In September 2013, NASA’s Lunar Atmosphere and Dust Environment (LADEE) spacecraft was launched with the main scope of collecting information about the lunar atmosphere. The spacecraft also hosts the Lunar Laser Communications Demonstration (LLCD), which has successfully demonstrated bidirectional communications based on 1550 nm laser technology and PPM Modulation.
Thanks to a technical assistance agreement with MIT and a cooperation agreement with ESA, DLR participated in the measurement campaign performed at ESA’s Optical Ground Station in Tenerife (see Figure 2 left), where optical links were established to LADEE spacecraft.
The Optical Communications Group in the Institute of Communication and Navigation contributed with power measurements and the estimation of the Fried parameter using focus camera images. The channel information obtained through these measurements is valuable because it characterizes the atmospheric turbulence during the first downlinks from Moon at 1550 nm.
The optical power measurement device was mounted at the focus of the 20 cm side telescope, whereas the focus images were captured by the tracking camera through the one meter diameter telescope. The measurements were performed at several elevation angles during several days.
In the right graphic of Figure 2, an example of measured received downlink power is plot. The measurement was performed after midnight and the influence of the background light from the moon surface can be clearly seen in the power vector. The mean received power of the data signal was around 20 pW, except before the end of the link, around one o’clock, when the transmitted power was increased by a factor of two.
The strength of the turbulence can be estimated from focus speckle images. In Figure 3, two focus images under different turbulence conditions are plot: the turbulence was stronger in the right image than in the left one. The data analysis is currently ongoing.
Figure 2. ESA-OGS 1 meter Telescope (left) and measured power vector (right)
Figure 3. Focus spot images at two different turbulence conditions