Jennifer M. Nappier

On the Physical Realizability of Hybrid RF and Optical Communications Platforms for Deep Space Applications

There are hefty constraints levied on all space systems, and some of the most fundamental of them are purely physical. Much of the literature published on optical communications for space data systems focuses on the increased capabilities that optical communication systems have to offer, but ignores the practical infusion strategies of the technology into an operable network. Significant resources have been spent over the better half of the last century across multiple governments to establish robust radio frequency (RF) infrastructure around the world, and any new spacecraft will include provisions to utilize that investment. Therefore, every optical communication system flown will be a functional subset of a larger communication architecture dominated by RF. The key is to optimize the architecture and integrated components into a system that can be used to support hybridized RF and optical communications within the same asset, throughout diverse atmospheric (weather) and in-space conditions. The focus of this paper is on the physical realizability of a hybrid optical and RF communications system, and its ability to increase mission capability without imparting extra burden to the host spacecraft in the form of either increased physical mass or power requirements or additional operational requirements. The space applications may feature a hybrid RF/optical aperture this conjunction of a telescope and an antenna has been dubbed the teletenna. The teletenna is a single co-boresighted aperture that combines the radio and optical beams together into one physical package. The tradeoffs associated with the teletenna optimization are discussed, as well as the pointing requirements and strategies. Initial optical terminals on spacecraft have utilized Earth-based beacons Aerospace Technologist in Telecommunications, Optics and Photonics Branch (LCP), 21000 Brookpark Road/Mail Stop 54-1, AIAA and Technical Committee Member Aerospace Technologist in Telecommunications, High Frequency Branch (LCH), 21000 Brookpark Road/Mail Stop 54-1 Senior Aerospace Technologist in Telecommunications, Architectures, Networks and System Integration Branch, (LCA), 21000 Brookpark Road/Mail Stop 54-1 Aerospace Technologist in Data Systems, Information and Signal Processing Branch (LCI), 21000 Brookpark Road/Mail Stop 54-1 Research Aerospace Technologist in Data Systems, Networking and Architectures Branch (LCA), 21000 Brookpark Road/Mail Stop 54-1 Research Aerospace Technologist in Telecommunications, Information and Signal Processing Branch (LCI), 21000 Brookpark Road/Mail Stop 54-1 Aerospace Technologist in Mechanical Components, (LMT), 21000 Brookpark Road/Mail Stop 86-2 to establish beam tracking, but eventually beacon-less pointing will be developed. Investments in technologies to implement autonomous on-board navigation (and maneuvering) will permit a reduction in dependence on ground-based tracking, ranging, trajectory/orbit/attitude determination and maneuver planning support functions resulting in increased intelligence and autonomy of the complete payload. Innovative approaches include exploiting the optical communications terminal to perform navigational measurements such as star sighting, or a star tracker technology developed to process images at very high data rates. This paper discusses how such a beaconless strategy may be incorporated to work with the proposed hybrid communications system, and highlights the synergistic benefit to both communication and navigation functions for the purpose of decreased size, mass and power burden to users. A common constraint between both deep space and near Earth scenarios is mass. As elegant as a proposed optical communications solution may be, it must be shown that the final product is not so cumbersome as to outweigh the benefits, and that includes the necessary RF technology to complete the full communications payload. We establish a stringent goal of compressing the aforementioned highly capable hybrid RF/optical system into a package which installs within a comparable size, mass, and power envelope as the telecommunications subsystem of Mars Reconnaissance Orbiter (MRO). The goal of this paper is to demonstrate that with modern technology our goal is feasible and therefore has a place in today’s space data systems as well as building a bridge to tomorrow’s.

Dual-pulse pulse position modulation (DPPM) for deep-space optical communications: Performance and practicality analysis

Due to its simplicity and robustness against wave-front distortion, pulse position modulation (PPM) with photon counting detector has been seriously considered for long-haul optical wireless systems. This paper evaluates the dual-pulse case and compares it with the conventional single-pulse case. Analytical expressions for symbol error rate and bit error rate are first derived and numerically evaluated, for the strong, negative-exponential turbulent atmosphere. The capacity of the turbulent FSO channel modeled as a Z-channel is evaluated, and throughput, bandwidth efficiency and energy efficiency of Dual-pulse and single-pulse PPM are subsequently assessed. It is shown that, under a set of practical constraints including pulse width and pulse repetition frequency (PRF), dual-pulse PPM enables a better channel utilization and hence a higher throughput than its single-pulse counterpart. This result is new and different from the previous idealistic studies that showed multi-pulse PPM provided no essential information-theoretic gains over single-pulse PPM.

