Douglas S. Abraham

Prospects for a Next-Generation Deep-Space Network

A next-generation deep-space network is currently under consideration by the National Aeronautics and Space Administration. Building upon its many past successes, this network will be required to meet the needs of current and planned missions. These will, no doubt, include the familiar suite of telemetry, command, tracking, and navigation services, with performance levels derived from analysis of the probable future mission set. Additionally, it will be expected to provide enabling capabilities for missions still on the drawing boards. Traditionally, the network serves the robotic deep-space exploration fleet. However, at this time, consideration of the special needs of planned future human lunar missions is appropriate, as well as the evolution to the eventual human exploration of Mars.

Integrated network architecture for sustained human and robotic exploration

The National Aeronautics and Space Administration (NASA) Exploration Systems Mission Directorate is planning a series of human and robotic missions to the Earths Moon and to Mars. These missions will require telecommunication and navigation services. This paper sets forth presumed requirements for such services and presents strawman lunar and Mars telecommunications network architectures to satisfy the presumed requirements. The paper suggests that a modest ground network would suffice for missions to the near-side of the Moon. A constellation of three Lunar Telecommunications Orbiters connected to a modest ground network could provide continuous redundant links to a polar lunar base and its vicinity. For human and robotic missions to Mars, a pair of areostationary satellites could provide continuous redundant links between a mid-latitude Mars base and Deep Space Network antennas augmented by large arrays of 12-m antennas

Exploration of Pluto

Abstract Pluto is the last known planet in our Solar System awaiting spacecraft reconnaissance. In its eccentric orbit taking it 50 AU from the Sun, Pluto presently has a thin atmosphere containing methane, which is projected to “collapse” back to the icy planets surface in about three decades, following Plutos 1989 perihelion pass at 30 AU. Based on ground and Earth-orbit-based observing capabilities limited by Plutos small size and extreme distance, present top-priority scientific questions for the first mission concern Pluto and Charons surface geology, morphology and composition, and Plutos neutral atmosphere composition. Budgetary realities preclude a large, many-instrument flyby spacecraft, while distance and launch energy requirements preclude any but the smallest orbiter using presently available launch vehicles and propulsion techniques. A NASA-sponsored Pluto Mission Development activity began this year. Two alternative cost-constrained mission implementations are described, based on which a primary implementation will be chosen. The Pluto Fast Flyby (PFF) mission utilizes an 83 kg (dry) spacecraft launched in 1998 aboard a Titan IV(SRMU)/Centaur for an ∼7 year direct trajectory to Pluto. Instruments described are an integrated CCD-imaging/ultraviolet spectrometer, with a possible integrated infrared spectrometer. The larger Pluto-350 spacecraft, ∼316 kg, carries a broader instrument set, greater redundancy, and requires > 11 year flight time launching in 2001 aboard a Delta or Atlas, toward Earth and Jupiter swingbys to provide the energy to reach Pluto. Launch by Proton is under consideration. Both mission implementations store data during the brief encounter, to be played back over several months. Cost is the primary design driver of both alternatives, with major tradeoffs between spacecraft development, launch services, radioisotope thermoelectric generator procurement and launch approval, and mission operations. Significant benefits are apparent from incorporating “microspacecraft” technologies from Earth orbiters.

Deep-space optical communications

Current key initiatives in deep-space optical communications are treated in terms of historical context, contemporary trends, and prospects for the future. An architectural perspective focusing on high-level drivers, systems, and related operations concepts is provided. Detailed subsystem and component topics are not addressed. A brief overview of past ideas and architectural concepts sets the stage for current developments. Current requirements that might drive a transition from radio frequencies to optical communications are examined. These drivers include mission demand for data rates and/or data volumes; spectrum to accommodate such data rates; and desired power, mass, and cost benefits. As is typical, benefits come with associated challenges. For optical communications, these include atmospheric effects, link availability, pointing, and background light. The paper describes how NASAs Space Communication and Navigation Office will respond to the drivers, achieve the benefits, and mitigate the challenges, as documented in its Optical Communications Roadmap. Some nontraditional architectures and operations concepts are advanced in an effort to realize benefits and mitigate challenges as quickly as possible. Radio frequency communications is considered as both a competitor to and a partner with optical communications. The paper concludes with some suggestions for two affordable first steps that can yet evolve into capable architectures that will fulfill the vision inherent in optical communications.

The impact of traffic prioritization on Deep Space Network mission traffic

A select number of missions supported by NASAs Deep Space Network (DSN) are demanding very high data rates. For example, the Kepler Mission was launched March 7, 2009 and at that time required the highest data rate of any NASA mission, with maximum rates of 4.33 Mb/s being provided via Ka band downlinks. The James Webb Space Telescope will require a maximum 28 Mb/s science downlink data rate also using Ka band links; as of this writing the launch is scheduled for a June 2014 launch. The Lunar Reconnaissance Orbiter, launched June 18, 2009, has demonstrated data rates at 100 Mb/s at lunar-Earth distances using NASAs Near Earth Network (NEN) and K-band. As further advances are made in high data rate space telecommunications, particularly with emerging optical systems, it is expected that large surges in demand on the supporting ground systems will ensue. A performance analysis of the impact of high variance in demand has been conducted using our Multi-mission Advanced Communications Hybrid Environment for Test and Evaluation (MACHETE) simulation tool. A comparison is made regarding the incorporation of Quality of Service (QoS) mechanisms and the resulting ground-to-ground Wide Area Network (WAN) bandwidth necessary to meet latency requirements across different user missions. It is shown that substantial reduction in WAN bandwidth may be realized through QoS techniques when low data rate users with low-latency needs are mixed with high data rate users having delay-tolerant traffic.1 2

Deep Space C3: High Power Uplinks

The uplink transmitters of the Deep Space Network (DSN) perform three key functions in support of space missions: navigation, command uplink, and emergency recovery. The transmitters range in frequency from S‐band to Ka‐band, and range in RF transmit power from 200W to 400kW. Future improvements to the uplink transmitters will focus on higher frequency transmitters for high data rate communications, high power X‐band uplinks for emergency recovery, and/or in‐phase uplink arraying for either application.

