Characterizing the Performance of Haleakala as a Ground Site for Laser Communications

Radio Frequency (RF) signals have been relied on exclusively and successfully to communicate with spacecraft since satellite communications began nearly 60 years ago. However, missions now demand higher data rates to meet their data collection requirements. In response to this need, several organizations have begun to take steps to increase the data capacity of future missions by developing laser communications terminals and operational concepts for future missions. For example, NASA’s Lunar Laser Communications Demonstration (LLCD) successfully demonstrated high data rate communications links to and from the LADEE satellite orbiting the moon during the Fall of 2013. As a next step, the Laser Communication Relay Demonstration (LCRD) will build upon the experience gained from LLCD and perform multi-year testing of Free-Space Optical Communications (FSOC) from geosynchronous orbit. Planning for these missions has included identifying candidate ground station locations, quantifying the impacts of the atmosphere on the data links, and developing operational concepts for mitigating transmission losses due to clouds, turbulence, and aerosols. Since space-to-ground optical communications are adversely affected by the presence of clouds, turbulence, and other atmospheric phenomena, it is important to study the effects of the atmosphere on the communications link. To support this, Northrop Grumman is leading a campaign to measure and model the atmospheric effects on the link between a ground station on the summit of Haleakala and a satellite in geostationary orbit. Part of this effort involves using a modified version of the Weather Research & Forecast (WRF) model to generate long-term climatologies of optical turbulence parameters as well as to characterize the atmosphere along line-of-sight (LOS) from the ground station to the satellite during operations to be used as a link diagnostic tool. While ground-based instruments can be used to measure the effects of turbulence integrated along the entire LOS, they cannot generally be used to identify the vertical structure of turbulence. In this work, WRF is used to generate a three-dimensional representation of Cn and other atmospheric parameters in both the planetary boundary layer and the free atmosphere. This allows for the characterization of Cn along the entire portion of the LOS below 20-km above mean sea level along with estimates of the Fried Coherence length (r0) and other seeing parameters along the LOS. In addition, a suite of ground-based sensors will be deployed, including a meteorological station, a whole-sky imager, and a ceilometer. Their measurements will be combined with output from WRF to support mission planning and the development of operational concepts for mitigating link outages. In particular, the in situ cloud data will be used along with multispectral geostationary satellite imagery and WRF model soundings to characterize and predict cloud heights and cloud encroachment over the summit of Haleakala. For this work, the WRF model is configured to run at 1-km horizontal resolution over a domain that includes the major observatories on the Big Island of Hawaii as well as Haleakala on Maui. Results from this work will be used to quantify the effects of the atmosphere on FSOC communications, diagnose link disruptions, and to develop atmospheric mitigation strategies.