Christopher B. Haskins

Multi-band software defined radio for spaceborne communications, navigation, radio science, and sensors

Demanding mass and power requirements across many low-cost NASA mission sets (Discovery, New Frontiers, Mars Scout, SMEX, MIDEX, and others) place a premium on lightweight, efficient, and versatile radios.1,2 A low power, low mass, modular, multi-band software-defined radio (SDR) has been developed by JHU/APL, under the name Frontier Radio, for use in communications, navigation, radio science, and sensor applications for a variety of NASA missions. The current SDR implementation features communications and Doppler navigation modes, and provides a highly capable platform to build upon for future technology enhancements. Features such as in-band channel assignment, bit rate, modulation format, turnaround ratio, loop bandwidths, and coding formats are reconfigurable in flight. Modularity within the core hardware and firmware platforms enable infusion of new technology with minimal non-recurring engineering (NRE) costs. Current configurations operate within the NASA S, X (under development), and Ka-bands (26 and 32 GHz), though alternate RF slices may be added and/or substituted for other bands or sensor applications. This SDR is currently capable of transmit data rates up to 25 Mbps (and higher with 8/16 PSK/QAM) and receive data rates up to 1.3 Mbps via QPSK, with significantly higher capability under development. Compatibility with NASAs STRS architecture helps promote the use of this SDR throughout the NASA community. Along with its low power (5 W receive mode w/internal ovenized oscillator and 28V bus power) and low mass (1.8/2.1 kg, single/dual band configuration), this SDR offers missions a combination of capabilities and efficiency. The NASA Radiation Belt Storm Probes (RBSP) mission is currently developing a flight implementation of this SDR (S-Band only), with launch planned for the year 2012.

Demonstrating TRL-6 on the JHU/APL Frontier Radio for the Radiation Belt Storm Probe mission

The NASA Radiation Belt Storm Probes (RBSP) mission, launching in 2012, features two spacecraft that will measure the dynamics of the radiation belts as they proceed in nearly identical orbits around the Earth.12 The Radio Frequency (RF) telecommunications subsystem will utilize an S-Band version of the Johns Hopkins University Applied Physics Laboratorys (JHU/APL) Frontier Radio. The Frontier Radio is a low-power, low-mass, modular Software Defined Radio (SDR) platform designed for communications, navigation, radio science, and sensor applications. The RBSP Frontier Radio is the first of this platform with a full software implementation to demonstrate a Technology Readiness Level (TRL) of 6. The Engineering Model (EM), with mature packaging, hardware, firmware, and software implementations has successfully completed environmental and compatibility testing designed to simulate the space environment expected for the RBSP mission. The ground support equipment (GSE) utilized a ground station modem with turbo and convolutional decoding and internal bit and frame error rate (BER/FER) testing capabilities. The modem, JHU/APL-built equipment for emulating the spacecraft interfaces, and the use of EM hardware for other RF subsystem components enabled test-as-you-fly scenarios that were used throughout testing. Several novel packaging and process technologies were also qualified during the development of this radio. These include a high-reliability assembly process for installing Quad Flat Pack No-Lead (QFN) integrated circuits and a 152-pin stacking connector that serves as the main interconnect between the functional blocks of the radio. The RBSP radio is the first Frontier Radio to be qualified for a spaceflight mission. This demonstration of a high reliability, low power, and low mass radio further validates the platform for use in future space missions.

The Frontier software-defined radio for the solar probe plus mission

The latest adaptation of the Frontier Radio, an X/Ka-band deep space implementation, has been transitioned into a finished product for Solar Probe Plus (SPP) and future missions. Leveraging the technology readiness level (TRL) 9 software-defined radio (SDR) platform successfully flown on the Van Allen Probes (VAP) mission, the Frontier Radio now brings a low-power, low-mass, yet highly radiation-tolerant and robust SDR to deep space applications. This implementation brings with it a suite of enhanced capabilities and improvements to the Frontier Radio platform. The core deep space software implementation is designed to match or improve upon the signal acquisition and tracking performance, as well as improve the receive and transmit implementation losses of its predecessors (JHU/APL and industry). The deep space radio operates using less than 6W at 30V in receive mode, and approximately 10W with either the X- or Ka-band exciter enabled and operating in two-way coherent duplex mode. The power consumption in these modes can be further reduced to as low as 3W and 9W respectively, depending on the spacecraft bus and mission requirements. In addition to providing standard deep space navigation features such as two-way Doppler, two-way ranging, and differential one-way ranging (DOR), firmware and software enhancements were made to improve the receiver acquisition and tracking robustness. A software enhancement was also essential in 1) reducing the effects of turnaround noise on the Ka-band link and 2) reducing the impact on downlink frame error rates while operating in a coherent turnaround mode. These improvements enable simultaneous science return and navigation over the Ka-band link with minimal implementation loss. A number of enhancements to the hardware and test platforms have been made to improve manufacturability, reduce manufacturing and test cost and turnaround time, improve portability across multiple frequency bands and applications, and increase processing capacity. The parts selection provides for a total ionizing dose (TID) tolerance of at least 100krads, subject to the parts purchased at time of manufacturing, without spot or bulk shielding. The Frontier Radio provides a robust selection of single event mitigation and fault protection techniques. Future deep space missions such as Europa Clipper plan to utilize the Frontier Radio. A single board version of the Frontier Radio is also under development for CubeSat and other small form factor spacecraft, with a current best estimate (CBE) of 1W receive mode and less than 5W duplex mode with a 1W power amplifier; this implementation leverages the same robust parts selection as the parent product, with a streamlined hardware implementation that leverages advancements in high speed signal conversion and processing. This paper describes the current capabilities of the Frontier Radio for deep space and provides a short discussion of future efforts related to the platform.

