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 it’s 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 want to shed some light on the limited memory on-board a spacecraft.

The satellites I presently work with — Metop (Meteorological Operational satellites) — are weather and climate monitoring polar satellites orbiting 800 km above the Earth’s surface in Low Earth Orbit (LEO). They carry 11 scientific instruments that take measurements to improve Numerical Weather Prediction (NWP) models. In the short term, this leads to more reliable weather forecasts; in the long term, it helps monitor climate change more accurately.

A small side note to explain where I fit into all this:
As the spacecraft operations engineer responsible for the Attitude and Orbit Control System (AOCS), my role is to keep the platform stable and ensure that the satellite is oriented correctly, so the instruments can look where they’re supposed to and provide the data needed 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!), completing one orbit around Earth’s poles in about 100 minutes. That’s 14 or 15 revolutions per day.

Their geostationary (GEO) siblings, by contrast, move at the same speed as Earth’s rotation — one orbit every 24 hours — and enjoy continuous visibility from the same spot on Earth (hence the name geostationary). These GEO satellites are positioned much higher, about 36,000 km above the Earth’s surface.

Metop, being a polar LEO satellite, manages to traverse nearly the whole globe in the course of a day. This means the best candidates for ground stations (to send telecommands and receive telemetry) are located near the poles. For Metop, these are in Svalbard (near the North Pole) and Antarctica (near the South Pole).

The satellite passes over these ground stations every orbit, and the pass duration ranges from 7 to 13 minutes, averaging around 10 minutes.

So essentially, the satellite is only visible from the ground for about 10 minutes every 100 minutes — assuming we’re using just one of the polar sites.

The satellite’s scientific instruments are taking continuous measurements, day and night. These are stored in an onboard Solid State Recorder (SSR), which has limited memory — enough to hold about one and a half orbits’ worth of data. After that, it starts overwriting older data.

That’s why it’s crucial that each time the satellite flies over a ground station, the science data from the last orbit is dumped to the ground, transmitted to the Mission Data Centre, played back, processed, archived, and finally disseminated to users.

Another side note:
Those 10 minutes are also crucial for operations. If something on the satellite breaks, that’s your only window to try and fix it. If you’re too late or miss something, you have to wait another 100 minutes to talk to the satellite again.

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

My Earliest Satellite Memory

Well, since I’m in a chatty mood, I thought I’d also write about my earliest satellite memory.

It was 12 October 2011 — the launch of Meghatropiques, a LEO satellite and a collaborative mission between ISRO (Indian Space Research Organisation) and CNES (French National Space Agency). It was the very first time I was inside a mission control centre during LEOP (Launch and Early Operations Phase) for a satellite.

I was a young spacecraft engineer, barely a month into my exciting new job at ISRO. Until then, I had only watched launches on live TV, where the broadcast would stop once the satellite separated from the launcher.

What I realized that day was — the real action begins after the TV broadcast ends.

After separation, the satellite initiates an automated sequence that deploys solar panels and orients itself in a favourable attitude to start generating power. Then its equipment is switched on one by one, followed by orbit corrections, and eventually the satellite begins its scientific duties.

One of our colleagues gave us a hands-on training that day, explaining what the next few hours and days would look like — both for the satellite and for the team responsible for it.

Since I was brand new, I didn’t really do anything meaningful that day. I just watched from afar — but I remember feeling like a little happy kid in a candy shop, staring at those massive screens I had only seen in space movies like Apollo 13.

During my 8 years at ISRO, I got to contribute to several LEOPs of various satellites in LEO, GEO, and even beyond. But I will never forget that day with Meghatropiques.

Whether it’s the solid state memory of a satellite or the unforgettable memory of my first LEOP, both highlight how precious and fleeting moments can be — and why it’s so important to capture and share them.

Thanks for reading. 🚀