Constellation design for the NASA TROPICS mission

The Time-Resolved Observations of Precipitation structure and storm Intensity with a Constellation of Smallsats (TROPICS) is a NASA Earth Venture Instrument mission to be launched in 2020. It consists of a constellation of six 3U CubeSats, each carrying two microwave sounders with a combined 12 channels spanning water vapor and oxygen absorption features as well as imaging bands across the millimeter-wave region from 90GHz to 205 GHz.

As a member of the TROPICS Constellation and Instrument Science Team, our lab performed some mission analysis work early on that informed the selection of the final constellation design.

It is important to note that a single metric such as average revisit time is not sufficient to capture coverage performance of a constellation. For example, average revisit time is often not sensitive to the number of planes. Instead, it is important to consider the whole picture given by the cumulative distribution function of gap durations.

CDF of gap durations for 4 constellations with different inclinations

We conducted a large trade study to explore a space of 4,400 different constellation designs, with varying number of satellites, planes, altitudes and inclinations. For each constellation design, we used the lab software to estimate various coverage metrics as well as a relative lifecycle cost metric. The figure below shows a view of the overall trade space.

Weighted Mean Revisit Time vs Horizontal Spatial Resolution and Cost for 4,400 different constellation designs for TROPICS, color-coded by altitude.

In addition, we conducted a study to assess the robustness of different constellations to loss of one or more spacecraft or launch vehicles.

Various latitude-weighted coverage metrics for different constellations with one or more spacecraft or launch failures.

Combined, these results helped the TROPICS team decide on the final baseline and threshold architectures.

You can read more details about this study in the paper below:

Buzzi, P. G., Selva, D., Hitomi, N., & Blackwell, W. J. (2019). Assessment of constellation designs for earth observation: Application to the TROPICS mission. Acta Astronautica.