Ka-Band Link Study and Analysis for a Mars Hybrid RF/Optical Software Defined Radio

The integrated radio and optical communications (iROC) project at the NASA Glenn Research Center (GRC) is investigating the feasibility of a hybrid RF and optical communication subsystem for future deep space missions. The hybrid communications subsystem enables the advancement of optical communications while simultaneously mitigating the risk of infusion by combining an experimental optical transmitter and telescope with a reliable Ka-band RF transmitter and antenna. The iROC communications subsystem seeks to maximize the total data return over the course of a potential 2-year mission in Mars orbit beginning in 2021. Although optical communication by itself offers potential for greater data return over RF, the reliable Ka-band link is also being designed for high data return capability in this hybrid system. A daily analysis of the RF link budget over the 2-year span is performed to optimize and provide detailed estimates of the RF data return. In particular, the bandwidth dependence of these data return estimates is analyzed for candidate waveforms. In this effort, a data return modeling tool was created to analyze candidate RF modulation and coding schemes with respect to their spectral efficiency, amplifier output power back-off, required digital to analog conversion (DAC) sampling rates, and support by ground receivers. A set of RF waveforms is recommended for use on the iROC platform.

Unique challenges testing SDRs for space

This paper describes the approach used by the Space Communication and Navigation (SCaN) Testbed team to qualify three Software Defined Radios (SDR) for operation in space and the characterization of the platform to enable upgrades on-orbit. The three SDRs represent a significant portion of the new technologies being studied on board the SCAN Testbed, which is operating on an external truss on the International Space Station (ISS). The SCaN Testbed provides experimenters an opportunity to develop and demonstrate experimental waveforms and applications for communication, networking, and navigation concepts and advance the understanding of developing and operating SDRs in space.

SDR input power estimation algorithms

The General Dynamics (GD) S-Band software defined radio (SDR) in the Space Communications and Navigation (SCAN) Testbed on the International Space Station (ISS) provides experimenters an opportunity to develop and demonstrate experimental waveforms in space. The SDR has an analog and a digital automatic gain control (AGC) and the response of the AGCs to changes in SDR input power and temperature was characterized prior to the launch and installation of the SCAN Testbed on the ISS. The AGCs were used to estimate the SDR input power and SNR of the received signal and the characterization results showed a nonlinear response to SDR input power and temperature. In order to estimate the SDR input from the AGCs, three algorithms were developed and implemented on the ground software of the SCAN Testbed. The algorithms include a linear straight line estimator, which used the digital AGC and the temperature to estimate the SDR input power over a narrower section of the SDR input power range. There is a linear adaptive filter algorithm that uses both AGCs and the temperature to estimate the SDR input power over a wide input power range. Finally, an algorithm that uses neural networks was designed to estimate the input power over a wide range. This paper describes the algorithms in detail and their associated performance in estimating the SDR input power.

An Optical Receiver Post-Processing System for the Integrated Radio and Optical Communications Software Defined Radio Test Bed

The Integrated Radio and Optical Communications (iROC) project at the National Aeronautics and Space Administrations (NASA) Glenn Research Center is investigating the feasibility of a hybrid radio frequency (RF) and optical communication system for future deep space missions. As a part of this investigation, a test bed for a radio frequency (RF) and optical software defined radio (SDR) has been built. Receivers and modems for the NASA deep space optical waveform are not commercially available so a custom ground optical receiver system has been built. This paper documents the ground optical receiver, which is used in order to test the RF and optical SDR in a free space optical communications link.

Development of an optical slice for an RF and optical software defined radio

A key component in the Integrated Radio and Optical Communications project at the National Aeronautics and Space Administration’s (NASA) Glenn Research Center (GRC) is the radio frequency (RF) and optical software defined radio (SDR). A NASA RF SDR might consist of a general purpose processor to run the Space Telecommunications Radio System (STRS) Architecture for radio command and control, a reconfigurable signal processing device such as a field programmable gate array (FPGA) which houses the waveform, and a digital to analog converter for (DAC) transmitting data. Prior to development, SDR architecture trades on how to combine the RF and optical elements were studied. A modular architecture with physically separate RF and optical hardware slices was chosen and the optical slice of an SDR was designed and developed. The Harris AppSTARTM platform, which consists of an FPGA processing platform with a mezzanine card targeted for RF communications, was used as the base platform in prototyping the optical slice. A serially concatenated pulse position modulation (SCPPM) optical waveform was developed. The waveform follows the standard described in the Consultative Committee for Space Data Systems (CCSDS) Optical Communions Coding and Synchronization Red Book. A custom optical mezzanine printed circuit board card was developed at NASA GRC for optical transmission. The optical mezzanine card replaces the DAC, which is used in the transmission of RF signals. This paper describes RF and optical SDR architecture trades, the Harris AppSTARTM platform, the design of the SCPPM waveform, and the development of the optical mezzanine card.

A COTS RF/Optical Software Defined Radio for the Integrated Radio and Optical Communications Test Bed

The Integrated Radio and Optical Communications (iROC) project at the National Aeronautics and Space Administration (NASA) is investigating the merits of a hybrid radio frequency (RF) and optical communication system for deep space missions. In an effort to demonstrate the feasibility and advantages of a hybrid RFOptical software defined radio (SDR), a laboratory prototype was assembled from primarily commercial-off-the-shelf (COTS) hardware components. This COTS platform has been used to demonstrate simultaneous transmission of the radio and optical communications waveforms through to the physical layer (telescope and antenna). This paper details the hardware and software used in the platform and various measures of its performance. A laboratory optical receiver platform has also been assembled in order to demonstrate hybrid free space links in combination with the transmitter.