The Secrets of SAR Remote Sensing

Few remote sensing methods are going through a fast development as radar remote sensing. Also known as Synthetic Aperture Radar (SAR), these systems use an active radar beam that bounce back from the surface of the Earth to create images. This so-called ‘active’ remote sensing concept allows SAR satellites to observe in day and night, and also through cloud cover, making them useful around the clock. In contrast, ‘passive’ optical instruments depend on (reflected) daylight and cannot penetrate cloud cover.

Because of the specific (invisible) bandwidth that radar waves are using, and because of the ‘bounced back’ images, SAR images do not look like regular optical satellite photos. Instead, there is a lot of calculation that needs to be done to make radar images ‘visible’ to the human eye.

It is this active radar system and its perceived complex mathematics to turn reflected signals into images that shies many users away from SAR data. And even for experienced SAR specialists it is not always easy to explain how SAR works to untrained clients.

Very high resolution

Until recently, SAR images were very coarse, stripy, and showed little detail, but this is where things have progressed very rapidly in the last few years. Recently, Finnish SAR company ICEYE announced an unprecedented 25 centimetre resolution of its latest remote sensing satellites, which is even better than most commercially available optical systems, including Copernicus Sentinel 2 and 3.

Even more interesting is that the ICEYE satellite isn’t a very big satellite, so most people are probably wondering why it can capture images with such a high resolution. The reason for this is that in the case of SAR, smaller antennas actually cause a higher spatial resolution, in contrast to other antenna-related technology in space, where ‘bigger is always better’.

This article on Japanese remote sensing blog ‘Sorabatake.jp‘ explains in simple terms how this works (you can read the entire article here):

Short radar pulses

In order to understand the most basic principle behind SAR resolution, we first need to understand what kind of waves are being sent for SAR.

Unlike communication satellites, SAR doesn’t emit a continuous stream of waves over a long period of time. Instead, SAR uses a high output signal called a “pulse”, which is sent out only for short periods of time. Since waves spread out like radiation, they bounce back from the point they hit on the earth’s surface.

When this happens, there are two points on the earth from where these waves bounce back at slightly different timings. When the difference in distance is great enough to detect, these two points are considered to be “resolved”. This is the most fundamental way to determine resolution in SAR images.

So what can we do to increase resolution, and increase it even when the two points are close together? All you have to do is make the pulse as short as possible. Making a pulse shorter is called “pulse compression”. The method used to compress SAR pulses is actually kind of ridiculous, so let’s take a minute to break it down.

So what can we do to increase resolution, and increase it even when the two points are close together? All you have to do is make the pulse as short as possible. Making a pulse shorter is called “pulse compression”. The method used to compress SAR pulses is actually kind of ridiculous, so let’s take a minute to break it down.

Pulse compression

To compress a pulse, you might well guess that you just turn the wave emission on and off really quickly.

The issue with doing that is there is a technical limitation as to how much you can do it. To be more specific, there is a hardware limitation to how quickly electrical currents can be switched on and off.

This is where the pulse compression technique comes into play.

Using the pulse compression technique, we send the pulses with frequency-changing radio waves, unlike the square waves (the ones shaped like squares) we have shown up until now.

The technical term for this is “chirp modulation”. This modulation is the same one used in frequency modulation, or more commonly known as FM, as in FM radio. You listen to different channels on FM radio by changing its frequency.

This is where it gets really technical (read it here), but it can be summarised as follows:

In summary

1. SAR resolution increases as the pulse signals are sent at short amounts of time.
2. To compress pulse signals even more, it required a pulse compression technique which uses the convolution integral.
3. To compress pulses by using the convolution interval, you need to broaden the FM spectrum.
   In other words, SAR resolution is closely tied to the FM spectrum!
4. To broaden the spectrum for the azumith direction, you need to increase irradiation time.
5. To increase irradiation time, you need to increase the beam width.
6. A smaller antenna creates a wider beam, so smaller antennas do increase resolution.

Series on SAR remote sensing

You can read more about how SAR remote sensing works, how you can get to this data (Sentinel-1) and which applications in real life this type of data has here.

It is also worth mentioning that the people of Ukraine recently purchased one of the ICEYE satellites to support the war efforts against Russian invading troops. We wrote about this ‘People’s Satellite’ earlier:

The ‘People’s Satellite’