In response to the requirements of the Space-Based Relay Study (SBRS) undertaken by NASA in 2013, as well as the Integrated Radio and Optical Communications (iROC) project being conducted by NASA Glenn Research Center (GRC) beginning in 2012, a calculation procedure was required to rapidly assess the operation of optical communication links originating from within deep space (in particular, from around Mars in the case of iROC), as well as within the near-Earth scenarios of LEO (low-Earth orbit) and GEO (geostationary Earth orbit) in the case of SBRS. Such an assessment included the specification of the design components of the optical system to achieve reliable communications as prescribed by one or several metrics that indicate overall system operation.
Additionally, it was also desired to be able to dynamically evaluate such optical link operation as the satellite/Earth orbital positions evolve during the mission lifetime. This provided the motivation of the creation of the link budget software. Although there are similar link budgets that have been developed, no such software was made available to NASA GRC for this purpose, so the decision was made to create a software tool that would satisfy the requirements and, most importantly, be flexible enough to update and add new capabilities as the needs arose. The unique desire of wanting the optical links to dynamically evolve as the mission orbits evolve necessitated the software to be capable of being interfaced to Satellite Tool Kit (STK).
The calculation procedure has been designed and implemented in software that enables the specification and design of a space-based optical communications system. The resulting optical link budget essentially accounts for the communication power flow through the entire optical communications channel from the transmitter to the receiver, and yields design specifications for the optical system necessary to assure reliable data transmission with desired operational metrics such as data rate, link margin, etc. The link budget accounts for PIN diode, avalanche photodiode, as well as nanowire optical detectors, differential phase-shift keying and pulse-position-modulation formats, and various levels of weak and strong atmospheric turbulence to determine the overall link bit error rates. The required uncoded and coded optical power levels are calculated from channel capacity considerations. The link budget has the capability of being interfaced with the orbital element calculations of STK to allow the dynamic description and evolution of optical link operation from any space-borne satellite within the solar system to and from the Earth.