Dipak K. Srinivasan

Radiation Belt Storm Probe spacecraft and impact of environment on spacecraft design

NASAs Radiation Belt Storm Probe (RBSP) is an Earth-orbiting mission scheduled to launch in September 2012 and is the next science mission in NASAs Living with a Star Program. The RBSP mission will investigate, characterize and understand the physical dynamics of the radiation belts, and the influence of the sun on the earths environment, by measuring particles, electric and magnetic fields and waves that comprise the geospace. The mission is composed of two identically instrumented spinning spacecraft in an elliptical orbit around earth from 600 km perigee to 30,000 km apogee at 10 degree inclination to provide full sampling of the Van Allen radiation belts. The twin spacecraft will follow slightly different orbits and will lap each other 4 times per year; this offers simultaneous measurements over a range of spacecraft separation distances. A description of the spacecraft environment is provided along with spacecraft and subsystem key characteristics and accommodations that protect sensitive spacecraft electronics and support operations in the harsh radiation belt environment.

The APL 18.3m station upgrade and its application to lunar missions

The Johns Hopkins University Applied Physics Laboratory has conducted an upgrade to its 18.3 meter ground station.1 2 These upgrades include adding a high-power S-band uplink, dual polarization S- and X-band downlink, multiple parallel downlink channel capability and software-defined telemetry, tracking and command (TT&C) units. The 18-m antenna size provides full-disk lunar coverage in S-band. This coverage, in combination with the parallel downlink capability and APLs coherent transceiver technology, affords the ability to track multiple lunar targets simultaneously. This paper presents the upgraded station architecture, its performance for current lunar missions, and discusses its application for different lunar mission scenarios.

In-flight contour radiometric performance

This paper provides a detailed description of the flight experience obtained during the first operational use of a noncoherent Doppler and ranging system. This experience was gained from the Comet Nucleus TOUR (CONTOUR) spacecraft between its launch on July 3 and the solid rocket motor firing on August 15, 2002. The data obtained during these 43 days include Doppler velocities and range measurements made through NASA’s Deep Space Network. Onboard the CONTOUR spacecraft was an X-Band transceiver system. In order to support Doppler velocity measurements that are not affected by bias and drift of the spacecraft frequency reference, the spacecraft carried a small amount of additional hardware that placed radiometric data directly into the telemetry stream. A software process on the ground used this telemetry to convert the Doppler observables to those that would have been obtained with a transponder. In order to support ranging measurements, the uplink Doppler effects were removed through the use of programmed uplink fi-equency ramps. This paper describes the process of performing Doppler and range measurements and presents samples of the results obtained. Additional information, such as precise spacecraft oscillator monitoring and receiver fi-equency monitoring that are provided by such a system, is also presented. 0-7803-7651-X/03/

Spacecraft-level verification of the Van Allen Probes' RF communication system

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STEREO superior solar conjunction mission phase

This paper presents the verification process, lessons learned, and selected test results of the radio frequency (RF) communication system of the Van Allen Probes, formerly known as the Radiation Belt Storm Probes (RBSP). The Van Allen Probes mission is investigating the doughnut-shaped regions of space known as the Van Allen radiation belts where the Sun interacts with charged particles trapped in Earths magnetic field. Understanding this dynamic area that surrounds our planet is important to improving our ability to design spacecraft and missions for reliability and astronaut safety. The Van Allen Probes mission features two nearly identical spacecraft designed, built, and operated by the Johns Hopkins University Applied Physics Laboratory (JHU/APL) for the National Aeronautics and Space Administration (NASA). The RF communication system features the JHU/APL Frontier Radio. The Frontier Radio is a software-defined radio (SDR) designed for spaceborne communications, navigation, radio science, and sensor applications. This mission marks the first spaceflight usage of the Frontier Radio. RF ground support equipment (RF GSE) was developed using a ground station receiver similar to what will be used in flight and whose capabilities provided clarity into RF system performance that was previously not obtained until compatibility testing with the ground segments. The Van Allen Probes underwent EMC, acoustic, vibration, and thermal vacuum testing at the environmental test facilities at APL. During this time the RF communication system was rigorously tested to ensure optimal performance, including system-level testing down to threshold power levels. Compatibility tests were performed with the JHU/APL Satellite Communication Facility (SCF), the Universal Space Network (USN), and the Tracking and Data Relay Satellite System (TDRSS). Successful completion of this program as described in this paper validated the design of the system and demonstrated that it will be able to meet all of the Van Allen Probess communications requirements with its intended ground segments.

