Robert S. Bokulic

Optical communications development for spacecraft applications: recent progress at JHU/APL

Free-space optical communication systems for deep space as well as near terrestrial space environments are now under development for deployment aboard spacecraft within the next few years. Ever-increasing requirements for high data-rate communications are driving significant investments by NASA and DoD in critical technology readiness for spaceflight. One of the key NASA requirements is science data retrieval at rates much higher than heretofore possible with RF systems, for missions as far out as interstellar space and as close as geosynchronous Earth orbit (GEO). Recent efforts at Johns Hopkins University Applied Physics Laboratory (JHU/APL) are summarized that are focused on these requirements and challenges. We are developing a spacecraft optical communications terminal architecture initially using commercial off-the-shelf components while accelerating the development of state-of-the-art replacement components, which minimize mass and prime power while maintaining or improving performance. Recent technology development efforts will be summarized that include pulse position (PPM) modulator/demodulator chip development, compact optical beamsteering technology, including micro-electromechanical systems (MEMS), an ultra-lightweight deployable dual-band antenna concept, and a low-mass low-power optical downlink terminal design intended for deployment on a realistic interstellar explorer (RISE) mission

A two-way noncoherent ranging technique for deep space missions

The Johns Hopkins University Applied Physics Laboratory (APL) has adopted an integrated electronics module (IEM) approach for many of its spacecraft programs. As a result APL has developed a relatively simple X-band transceiver that allows the telecommunications subsystem to be manufactured on plug-in cards that fit into the IEM. An issue with the transceiver approach is that the downlink frequency is not related to the uplink frequency. The noncoherent relationship between the uplink and downlink signals has implications in both Doppler tracking and ranging. APL has developed a method for performing highly precise noncoherent Doppler tracking (J.R. Jensen and R.S. Bokulic, IEEE Trans. Aerospace and Electronic Sys., vol. 35, no. 3, pp. 963-973, 1999). This paper addresses a technique for performing accurate ranging with a noncoherent system. Comet Nucleus Tour (CONTOUR) is the first deep space mission to employ a transceiver and rely on the noncoherent ranging technique. Analysis and testing of the technique implemented for the CONTOUR mission is presented. Tests of the noncoherent ranging technique using the CONTOUR communications hardware at the Deep Space Networks (DSN) Development and Test Facility (DTF21) verified that the technique will provide ranging measurements that meet its navigation requirements.

MESSENGER mission: first electronically steered antenna for deep space communications

The MESSENGER mission to orbit the planet Mercury poses significant design challenges for its deep space communication system. These challenges include a wide pointing range, tight packaging, and a high temperature environment. To meet these challenges, the spacecraft incorporates the first steerable phased array antenna flown for deep space communications. The invention of a method for achieving circular polarization in a high-temperature (+300/spl deg/C) environment has doubled the science return of the mission relative to its initially proposed implementation. Cross-strapping between the phased array antennas and solid state power amplifiers (SSPAs) enables both amplifiers to be turned on when sufficient power is available, enhancing the scientific return at the planet. A science return of 25 Gbits/year is achieved using only one SSPA in Mercury orbit. The science return increases to 100 Gbits/year if both SSPAs are used in the orbital phase.

Mission to the sun: The solar pioneer

Abstract The Solar Pioneer is a mission concept developed by the Johns Hopkins University Applied Physics Laboratory (JHU/APL) to do exploratory science in the inner heliosphere and outer solar corona. The concept is derived from the Near Earth Asteroid Rendezvous (NEAR) spacecraft now being built by JHU/APL for NASA. The purpose of the Solar Pioneer is to deliver a payload of scientific instruments to approximately 4 solar radii ( R s - 3 radii above the visible solar surface or photosphere). The Solar Probe concept has evolved since its original introduction to the scientific community in 1978 1 while the key science questions to be answered have changed little. The primary purpose, to conduct an exploratory in situ basic science investigation, has been focused to emphasize in situ particles and fields measurements of the outer solar corona and inner solar wind region. Key science goals of the probe mission include the determination of: (1) how the solar wind is heated and accelerated, (2) where different types of solar wind come from, (3) how solar energetic particles (SEP) are accelerated, and (4) the nature of the unique plasma turbulence near the sun. The basic characteristics of the JHU/APL baseline concept provide for delivery of 42 to 60 kg of scientific payload to 4 R s while maintaining a high data rate of 30 kilobits per second (kbps) at 1 astronomical unit (AU) (X-band downlink to 70 meter antenna of the Deep Space Network). The baseline design draws upon the NEAR spacecraft and subsystem design to the maximum extent possible. The baselined launch vehicle is an Atlas IIA with a Star48 insertion stage system. Life-cycle cost control is obtained by: (1) focusing on prime science objectives, (2) applying advanced technology where it makes sense, and (3) capitalizing on developed subsystems derived from the NEAR spacecraft The Solar Pioneer is designed to solve questions of basic physics about the solar environment, enabling the solution to applied questions of the effects of the sun and solar cycle on the Earth and human society. By implementing a focused strategy from science goals through all stages of program management, such a mission can be carried out for less than one quarter of previous Solar Probe mission cost estimates 2,3

Detection of very weak transmissions from deep space

Abstract Designers of future planetary missions often reduce spacecraft transmitter power and oscillator stability requirements to decrease mission cost. Unfortunately, these reductions can make it impossible to detect weak signals from deep space using conventional demodulation techniques. The new Block V receiver being installed in the Deep Space Network (DSN) can recover suppressed carrier signals and can utilize very narrow loop bandwidths — as narrow as 0.1 Hz. However, operations at very narrow tracking loop bandwidths are quite sensitive to spacecraft oscillator stability. Low-cost oscillators planned for future missions can force the use of wide tracking loop bandwidths, leading to reduced carrier tracking performance. This reduced carrier tracking performance can, in turn, lead to a significant increase in required spacecraft transmitter power. This paper presents the theory behind coherent detection of very weak telemetry from deep space. It then briefly recounts tests characterizing Block V performance for the NEAR spacecraft in two-way coherent operations and presents recommendations for mission designers.