# What makes Gaofen-4 “the most powerful GEO spy satellite”?

Does Popular Science's GAOFEN 4, The world's most powerful GEO spy satellite, continues China's great leap forward into space label as most powerful GEO spy satellite refer to optical resolution, electrical power, or ability to knock out satellites on the other side of the GEO orbit with laser beams? That last one is meant in jest.

If it is optical, and the resolution at Earth's surface is 50 meters (as mentioned in the article) at 36000 km that's 1.6E-06 rad. With a wavelength of 500 nm $\displaystyle 1.22 \frac{\lambda}{\theta}$ suggests an aperture of 40 cm.

That's about the same size as the cameras on deep space or planetary science missions, LEO spy satellites have apertures of 1 or 2 meters (or even more perhaps). Here's what a 1.5 meter spysat looks like. Even Hubble in LEO is 2.4 meters and if it were just a 21st century technology telescope and imager with a lighter mirror and none of the other science packages, I think it could have been lofted to GEO.

Is it really a roughly 40 cm camera (16 inches) that is making Gaofen-4 the most powerful GEO spy satellite? It certainly looks like a 40cm telescope in this photo:

above: from Spaceflight 101's Gaofen-4 Earth-Watching Satellite put into on-target Orbit by Long March Rocket. Photo credit: CAST via Xinhua

Gaofen-2 employs CS-L3000A bus, configured with one 1 meter panchromatic/4m multi-spectral camera, with designed lifespan of over 5 years. (my emphasis)

The payload fairing is 4.2 meters in diameter. If Gaofen-4's multispectral camera is using a 4 meter aperture, that sounds powerful to me, but I'm not sure why it would be limited to 50 meter resolution on Earth. And if 40cm is in fact the most powerful camera in GEO, I'll also be surprised.

edit: the answer was clear, I just needed reminding that this part was about Gaofen-2 and not Gaofen-4.

To public knowledge, it is the highest-resolution imager in GEO. The 2nd place is occupied by weather sats with instruments like GOCI (https://en.wikipedia.org/wiki/Geostationary_Ocean_Color_Imager), with resolutions around 500m.

The thing is there is a compromise to be made between field of view and resolution: at 50m resolution, GF-4 has a 400km field of view (source https://i.imgur.com/3g6vuSv.jpg). That makes it useful for spotting ships at sea, over a wide area. If it had a 2.4m mirror, it would have a 8m resolution and a much lower field of view. You'll notice that 8m resolution is still not great, and it is not enough to detect vehicles for instance, so the increased resolution does not make the satellite much more useful. On the contrary, it makes it heavier, slower to orient, and less productive due to the lower FOV.

For an overview of the surveillance capabilities from high orbits, you can have a look at https://satelliteobservation.wordpress.com/2016/10/17/persistent-surveillance/

Note that regarding GF-2, "4m multi-spectral" means 4m resolution, not aperture. The aperture is much smaller.

• Great answer, thank you! I don't know what "4m resolution" would mean for a multi-spectral imager. 4 megapixels? Four filters? 4nm spectral bandpass per wavelength channel? – uhoh Feb 11 '17 at 9:52
• @uhoh Each pixel of the image corresponds to 4 meters on the ground – gosnold Feb 11 '17 at 9:55
• 50 meter resolution on the ground is consistent with a 40cm aperture. 4 meter resolution on the ground needs a 5 meter aperture! How can the multi-spectral camera have a 4 meter resolution with a ~40cm aperture? – uhoh Feb 11 '17 at 10:04
• @uhoh You misread the article. In this passage it talks about Gaofen 2, not Gaofen 4. – gosnold Feb 11 '17 at 10:05
• Many thanks! Now I can sleep easier tonight. :) I've noted my oversight in the question. – uhoh Feb 11 '17 at 10:09