TerraSAR-X and TanDEM-X (Identified)

I recently managed to capture something special. The German radar satellite duo TerraSAR-X and TanDEM-X passing overhead in their famously tight formation. Below is the animation I put together from the sequence.

Animation showing the two satellites tracking across the sky together.

And here's a still frame from the same pass. You can clearly see both spacecraft as separate points of light, holding their formation as they cross the field of view.

A still frame showing both satellites side by side.

I also put together a short video of the pass. You can watch it below:

Identifying what I'd caught

I didn't know exactly what I'd filmed at the time. To work it out, I referenced the time and location and searched Stellarium and matched the tracking against the predicted satellite passes. Here's the wider view, showing the pair against the surrounding sky:

Stellarium view, zoomed out, showing the broader area of sky.

Zooming in confirmed it. Two satellites sitting almost on top of each other, labelled as TerraSAR-X and TanDEM-X. That formation signature is what made the identification obvious.

Stellarium zoomed in, with both satellites resolved as a tight pair.

A little about the pair

TerraSAR-X and TanDEM-X are German radar Earth-observation satellites operated by the DLR (German Aerospace Center). TerraSAR-X launched in 2007 and was joined in orbit by its near-identical twin TanDEM-X in 2010. Together they act as the world's first configurable single-pass synthetic aperture radar (SAR) interferometer in space.

What makes them remarkable, and what makes them such a fun target to film, is just how close they fly. The pair orbits at around 514 km altitude, but the separation between the two satellites is typically only 250 to 500 metres. That's a couple of city blocks, at orbital velocity. The formation is precisely controlled so that the two radar antennas can image the same patch of ground simultaneously from slightly different angles, which is what allows them to build extremely accurate global Digital Elevation Models (the WorldDEM dataset).

Why it's a nice catch

Visually, the close formation is what makes this pass interesting. Most satellites you film are solo objects, a single moving point of light. With TerraSAR-X and TanDEM-X you get two points of light moving in perfect lockstep, almost like a binary star drifting across the sky. They're not especially bright satellites, so you need a clear sky and a sensitive setup, but when conditions are right the result is unmistakable.

It's a small reminder of just how precisely modern satellite formations are flown. Two 1,300 kg spacecraft, hundreds of kilometres up, holding station within a few hundred metres of each other for over a decade.