GASPACS Picture Transmission

Alex Allgrunn, Tom Christensen
08/02/2021

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boom deploy test on campus
GASPACS boom deploy and picture test on campus

One of the main goals of the GASPACS satellite is to transmit a picture of the deployed payload to the ground station, which helps us to determine the success of the mission. Because pictures are so important for us, we have done a vast amount of work to ensure that our picture system is both robust and efficient. While there have been many challenges to overcome, it has been an exciting process filled with learning opportunities.

On the satellite, we take both a high and low quality picture of our deployed payload. As we take more and more pictures through instructions from the ground station, we save more pictures on the satellite. These pictures are compressed and prepared for transmission when they are requested from the ground station. They are then transmitted down, received, and processed on our ground station so they can be viewed. Two of the problems we have encountered are that pictures can get corrupted and that picture may need to be sent across multiple transmissions.



 

Transmisison Basics

When the pictures are prepared for transmission, they are compressed by an open source program called Slow Scan Digital Video (SSDV). This turns each picture into a collection of packets which can each be sent down over the radio to the ground station. Using packets is very beneficial because if when transmitting an entire picture a piece gets lost or corrupted, the whole picture is now unviewable. When packetized, the SSDV software will piece the picture back together on the ground station. This means that we will only lose small pieces of our picture if they are lost or corrupted. Using SSDV helps us to ensure mission success by making our transmission system more reliable.

Transmission Issues

Once the pictures are on the ground, there are other problems that arise. One major obstacle was that while low quality pictures take a minute or less to transmit, high quality pictures can take much longer to send. Each day, our ground station only gets 2-3 short windows to interact with the satellite, each only being about 10-15 minutes long. If we need to send down a picture over multiple transmissions, we need to be able to piece it together across multiple transmissions.


Transmission Protection

In order to solve this problem, we had to be very creative. One hurdle was that the data is in a different format when it comes in, when it is processed, and when it’s stored. However, we figured out a way to read old data and place everything into a list of packets. When a new packet comes in, it gets added to the list in the correct place based on its packet number. This means that we can both continue to add to a previous picture and fill in holes in the picture if we transmit the picture again from the beginning.

transmitted low quality picture missing 1 packet

transmitted low quality picture missing half of it's packets

transmitted low quality picture missing packets with a portion recovered

 

While there have been many challenges, it has been fun and fulfilling to be a part of a mission critical part of the GASPACS satellite and we look forward to getting pictures from orbit after our launch!

High altitude picture test facing towards earth