Norman H. Adams

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.

Comparison of Ka-band link design strategies for solar probe plus

This study compares different strategies for planning and controlling the Ka-band downlink for NASAs upcoming Solar Probe Plus mission. This downlink provides the science data return: as such the availability of a specific pass is of less importance than the average performance of the link over an entire orbit. Three options for setting the link data rate were considered in this study: 1) a single data rate optimized for maximum effective data rate at the beginning of the pass, 2) a single data rate set to optimize return over the entire pass, and 3) a stepped data rate which optimizes the return over consecutive portions of the pass. In each case, the data return is characterized using a statistical model for the Ka-band link parameters, including weather and elevation-dependent G/T, yielding a cumulative distribution function for available data rate. It is important to the mission to minimize the contact time needed to meet the missions science data requirement. Hence we propose a downlink methodology that minimizes contact time subject to constraints on data delay and complexity.

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].

Telemetry ranging using software-defined radios

Telemetry ranging is a technique that inserts ranging data measured by the spacecraft into the downlink telemetry stream, thereby avoiding the need to allocate downlink power for a ranging signal. This technique has many benefits depending on the mission profile, including increased data return, operational simplification, and spectrum efficiency. The present study considers a variation to the ranging technique presented in [1] in order to facilitate implementation in a software-defined radio (SDR). This implementation tracks an uplink PN range code and measures the code phase coincident with the start of downlink telemetry frames. The phase is then embedded in subsequent telemetry frames. The method is implemented in the JHU/APL Frontier Radio and leverages the PN ranging design from the NASA New Horizons communications system. Initial test results are summarized and indicate that the method is viable for space exploration.

Autonomous loop switching: Interpreting and modifying the internal state of feedback tracking loops

Receiver tracking loops are implemented in software in modern space-borne radios. Software implementation allows loop designs to be modified in flight. Not only filter coefficients and gain, but also loop order and type can be modified. This flexibility enables new cognitive and autonomous capabilities. Loop designs can be optimized for each mission phase, and separate loops can be used for acquisition and tracking. Furthermore, the loops can be automatically adapted based on changing signal dynamics or SNR. However, if the loop has tracked away from its quiescent state, the loop will lose lock when the switch occurs unless the internal state of the loop is translated appropriately. This paper considers the internal state of feedback tracking loops. In particular, a physical interpretation of loop state is derived that enables translating the loop state from one design to another. Limitations to autonomous switching, including high-order signal states and noise, are described, and several examples are simulated. Practical applications for both near-earth and deep-space missions are discussed.

Flight software verification methods in Frontier Radio for solar probe plus mission

Success of deep space missions requires comprehensive performance verification for all hardware and software systems on the spacecraft over a broad scope of conditions and configurations, including the telecommunications subsystem. NASA Solar Probe Plus mission uses a software-defined radio for its telecommunications; thus a dedicated suite of tests are required for verification of the radio software in addition to traditional hardware verification procedures. Frontier Radio, developed by Johns Hopkins University Applied Physics Laboratory with near-Earth mission flight heritage, uses its software as a means to enhance its digital signal processing capabilities with no loss of robustness; thus, the testing required to validate its features is commensurately more complex. For this reason, a suite of tests were developed to interface with the radio software via a proprietary PC-based software platform to exercise the full scope of radio software functionality through a methodical and repeatable process. Automated logging functions embedded in the tests ensured that tests were documented with minimal operator intervention for greater accuracy and consistency than manual procedures used for hardware-only radios. Similarly, software-based interfaces to ground station equipment were developed to minimize the probability of operator error when testing the radio under varying signal conditions simulated by external instruments. The testing process identified several issues with radio software whose discovery would have been unlikely without the strenuous operating conditions created by the test procedure; the issues were subsequently mitigated resulting in a later software upgrade. Repeatability, and consequently the reduction of testing costs, was demonstrated through the execution of regression testing when the upgraded radio software was released. Results of the tests were examined and verified both internally and by external resources, namely NASA Independent Verification & Validation (IV&V), and the test procedures and results have been documented to the extent required for its reuse with future adaptations of Frontier Radio for other space mission applications. Additional software testing was performed that verified radio receiver functionality to a greater depth and complexity than that defined by formal requirements; the results were nominal as determined by the process of internal documentation and review.

Outer planets proximity link protocol

A directional X-band proximity link protocol derived from the Consultative Committee for Space Data Systems (CCSDS) Proximity-1 protocol that accommodates link dynamics and end-to-end file transfer considerations is presented. This implementation is targeted for the NASA Europa Multiple Flyby (aka Clipper) and Lander Missions currently under study, and is intended to be extensible for other outer planetary missions. A variant of the Proximity-1 protocol has been successfully used at Mars for UHF communications between Mars rovers and relay spacecraft. Mission designs currently under study for the Europa Multiple Flyby and Europa Lander missions include ranges that do not support high data rate, omni-directional UHF relay links. Directional X-band links are an attractive alternative given the presence of X-band telecommunications systems on many outer planetary spacecraft such as Europa Multiple Flyby and the Europa Landers Carrier spacecraft. Relay link concepts of operations and implementation of hardware, firmware and software in the Johns Hopkins University Applied Physics Laboratory (JHU/APL) Frontier Radio are discussed. The proposed tailoring of Proximity-1 for these Europa missions will be described for the Physical layer and Coding & Synchronization sub layer (C&S). An analysis of the interfaces between the protocol and the spacecraft command and data handling system is described. After the Europa Lander has completed its primary mission, the Europa Multiple Flyby and Lander Carrier spacecraft could potentially both serve as long-term relay assets for future Jovian missions.

A unified method for solving the frequency plan of space-borne software-defined radios

Software-defined radios (SDR) give space missions great flexibility by supporting diverse waveforms and mission scenarios. To configure and control this range of capability requires a model of the processing elements, both analog and digital. In this work we describe a unified method for solving and implementing the receiver and exciter frequency plans for a wide variety of hardware architectures. The flexibility of an SDRs frequency plan is enabled with programmable synthesizers and oscillators (e.g. DDS). Careful arrangement of these elements and band-selection filters is central to the hardware design. In the hardware design, up conversion and down conversion stages characterize the frequency plan for receive and transmit chains. Individual up conversion and down conversion stages can be segmented into a set of series or parallel blocks that completely represent the hardware design. Each stage can be fully characterized using a generalized converter block. We then formulate the frequency plan as a set of linear equations and solve the frequency plan with a single matrix operation. The converter stages also manage constraints on bandwidth and preferentially chose parameters to minimize spurious output. A block can also represent a reference oscillator or tunable DDS as part of a feedback control loop. The unified method can also accommodate a shared local oscillator between the receive and transmit paths. The end user can specify desired receive and transmit frequencies, and the method solves for all internal LO frequencies, or vice versa. The frequency plan solution is implemented in software and can be applied on any SDR platform. A notional configuration of the Frontier Radio for NASAs Europa mission is used as an example to demonstrate the methods.