Kar-Ming Cheung

Link-capability driven network planning and operation

Current deep space planning, scheduling, and operation are based on spacecraft-ground-view period and criteria such as mission priority and science returns. There are, however, no network models that take into consideration the real-time link capabilities and communication performances. As a result, many communication resources are under-utilized. In this paper, we develop a network planning and scheduling concept that takes into account communication link capabilities and telecom performances. Valuable link-driven information such as favorable telecom configurations (antenna pattern, frequency, temperature, etc.) and geometry (range, elevation angle, etc.) can be translated into higher supportable data rates during each pass. Therefore larger data volume can be transmitted or received within a period and mission tracking time can be shortened. This approach enables us to maximize the number of missions that the network resources can support. A mathematical framework for sample link models and optimization algorithms along with solutions are presented.

Orbit design based on global maps of telecom metrics

This paper describes a tool to aid orbit design called the Telecom Orbit Analysis and Simulation Tool (TOAST). By specifying the six orbital elements of an orbit, a time frame of interest, a horizon mask angle, and some telecom parameters such as transmitter power, frequency, antenna gains, antenna losses, required link margin, and received threshold powers for the rates, TOAST enables the user to view orbit performance as animations of two- or three-dimensional telecom metrics at any point on the planet (i.e., on global planetary maps). Supported metrics include: (i) number of contacts; (ii) total contact duration; (iii) maximum communication gap; (iv) maximum supportable rate; and (v) return data volume at a best single rate or with an adaptive rate, along with; (vi) the orbiters footprint and (vii) local solar times. Unlike other existing tools, which generally provide geometry, view periods and link analysis for an orbiter with respect to a single location on the planet, TOAST generates telecom performance metrics over the entire planet. The added capabilities provide the user an extra degree of freedom in analyzing orbits and enable the user to focus on meeting specific mission requirements, such as what data rates can be supported, what data volume can be expected, and what the time gap will be between communication periods. Although TOAST can be used to study and select orbits about any planet, we describe here its use for missions to Mars. TOAST is being used to analyze candidate orbits for the 2009 Mars Telecommunications Orbiter mission. Telecom predicts generated by TOAST for MTO orbit candidates are laying a foundation for selecting the MTO service orbit. This paper presents numerical simulations and telecom predicts for four candidate MTO orbits.

Fast eigen-based signal combining algorithms for large antenna arrays

A large array of small antennas can be used to enhance signals with very low signal-to-noise ratio and can also be used to replace large apertures. In this paper, a fast combining algorithm is proposed and analyzed to maximize the combined output signal-to-noise ratio. Our approach does not assume any sequence of trained symbols and is a blind combining technique, which does not require a priori knowledge of spacecrafts or the arrays spatial information. Our method for computing the optimal weight is based on the generalized Eigen theory and the algorithms are built upon the Power method. Unique advantages of our proposed algorithm include (i) no formation of covariance matrices and hence less storage is required (ii) the optimal weight is obtained with significant less efforts and thus the optimal weight can be attained more quickly (iii) our proposed algorithm is capable of handling the case when the symbol signal-to-noise-ratios at the receivers are very weak. Mathematical framework for large antenna arrays using the Eigen-based signal combining techniques along with detailed performance analysis, numerical algorithms and computer simulations are presented.

Statistical Risk Estimation for Communication System Design

Spacecraft is complex systems that involve different subsystems and multiple relationships among them. For these reasons, the design of a spacecraft is an evolutionary process that starts from requirements and evolves over time across different design phases. During this process, a lot of changes can happen. They can affect mass and power at component, subsystem, and system levels. Each spacecraft has to respect the overall constraints in terms of mass and power: for this reason, it is important to be sure that the design does not exceed these limitations. Current practice in the system model primarily deals with this problem by allocating margins on individual components and on individual subsystems. However, a statistical characterization of the fluctuations in mass and power of the overall system (i.e., the spacecraft) is missing. This lack of an adequate statistical characterization would result in a risky spacecraft design that might not fit the mission constraints and requirements, or in a conservative design that might not fully utilize the available resources. Due to the complexity of the problem and due to the different expertise and knowledge required to develop a complete risk model for a spacecraft design, this research is focused on risk estimation for a specific spacecraft subsystem, the communication subsystem. The current research aims to be a “proof of concept” of a risk-based design optimization approach, which can then be further expanded to the design of other subsystems as well as to the whole spacecraft. The objective of this paper is to develop a mathematical approach to quantify the likelihood that the major design drivers of mass and power of a space communication system would meet the spacecraft and mission requirements and constraints through the mission design lifecycle. Using this approach the communication system designers will be able to evaluate and compare different communication architectures in a risk tradeoff prospective. The results described in the presentation include a baseline communication system design tool, and a statistical characterization of the design risks through a combination of historical mission data and expert opinion contributions. An application example of the communication system of a university spacecraft is presented.

