Cyclone Global Navigation Satellite System (CYGNSS) Calibration Methodology

The Cyclone Global Navigation Satellite System (CYGNSS) will launch eight new satellites in October of 2016 which will enable unprecedented retrieval of ocean surface wind speeds during the development of tropical cyclones in order to facilitate significant advancements in the understanding of ocean-atmosphere interactions, and ultimately to increase skill at forecasting hurricane track and intensification.

Here, we address the calibration of the CYGNSS science data products. Specifically, we present a methodology for determining which zenith antenna look directions are contaminated by the brightness of the galactic plane of the Milky Way, and thus unavailable to perform required calibration. For determining the location of the galactic plane, we employ a map of L-band equivalent brightness temperatures (TB) of the sky compiled from radio astronomy surveys [1]. Within this data is a band of high TB associated with the galactic plane.

After applying a 2D Gaussian smoothing filter to the data, we define a threshold of 4 K, above which are the bright Milky Way values that interfere with calibration (M), and below which are acceptable cold sky values (C). We represent the look direction of the zenith antenna, as well as the location of all M and C pixels, as unit vectors in Earth-Centered Inertial coordinates. We measure the angles between the look direction and all M and C pixels. Angles below 90 degrees correspond to points that are within the antenna's view. The cosine of these angles are evaluated in order to weight their significance owing to antenna gain. Ultimately, we use the ratio of the weight of the Milky Way points compared to that of the total sky (Milky Way plus cold sky) as a robust evaluation of how much the Milky Way contaminates the antenna view for the given look direction.

[1] Dinnat, E.P., Le Vine, D.M., Abraham, S., & Floury, N. (2010). Map of sky background brightness temperature at L-band. Retrieved from http://oceancolor.gsfc.nasa.gov/AQUARIUS/DinnatEtAl2010/.