A.

Detector and electronics

The first category is the detector chain, which relates to all electronic and detector related aspects. For this characterization mainly the instrument internal sources are used. To determine the linearity of the detector chain response the internal LED’s close to the detector are used. The LED will illuminate the complete detector homogenously. In a stable environment LEDs prove to be very stable in output. In this case the instrument is temperature stabilized, which guarantees a stable environment for the LED (after warm-up). For the measurement the LED output is kept stable and the just exposure time of the detectors is varied creating different filling levels of the detector pixels.

The aim is to not only characterize the level of linearity but also to obtain a correction to the level of 0.1%. This correction shall be used for the full in orbit dynamic range as determined by the Earth and solar spectra. Due to the LED low radiance levels, this approach requires that the detector exposure times can be set to levels which are much different from the values used for Earth / sun observation. To cover the large dynamic range, in case of the CCD’s, this requires very long exposure times, leading to long in-orbit measurements. To reduce the measurement time, the full dynamic range is therefore split over 2 LED’s with a factor 10 difference in radiance.

For the CMOS detectors this approach is not necessary because the exposure times of the CMOS can be set much shorter.

Another very important parameter is the pixel response non-uniformity (PRNU). This parameter needs to be corrected for as its dynamics can be comparable of the science product. The PRNU is caused by tolerances of the detector manufacturing process and is wavelength dependent. This wavelength dependency rules out the use of the LED measurements since these measurements are not via the spectrometer and thus the wavelength reaching the pixel is different from the wavelength that pixel will see from the earth observation scene. Therefore the instrument is equipped with a white light source delivering a smooth spectrum via the spectrometer. Knowing that the WLS has a smooth spectrum, high frequency features in the measurement will be classified as PRNU. During the C&C campaign, also a WLS as part of the OGSE will be used for this measurement.

B.

Spectral

The second category defined by ESA is the spectral calibration. Two main parameters are to be determined; the wavelength assigned to each pixel and the instrument spectral response function (ISRF) of each pixel. The latter will also give the full width half maximum of the spectral band measured by the pixel.

At first the spectral assignment was foreseen to be measured with spectral emission lamps and absorbing gas-cells. This approach has been applied to all Sentinel 5’s predecessors (GOME, SCIAMACHY, OMI, GOME-2, TropoMI). A disadvantage of this approach is that a good distribution of the spectral calibration lines is not always possible. This is illustrated in figure 3 for the FPA-3 (400-600nm) band of the GOME-2 instrument. As can be seen there is a substantial gap between pixels 200 and 400. In the picture it might not seem harmful but consider that the required wavelength allocation accuracy is 0.04 pixel.

Fig. 3.

Dispersion curve determined for the FPA-3 of the GOME-2 instrument. The diamonds indicate the spectral emission lines of the source used as fitting points.

As it is nowadays possible to determine the wavelength of the laser output with sufficiently high accuracy, this limitation can be overcome, using the tuneable laser wavelength scans required for the ISRF characterization. Since for the ISRF there will be several lines measured per pixel there will be more than enough lines available for the spectral assignment.

C.

Radiometric

The third category defined by ESA is the radiometric calibration. The approach foreseen is rather traditional using a calibrated source from NIST (1000W FEL lamp). The source, in combination with a calibrated external diffuser (ODIFF), is primarily used for the absolute radiance response at nadir. This absolute radiance response is then transferred to all other viewing angles and to the sun irradiance response using a combination of measurements with an integrating sphere and a sun simulator. How the different measurements are combined is depicted in figure 4.

Fig. 4.

Determination of the radiometric key-data from measurement data.

The absolute radiance response at nadir is determined with the FEL via ODIFF measurement. This product is combined with the relative radiance response, based on measurements with an integrating sphere, to the radiance response over the complete telescopes field of view.

