Matthew P. Angert

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.

Ka-band I/Q modulator Multi-Chip Module for high data rate communications

This paper describes the development and test results of a flight qualified, hermetic I/Q modulator Multi-Chip Module (MCM) with X-band input and Ka-band output for either Tracking and Data Relay Satellite System (TDRSS) or Deep Space Network (DSN) high data rate communications applications. The module is 0.9″ wide by 1.7″ long without the attachable coaxial connector. It draws <500 mW of DC power. A Ka-band modulator MMIC previously developed by the Johns Hopkins University Applied Physics Laboratory (JHU/APL) provides the direct I/Q modulating function in the module. A Commercial Off The Shelf (COTS) MMIC frequency multiplier provides either a multiply-by-three or a multiply-by-four function for the upconversion of the X-band carrier to one of the Ka-band output frequencies of choice. The multi-chip module is configurable for the desired frequency band during fabrication by installing the corresponding band-pass filter. This filter, along with the biasing of the multiplier MMIC, passes the upconverted carrier to the I/Q modulator. A COTS MMIC broadband amplifier is used following the modulator to boost the output signal level to approximately +10dBm range, suitable for driving an external SSPA or TWTA. Chip attenuators are located between each function block for level setting as well as temperature compensation. An innovative package was designed to house the circuits described. Both the TDRSS and DSN versions of this upconverting direct modulation module have been fully tested and successfully implemented in the Ka-band Exciter of both the S/Ka-band and X/Ka-band Frontier Radio developed by JHU/APL for NASA. A data rate of up to 200 Mbps has been demonstrated during radio test, with current efforts targeting beyond 300 Mbps. 1 2

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.

Advancements in hardware design for the frontier radio used for the solar probe plus mission

The Frontier Radio for the Solar Probe Plus mission offers a host of hardware design and manufacturing improvements. These improvements build on the technology readiness level (TRL)-9 radio platform that was flown on the Van Allen Probes mission in a duplexed S-band configuration and several development tasks funded by NASA Headquarters. Prior RF slice designs consisted of two separate circuit boards: one for lower frequencies and one for high-frequencies; advances in technology enabled the use of a high-frequency multilayer laminate with highly integrated miniature components to create a single circuit board, thereby simplifying manufacturing. This change also enabled an improved circuit topology in the upconverter in both exciters producing lower phase noise and better I/Q modulation accuracy. RF shielding performance was improved using compartmentalized plates and Spira-Shield gaskets. Use of a magnesium alloy for the slice packaging reduced the overall radio mass. A design-variant approach was implemented to facilitate flexibility and re-configurability across multiple missions and applications. For example, one circuit board artwork supports S, X, and Ka-band configurations of the exciter slice, and the digital signal processing (DSP) slices board artwork facilitates numerous alternate configurations for a variety of mission applications. Advances made to the DSP slice include dual footprint circuits and an interposer for the field programmable gate array (FPGA) to minimize layout changes, a change to a radiation-hardened magnetoresistive random access memory (MRAM) to replace the programmable read-only memory (PROM), and new test and debugging circuits including a serializer-deserializer (SERDES), a SpaceWire debug link, and small footprint soft-touch connectors to access internal signals. A multi-chip module (MCM) was qualified for use in space that converts X-band to Ka-band and I/Q modulates directly at Ka-band. This enables a first for a deep-space mission: primary science data downlink with simultaneous data and navigation over Ka-band. Manufacturing techniques such as the use of a pick-and-place machine with part value validation, standardized circuit board sizes, and fewer overall circuit boards to integrate all contributed to faster and more reliable hardware assembly. The flight radio hardware is fully fabricated and has completed proto-flight testing for a planned 2018 spacecraft launch.

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].

Development of a flight qualified Ka-band multi-chip module for the solar probe plus mission

The Johns Hopkins University Applied Physics Lab (JHU/APL) has developed a flight qualified, hermetically sealed, I/Q modulator Ka-band Multi-chip Module (MCM). Prototypes of this device have been developed over the years, but Solar Probe Plus (SPP) will be the first mission to use a flight qualified version of the MCM. This MCM enables a first for a deep-space mission: primary science data downlink with simultaneous data and navigation over Ka-band. SPP will also be the first JHU/APL mission to use Ka-band for downlink. The MCM contains three gallium arsenide (GaAs) monolithic microwave integrated circuit (MMIC) die, two of which are commercial off the shelf (COTS) parts, and the third is a custom die designed at JHU/APL. The MCM takes an X-band input, multiplies the signal up to Ka-band, modulates I/Q data directly onto the Ka-band carrier and outputs a signal in the +10dBm range, capable of driving an external SSPA or TWTA. Improvements made over previous prototype designs include a revision of the custom I/Q modulator MMIC on a different foundry process, development of automatic wire-bonding for large quantity (>30) flight unit assembly, test automation, and a novel calibration routine to create a linear I/Q space. All flight units went through a NASA approved screening and qualification process. A full lot of 30 MCM units were screened and qualified, producing 12 usable flight units, of which 2 were selected to be used in the 2 flight Frontier Radios on the Solar Probe Plus mission.

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.