System design and demonstration of a CCD-based solar spectroradiometer

Abstract

The Atmospheric Remote Sensing Lab at the University of Arizona's Electrical and Computer Engineering Department has been involved with the study and measurement of atmospheric gases and aerosols for many years. The research has been conducted using instruments designed and constructed by the lab. This dissertation presents a system design for the next step in the evolution of spectroradiometers designed by the Atmospheric Remote Sensing Lab. The design draws upon the lessons learned from previous generations of radiometers and from the requirements of ongoing research. The proposed spectroradiometer uses an inexpensive CCD as the detector and takes advantage of modern processors and re-programmable CPLDs. The new design employs an embedded DSP in a novel way; it provides high level control over the CCD detector, receives serial ADC data and communicates with a Host computer. Through the use of one of the serial channels, the DSP identifies when to accumulate charge in the CCD and when to dump it. This controlled sampling allows charge to accumulate from adjacent cells internal to the CCD, improving the SNR in regions of poor spectral transmission. Since the charge accumulate/reset is controlled by the DSP through software, the sequence is programmable using the host computer interface and can be dynamically re-programmed to accommodate changing atmospheric conditions. A re-programmable CPLD isolates the DSP from the detector hardware and provides low level control of the detector assembly. The CPLD accepts high level commands from the DSP and generates the low level clocks and control signals used by the CCD and ADC. This capability permits the CPLD to be re-programmed to accommodate various CCDs and ADCs available today and in the future without altering the Host communication, control or analysis software. The capabilities of the instrument can be altered by downloading new software to the embedded DSP. Provisions have been made to download software or configuration data to the instrument and execute from RAM. Once correct operation of the software has been verified, it can copied to non-voltile memory.