradation (at 2-sigma) in all the reflective 
spectral bands of up to -0.22%/year since July 2003. For the 
TM, the PICS were instrumental in updating the calibration 
in 2007 and now suggest two bands may require another 
update. The data show a statistically significant degradation 
(at 2-sigma) in Bands 1 and 3 of -0.27 and -0.15%/year, 
respectively, since March 1999. The data filtering and 
processing methods are currently being reviewed but these 
PICS results may lead to an update in the reflective band 
calibration of both Landsat-7 and Landsat-5. 
 
Index Terms— Landsat, reflective bands, radiometric 
calibration, pseudo-invariant calibration sites, ETM+, TM 
 
1. INTRODUCTION 
 
The two operational Landsat instruments, the Enhanced 
Thematic Mapper+ (ETM+) and the Thematic Mapper (TM) 
both have on-board calibration (OBC) systems. The TM 
OBC consists of an internal lamps and a shutter, which 
provides a per-scan estimate of gain and bias [2]. The 
ETM+ has a similar lamp system, as well as a full aperture 
solar diffuser panel, which is used monthly [4]. While the 
OBC methods provide very precise gain estimates (on the 
order of tenths of a percent), they have limitations due to 
their own instability. When changes are observed in the 
instrument’s response to an OBC, it is often difficult to 
determine whether the change is due to the calibrator or 
from the imager. Vicarious calibrations assist in verifying 
the on-board calibration but are much lower precision and 
are generally labor intensive. Teams from the University of 
Arizona and South Dakota State University have been 
collecting data to validate the calibration of both TM and 
ETM+ since 1999, though they only average about six 
collects per year for each team [7].  
 
Since 2001, the Landsat calibration has also been monitored 
using Pseudo-Invariant Calibration Sites (PICS) 
recommended by Cosnefroy [1]. Though not a source of 
absolute calibration, the PICS provide a long-term full-
aperture stable source. This effort has led to an update of the 
Landsat-5 TM reflective band calibration in 2007 [2] and 
increased confidence that the changes seen in the response 
of Landsat-7 ETM+ to its on-board calibration system are 
due to the calibrators and not the imager [5]. 
 
2. LANDSAT PSEUDO-INVARIANT  
CALIBRATION SITES 
 
Previous studies evaluated largely vegetation-free desert 
regions, including portions of the Saharan and Arabian 
deserts for use as calibration sites for optical sensors [1]. 
The list of sites established by Cosnefroy et al served as a 
starting point in 2001 for the Landsat effort to incorporate 
Pseudo-Invariant Calibration Sites (PICS) into the 
monitoring system. Landsat data covering these sites were 
ordered and assessed for uniformity, cloud-cover and 
availability in the archive. From the 20 initial Cosnefroy 
sites, four were chosen for long-term analysis. In the 
Landsat WRS-2 Path/Row coordinate system, they are 
181/40, 177/45, 165/47 and 201/46 which are located in 
Libya, Epypt/Sudan, Saudi Arabia, and Mauritania 
respectively. 
  
In subsequent years, the PICS analysis was extended to 
include Landsat-5 TM. The 181/40 site was the only one of 
the four sites for which there was significant coverage in the 
global archive so the Landsat-5 analysis was initially based 
https://ntrs.nasa.gov/search.jsp?R=20120014255 2019-08-30T22:00:09+00:00Z
on data from a single site. An additional site, 192/39, was 
added to the analysis after several years, because the 181/40 
imagery was generally saturated in the TM band 5 (SWIR1). 
 
A 90x90km region of interest in the center of the each scene 
was established to eliminate some non-uniform features at 
the edges of some sites. This same size region was used for 
all sites regardless of scene content. For ETM+, the region 
was centered on the nominal scene center for the individual 
Path/Row. This means that as the satellite moves east/west 
across the path, the same region of the PICS is used every 
time. However, the geometric knowledge for TM was less 
precise when this analysis was started, so the region of 
interest is based on the pixel center of the image, not on a 
specific geographic location.   
 
For ETM+, the data were processed using standard 
processing techniques to calculate radiance. For the TM 
analysis, the 90x90km regions were extracted as bias-
subtracted digital counts and the calibration gains and biases 
are applied outside of the standard processing software to 
convert to radiance1. The top-of-atmosphere radiances were 
converted to top-of-atmosphere reflectances [3].  An 
empirical correction for solar elevation angle is applied to 
the reflectances (Figure 1), to attempt to account for some of 
the systematic seasonal variations most likely due to a scene 
bidirectional reflectance distribution function (BRDF) 
effect. Studies are ongoing to generate corrections for 
changing atmospheric conditions and a physical model of 
the BRDF. The average reflectances of the region of interest 
are scaled to gain for comparison to measurements made by 
the on-board calibrators.  For ETM+, the data are scaled to 
the prelaunch gains and for TM, the gains are scaled to the 
gain from ETM+/TM cross-calibration [6]. 
 
