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''Figure 6. Typical example constellation definition, in this case for a proposed Chinese constellation.''

The output of this tool could then be passed to PassPredict or a similar algorithm to model the observability of the constellation at a given observatory and date. In an advanced mode, we should also predict the satellite brightness. Simulations should include cases for professional observations and also of the effect on the naked-eye sky. As to existing software, various unpublished research codes exist but would need to be modified to generate the ephemeris database output and improved to be robust enough for general use.

3.3.1.1. PassProbability

We can use the approach of EphemSimulate in a statistical mode to predict the average density of satellite passes on the sky in a given situation. We call this tool PassProbability. This can be used for longterm impact studies or as a quick alternative to the computation-intensive PassPredict to estimate the probability that a given observation will be affected by a satellite trail. Bassa et al. (2021, in preparation) have generated an analytic satellite density model that can be used to go from the constellation definitions to probability sky maps and calendars. The probability that an observation will be affected varies by orders of magnitude with only slight changes in the observation parameters.

We can consider two modes of output: Rh
 * 1) A Probability Map (Figure 7) which shows the fraction of exposure lost due to satellite trails as a function of celestial position. A value greater than 1 indicates that the exposure is entirely lost.
 * 2) A Probability Calendar (Figure 8). Here we calculate the probability of exposure loss for a specific target (in this case, the Large Magellanic Cloud) as a function of time of night and observation date. The idea is to plan the optimal dates for an observing run. The solar elevation is indicated by the blue contours; yellow-green slanted lines indicate the elevation of the target, with elevations below 20 degrees considered unobservable and shaded in gray. The color scale indicates the expected fraction of exposures lost.