Modeling a GNSS Satellite Constellation
Propagate a constellation of satellites in Simulink® using the Orbit Propagator block in Aerospace Blockset™ and load the logged ephemeris data into a satelliteScenario object for access analysis. The satelliteScenario object in Aerospace Toolbox lets you load previously generated, time-stamped ephemeris data into a scenario from a timeseries or timetable object. Data is interpolated in the scenario object to align with the scenario time steps, allowing you to incorporate data generated from a Simulink model into either a new or existing satellite scenario object. The satellite constellation can then be visualized using the Satellite Scenario Viewer. In the viewer, a line is drawn between each ground station and satellite during their corresponding access intervals.
Published: 26 Sep 2022
A satellite constellation, like the one shown here, is a group of satellites that work together as a system. Two constellations you may be familiar with are Galileo and GPS. They are both a specific kind of satellite system known as a GNSS, or global navigation satellite system.
GNSSes work by providing signals from space that transmit positioning and timing data to receivers. This technology allows receivers, like a cell phone, to know where on Earth you are located. This works through a process called trilateration. in order to achieve GNSS trilateration, the receivers need to have a line of sight access to at least four satellites.
Therefore, if you are working with a satellite constellation, it can be important to compare access of multiple ground stations to the constellation. This ensures each station has sufficient satellites at any given time.
Aerospace Blockset's Orbit Propagator block can be used to simulate a satellite constellation. You can then use the Satellite Scenario Viewer to visualize the constellation results.
In this example, we are going to be looking at a satellite constellation like Galileo, the European GNSS. Let's begin setting up the constellation.
We can start by defining the mission parameters, including the start date and duration. The constellation in this example is a Walker Delta constellation. Walker Delta constellations are a common solution for maximizing geometric coverage over Earth while minimizing the number of satellites required.
This example has an inclination of 56 degrees, with 24 total number of satellites. The satellites are located on three equally spaced geometric planes, and there's one space between satellites and adjacent planes.
We can then specify the initial conditions to reflect this. Next, we can begin looking at the Simulink model and configuring it. We can set the satellite initial conditions and configure the propagator.
We can then run the model and collect the positioning data. After, we can then extract position and velocity data from the model. Next, we can set the start date as we defined earlier in the mission parameters.
Next, we can create a satelliteScenario object for analysis. We can then add all 24 satellites to the satelliteScenario from the ecef position and velocity timeseries object using the satellite method.
After that, we can make some graphic adjustments for viewability. This includes hiding the satellite labels and ground tracks and setting satellites in each orbital plane to have the same orbit color.
Next, we can add the ground stations. In this example, we will use three MathWorks locations to compare total constellation access over the one-day analysis window to different regions of Earth. The ground stations are located in Natick in the USA; Munich, Germany; and Bangalore, India.
After, we can calculate line of sight access between the ground stations and each individual satellite using the access method. We can then set access colors to match orbital planes-- colors assigned earlier in the example. We can then view the full access table between each ground station and all of the satellites in the constellation.
After all that work, we can now view the constellation via the Satellite Scenario Viewer. As you can see, the viewer window contains all 24 satellites in each of the three orbital planes, as well as the ground stations. Also, you can notice a line drawn between each ground station and satellite during their corresponding access intervals.
Next, we can calculate access status between each satellite and ground station. We can plot the cumulative access for each ground station over the one-day analysis window.
Walker Delta constellations are evenly distributed across longitudes. Natick and Munich are located at similar latitudes and, therefore, have very similar access characteristics with respect to the constellation.
Bangalore is at a latitude closer to the equator. And, despite having a lower number of individual access intervals, it has the highest average number of satellites in view. It also has the highest overall interval time and the longest average interval duration by about 95 minutes. All locations always have at least four satellites in view, as is required for GNSS trilateration.
For more information on orbit propagation in satellite scenarios and how to use the Satellite Scenario Viewer, please see the Introduction to Orbit Propagation and Visualization video for an in-depth overview. This includes a more in-depth guide on using the Satellite Scenario Viewer, which can be found in Aerospace Toolbox.
Also, please see the Constellation Modeling with the Orbit Propagator Block documentation to try out the example covered in this video of yourself. Please check out these resources and more with the links provided.