For the absolute irradiance response a dedicated sun simulator (Susi) will be used. This source simulates the sun in the sense that the illumination geometry is comparable to the sun and not necessarily the irradiance level of the sun (unlike commercial sun simulators, which focus on high output flux). There are several reasons not to use the FEL source as irradiance source. The main reasons are the low UV flux of the FEL lamp and the limited calibration validity (50 burn hours), making it impossible to measure at several sun incidence angles. Since the Susi is not absolute calibrated, the irradiance calibration of the FEL is transferred using radiance measurements for both sources on nadir. The measurements with the Susi on the sun port will be performed for several angles such that the relative dependence on the sun incidence angle can be determined.

Finally the instrument BSDF needs to be determined. The instrument BSDF is the ratio between the absolute radiance response and the absolute irradiance response of the instrument. This ratio will be based on the Susi measurements for radiance and irradiance.

D.

Geometric

The fourth category is geometrical calibration. Here the viewing angle of the earth and sun port are meant as well as, for the earth port, the instrument spatial response function. The viewing angles of the sun port are determined by the sun port baffle. The sun diffuser itself does not have a field of view. The sun port baffle is designed such that for the calibration angle the sun diffuser can be fully illuminated by the sun. Outside those angles the sun diffuser can only be partly illuminated by the sun (or any other source) until at a certain angle the light is blocked completely by the baffle. For the on-ground calibration it is foreseen to only characterize and verify the clear field of view i.e. determine instrument response as function of Susi angle of incidence on the diffuser for those viewing angles where the Susi fully illuminates the diffuser. There will be a limited set of measurements going beyond the field of view to check performances. For the measurements themselves an extension (more incidence angles) of the sun simulator measurements over the sun port are foreseen.

The earth field of view measurements have to deliver input for both the line of sight with respect to the Instrument Alignment Cube (IAC) as well as the spatial response function. For this a field scan with a with a narrow field source in one direction (along track or across track) is foreseen. Tto cover the complete wavelength range, a White Light Source (WLS) will be used. For the across track viewing properties, the field will be narrow in across track direction and this will be scanned along the swath. In order to be able to measure several pixels in parallel, there will be a number of those narrow field lines across part of the swath. In the along track direction a similar scan will be made, but in this case only one narrow field is offered to the instrument. The approach is shown schematically in figure 5. Illuminating multiple pixels at the same time shortens the measurements time.

Fig. 5.

Field of view scan principle.

The measurement will give as results a spatial response function in two directions. From this function the centre of gravity will be determined to give the line of sight and the full width of half maximum will give the field of view.

Due to the presence of the polarisation scrambler, the viewing properties are expected to be polarisation dependent. Therefore, The scans will also be repeated for with polarised light to determine the polarisation dependence of the viewing properties.

E.

Others

The last category defined is “others”, which in case of Sentinel 5 covers stray-light and polarisation. For polarisation it is required to determine the first three elements of the Müller matrix. The first element of the Müller matrix is the absolute radiometric response. The second and third element will be determined relative to the first element with a scan of linear polarised light in front of the earth port.

Stray-light will be measured with a laser and with filter measurements. The laser measurements will be performed via the earth port and will result in a spot on the detector. Theoretically this is the best measurement, but in practice it is limited by the dynamic range of the detector. To overcome this a larger part of the detector has to be illuminated leading to more energy on the detector with respect to the not illuminated part (i.e. the part where the stray-light needs to be measured). This is done by using filters with a much broader wavelength band than the laser. The disadvantage of this approach is that stray-light ghost will be blurred. The filter measurements are most useful for the stray-light coming from scatter, which has a more uniform nature.

In addition to the measurements on the earth port also measurements on the sun port will be performed. These measurement are mainly to check and improve the earth port measurements since they will illuminate the complete swath, i.e. resulting in a line on the detector. Similar kind of checks will be performed with the earth port, using WLS based radiometric measurements. In this case the full spectrum is illuminated but only a small part of the swath.