Digital signal processing architecture design for gate array based software defined radios

A complete digital signal processing architecture for software defined radios (SDRs) has been developed for several NASA missions under the JHU/APL name Frontier Radio.1, 2 The design is completely integrated into a single field programmable gate array (FPGA), and includes all processing necessary to execute a variety of modulation and demodulation schemes, as well as abstract the radio hardware and control up to a human interface level. A balance in the inherent dichotomy between key mission requirements, low power and infinite flexibility, has been achieved with an eye towards FPGA and space flight qualified application specific integrated circuit (ASIC) commercial device trends. The Space Telecommunications Radio System (STRS) compliant modular design architecture allows for low cost reconfiguration, replacement, or addition of functionality. Trades made during the development process of this architecture are discussed: including different firmware development paths, FPGA integration vs. commercial device usage, and implementation of digital signal processing (DSP) mathematics. Several usage scenarios, STRS Waveforms, of the FPGA architecture are discussed together with tested performance metrics.

Regenerative PN ranging experience with New Horizons during 2012

The New Horizons mission to Pluto is the first deep space mission to include the capability of supporting regenerative PN ranging. During the current phase of the mission, sequential tone ranging supports the mission navigation requirements but regenerative ranging will expand the conditions (antenna selection, integration time, etc.) over which ranging will be successful during any extended mission following the Pluto fly-by, to objects in the Kuiper belt. Experience with regenerative ranging is being obtained now in preparation for its use in an extended mission. During most of 2012, New Horizons was in a hibernation state. Tracking was conducted between late April and early July. Six regenerative ranging passes were performed to bookend this interval; 2 at the beginning and 4 at the end. During that time, the distance between the spacecraft and Earth was in excess of 22 Astronautical Units (AU) and the Pr/No levels were below 15 dB-Hz. A seventh regenerative ranging pass was performed in May at a higher signal level in order to test the acquisition of the ranging code by the spacecraft during a variety of conditions. The consistency of the regenerative range measurements with the adjacent sequential tone ranging measurements has been demonstrated and serves as a check on the calibration of the regenerative ranging system conditions. The range measurement precision has been shown to follow the predictions that are based on the uplink and downlink signal power. The regenerative ranging system has been shown to acquire the uplink ranging code with and without a commanded reset and regardless of the noise bandwidth setting of the system. This paper will present the data that was obtained during 2012 and will describe the analysis results for the regenerative ranging experience during 2012.

First deep-space flight demonstration of Regenerative pseudo-noise ranging

Deep-space missions traditionally use sequential, turnaround ranging to measure the distance to the spacecraft. The spacecraft demodulates the uplinked ranging signal (composed of sequential frequency tones) and retransmits it (“turnaround”) onto the downlink. Since this operation is bent-pipe, noise on the uplink within the ranging channel bandwidth is also retransmitted. When the received uplink signal-to-noise ratio (SNR) is low, regenerating the ranging signal can increase the retransmitted ranging SNR by 30 dB or more, compared to the bent-pipe method. The Regenerative pseudo-noise (PN) ranging creates this gain by using a PN digital ranging signal on the uplink and detecting that signal in the spacecraft receiver. The PN signal is regenerated digitally, with no uplink noise, and retransmitted to the ground. We report the first deep-space flight demonstration of regenerative ranging on the New Horizons Mission to Pluto. We provide an overview of the technique, including the operational PN ranging capability deployed at NASAs Deep Space Network (DSN). We describe the regenerative ranging implementation on the New Horizons spacecraft. Finally, we report on the operational flight tests, compare their results to the current trajectory (and sequential ranging), and detail how this technique will aid this mission in the future.