Long-range planning for the Deep Space Network

Conduct of space exploration is undergoing a significant transformation. Initial reconnaissance missions are giving way to long duration observations with data-intensive instruments, in situ investigations and complex operations. To keep pace, a transformation in the Deep Space Network is in order. Downlink performance must increase by 1 to 2 orders of magnitude over the next decade and by a similar amount in uplink over the next 20 years. Comparable improvements in navigation precision will be required. Network topology will encompass a diversity of sites, including some owned by non-NASA agencies, others in Earth orbit, and still others deployed as local planetary infrastructure, particularly at Mars. Point-topoint links between nodes will evolve toward networked connectivity among nodes. Provision of services will move toward greater simplicity for the user backed by highly reliable systems. Finally, new types of high-level information services, enabled by high-capacity connectivity, will be developed so as to enable and enhance the next wave of space exploration.

Emergency communications for NASA's deep space missions

The ability to communicate with spacecraft during emergencies is a vital service that NASAs Deep Space Network (DSN) provides to all deep space missions. Emergency communications is characterized by low data rates (typically ∼10 bps) with the spacecraft using either a low-gain antenna (LGA, including omnidirectional antennas) or, in some cases, a medium-gain antenna (MGA). Because of the use of LGAs/MGAs for emergency communications, the transmitted power requirements both on the spacecraft and on the ground are substantially greater than those required for normal operations on the high-gain antenna (HGA) despite the lower data rates. In this paper, we look at current and future emergency communications capabilities available to NASAs deep-space missions and discuss their limitations in the context of emergency mode operations requirements. These discussions include the use of the DSN 70-m diameter antennas, the use of the 34-m diameter antennas either alone or arrayed both for the uplink (Earth-to-spacecraft) and the downlink (spacecraft-to-Earth), upgrades to the ground transmitters, and spacecraft power requirements both with unity gain (0 dB) LGAs and with antennas with directivity (> 0 dB gain, either LGA or MGA, depending on the gain). Also discussed are the requirements for forward-error-correcting codes for both the uplink and the downlink. In additional, we introduce a methodology for proper selection of a directional LGA/MGA for emergency communications

Enabling Affordable Communications for the Burgeoning Deep Space Cubesat Fleet

The low costs of development and launch, coupled with new propulsive technologies, have made cubesats increasingly popular for use in science investigations beyond geosynchronous orbit. As this deep space cubesat fleet grows in size, the challenge of trying to provide affordable communications for it grows commensurately. The mass, power, and volume constraints inherent to cubesats limit the antenna size and transmit power that they can use to close the deep space link. As a consequence, cubesats need to rely more heavily on ground antennas that are characterized by large aperture, low noise temperatures, and relatively high-power transmitters. Such antennas are not in great abundance, nor are they inexpensive to build. For this reason, NASA’s Deep Space Network has been advocating a three-pronged approach to meeting anticipated cubesat demand: development of simultaneous, shared-beam multi-spacecraft communications capabilities, development of large-antenna cross-support arrangements with other agencies and universities, and development of less uplink-intensive navigation techniques. This paper focuses on the pursuit of simultaneous, shared-beam multi-spacecraft communications capabilities. While the Multiple Spacecraft per Antenna (MSPA) technique has existed for over a decade, it has generally been limited to supporting downlink for just two in-beam spacecraft at a time. This limitation has largely been a function of the number and cost of available receivers. A relatively new technique that potentially overcomes this limitation is Opportunistic MSPA (OMSPA). Instead of relying on additional receivers, OMSPA makes use of a digital recorder at each ground station that is capable of capturing the intermediate frequency (IF) signals from every spacecraft in the antenna beam within the frequency bands of interest. When cubesat projects see one or more opportunities for their cubesat(s) to intercept the traditionally scheduled antenna beam of a “host” spacecraft, they can arrange for the cubesat(s) to transmit open loop during those opportunities. Via a secure Internet site, the cubesat mission operators can then retrieve the timeand frequency-relevant portions of the digital recording for subsequent demodulation and decoding, or subscribe to a service that does it for them. This “opportunistic” use of a host spacecraft’s ground antenna beam potentially enables cubesat projects to make use of large ground antennas for downlink without having to compete with bigger, better-funded missions for antenna time in the formal scheduling process. In so doing, it also potentially enables cubesat projects to avoid the aperture fees associated with formally scheduled downlink time – fees that factor into the “bottom-line” of competitivelybid NASA missions and that actually get charged to non-NASA missions. Taking advantage of these potential OMSPA benefits, however, will require cubesat projects to pursue mission designs that ensure at least periodic in-beam operations relative to a “host” spacecraft. In the case of a constellation of cubesats with inter-spacecraft distances that do not extend outside of the beam-width of the desired ground antenna at the given range, one cubesat can serve as the “host” and have a formally scheduled downlink while the rest of the cubesats can downlink essentially for “free” via OMSPA. Deep space cubesats, of course, will need uplink in addition to downlink. Beyond commanding, this need is driven by the use of two-way ranging and Doppler for navigation. While OMSPA may not directly facilitate uplink, it does have the potential to free up antennas for those spacecraft that periodically require formally scheduled links for commanding and two-way radio metrics. NASA is also exploring the physical feasibility of an in-beam, simultaneous multi-spacecraft uplink technique. As with OMSPA, if successful, it will require little new equipment, further enabling affordable deep space cubesat communications.