Exploring the impact of the Tracking and Data Relay Satellite System in Sedaro
Thirty-five thousand kilometers above Earth's surface sits a network of spacecraft called the Tracking and Data Relay Satellite System (TDRSS). These seven satellites congregate over the Atlantic, Pacific, and Indian oceans. Their orbits largely fix them over one longitude on Earth, moving slightly over the ground in a figure eight once a day but otherwise staying put for the life of their mission. Each of these bus-sized spacecraft is fitted with several communications devices, among them three large dishes and a flat array of helical antennas on one surface. The users of this network are (generally) not on Earth; instead, they are other satellites also in orbit around Earth.
Lower-orbiting spacecraft often struggle to find sufficient opportunities to send data to the ground. The lower the orbit is, the more likely it is that any particular ground station is below the horizon or otherwise obstructed by terrain. To solve this problem, the TDRSS network provides an always-available communication partner that can relay mission-critical data directly to ground. For 40 years, the TDRSS network has been parked up in geosynchronous orbit quietly routing data for Earth-orbiting satellites. Today, the network provides 100% coverage of orbits up to 3,000 kilometers in altitude and regularly supports over 25 missions [1, 2].
I spent some time doing operational navigation support for the TDRS constellation at NASA Goddard Space Flight Center, but I never got to take the time to look at the customer point of view. So, today I wanted to take the opportunity to quantify the benefits of TDRSS using Sedaro and see what the network looks like from a different perspective.
To start our analysis, let's think about what we would do if the TDRS system did not exist. Instead, we'll have to talk to receivers on the ground to transmit data. The Near Earth Network (NEN) is a good candidate.
The mission of the Near Earth Network (NEN) is to use ground-based antennas to provide “best value” direct-to earth communication services for NASA missions operating in the Near Earth regime (zero to two million kilometers from Earth).... The NEN provides Telemetry, Tracking and Command (TT&C) services for orbital missions and occasionally sub-orbital missions .
Our scenario follows the International Space Station (ISS) through one simulated day. I've mocked up a constant rate of mission data production in a central CPU component with onboard storage. When a link is available, the mock ISS will enter an operational mode and begin transferring the data through its modem and out to the target ground station. Once transmitted, the data is removed from storage.
I've also added 13 NEN ground stations approximating the regional coverage of NEN as a whole, taken from the Near Earth Network Users' Guide . In general, each of these locations has several antennas available for operations, but for simplicity I'm only adding one per site. This shouldn't impact our results since the additional antennas would be close together and all visible to the satellite at the same time.
Now, let's check out some results. The graphic below shows the operational mode history of our mock ISS. The "NEN Access" mode indicates at least one NEN antenna is in view and "No Access" means no antenna is visible. We can see that there are frequent access opportunities due to the geographic diversity of the network, but each communication window is fairly small.
Looking at the summary statistics of this same data, we see that we have access to the ground for only about 27% of the day. For many missions, this is level of coverage is adequate for downlinking data in a timely manner, but human space flight missions are subject to much more strict requirements. In an emergency situation, this level of access would be far from enough.
Looking in closely in the 3D view of our sim we can see why access is so limited. On average, the ISS is about 400 kilometers above earth. To make contact with a ground antenna, we almost need to pass directly overhead, and it's impossible (with our simulated network) to see multiple sites at the same time.
Now let's see how the TDRSS can help. For this next scenario, we'll have the same 13 NEN ground stations but also add the seven operational TDRS and three ground terminals. The relays in this scenario will listen for incoming data and immediately transmit anything it receives to the nearest ground terminal. This operations concept is often referred to as a "bent pipe". Since a TDRS always has access to a ground terminal, data never needs to be persisted onboard and can instead flow directly from the customer, through the TDRS, and then to the ground.
We'll use this CAD model to represent our TDRS. The model captures both single access antennas (the large dishes on either side), the multi-access phased-array grid on the main bus, and the smaller space-to-ground link (SGL) dish. For communicating with customer satellites, the TDRS uses the single- and multi-access antennas. While the multi-access array is fixed to the spacecraft, the two single access dishes can rotate to point at specific targets. The figure below shows the model in Sedaro playback with its fields of view (FOV) annotated.
The plot below shows the impact this change had on our operational modes. As advertised, the TDRS network provides full coverage for this altitude -- any time the ISS is not covered by a NEN station, it is covered by TDRS.
This is clearly a much better situation for maintaining constant contact with the space station. If we modify the operational modes in our scenario slightly we can see how robust this TDRS access actually is (figure below). Not only do we have full access to the TDRS network anywhere in the orbit, but we have at minimum two visible relays at any time! We'll likely never need five relays at once, but this broad coverage gives us confidence that the network will be there even in the event of multiple failures.
Now let's see the network in action! This video covers several orbits of simulation data from various perspectives to show a continuous stream of data flowing from the ISS to the ground.
Check out these scenarios in our live site right now, no account required:
These two scenarios cover the two cases that we've discussed above: the first shows 24 hours of ISS contacts with the NEN ground stations and the second link includes both NEN and TDRS communications. Each of these links will take you to the the scenario playback view where you can watch the interactions between the participants in the scenario evolve over time. Click on the Agents tab under Analyze to dive into the specifics for a particular spacecraft.
 NASA, "Space Network." https://esc.gsfc.nasa.gov/projects/SN?tab=capabilities (accessed Jul. 21, 2023).
 NASA, "Space Network Users' Guide", 2012.
 NASA, "Wallops Missions, Programs and Projects, https://www.nasa.gov/centers/wallops/missions/index.html (accessed Jul 21, 2023).
 NASA, "Near Earth Network (NEN) Users' Guide.", 2019.
 NASA, "Near Space Network." https://www.nasa.gov/directorates/heo/scan/services/networks/near_space_network/about (accessed Jul 21, 2023).