Dispersion of CubeSats in a “flock”

In a previous post I described the difficulty in distinguishing multiple CubeSats that are simultaneously deployed from the ISS. In this post I’ll describe how the satellites move away from the International Space Station (ISS) and drift apart from one another.

Four satellites were released on July 13, 2018 from a device on the ISS called the NanoRacks CubeSat Deployer. They were ejected at about 1.5 m/.s (3 mph.) Initially they were spaced about 10 to 50 cm (4 to 20 inches) apart. They quickly moved ahead of the ISS together in a cluster as the distance between them more gradually increased over time. EQUiSat, the satellite built by Brown Space Engineering (BSE) students, is the second from the right.

Four CubeSats shortly after deployment on July 13th
Four CubeSats shortly after deployment on July 13th. Credit: NASA

Below are maps showing the ground track of the small satellites as they orbit 400 km (250 miles) above the Earth. Also shown are the paths they take across the sky as they pass above the K1AD ground station at Ladd Observatory. Traveling at a speed of 27,600 km/h (17,100 mph) it takes only 92 minutes to orbit the Earth. The same 1998-067 object label suffixes are used as in the last post: NZ, PA, PB, and PC. EQUiSat is PA and EnduroSat is NZ. RadSat-g and MemSat failed to transmit signals and so they can’t be distinguished based on radio transmissions. I suspect that RadSat-g corresponds to the leading PC and MemSat is the trailing PB.

The first pair of diagrams shows the distance between the “flock” of satellites and the ISS on July 16th. This was the first day that orbital elements were published for the CubeSats. It took 3 days before they were separated from the ISS by enough distance that their position could be measured. They lead the ISS by about 400 km (250 miles. ) But they are too close together to be distinguished from each other and each individual satellite can not yet be tracked accurately enough for long term predictions. The tracking is good enough to predict passes above a ground station for the next day or so.

Ground track of the ISS and four CubeSats on July 16th
Ground track of the ISS and four CubeSats on July 16th. The CubeSats are too close together to distinguish.

Sky view of the path of the ISS and four CubeSats on July 16th
Sky view of the path of the ISS and four CubeSats on July 16th. The CubeSats move together across the sky.

The next pair shows the dispersion on August 23rd. This is when they were far enough apart to easily distinguish between them. They are all above the ground station at the same time but with a slight delay in rise and set times. The first one sets about 3 minutes before the last. The Doppler shift of the radio transmissions is also slightly different for each one. The distance between the first and last satellite is about equal to the distance between Providence and Chicago. The ISS is now half way around the world from them – over Australia.

Ground track of the ISS and four CubeSats on August 23rd
Ground track of the four CubeSats on August 23rd. The CubeSats have spread apart.
Sky view of the four CubeSats on August 23rd
Sky view of the four CubeSats on August 23rd. The CubeSats are setting at slightly different times.

The final pair shows the dispersion of the satellites on October 7th. They are still in very similar orbits but are no longer passing over the ground station at the same time. One is setting below the horizon as the next one rises. The distance between the leading and trailing satellites is now about equal to the distance between the BSE ground stations in Providence and Rome. It takes EQUiSat only 19 minutes to cover the distance between the stations.

Ground track of the four CubeSats on October 7th
Ground track of the four CubeSats on October 7th. The CubeSats continue to disperse.
Sky view of the four CubeSats on October 7th
Sky view of the four CubeSats on October 7th. One CubeSats is setting as the next one rises.

The current position and ground track of EQUiSat are shown on the BSE Mission Control page.

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