Is CASPR free?
Yes, CASPR was developed with funds from NASA, and is provided free of charge. All we ask is that you acknowledge it appropriately in any publications that result from its use. See the User's Guide for details on how to reference the software.On what computer platforms will CASPR run?
The code is written in IDL (Interactive Data Language) and C so it will run, in theory, on any computer that has a C compiler and the IDL environment. IDL is a commercial product, so it won't be available in as many as C. Additionally, the C components must be compiled in a way that allows them to be called from IDL. For most Unix platforms this is not a problem, and instructions on doing this are given in the User's Guide. For PCs, however, the procedures are under investigation.Are all the algorithms fully validated?
No. Most are, but the cloud optical depth and particle effective radius retrievals are experimental. The nighttime retrievals have the largest uncertainties. New procedures are currently under development. All algorithms arel described in the Reference Guide.CASPR does a lot of things well but doesn't do anything perfectly. There are three broad problems worth mentioning here. First, everything depends on cloud detection, which sometimes borders on being as much an art as a science when working in the polar regions with the AVHRR. We are, indeed, trying to squeeze water from a stone. We do not claim to have solved the cloud detection problem, but rather provide methods that work reasonably well most of the time. Second, there are fundamental difficulties in estimating some parameters at extreme viewing and illumination geometries common to the high latitudes. We're venturing into territory that is difficult to model so beware. Third, the retrieval of cloudy sky parameters requires temperature and reflectance values underneath the clouds. CASPR interpolates clear sky values to cloudy areas. This generally works but can result in large uncertainties in very cloudy areas. Cautionary notes are given throughout the Reference Guide. Please do not ignore them!
Is there a GUI (graphical user interface)?
Yes. IDL widgets have been constructed to aid in running CASPR, if desired. This is particularly useful for new users. However, you'll probably find the IDL procedure calls to be relatively straightforward after you work with them a few times.How much memory and disk space does it require?
CASPR requires at least 30 MB of memory to process a 512 x 512 pixel image, more if you want to use the time series cloud detection approach. More precise requirements are given here.What are the recent changes?
See the detailed revision history.Why are there areas indicated by the cloud mask as cloudy but without values for cloud properties and radiative fluxes?
There are a number of conditions for which retrievals are not done, so not all pixels identified as cloudy in the cloud detection step will have retrievals for other parameters. For example, with a very low sun and a bright surface cloud optical depth retrievals often fail and are set to the missing value. Similarly, the uncertainty in the surface albedo retrieval for solar zenith angles between 80 and 90 degrees is very high, so no retrieval is done. Of course, when any cloud or surface properties are missing, radiative fluxes cannot be calculated so they will also be missing.I see messages about floating point errors in the 'fill_clear' procedure. Should I be worried?'
No. Pixels for which floating point errors occur are set to missing. In 'fill_clear' this appears to be happening in IDL's kriging procedure, and it does not (appear to) affect the CASPR results.