Shervin Shambayati

Deep Space Ka-Band Link Management and Mars Reconnaissance Orbiter: Long-Term Weather Statistics Versus Forecasting

During the last 40 years, deep space radio communication systems have experienced a move toward shorter wavelengths. In the 1960s, a transition from L- to S-band occurred, which was followed by a transition from S- to X-band in the 1970s. Both these transitions provided deep space links with wider bandwidths and improved radio metrics capability. Now, in the 2000s, a new change is taking place: namely, a move to the Ka-band region of the radio frequency spectrum. Ka-band will soon replace X-band as the frequency of choice for deep space communications, providing ample spectrum for the high data rate requirements of future missions. The low-noise receivers of deep space networks have a great need for link management techniques that can mitigate weather effects. In this paper, three approaches for managing Ka-band Earth-space links are investigated. The first approach uses aggregate annual statistics, the second one uses monthly statistics, and the third is based on the short-term forecasting of the local weather. An example of weather forecasting for Ka-band link performance prediction is presented. Furthermore, spacecraft commanding schemes suitable for Ka-band link management are investigated. Theses schemes will be demonstrated using NASAs Mars Reconnaissance Orbiter spacecraft in the 2007-2008 period, and the demonstration findings will be reported in a future publication.

The Cassini May 2000 solar conjunction

Interplanetary spacecraft, which fly in the ecliptic plane, typically encounter solar conjunctions during their main missions. The communications link between an interplanetary spacecraft and Earth is affected by the charged particles that constitute the intervening solar corona and solar wind. As the Sun-Earth-probe (SEP) angle becomes small (usually <3/spl deg/ for X band or 8.43 GHz), the signal suffers increased degradation. The effects on the received signal include time delay and phase fluctuations due to the fluctuating columnar electron density, which in turn cause carrier lock problems and telemetry data loss. Because of these effects, studies of solar corona charged particle effects on spacecraft signals were conducted to determine strategies for optimizing data return during these periods. The first solar conjunction of the Cassini spacecraft occurred between May 8, 2000 (2000/129) and May 18, 2000 (2000/139). During this period, the Cassini spacecraft was within 3.2/spl deg/ of the Sun as seen from Earth with the minimum SEP angle of 0.56/spl deg/ occurring on May 13 (2000/134). This solar conjunction occurred prior to the expected peak of the current solar cycle. Coherent dual-frequency X band (8.43 GHz) and Ka band (32 GHz) data were acquired from 3.2/spl deg/ to near the minimum SEP angle at 0.6/spl deg/ for both ingress and egress. The measurements of amplitude scintillation, spectral broadening and phase scintillation were examined as a function of SEP angle. As expected, these solar effects are significantly less at Ka band than at X band for the same SEP angle. This studys results will be combined with those of other spacecraft solar conjunctions in order to build a statistical database of solar effects as a function of solar elongation angle and phase of the solar cycle. Such studies are useful in the design of telecommunications systems for future spacecraft missions, which may have stringent communication requirements during their solar conjunction phases.

On the benefits of short-term weather forecasting for Ka-band (32 GHz)

Due to spectrum limitations at lower frequencies, NASAs Deep Space Network is currently implementing Ka-band (32 GHz) tracking capabilities at all of its deep space communication complexes (DSCCs). Since weather effects and increases in the atmospheric noise temperature associated with them are the biggest uncontrollable factors in the performance of a Ka-band deep space telecommunications link, use of algorithms to forecast the atmospheric noise temperature for a pass is desirable. In this paper, an analytical method for comparing the performance of an ideal forecasting algorithm to the best statistical methods in terms of average data return is derived. This methodology is applied to two different cases. In the first case, the spacecraft cannot change its data rate during the pass. In the second case, the spacecraft can continuously vary its data rate. This methodology is applied to four different elevation profiles whose maximum elevation varies from less than 30 degrees to greater than 80 degrees for Goldstone, Madrid and Canberra DSCCs. This analysis shows that for the fixed data rate case, while the forecasting does not significantly increase the average data return on the link (between 0.2 dB and 0.4 dB, depending on the DSCC and the elevation profile) it does improve the reliability of the link significantly (in ideal case to 100%). For the continuously variable data rate case, forecasting improves both the average data return (by between 1 dB and 1.9 dB depending on the elevation profile and the DSCC) and the reliability of the link (in ideal case to 100%).

Link design and planning for Mars Reconnaissance Orbiter (MRO) Ka-band (32 GHz) telecom demonstration

NASA is planning a Ka-band (32 GHz) engineering telemetry demonstration with Mars Reconnaissance Orbiter (MRO). Capabilities of Ka-band for use with deep space mission are demonstrated using the link optimization algorithms and weather forecasting. Furthermore, based on the performance of previous deep space missions with Ka-band downlink capabilities, experiment plans are developed for telemetry operations during superior solar conjunction. A general overview of the demonstration is given followed by a description of the experiment planning during cruise, the primary science mission and superior conjunction. As part of the primary science mission planning the expected data return for various data optimization methods is calculated. These results indicate that, given MROs data rates, a link optimized to use of at most two data rates, subject to a minimum availability of 90%, performs almost as well as a link with no limits on the number of data rates with the same minimum availability requirement. Furthermore, the effects of forecasting on these link design algorithms are discussed

