# How can ICEYE-X1 capture 2D high resolution SAR images in "tens of seconds"?

The PRNewsWire release First ICEYE-X1 Radar Image from Space Published says:

The full image transmitted to the ground from ICEYE-X1 exceeded 1.2GB of raw data and spans an area of roughly 80 x 40 km on the ground. ICEYE-X1 obtained the image in the span of ten seconds, traveling at a speed of more than 7.5 km/s and at an altitude exceeding 500 km. Matching what ICEYE simulated prior to the launch, the final data resolution from the first satellite reaches 10 x 10 meters.

Synthetic Aperture Radar works by mathematically combining radar data recored from a number of different physical locations, effectively synthesizing a large effective aperture defined by the cluster of different locations. For imaging the Earth from space, this is done using the movement of the spacecraft itself along-track, and if I understand correctly, for full 2D imaging of the surface the cross-track sampling comes either from a large physical span of antennas (see for example this question) or from closely-spaced ground tracks of completely different orbits.

The quoted passage says that the image was taken in only tens of seconds, which I believe means that there are not multiple passes used here. Since the full width of the spacecraft is only about 3.2 meters, and the wavelength is probably not much less than a centimeter, the cross-track resolution at an altitude of 500 km should be about 1km.

But the released 1600 x 1600 pixel image seems to be getting fairly close to 1 pixel resolution. I don't think this is a 1600 x 1600 km patch of Alaska.

Question: How can ICEYE-X1 capture 2D high resolution SAR images in "tens of seconds"?

edit: Looking a the images, edges facing towards the top are much brighter than those facing the bottom, suggesting reflections were recorded obliquely rather than top-down (nadir). I'm wondering if the rate of change of path length being different for different cross-track positions might be the distinguishing feature producing resolution in that direction.

below: "First Light" image from the press release. Open in new window to see full size. The SatCat hasn't even sorted out the payload objects of 2018-004, and one of them is already "Tweeting" images back to Earth!

caption: First ICEYE-X1 Radar Image from Space of Noatak National Preserve, Alaska

ICEYE-X1 does not have a vertically large antenna, so it can only provide 2D data. To interpret the image correctly, you can imagine the satellite crossing over the top of the image from left to right, looking sideways. The axis are:

• x-axis: position along track
• y-axis: slant range
• color axis: reflectivity

None of these measurements requires a large antenna. Resolution along track is provided by the synthetic aperture and resolution in distance is naturally good with any radar (it only depends on bandwidth).

A 3D-capable radar would be able to give ground distance and height instead of just slant range.

• Yes, this is exactly the answer to the question "How can..." that I am looking for. After thinking a bit I now understand what you mean by the distinction between 2D and 3D. For a single raw image, the range data does not tell you the real location in 3D space, so a given delay is a combination of distance along the geoid surface (for example) plus the altitude above the geoid. However, images do not exist in a "vacuum" and so a single image can be interpreted in the context of other images and known data. Thank you for this complete answer!
– uhoh
Feb 4 '18 at 2:14

A few issues with what you included:

1. The resolution is stated as 10m in the bit you included.
2. The size of the image was 80 km/ 40 km
3. The frequency is 10 GHz, meaning a wavelength of about 3 cm.
4. The SAR antenna is deployable, meaning the effective beam width can be greater then the satellite. Of course, the total width is about 3.2 m, as you indicated.

The actual theoretical resolution of an image can be found from this source, and is actually the size of the antenna, meaning a smaller antenna will give higher resolution. Thus, the maximum resolution is actually 1.6 m, or half of the antenna size.

It also seems that the actual beamwidth might be 200 km. If that is the case, it seems likely that they concentrate it on the 80x40km section, but have increased resolution otherwise.

Note that repetition time is 6 kHz, meaning that 6k images are used per second, or at least 60k during the time period over which the satellite passes overhead.

• I believe the key to the answer lies in the oblique view, as I've mentioned in the question. After I first posted the question, I noticed the asymmetry in the image. By looking to one side only, the cross-track dimension of the image correlates with return time (range-resolution). I think a fairly simple explanation of the use of return time for cross-track, plus spacecraft motion for along-track would be good enough here, albeit a simplification. The key is that neither one of those requires a large antenna, nor closely spaced multiple passes, which is what I've asked about.
– uhoh
Jan 21 '18 at 18:09
• @RoryAlsop OK you are probably right. Thanks!
– uhoh
Jan 21 '18 at 18:25

Full disclosure: ICEYE employee, I'll try my best to answer. Thank you for asking and being interested in ICEYE-X1!

The "amount of seconds" refers to the amount of time the satellite is actively imaging data for an individual frame.

I would not recommend referring to the current Spaceflight101.com article for technical specifications on ICEYE-X1.

The original Alaska image in the CISION/PR Newswire press release has unfortunately been upscaled by the service itself to 1600x1600 pixel size, which may have caused unnecessary confusion. The "original" released image on the ICEYE website press release section is the correct one (925x925 pixel png file) to look into, but that file is compressed down from the true original.

More ICEYE-X1 SAR imagery at a far higher resolution has recently been made available on the company website (for free but requires filling in a form), which you may find to be informative and valuable to answer these questions.

• Thank you for clarifying this information! It certainly has relevance to this question about resolution. Spaceflight101 generally provides excellent technical data and explanations. If there is an issue in this case, it might be helpful to communicate to them directly.
– uhoh
Feb 1 '18 at 23:37
• @uhoh - Cheers! Spaceflight101 slightly improved, now. Some other sources still a bit off, but improving. Don't hesitate to ask more, thanks! Feb 3 '18 at 22:57
• That's great! OK I'll be keeping an "eye on" ICEYE!
– uhoh
Feb 4 '18 at 2:17
• @uhoh it's cool to see this thread continues to be updated! Responses on the above from ICEYE: bbc.com/news/science-environment-51049746 Jan 30 '20 at 8:18
• ha! I've cited that one in How do radio astronomers avoid having their receivers burned out by ground-imaging radar from satellites? I'm glad you receive a notification of my comment, thank you for staying in touch with SE!
– uhoh
Jan 30 '20 at 9:25