Development of Ka-band frequency translators for high data rate communications

As future communications in space require increasing data rates, Ka-band technology is needed to enable wider bandwidths and higher data rates. Linear frequency translation is essential in enabling higher order modulation schemes for high data rates, which in turn enhances flexibility and software defined capability. This paper summarizes the key design features and data results of Ka-band forward and reverse link frequency translators compatible with the Tracking and Data Relay Satellite System (TDRSS) bands (26 GHz); future work could expand operation to the Deep Space Network Ka-band. The technology developed was a set of Ka-band transmit and receive multi-chip module (MCM) frequency translators with associated low phase noise local oscillator (LO) synthesizer boards. This set of hardware converts between S-band and Ka-band. The translators are designed for a 1 GHz bandwidth and for a center frequency that is re-tunable in this 1 GHz range. The MCMs offer a versatile design that can be used for both exciter and receiver translators and provide areas for future expansion. Furthermore, the MCM and LO synthesizer form a low power system with 1.6W of power consumption for the exciter hardware and 1.8W for the receiver portion. Finally, the LO synthesizer has low phase noise under 2.5 degrees rms from 100 Hz to 40 MHz which enables high data rates and high order modulation schemes. 1 2

Wideband, oversampled I/Q modulation architecture of the JHU/APL Frontier Software Defined Radio

The modulation architecture in the Frontier Software Defined Radio platform supports a wide variety of modulation formats with a single low-power hardware/firmware circuit: PSK, QAM, subcarrier BPSK and ranging. The wideband hardware supports downlink data rates from 1 sps to 150 Msps, yet the firmware circuitry can generate tightly controlled narrow-band modulation. This flexibility is achieved with oversampled waveform generation, including sine-wave subcarriers and interpolated turnaround ranging. For phase modulation, a composite modulation signal is converted to parallel I/Q signals that modulate the RF carrier. Due to imperfections in the analog I/Q modulator, harmonic upcon-verter and DACs, calibration of the digital phase-to-I/Q conversion is necessary to accurately generate the desired RF signal. The calibration procedure optimizes a discrete PSK constellation, and then interpolates the constellation for applications that require continuous phase modulation, such as sine-wave subcarrier or turnaround ranging. For S-band and Ka-band exciters, the modulation is applied at the RF frequency, and the I/Q conversion subtends 360°. In contrast, for X-band exciters, the modulation is applied at 1/4th the RF frequency, and the I/Q conversion subtends only 90°. In summary, this architecture can generate wide-band high-rate downlink when sufficient link margin and bandwidth are available, and can also generate tightly controlled narrow-band signals when only low-rate links are available or limited bandwidth can be allocated.

Advanced tracking loops to support low rate coded uplinks

Conventional second-order Doppler tracking loops impose a tradeoff between phase noise and tracking performance. In order to maintain adequate bit-error rate, the loop bandwidth must be narrow enough that phase noise is small. However, this limits the track range, track rate, and acquisition time of the loop. The tradeoff between bandwidth and tracking capabilities can prevent communication during intervals of strong Doppler dynamics. Furthermore, this tradeoff can also be problematic for deep-space scenarios in which carrier to noise ratio is poor and loop bandwidth must be extremely narrow. In such scenarios, tracking even modest Doppler is prohibitive. This limitation is one reason that coding has not been applied to emergency-mode uplinks. High-order loops can mitigate this tradeoff. Typically, high-order loops are typically only employed for downlink communications. The JHU/APL Frontier Software Defined Radio is capable of implementing both very narrow loops, as well as high-order loops, within the existing hardware and firmware architecture; only software changes are necessary. The present paper describes a case study and preliminary results for carrier tracking loops that can support coded emergency-mode uplinks.

Enabling coherent Ka-band downlink with a software-defined radio

The migration to Ka-band for science downlink on deep space missions increases data rates significantly, but also presents new challenges to radio and RF system designers. One challenge is to maintain low carrier phase noise on a coherent downlink. Thermal noise on the X-band uplink that is within the bandwidth of the carrier recovery process modulates the phase of the coherent downlink. For missions that use X-band for command uplink and Ka-band for science downlink, such as the NASA Solar Probe Plus mission, the ratio of downlink to uplink frequency acts as a phase noise multiplier on the coherent downlink. Analysis and prototype tests revealed that the additional phase noise degraded both telemetry and navigation performance significantly. Accordingly, an additional software filter is inserted into the Ka-band coherent turnaround path. This filter constrains the phase noise sufficiently to meet all communication and navigation requirements. In this paper we describe the phase noise on a coherent downlink due to additive noise that is tracked by the uplink carrier recovery process. We present simulated and measured phase noise performance, with and without the additional filter. Measured frame-error rate performance is presented and the impact on radio navigation due to increased delay through the turnaround channel is discussed. This paper describes the filter implementation and results obtained with an engineering model of the SPP Frontier Radio. A companion paper describes the analytic formulation and considers other phase noise contributions such as solar scintillation [1].