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12Spacecraft are complex systems that involve different subsystems and multiple relationships among them. The design of a spacecraft is an evolutionary process that starts from requirements and evolves across different design phases. During this process, a lot of changes can happen and affect mass and power at component level, at subsystem level, and even at system level. Each spacecraft has to respect overall constraints in terms of mass and power: for this reason, it's important to be sure that the design does not exceed these limitations. Current practice in system modeling primarily deals with this problem allocating margins. However, a statistical characterization of the fluctuations in mass and power in the early design phases is missing. This lack of statistical characterization would result either 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. The authors have previously developed a mathematical approach ( [1] [2] [3] [4]) 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. The methodology uses a combination of statistics based on data and probabilities assessed by experts in the field. Additionally, the methodology is innovative in the approach developed to elicit experts' opinions [5] which allows the analyst to identify biases or mis-calibrations which can affect the final estimation. The work presented in this paper extends the approach developed to the entire spacecraft design, focusing on small spacecraft (less than 50 Kg mass) due to the recent interest in the space community in such platforms. The spacecraft is analyzed in its subsystems and related components. A database is constructed to compute data statistics. An interview process to elicit expert opinions in all the areas of spacecraft design is developed and the results are presented. A test case made of a small satellite mission (CASTOR) is presented and the results are described.

CubeSats and small satellites have potential to provide means to explore space and to perform science in a more affordable way. As the goals for these spacecraft become more ambitious in space exploration, moving from Low Earth Orbit (LEO) to Geostationary Earth Orbit (GEO) or further, the communication systems currently implemented will not be able to support those missions. One of the bottlenecks is the antennas’ size, due to the close relation between antenna gain and dimensions. Current antennas for CubeSats are mostly dipole or patch antennas with limited gain. Deployable (not inflatable) antennas for CubeSats are currently being investigated, but these solutions are affected by the challenge of packaging the whole deployable structure in a small spacecraft. The work that we propose represents the first attempt to develop an inflatable antenna for CubeSats. Inflatable structures and antennas can be packaged efficiently, occupying a small amount of space, and they can provide, once deployed, large dish dimension and correspondent gain. Inflatable antennas have been previously tested in space (Inflatable Antenna Experiment, STS-77). However they have never been developed for small spacecraft such as CubeSats, where the packaging efficiency, the deployment, and the inflation represent a challenge. Previous works developed by the authors described trade-off analysis, preliminary design and radiation model for the antenna. The research presented in this paper is focused specifically on implementation and testing. Details of the antenna’s fabrication and related issues are illustrated as well as the mechanism to fold and deploy the antenna in space. Finally, results of the experimental tests (vacuum chamber and anechoic chamber) are described. Future work in the development of the antenna will include the improvement of the fabrication process and the design of a 3U CubeSat mission to be proposed as a technical demonstration.

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.