 
3. RESULTS 
 
Figures 2 and 3 illustrate the utility of the PICS sites in the 
calibration of both ETM+ and TM. Use of the PICS has 
allowed confirmation that the decreasing gains estimated by 
the ETM+ solar panel is likely due to changes in the panel 
and not due to a change in the detectors (Figure 2). Table 1 
compares the estimated change in calibration gain since July 
2003 as determined by the solar panel and PICS. The PICS 
estimate is calculated from the weighted average of the gain 
change over time estimated by each of the four sites (192/39 
is not included). The estimate of gain change is only for data 
acquired since July 2003 as that is when the degradation of 
                                                
1 The standard processing system was originally not used for TM 
processing because the calibration gains were not trusted and the 
calibration gain could not be adjusted within the standard software. 
Since the 2007 update of the calibration gains, the processing of 
the TM PICS data could be run through standard processing but the 
switch has not been made yet. 
the solar panel changed characteristics, and the thought was 
perhaps the PICS and panel data would agree. However, 
while the ETM+ response to the solar panel is changing by 
as much as 0.7%/year, the ETM+ response to the PICS only 
changing by as much as -0.22%/year. The change is 
statistically significant in all bands at a 2-sigma level. 
Currently, a review of the data filters and corrections is 
underway, but these results could lead to an update in the 
calibration gains for the whole history of the ETM+. 
 
For Landsat-5, the PICS data were used to revise the whole 
calibration history of the TM reflective bands. After one 
calibration update based on internal calibrator data in 2001, 
the PICS study revealed that the trend predicted by the 
lamps was due to both instrument and lamp degradation. An 
update was made to the calibration gains in 2007, based 
almost entirely on the PICS data to remove the effect of the 
lamp degradation from the calibration [2]. Since the 2007 
update, the PICS continue to indicate that the TM reflective 
bands are relatively stable (Figure 3), while the lamp system 
has become so erratic as to be unusable in most bands. Table 
1 lists the gain change over time as predicted by the 181/40 
site when processed using the 2007 calibration gains for 
data acquired after March 1999. March 1999 was chosen as 
the starting point because that is when the degradation that 
started just after launch appeared to level off. However, the 
trends show that the -0.27% and -0.15%/year change in 
Bands 1 and 3 is statistically significant and hence the 2007 
Calibration Gains may need to be updated to reflect the 
continuing decline. However, the data populations are not 
evenly distributed over the time period since March 1999; 
from 1999 through 2001, the US archive only holds summer 
data of 181/40, so the whole annual distribution of sun 
angles is not available yet. An attempt will be made to fill in 
the calibration record before making any changes to the 
calibration. 
 
 
Table 1. Landsat-7 ETM+ and Landsat-5 TM gain change over 
time as estimated by the PICS and the ETM+ solar panel. The 
ETM+ results are only calculated on data since July 2003 (~4.5 
year since launch), when the scan line corrector failed. The TM 
results are for data acquired since March 1999 (~15 years since 
launch) for 181/40 only; the trend for the second site (192/39) has 
not been fully populated yet. 
 
Landsat-7 
ETM+ 
Solar Panel 
[%/year ±  
2-sigma] 
Landsat-7 
ETM+ 
PICS 
[%/year ±  
2-sigma] 
Landsat-5 TM 
PICS (181/40 
only) 
[%/year ±  
2-sigma] 
Band 1 -0.51 ± 0.02 -0.22 ± 0.06 -0.27	  ±	  0.05 
Band 2 -0.46 ± 0.02 -0.10 ± 0.05 0.03	  ±	  0.06 
Band 3 -0.52 ± 0.04 -0.10 ± 0.05 -0.15	  ±	  0.06 
Band 4 -0.68 ± 0.04 -0.11 ± 0.09 -0.03	  ±	  0.12 
Band 5 -0.11 ± 0.01 -0.17 ± 0.05 N/A 
Band 7 0.13 ± 0.02 -0.20 ± 0.10 0.02	  ±	  0.18 
Band 8 -0.56 ± 0.02 -0.09 ± 0.08  
 
 
4. CONCLUSIONS 
 
Though not an absolute calibration system, the PICS 
continue to be used to monitor the calibration of Landsat-7 
ETM+ and Landsat-5 TM in a relative sense and to 
corroborate or refute changes seen in the on-board 
calibration systems. The PICS have already been used to 
update the calibration of the TM in 2007.   
 
