TUZ GOLÜ: NEW ABSOLUTE RADIOMETRIC CALIBRATION TEST SITE 
S. Gurol *, H. Ozen, U. M. Leloglu, E. Tunali 
TUBITAK UZAY, ODTU Kampusu 06531 Ankara, Turkey (sdirne.gurol, hilal.ozen, leloglu, 
tunali)@uzay.tubitak.gov.tr 
Commission I, WG 1/1 
KEY WORDS: Remote Sensing, Radiometric Accuracy, Radiometry, Satellite Sensor, Calibration, Quality 
ABSTRACT: 
Calibration, absolute radiometric calibration in specific, is an important process for continuity and reliability of optical satellite data, 
since it puts the data on a standard scale and makes it compatible and comparable with the data acquired from different sensors. 
Therefore, the importance of performing a calibration campaign increased significantly. Being aware of this significance, TUBITAK 
UZAY (Space Technologies Research Institute, The Scientific and Technological Research Council of Turkey), by taking advantage 
of having such a suitable site in the country, decided to establish a new absolute radiometric calibration test site on Tuz Gold (Salt 
Lake) for the calibration and validation requirements of cal/val community in general and of the country, specifically. In order to 
evaluate the usability of the site and determine the most usable area, we first compiled a list of properties that a 
calibration/validation test site should have. We have seen that Tuz Gold fulfils most of those requirements. Later, we performed 
homogeneity analysis using Getis Statistics on MODIS satellite images of July and August 2004-2007. As a result of analysis, we 
obtained approximate usable area to be 324.026 km 2 . We also observed that, the longer duration the region is covered with water, 
the more homogeneous it is. The site will be visited annually and further analysis and field studies will be conducted. The test site is 
considered to be useful to scientists all over the world. 
1. INTRODUCTION 
Calibration is an important process for continuity, reliability 
and, hence, widespread use of satellite images from different 
sensors for earth observation applications. Although calibration 
has been performed before launch in most cases, on-orbit 
calibration should be done especially for the stable radiometric 
calibration which plays a critical role in consistent long period 
data analysis, because the characteristics of camera electronics, 
optics and detectors can shift over time. Absolute radiometric 
calibration puts the data on a standard scale and makes it 
compatible and comparable with the data acquired from 
different sensors. 
For absolute radiometric calibration reflectance-, irradiance-, 
and radiance-based techniques were developed at the University 
of Arizona in late 1980s. The main advantage of these methods 
is that they provide another effective way of calibration in 
addition to preflight and onboard calibration methods. 
The reflectance based technique known as vicarious calibration 
depends on measurements of the surface reflectance of a test 
site at the time of sensor overpass. Therefore; scene acquisition 
time and satellite attitude should be determined for the satellite 
for the specific test field. At the same time of the scene 
acquisition, ground data collection is performed by field 
spectrometers and atmospheric parameters are measured to 
obtain reflectance, BRDF, aerosol loading, aerosol size 
distribution, column ozone, column water vapor etc. From these 
measurements top of the atmosphere radiance at the sensor 
within its spectral bands are computed. Sensor DN values and 
at-sensor-radiance is then used to calculate radiometric 
calibration parameters. 
Up to date, many such test campaigns in different regions were 
held and the requirements of this process were reported in the 
literature. Especially CEOS (Committee on Earth Observation 
Satellites) WGCV (Working Group on Calibration and 
Validation) listed out the requirements and started for the 
standardization studies (best practices) of calibration test sites 
including characterization, methodology, and instrumentation to 
be helpful to the cal/val researchers. Also, USGS prepared a 
catalogue of World-wide test sites for sensor characterization 
(Chander, 2008) and (Goryl, and Danne, 2008) introduced 
cal/val portal which is also very useful for the cal/val 
community. 
In this article, a study towards establishing a new absolute 
radiometric calibration test site, Tuz Gold, specifically, initial 
studies done to assess its usability is presented. Firstly site 
selection criteria will be mentioned in Section 2. General 
information about Tuz Gold and its characteristics will be 
evaluated as mentioned in the key properties list of CEOS 
WGCV template, in Section 3. In Section 4, the analysis of 
homogeneity, an important criteria for determining the test site 
on the lake, will be given. Homogeneity analysis, also taking 
into consideration monthly water coverage, will be presented. 
Finally, conclusions will be given in the last section. 
2. SITE SELECTION CRITERIA 
Considering test site selection criteria stated by cal/val 
community up to date (Morain, and Budge, 2004; Teillet et. al., 
2007; Thome, 2002; Scott et. al., 1996), the following list has 
been compiled: 
Corresponding author.