A.3 Laboratory calibration
The laboratory calibration of TRMM LIS is discussed in detail inKoshak et al. [2000], and this was the same calibration approach applied to the spare LIS unit (i.e., ISS LIS). The calibration consisted of four main efforts: (1) a static response test, a (2) transient response test, (3) a spectral test, and (4) an FOV test. Additional elements of the calibration pertinent to LIS performance characteristics are discussed in Boccippio et al. [2002].
The calibration tests, which are referred to here as the original calibration (OC), were carried out on both the original TRMM LIS and the spare unit (the present ISS LIS) in the summer of 1997. TRMM LIS was subsequently launched to orbit, while the spare unit was stored in a safe box in an environmentally controlled facility for many years, until it was integrated on STP-H5. In the summer of 2014 and prior to the integration on STP-H5, a retest calibration (RC) was performed on the spare unit to determine if there were any significant changes in the OC given the many years that the unit was in storage. The RC instrumentation and procedures employed were made as similar as possible to those employed in the OC, but unfortunately the OC and RC methodologies were not identical.
A brief overview of the OC tests applied to both the TRMM LIS and ISS LIS are provided below:
As mentioned previously, the RC methodology and equipment were not identical to the OC. In the fall of 2013, prior to beginning the RC, it was deemed necessary to upgrade much of the equipment employed in the OC, since these were out of date and were no longer compatible with current technology. For the static response test, two integrating sphere systems were procured. The first was a 12-inch sphere with a large aperture which was intended to allow uniform illumination of a larger portion of each quadrant of the ISS LIS instrument. The second was a 6-inch sphere that was comparable but not the same size as the 8-inch sphere used in the OC. This second sphere was acquired for the purposes of transient testing, as it had a removable port in the back for attaching an LED.
Ultimately the 12-inch sphere was deemed unusable for the static response test because the large opening allowed for “hotspots” where the luminosity was greater surrounding the tungsten bulbs. The 6-inch sphere worked for testing the static response of the instrument; however, it only covered a fraction of each of the quadrants. After comparing the results with the legacy calibration, it was noticed that there was a form factor difference, so the legacy 8-inch sphere was then used to determine if the values were still the same as the legacy calibration. Using the legacy sphere as a one-to-one comparison with the previous calibration was a success.
Use of the 6-inch sphere for transient testing was originally the plan; however, after inserting the newly painted LED insert there were noticeable differentials in the reflectivity of the new insert, especially around the edges. It was decided to use the LED as the source for the backgrounds for each pixel and then on top of that send the pre-programmed transient signal. This allowed for a very controllable and fluid process for the transient calibration.
For the spectral test, the grating monochromator used in the OC could no longer be used because the controller was no longer available, and the connections were obsolete. Instead, a new monochromator was employed for the RC. The FOV test for the RC was performed in much the same way as in the OC, and the results were essentially identical.
ISS LIS alignment measurements were conducted in the RC. The alignment measurements were obtained by illuminating different sides of a mirror-faced alignment cube (attached to the outside of the LIS sensor head assembly) with a theodolite; this allowed determination of the overall rigid alignment of the LIS lens/CCD systems with the STP-H5 module and ISS platform.
While the RC procedure was acceptable, it was not optimal. The intent of the RC was to ensure that nothing significantly changed with the instrument during the years in storage. For expediency, and because the RC results showed no significant changes from the OC results for TRMM LIS (Fig. A1), the OC results for the TRMM LIS were applied in the ISS LIS processing code, which produces the version 1 dataset available at the GHRC.
However, a plan is being implemented to replace the TRMM LIS OC results in the ISS LIS processing code with the ISS LIS OC results, since identical procedures could not be followed due to the passage of time. In particular, the RC static response test was incomplete because it did not cover all pixels in the CCD array; it only illuminated a small circular portion in the center of each quadrant. This leaves some lingering calibration uncertainties in the version 1 ISS LIS dataset. The ISS LIS science team is working to retrieve the digital OC calibration data for the spare unit, and before updating the ISS LIS processing code the ISS OC and RC results will be compared in detail, as well as with the OC results for TRMM LIS. Based on initial analysis, an improvement of 2-5% in key instrument performance parameters (e.g., flash detection efficiency, flash false alarm rate, geolocation accuracy, and optical amplitudes) is expected after a future switch to the ISS LIS OC.