The latest analysis indicates degradation in both ETM+ and 
TM.  In ETM+, all bands are showing statistically 
significant degradation (at 2-sigma) of between -0.10 and -
0.22%/year. The data filtering and corrections will be 
reviewed thoroughly before making any changes, but a 
change to the lifetime calibration of ETM+ is possible.   
 
In the TM, two bands are showing statistically significant 
degradation (at 2-sigma) of -0.27 and -0.15%/year, but the 
distribution of the data over time may be contributing to the 
significance. An attempt will be made to fill out the history 
completely before making any changes. 
 
5. REFERENCES 
 
[1] H. Cosnefroy, M. Leroy, and X. Briottet, “Selection and 
characterization of Saharan and Arabian desert sites for the 
calibration of optical satellite sensors,” Remote Sensing of 
Environment, Vol. 58, pp. 101-114, 1996. 
[2] D.L. Helder, B.L. Markham, K.J. Thome, J.A. Barsi, G. 
Chander, and R. Malla, “Updated Radiometric Calibration for the 
Landsat-5 Thematic Mapper Reflective Bands,” IEEE 
Transactions on Geoscience and Remote Sensing, Vol. 46, No. 10, 
pp. 3309-3325, Oct 2008. 
[3] Landsat 7 Science Data Users Handbook, Section 11.3.2 
http://landsathandbook.gsfc.nasa.gov/data_prod/prog_sect11_3.html, 
Accessed Jun 1, 2011. 
[4] B.L. Markham, J.L. Barker, E. Kaita, J. Seiferth, and R. 
Morfitt, “On-Orbit performance of the Landsat-7 ETM+ 
radiometric calibrators,” International Journal of Remote Sensing, 
Vol 24, No 2, 2003. 
[5] B.L. Markham, K.J. Thome, J.A. Barsi, E. Kaita, D.L. Helder, 
J.L. Barker, P.L. Scaramuzza, “Landsat-7 ETM+ on-orbit 
reflective-band radiometric stability and absolute calibration,” 
IEEE Transactions on Geoscience and Remote Sensing, Vol 42, 
No 12, pp. 2810-2820, Dec 2004. 
[6] P.M Teillet, J.L Barker, B.L Markham, R.R Irish, G 
Fedosejevs, J.C Storey, Radiometric cross-calibration of the 
Landsat-7 ETM+ and Landsat-5 TM sensors based on tandem data 
sets, Remote Sensing of Environment, Volume 78, Issues 1–2, 
October 2001, Pages 39-54, ISSN 0034-4257, 10.1016/S0034-
4257(01)00248-6. 
[7] K.J. Thome, D.L. Helder, D. Aaron, J.D. Dewald, “Landsat-5 
TM and Landsat-7 ETM+ Absolute Radiometric Calibration Using 
the Reflectance-Based Method,” IEEE Transactions on Geoscience 
and Remote Sensing, Vol 42, No 12, pp. 2777-2785, Dec 2004. 
 
 
 
 
Figure 1. The empirical correction to reflectance for solar elevation angle. The figure on the left shows the gain vs. solar elevation angle for 
the original data and the corrected data. The figure on the right is the ratio of the corrected to the original. The data are adjusted to a 
reference solar elevation angle of 52.5 for ETM+ and 58.5 for TM using a linear regression of gain vs. angle (different angles were used 
because originally, the TM data did not cover an entire annual cycle). The correction is as large as 3.5% for some sun angles in ETM+ 
Band 7. The correction removes some of the annual cyclical pattern, but not all the systematic patterns are removed. Studies to characterize 
and correct for the remaining non-random variation are ongoing: BRDF and atmospheric effects are being investigated. 
  
 
  
 
Figure 2. The Landsat-7 ETM+ Band 3 calibration gains estimated by the PICS and the full-aperture solar calibrator over the life of the 
mission. The changing response to the panel is attributed to ultra-violet exposure rather than a change in the instrument since the response 
to the PICS has been more stable. The weighted average of the gain change of each of the four sites estimates that this band is degrading by 
-0.10%/year. 
 
 
 
Figure 3. The Landsat-5 TM Band 3 calibration gains as estimated by the PICS for the lifetime of the mission. The internal calibration on-
board TM is too unstable to use for calibration now, so the PICS are the primary source for relative long term calibration. Two sites are 
included on the figure, though not enough data has been assessed for 192/39 to use it as a source yet. The PICS data estimate that this band 
is degrading by -0.15%/year. 
 


Continued Monitoring of Landsat Reflective Band Calibration Using Pseudo-Invariant Calibration Sites

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