Measurements of ocean-water optical properties that are directly relevant to airborne oceanographic lidar (AOL) systems are rare. The two most important water optical properties to AOL systems are the volume scattering function (VSF) at 180 degrees and the lidar beam attenuation coefficient. An instrument has been developed, called Beta Pi, which measures, in situ, these two optical parameters. It is a self-calibrating instrument that provides an accurate measurement of both the VSF at 180 degrees and the lidar attenuation coefficient. Results from a deployment in the Gulf of Mexico near Panama City, Florida, during a Navy lidar test, are given. It was found that the VSF increases by more than 50 percent from 179 degrees to 180 degrees. A sharp enhancement, possible due to coherent backscattering, was also observed at 180 degrees with an angular width of about 0.03 degrees. Measurements made at six discrete wavelengths from 457 to 532 nm revealed that the spectral dispersion of backscattering at and near 180 degrees was substantially greater than the dispersion at 135 degrees measured with multispectral bi-static backscattering sensors. All of these results have important implications to the sea truth calibration of AOL systems.