  We propose a tunable laser-based satellite-mounted spectrophotometric and
absolute flux calibration system, to be utilized by ground- and space-based
telescopes. As spectrophotometric calibration may play a significant role in
the accuracy of photometric redshift measurement, and photometric redshift
accuracy is important for measuring dark energy using SNIa, weak gravitational
lensing, and baryon oscillations, a method for reducing such uncertainties is
needed. We propose to improve spectrophotometric calibration, currently
obtained using standard stars, by placing a tunable laser and a wide-angle
light source on a satellite by early next decade (perhaps included in the
upgrade to the GPS satellite network) to improve absolute flux calibration and
relative spectrophotometric calibration across the visible and near-infrared
spectrum. As well as fundamental astrophysical applications, the system
proposed here potentially has broad utility for defense and national security
applications such as ground target illumination and space communication.