CDMA communications systems with constant envelope modulation for CubeSats

In this paper a communication system for CubeSats in formation to operate in the vicinity of the Lunar Lagrangian L1 is proposed. CubeSats will collect lunar scientific data and will perform surface observations. An improved low complexity CDMA system for CubeSats for communications between the Lunar L1 and Earth station is considered. The complexity of a coded CDMA transmitter is lower than the complexity of the CDMA receiver with decoder therefore for downlink communications it makes sense to use encoders such as space standard LDPC code followed by a spread spectrum transmitter for CDMA systems for CubeSats. For the uplink an uncoded CDMA system is chosen since the uplink transmit power is expected to be high enough to support the use of uncoded CDMA system. The uncoded CDMA yields receivers for CubeSats that have low complexity implementation. For uplink since there would be no multipath the use of orthogonal spreading codes is more appropriate. The choice of orthogonal codes would reduce the multiuser interference at CubeSats. For the downlink, based on the available bandwidth, and the data rates, a reasonable processing gain could be obtained. Thus the multiuser interference degradation due to the other CubeSats could be made small at the Earth station. In this paper we analyzed and simulated the proposed improved CDMA system for a concept Constellation of CubeSats. All system simulations are done using Simulink Matlab platform. For highly efficient nonlinear power amplifiers we use a filtered offset QPSK with phase modulation which is a CCSDS standard for constant envelope signaling. This allows a nonlinear amplifier at CubeSat to operate at saturation point for the highest efficiency. We compare the difference in performance with our current pulse shaped BPSK (using half-sine wave). Certainly if there is no bandwidth limitation (due to the spectral standard masking) we could as well use unfiltered rectangular pulses which would produce constant envelope signaling. But it seems that rectangular pulses will not satisfy the bandwidth limitation imposed by the spectral standard. Filtered offset QPSK with phase modulation is much more bandwidth efficient scheme.

Risk analysis for nondeterministic mission planning and sequencing

In this paper, we address the dilemma of planning in the presence of uncertainty - the problem of scheduling events where some events might have nondeterministic durations. Real world planning and scheduling problems are almost always difficult. Planning and scheduling of events with a mixture of deterministic and nondeterministic durations is particularly challenging. The idea of scheduling events into a conflict-free plan becomes obscure and intangible when event durations are not known in advance - there is no guarantee that when the plan is executed, the scheduled events would not violate any pre-defined rules and constraints, and the resource usages would not exceed their maximum allowable limits. This dilemma of not being able to a priori quantify the likelihood of achieving a conflict-free plan in the presence of uncertainty usually results in an overly conservative plan where resources are under utilized. Making use of some standard communication link analysis techniques to characterize communication system performance, to support tradeoffs, and to manage the operational risks associate with the link usage, we instigate a probabilistic description of event durations and introduce the notion of risk in terms of probability that the plan fails to execute successfully, which we denote as Pp. We attempt to define a rational and systematic approach to weight risk against efficiency by iteratively applying constrained optimization algorithms and Monte Carlo simulations to the plan. We also derive a simple upper bound of PF for a given plan, which is independent of the optimization algorithm. This risk management approach allows planners to quantify the risk and efficiency tradeoff in the presence of uncertainty, and help to make forward-looking choices in the development and execution of the plan. Another emphasis of this paper is to demonstrate that the general criteria of optimality and rules and constraints for event planning can be described mathematically in terms of linear and non-linear functions and inequalities. This allows the use of customized and commercial off-the-shelf (COTS) constraint optimization algorithms to generate conflict-free plans. The results described in this paper are applicable to many general planning and scheduling problems. However the emphasis of this work is on mission planning and sequencing of spacecraft events with a mixture of deterministic and nondeterministic durations. Mission planning and sequencing is a critical component for mission operations. It provides a mechanism for scientists and engineers to operate the spacecraft remotely from the ground. It translates the science intents and spacecraft health and safety requests from the users into mission plans and sequences. After a rigorous process validating the plan, the plan will be transmitted to the spacecraft for its execution. Usually mission planning and sequencing and its validation are time consuming and costly operations. We apply the aforementioned methodology for formulating and optimizing both deterministic and nondeterministic sequence events planning. We demonstrate this approach with examples of scheduling science and engineering activities for mission operations.

Link analysis for space communication links using ARQ protocol

In space communications, standard link analysis assumes that messages are sent once. For a communication link that uses an error-correction coding scheme, bit-error-rate (BER) or frame-error-rate (FER), and link margins are common metrics that characterize the quality of a link, and they are used to determine the supportable data rate. With the advent of Automatic Repeat-reQuest (ARQ) protocols, when messages are corrupted during transmission, they can be resent multiple times automatically until they are correctly received and acknowledged. The concept of BER, FER, and link margin cannot be directly applied, and the link analysis approach for ARQ links needs to be re-examined. In [1] we described the problem formulation and defined the evaluation metrics to analyze the performance of ARQ links, and derived analytical models that describe the statistical behavior of the space links that use ARQ. In this paper, we show that by integrating these analytical ARQ protocol models into the standard link analysis, we bypass the need to simulate or emulate the ARQ protocol operations, and generate analytical models on effective data rate, effective throughput, latency, and FER. We demonstrate this approach using the Lunar L2 Flyby Mission communication scenarios, and discuss the insights and trades between link efficiency, latency, and error rate.

