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 and GEO (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 for the creation of this link budget software. Although there are similar link budgets that have been developed at government expense, 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 the Satellite Toolkit (STK).
A 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 assessment tool takes into account all the sources of deleterious noise that enter into the communications process, such as electronically generated noise in the optical detector, and stray optical irradiance from external sources. The link budget can be interfaced with the orbital element calculations of the STK to allow the dynamic description and evolution of optical link operation from any spaceborne satellite within the solar system to and from the Earth. The current version implements PPM and DPSK modulation types that can use PIN, APD, or nanowire optical detectors in the presence of atmospheric turbulence. Coded and un-coded link power margins are provided.
This software evaluates the operation of a space-based optical communications system based on the design specifications of the satellite and ground station. By providing an accurate accounting of the power flow through such a communications system, it can be used for the design as well as specification of optical satellite systems. With the capability of its interface to STK, it can be used to evaluate such optical communications systems placed anywhere in the solar system. Whereas Version 1 of this methodology can be used to give a first order analysis of optical communications scenarios, this Version 2 provides much more fidelity to the assessment of such systems.
The assessment tool provided here transcends the first version of this tool in that it calculates for three different optical detector types (PIN, APD, and nanowire), calculates for two optical modulation types (PPM and DPSK), incorporates the effect of atmospheric turbulence, provides for many intermediate system metrics, and discerns between uncoded and coded channel power margins through the use of channel capacity considerations.