Ka-band link optimization with rate adaptation

On-going development of Ka-band capability for the deep space networks (DSN) will radically increase the bandwidth available to support advanced mission concepts envisioned for future robotic as well as human exploration of Mars and beyond. While Ka-band links can operate at much higher data rate than X-band, they are much more susceptible to fluctuating weather conditions and manifest a significant trade-off between throughput and availability. If the operating point is fixed, the maximum average throughput for deep space Ka-band link is achieved at about 80 percent availability, i.e., weather-related outages will occur about 20 percent of the time. Low availability increases the complexity of space mission operation, while higher availability would require additional link margins that lowers the overall throughput. To improve this fundamental throughput-availability tradeoff, data rate adaptation based on real-time observation of the channel condition is necessary. In this paper, we model the Ka-band channel using a Markov process to capture the impact of the temporal correlation in weather conditions. We then develop a rate adaptation algorithm to optimize the data rate based on real time feedback on the measured channel conditions. Our algorithm achieves both higher throughput and link availability as compared to the constant rate scheme presently in use

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.

Low cost deep space hybrid optical/RF communications architecture

This paper reports on a study of hybrid optical/Radio Frequency (RF) architectures for deep space missions. Previous proposed optical deep space communication architectures were generally designed to achieve 90% or better availability 24/7. This study, instead, considered alternative metrics and architectures. It focuses on a strategy to use RF links and existing RF infrastructure for navigation and for communications requiring high availability, and optical communication links only for high volume downlink data. The optical link can then be designed to maximize data volume rather than availability. Utilizing Automatic Repeat Request (ARQ) with this strategy, a high level of completeness is possible even with low link availability - though with an increase in latency and spacecraft memory requirements. This strategy is suitable for deep space missions whose high volume links are dominated by science data that can tolerate long delays. The study found that with this optical downlink strategy, a single ground telescope can provide the principal expected benefit of optical communications (high data volume) at much lower cost than optical infrastructures designed to provide 90% availability 24/7. The study found also that data volume can, in some cases, be maximized by arraying all ground telescopes at a single site so that they have identical weather statistics. This low cost architecture, here named Single Optical Site (SOS), can eventually be augmented with multiple sites to provide high optical availability.

MRO Ka-band Demonstration: Cruise Phase Lessons Learned

NASAs mars reconnaissance orbiter (MRO) was launched on August 12, 2005 and was inserted into Mars orbit successfully on March 10, 2006. From September 2005 through January 2006, the Ka-band signal from the spacecraft was tracked several times by NASAs deep space network (DSN). During these tracks a number of Ka-band functions for both the spacecraft and the ground systems were tested. These tests were performed to assess the readiness of the ground system and spacecraft to support Ka-band demonstration activities during MROs primary science phase (PSP). As a result of these tests a number of important lessons were learned regarding the performance of the Ka-band link and the performance of the ground equipment. This paper presents the performance of the ground antenna pointing, delta differential one-way ranging (DeltaDOR) performance and the telemetry performance of the Ka-band link. These results indicate that the extra effort for pointing calibration of the ground antennas at Ka-band is extremely useful; Ka-band shows potential for improved DeltaDOR performance and the models for Ka-band performance of the ground station microwave noise temperature are very accurate.

The Struggle for Ka-band: NASA's Gradual Move Towards Using 32-GHz Ka-band for Deep Space Missions

32-GHz Ka-band was first considered for deep-space use in 1976. In 1979, 1 GHz of spectrum at 32-GHz Ka-band was allocated for deep space use. Since then NASAs Jet Propulsion Laboratory (JPL) has been developing technologies and architectures necessary to support Ka-band planetary missions. This paper is a survey of JPLs effort. This survey includes a summary of early paper studies done in the 1980s and 1990s, development of the 34-m beam waveguide (BWG) antennas at the deep space network (DSN), and Ka-band experiments on Mars Observer, Mars Global Surveyor, Deep Space 1, Cassini and Mars reconnaissance orbiter spacecraft. The focus of this paper is on the technological and architectural challenges that 32-GHz Ka-band operations have presented throughout this long history. These include challenges presented by the weather and tighter pointing requirements for the spacecraft as well as the need to use multiple data rates during a pass.

Ka-band link optimization with rate adaptation for Mars and lunar communications

On-going development of Ka-band capability for the Deep Space Networks (DSN) will radically increase the bandwidth available to support advanced mission concepts envisioned for future robotic as well as human exploration of Mars and beyond. While Ka-band links can operate at much higher data rate than X-band, they are much more susceptible to fluctuating weather conditions and manifest a significant trade-off between throughput and availability. If the operating point is fixed, the maximum average throughput for deep space Ka-band link is achieved at about 80% availability, i.e. weather-related outages will occur about 20% of the time. Low availability increases the complexity of space mission operation, while higher availability would require additional link margins that lowers the overall throughput. To improve this fundamental throughput-availability trade-off, data rate adaptation based on real-time observation of the channel condition is necessary. In this paper, we model the Ka-band channel using a Markov process to capture the impact of the temporal correlation in weather conditions. We then develop a rate adaptation algorithm to optimize the data rate based on real time feedback on the measured channel conditions. Our algorithm achieves both higher throughput and link availability as compared to the constant rate scheme presently in use. Copyright