Solid State Recorder – Bloganuary 2023

From my misc-pics-box here is a photo of a 1:1 Jason-3 satellite model outside EUMETSAT. It’s not the satellite I work on but is more photogenic than Metop I guess.

Welcome to Bloganuary , a WordPress Challenge I am participating in this month, that provides a daily writing prompt throughout January.

Day 3 – 3rd January 2023.

Today’s prompt is: What is the earliest memory you have?

In addition to writing every day during Bloganuary, I undertook an ancillary challenge to write more about space and satellites. Of course technical writing tends to be complicated but my aim here and with this blog is to declutter things and hopefully write an easily understandable interpretation.

So today, as per the Bloganuary prompt, I will talk about memory, but instead of digging deep into my own infinite memory, I would want to shed some light on the limited memory on-board a spacecraft.

The satellites I presently work with (Metop aka Meteorological Operational satellites) are weather and climate monitoring polar satellites orbiting 800 km above the Earth’s surface in a Low Earth Orbit (LEO). They carry 11 scientific instruments taking measurements that help in improving Numerical Weather Prediction models. In the short-term this leads to more reliable weather forecasts and in the long-term this helps in monitoring climate change more accurately.

A small side note to explain where I fit into all this; As the spacecraft operations engineer responsible for Attitude and Orbit Control System (AOCS), my role in particular is to keep the platform stable and to ensure that the satellite itself is oriented correctly, so that these instruments can look where they are supposed to and provide valuable data that is used by national weather agencies and meteorological organizations.

Like any other LEO satellite Metops move at a velocity of around 8 km per second (that is FAST!) making one orbit around the Earth’s poles in around 100 minutes, resulting in 14 or 15 revolutions of the Earth each day. Their geostationary (GEO) siblings move at the same speed as the Earth’s rotation about it’s own axis i.e. one orbit in 24 hours and enjoy continuous visibility due to this special orbit 36000 km above that keeps them ‘fixed’ over a point on the Earth (hence the name – geostationary). LEO satellites, however, move much faster and more or less manage to traverse the whole globe in the course of a day. Since these are polar satellites, the best candidates for a ground station to send telecommands to and receive telemetry from the satellite is at the poles. Metop’s ground stations are in Svalbard near the North pole and in Antarctica near the South pole and the satellite passes above these ground stations each orbit. The duration of this pass ranges between 7 to 13 minutes (average 10 minutes).

In short, the satellite is visible to us on ground only for around 10 minutes after every 100 minutes (considering only one of the polar sites). The scientific instruments take continuous measurements day and night. These are stored in a Solid State Recorder (SSR) which has a limited memory and can record up to one and a half orbit of data, after which it would overwrite older stuff. That is why it is important that every time the satellite flies over the ground station, the recorded science data from the last orbit is dumped to the ground. It is then transmitted to the mission data centre, played back, processed, archived and finally disseminated to users.

Another side note; Those 10 minutes are also crucial because, if for some reason something on the satellite decides to break, it’s just those 10 minutes that you have to ‘fix’ whatever’s broken. If it’s too late, you need to wait another 100 minutes before you’re able to ‘talk’ to the satellite again.

I hope reading this gave you some idea about the memory of a satellite.

Well, since I am in a chatty mood, I thought I would also write about my earliest satellite memory. This was 12 October 2011 – the launch of Meghatropiques which was a LEO satellite as a collaborative mission between ISRO (Indian Space Research Organization) and CNES (French National Space Agency). That was the very first time that I was in the mission control centre witnessing Launch and Early Operations Phase (LEOP) for a satellite. I was a young spacecraft engineer and was very new to ISRO as I joined my exciting job hardly one month ago. Before this time, I had only watched launch telecasts on live TV and the broadcast used to stop when the launcher successfully injected the satellite in it’s initial orbit. Well what I understood that day was that right when the TV broadcast stops is when the action began for us. After the satellite is separated from the launch vehicle, an automated sequence is initiated that deploys the solar panels and orients the satellite in a favourabe attitude so that it can generate power and sustain itself. Later it’s equipment are sequentially switched on and made operational. After that it’s orbit is corrected, if required, and then it begins with it’s technical and scientific duties. One of our colleagues gave us a hands-on training on LEOP that day explaining us what the next few hours and days would look like, both for the satellite and for the team responsible for it. Since I was very new I was only there to watch the operations from afar and was not really doing anything meaningful. But I remember feeling like a little happy kid in a candy shop, staring at those massive screens that I had earlier only seen in space movies like Apollo 13. During my 8 years at ISRO I had the opportunity to contribute to several other LEOPs of various satellites in LEO, GEO and beyond. But I can never forget that day with Meghatropiques.