Telemetry ranging using software-defined radios

With its long duration and high gain antenna (HGA) feed thermal constraint; the NASA Solar-TErestrial RElations Observatory (STEREO) solar conjunction mission phase is quite unique to deep space operations. Originally designed for a two year heliocentric orbit mission to primarily study coronal mass ejection propagation, after 8 years of continuous science data collection, the twin STEREO observatories entered the solar conjunction mission phase, for which they were not designed. Nine months before entering conjunction, an unforeseen thermal constraint threatened to stop daily communications and science data collection for 15 months. With a 3.5 month long communication blackout from the superior solar conjunction, without ground commands, each observatory will reset every 3 days, resulting in 35 system resets at an Earth range of 2 AU. As the observatories will be conjoined for the first time in 8 years, a unique opportunity for calibrating the same instruments on identical spacecraft will occur. As each observatory has lost redundancy, and with only a limited fidelity hardware simulator, how can the new observatory configuration be adequately and safely tested on each spacecraft? Without ground commands, how would a 3-axis stabilized spacecraft safely manage the ever accumulating system momentum without using propellant for thrusters? Could science data still be collected for the duration of the solar conjunction mission phase? Would the observatories survive? In its second extended mission, operational resources were limited at best. This paper discusses the solutions to the STEREO superior solar conjunction operational challenges, science data impact, testing, mission operations, results, and lessons learned while implementing.

Telecommunications systems for the NASA Europa missions

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.

Maximizing data return for the Europa lander: A trade study in the application of CCSDS protocols

The telecommunications systems for two NASA deep-space missions to Jupiters moon Europa are presented. One mission, Europa Clipper, is a Jovian orbiter with multiple Europa flybys; the other mission, Europa Lander, includes a Carrier and Relay Spacecraft (CRS), Deorbit Stage, Descent Stage (DS), and a Lander. Both missions are designed to communicate to Earth via the NASA Deep Space Network (DSN) and other ground stations. For Lander communications, both the CRS and Europa Clipper spacecraft are equipped with store-and-forward relay communication capability. The heart of each spacecrafts telecommunications system is the high-TRL Johns Hopkins University/Applied Physics Laboratory Frontier Radio, based on the Solar Probe Plus design. Other key telecommunnications hardware developments across the two missions include a 3-m dualband (X/Ka) high gain antenna (HGA), a GaN-based solid state power amplifier (SSPA) and slot-array HGA to enable the Lander communication system. All components must operate in a high-radiation environment and meet planetary protection requirements.

Outer planets proximity link protocol

In support of NASA, Caltechs Jet Propulsion Laboratory and The Johns Hopkins Applied Physics Laboratory are studying concepts for two missions to explore Europa: a multiple flyby spacecraft and a surface lander. This paper analyzes the use of packetized, multi-hop, multi-path communications protocols for the Europa lander concept and assesses their potential for reducing power requirements while increasing data return. Analysis includes three protocols standardized by the Consultative Committee for Space Data Systems (CCSDS): the CCSDS File Delivery Protocol (CFDP), the Bundle Protocol (BP), and the Licklider Transmission Protocol (LTP). A spacecraft may implement a networking stack of one or more of these protocols, with each of these stacks exhibiting different strengths and weaknesses. We present heuristic and analytical methods for evaluating protocol performance including a priori computations of protocol overheads, Monte-Carlo analysis across bit error rates and packet sizes, and high fidelity simulations. Quantitative metrics such as retransmission efficiency, packet overhead, and end-to-end transaction duration characterize individual protocol options. Qualitative metrics such as cost of ownership, mission operations complexity, and computational processing load characterize the mission impacts of various networking stacks. We generate results using anticipated mission link characteristics, data volumes, and network geometries and provide recommendations relating to the value of software protocols and multi-protocol networking stacks. Results demonstrate that each candidate protocol combination can be tuned to within 15% of optimal performance over links of up to 10−4 bit error rate, although achieving this efficiency with solely CFDP incurs up to 800% greater computational processing load versus the other stacks. We conclude multiprotocol stacks separate concerns when optimizing performance for multiple stakeholders. A CFDP/BP/LTP networking stack solves a joint optimization problem where CFDP can be tuned for onboard data operations, BP can be used to provide standardized priority and store-and-forward operations, and LTP can be tuned for retransmission and acknowledgement. This approach enables efficient end-to-end communications for the Europa lander concept that maximizes data return with minimal power requirements.

The MESSENGER Spacecraft

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.