Autonomous vicarious calibration based on
automated test-site radiometer
Ganggang Qiu (邱刚刚)
1,2
, Xin Li (李 新)
1,
*, Xiaobing Zheng (郑小兵)
1,
**,
Jing Yan (闫 静)
1
, and Yangang Sun (孙彦港)
3
1
Key Laboratory of Optical Calibration and Characterization, Anhui Institu te of Optics and Fine Mechanics,
Chinese Academy of Sciences, Hefei 230031, China
2
University of Sciences and Technology of China, Hefei 230031, China
3
Shanghai Maritime University, Shanghai 201306, China
*Corresponding author: xli@aiofm.ac.cn; **corresponding author: xbzheng@aiofm.ac.cn
Received August 12, 2016; accepted October 21, 2016; posted online November 23, 2016
We employ the in-site automated observation radiometric calibration (AORC) approach to perform vicarious
calibration, which does not require the manual efforts of a field team to measure the surface conditions. By using
an automated test-site radiometer (ATR), the surface radiance at any moment in time can be obtained. This
Letter describes the AORC approach and makes use of data to compute top-of-atmosphere radiance and com-
pare it to measurements from the Moderate Resolution Imaging Spectroradiometer. The result shows that the
relative deviation is less than 5% and the uncertainty is less than 6.2%, which indicates that the in-site AORC
maintains an accuracy level on par with traditional calibration.
OCIS codes: 120.0280, 010.0280, 030.5620.
doi: 10.3788/COL201614.121201.
Vicarious calibration, in place of laboratory calibration
[1,2]
,
is the process of determining the radiome tric calibration
of an on-orbit or aircraft sensor using an external
source. During a sensor overpass, personnel are present
at a test-site to make in situ measurements of the atmos-
pheric and surface conditions
[3–5]
. These data are inputed
to a radiative transfer code, which then computes top-of-
atmosphere (TOA) spectral radiances. Surface inhomo-
geneity, surface bi-directional reflectance distribution
function (BRDF), clouds, haze, and the radiative transfer
relationship between the surface and aperture are the
major error contributors to the process
[6,7]
. These limit
vicarious calibration accuracy to a 3%–7% level
[2,3–5]
.
Nevertheless, vicious calibration is useful to supplement
on-board calibration and to provide an independent
assessment of radiometric accuracy
[8,9]
.
A drawback of the traditional approach to vicarious cal-
ibration is the need to deploy a ground truth team to
collect measurements at the time of sensor overpass
[10,11]
.
The field campaigns are limited by the remote locations
of the calibration sites, personnel availabil ity, foul
weather, and equipment failures
[7]
. The act of driving or
even walking on the calibration site can change its reflec-
tance
[2,7]
. These difficulties lead to lack of temporal data,
making it impossible to establish long-term trends in
sensor performance. With the sharp increase in satellite
numbers and species, this difficulty is exacerbated
[8]
. The
concept of performing the automated vicarious calibration
approach was developed to address these concerns.
The automated approach to the reflectance-based
method aims at collecting data with a greater temporal
sampling rate in the absence of ground personnel, while
maintaining radiometric accuracy on par with manned
field campaigns
[2,7,8]
. We employ permanent automated
instruments, such as an automated test-site radiometer
(ATR), as shown in Fig.
1(a), for surface radiance
[12]
,
a Cimel sun photometer
[13–15]
, as shown in Fig. 1(b), for
atmospheric characteristics
[16,17]
, and an automated
diffuser-to-globe irradiance meter, as shown in Fig.
1(c),
for the field diffuser-to-globe factor, as a prototype of
the automated vicarious calibration system (AVCS).
These were deployed on the edge (94.41°E, 40.09°N) of
the China Radiometric Calibration Site (CRCS), situated
on a homogeneous section of alluvial fan at the west of
Dunhuang city in August 2015, to determine the feasibil-
ity of implementing automated vicarious calibration. This
Letter documents the use of AVCS and the process of
automated observation radiome tric calibration (AORC)
compared to the reflectance-based method and reports
the TOA radiance in comparison with Moderate Resolu-
tion Imaging Spectroradiometer (MODIS) imagery.
The ATR was developed and used to measure the sur-
face radiance, which samples the spectrum at 8 indepen-
dent bands, with interference filters with bandwidth
ranging from about 20–40 nm, coupled with silicon and
Fig. 1. AVCS. (a) ATR, (b) Cimel sunphotometer, and (c) auto-
mated diffuser-to-globe irradiance meter.
COL 14(12), 121201(2016) CHINESE OPTICS LETTERS December 10, 2016
1671-7694/2016/121201(5) 121201-1 © 2016 Chinese Optics Letters