Challenges for In-Flight Calibration of Thermal Infrared Instruments for Earth Observation

Satellite instruments operating in the thermal infrared wavelength range >3 µm provide information for applications such as land surface temperature (LST), sea surface temperatures (SST), land surface emissivity, land classification, soil composition, volcanology, fire radiative power, cloud masking, aerosols, and trace gases. All these instruments are dependent on blackbody (BB) calibration sources to provide the traceability of the radiometric calibration to SI (Système International d’Unités). A key issue for flight BB sources is to maintain the traceability of the radiometric calibration from ground to orbit. For example, the temperature of the BB is measured by a number of precision thermometers that are calibrated against a reference Standard Platinum Resistance Thermometer (SPRT) to provide the traceability to the International Temperature Scale of 1990 (ITS-90). However, once calibrated the thermometer system is subject to drifts caused by on-ground testing, the launch and space environments. At best the uncertainties due to thermometer ageing can only be estimated as there is no direct method for recalibrating. Comparisons with other satellite sensors are useful for placing an upper limit on calibration drifts but do not themselves provide a traceable link to the SI. In this paper, we describe we describe some of the technology developments, including phase change cells for use as reference standards, thermometer readout electronics and implementation of novel coatings, that are in progress to enhance the traceability of flight calibration systems in the thermal infrared.

EFFECTS OF VARIATION IN MEASUREMENT CHAIN ON TEMPERATURE MEASUREMENT CALIBRATION WITH RESISTANT TEMPERATURE SENSORS

Temperature is one of the most important key parameter to consider in measurement and mechanical engineering, because every measurement has to be conducted with reference to standard temperature conditions (20 °C, ISO 1). Strictly speaking, almost every measurement depends on the accuracy of the temperature measurement, which requires proper calibration. Therefore, standards list detailed criteria to fulfil temperature calibration with high precision. In fact, any calibration is only valid, if the whole measurement chain is taken into account. This would make recalibration necessary with each variation of the components in the measuring set-up (varying cable length, different measurement channel etc.), which is time-consuming or even impossible in practice. For that reason, this paper presents a practicable calibration strategy, which specifies each component individually and later combines the calibration results according to the composition of the measurement chain. This provides a fast and useful way to achieve the required accuracy of temperature measurement. The examined, exemplary measurement chain consists of an industrial platinum resistance thermometer (IPRT), cables with different lengths, an electrical amplifier and a reference temperature calibrator.

EVALUASI NON-LINIEARITAS JEMBATAN TAHANAN PADA SISTEM PENGUKURAN TERMOMETER TAHANAN PLATINA STANDAR

<p>Evaluasi non-liniearitas Jembatan Tahanan MI 6010C telah dilakukan di Puslit Metrologi LIPI menggunakan sebuah <em>resistance bridge calibrator</em> (RBC) dengan tujuan untuk menentukan nilai kontribusi ketidakpastian tipe B karena penggunaan indikator Jembatan Tahanan ini dalam proses kalibrasi <em>standard platinum resistance thermometer</em> (SPRT). RBC tersebut dihubungkan dengan lengan penghubung Rx dari Jembatan Tahanan. Selanjutnya, nilai tahanan dari RBC—sebanyak 34 kombinasi tahanan—dibandingkan dalam bentuk rasio dengan nilai tahanan dari resistor standar 100 Ω, yang dihubungkan dengan lengan penghubung Rs. Nilai rasio dari seluruh kombinasi tahanan kemudian diperoleh dan residu—selisih antara nilai rasio terukur dan terhitung—akan didapatkan untuk dinyatakan sebagai kesalahan penunjukan rasio dari Jembatan Tahanan. Selanjutnya metode <em>fitting liniear</em> digunakan untuk mengoreksi residu sehingga varian dari residu tersebut bernilai minimum. Akar kuadrat dari varian dinyatakan sebagai non-liniearitas Jembatan Tahanan. Hasil pengukuran menunjukkan bahwa Jembatan Tahanan tersebut masih laik untuk digunakan dengan nilai non-liniearitas sebesar 0,7 ppm, yang melingkupi rentang pengukuran rasio tahanan dari 0,16 hingga 1,16. </p>