# _Sci_
## 2 Materials and Methods
### History of Laser Employment as Link Source
#### 2.1.2 Early Twenty-First Century  
The advanced relay and technology mission satellite laser communications capability was supplied by the optical payload laser experiment and the laser-utilizing communications equipment. Both systems provided source laser beams, at ten milli-watts and forty milli-watts, respectively. However, a lesson learned from the semiconductor intersatellite link experiment was that even with the laser-utilizing communications equipment beam's quadruple power, it failed to provide sufficient irradiance at ground-to-geosynchronous orbit ranges to perform initial acquisition [37]. However, the semiconductor intersatellite link experiment demonstrated a downlink bit error rate of \(10^{-10}\)[37]. The semiconductor intersatellite link experiment did, however, reinforce a foundational advantage of laser communications over radio frequency communications: that laser communications requires less size, weight, and power for a given data rate [36]. Demonstrated years later was a 2.5 giga-bits per second communications link with a roughly three decibel link margin across a range of forty-two-thousand kilometers, a capability that can be made using a two-point-two meter antenna for Ka-band (weighing one-hundred fifty-three kilograms), a one-point nine meter antenna for a millimeter band (weighing one-hundred thirty-two kilograms), and a ten centimeter optical system for a 1.55 micrometer wavelength (weighing sixty-five kilograms) [37]. In 2013, the U.S. national air and space administration began examining laser communications in earnest, focused in support of the further exploration of Mars and beyond. That year, the administration launched the lunar laser communications demonstrator [42; 43], a high-rate demonstration of space laser communications [42]. _\"The Lunar Laser Communication Demonstration (LLCD) was conducted on NASA's Lunar Atmosphere and Dust Environment Explorer (LADEE) satellite that launched in late 2013 [41]. The LLCD payload demonstrated optical communication in the 1.5 \(\mu\)m band utilizing pulse position modulation (PPM) with 16 slots (16-PPM) to downlink data from the moon to a receiver on Earth at 622 Mbps. The uplink from the optical ground terminal on Earth utilized 4-PPM to uplink data at 20 Mbps to LLCD [40]. LADEE [40; 41] was a small satellite that weighed 383 kg at launch and the entire spacecraft consumed 295 W of power during its mission\" [39]  
Figure 1: Satellite uplink and downlink with terrestrial stations from [8]: 0.6 m transmitter uplinking at 514.5 nanometers and 1.2 m receiver downlinking at 830 nanometers. The lunar laser communications demonstrator demonstrated is shown in Figure 2, a six-hundred-hwenty-two mega-bits per second datalink at a range of four-hundred-thousand kilometers; an achievement at ten times the approximate range of a ground-to-geosynchronous orbit link while overcoming a one-hundred fold link loss [43]. Late in 2013, for thirty days the lunar laser communications demonstrator communicated in duplex from a lunar orbiting satellite to both earthbound locations, as well as the lunar atmosphere and dust environment explorer. This feat is illustrated one of the benefits of laser communications and very high data rates, since the achievement was hailed as both the highest data rate and the longest distance laser communications had yet achieved. Lunar laser communications demonstration demonstrated an order of magnitude higher data rate uplink, twenty mega-bits per second (error-free), than the best Ka-band radio system [43]. NASA's follow-on to lunar laser communications demonstration is the laser communications relay demonstration mission. The laser communications relay demonstration will use laser communications to transmit at rates from ten to one-hundred-times faster than radio frequency communications [46; 47; 48].