Wallops' low elevation link analysis for the Constellation launch/ascent links

Prior to the redirection of the Constellation Program, the Wallops 11.3-meter ground station was tasked to support the Orions Dissimilar Voice (DV) link and the Aress Development Flight Instrument (DFI) link. Detailed analysis of the launch trajectories indicates that during the launch and ascent operation, the critical events of Orion-Ares main engine cut off (MECO) and Separation occur at low elevation angle. We worked with engineers from both Wallops Flight Facility (WFF) and Johnson Space Center (JSC) to perform an intensive measurement and link analysis campaign on the DV and DFI links. The main results were as follows: (1) The DV links have more than 3 dB margin at MECO and Separation. (2) The DFI links have 0 dB margin at Separation during certain weather condition in summer season. (3) Tropospheric scintillation loss is the major impairment at low elevation angle. (4) The current scintillation models in the Recommendation ITU-R P.618 (Propagation data and prediction methods required for the design of Earth-space telecommunication systems), which are based on limited experimental and theoretical work, exhibit idiosyncratic behaviors. We developed an improved model based on the measurements of recent Shuttle mission launch and ascent links and the ITU propagation data. (5) Due to the attitude uncertainty of the Orion-Ares stack, the high dynamics of the launch and ascent trajectory, and the irregularity of the Orion and Ares antenna patterns, we employed new link analysis approach to model the spacecraft antenna gain.

Proximity link design and performance options for a Mars areostationary relay satellite

Current and near-term Mars relay telecommunications services are provided by a set of NASA and ESA Mars science orbiters equipped with UHF relay communication payloads employing operationally simple low-gain antennas. These have been extremely successful in supporting a series of landed Mars mission, greatly increasing data return relative to direct-to-Earth lander links. Yet their relay services are fundamentally constrained by the short contact times available from the selected science orbits. Future Mars areostationary orbiters, flying in circular, equatorial, 1-sol orbits, offer the potential for continuous coverage of Mars landers and rovers, radically changing the relay support paradigm. Achieving high rates on the longer slant ranges to areostationary altitude will require steered, high-gain links. Both RF and optical options exist for achieving data rates in excess of 100 Mb/s. Several point designs offer a measure of potential user burden, in terms of mass, volume, power, and pointing requirements for user relay payloads, as a function of desired proximity link performance.

Code division multiple access communications systems for CubeSats at Lunar Lagrangian L1

Interplanetary Cubesats would enable low-cost missions for high-quality scientific and exploration programs. In particular cubeSats in formation have been proposed to operate in the vicinity of the Lunar Lagrangian L1 to collect lunar scientific data and to perform surface observation. In this paper we present a low complexity CDMA system for CubeSats (M small spacecraft) for communications between the Lunar L1 and Earth station. It is well known that the complexity of a CDMA transmitter is much lower than the complexity of the CDMA receiver. Moreover, the complexity of a channel encoder is always much lower than the complexity of the channel decoder. So for downlink communications it makes sense to use encoders for modern codes such as Turbo and LDPC followed by a spread spectrum transmitter for CDMA systems for CubeSats. Here we used an LDPC coded CDMA with BPSK modulation with rectangular and half-sine pulse shaping. Except for the PN generator seed numbers, the communication structure of all CubeSats would be identical and operating at one single RF frequency. For the uplink we may choose an uncoded CDMA system since the uplink transmit power is expected to be high enough to support the use of uncoded CDMA system. In addition since there would be no multipath for the uplink (broadcast channel) the use of orthogonal spreading codes such as Walsh codes is appropriate. The choice of orthogonal codes would reduce the multiuser interference. However due to some limitation (bandwidth, data rates, and M) we may be forced to use nonorthogonal PN codes. In addition, one of the spreading codes will not carry any data, which acts as an unmodulated pilot to reduce the complexity of synchronization. The proposed uncoded CDMA yields receivers for CubeSats that have low complexity implementation. Each component of CubeSats could easily extract its own received data with almost no interference from other users in case of orthogonal spreading codes. For the downlink, depending on the available bandwidth, and the data rates, a large processing gain could be obtained if the N is not large. Thus the multiuser interference degradation due to the other CubeSats could be made small at the Earth station. If N is large, and the bandwidth and data rates do not allow large processing gains then the multiuser interference could be high. In such cases we could use a simple parallel interference cancellation method with two stages that dramatically improves the system performance for the downlink. In this paper we accurately analyzed and simulated the proposed CDMA system for a concept Constellation of 20 CubeSats (M=20). All system simulations are done using Simulink platform.