Analytical constellation design and link budget computation tool for EO missions

The recent trend for Earth Observation (EO) is to use constellations of small satellites for missions so far enabled only by large platforms, with relevant overall mission cost savings due to technology improvement and component miniaturization. The availability of a computationally light tool, capable of performing a parametric analysis of constellation performance as a function of number of satellites and orbital parameters and of estimating the link budget in a reliable way, is of fundamental importance in the preliminary phase of constellation design. Such a tool was recently developed in collaboration between SITAEL SpA and the University of Pisa and is presented and discussed in this paper. The code developed consists of three inter-related modules. The first module is dedicated to constellation design. Assumption the use of sun-synchronous repeating ground track orbits, this tool needs as input only the instrument field of view (FOV), the mean latitude and the characteristic size of the area of interest. For repeat cycles (RC) between 1 to 30 days, the altitudes are iteratively computed along with the number of satellites needed to cover the area of interest. For a constellation in a single plane, the module also provides the satellites phasing to cover the area adjacent to the area covered by the previous satellite; for a multi-plane constellation, the number of orbital planes and their relative phasing is provided. The propellant mass needed for drag compensation is estimated at each altitude using the NRLMSISE-00 model. The constellation robustness, i.e. the of area covered in the case of failure of one or more satellites, is assessed considering a mean value of the ground track length in the area of interest. The second module performs post-processing of data from NASA’s General Mission Analysis Tool (GMAT) to analyze coverage and link performance among satellites and ground stations, as well as inter-satellite links. Variables imported from GMAT include position, velocity and temporal information. Coverage status of user-specified targets is evaluated based on the optical instrument parameters. Considering the user specified points as ground stations (GS), the link budget is then computed. For specified antenna types, the position of the sub-satellite points are calculated relative to the electric field contour levels around a user-defined area. This approach therefore uses varying GS power output with fixed (maximum) gain, thus giving a range for uplink and downlink margins in terms of SNR and Eb/N0. Losses due to rain, cloud and atmospheric gases are accounted for from ITU-R models, while transmission losses in cables are calculated based on look-up tables for both the ground and the space segment. For the SNR method, the user specifies the bandwidths of the receivers or transmitters and the required SNR. For the Eb/N0 method, typical selectable modulation/demodulation schemes are incorporated in the tool. The third module computes visibility among all members of the satellite constellation, allowing for the computation of inter-satellite link budgets under the assumption that the satellites are pointing towards each other in the direction of maximum gain. In the paper, we present a number of selected EO mission test cases to show how the tool is used for the constellation design and the capabilities to performs link budgets in realistic mission scenarios, as well as the results of validation tests performed using the commercial STK software package. The advantages of using a simple, integrated tool for preliminary mission analysis, instead of a number of different software programs, are illustrated and discussed.