FLM

What on earth is Remote Sensing?

Hans de Zwart and I write a monthly series titled: Parallax. We both agree on a title for the post and on some other arbitrary restrictions to induce our creative process. For this post we agreed to write about a new technology using Linux Format‘s “What on Earth is …?” style (see example on Android). We did not agree on a particular technology and we would get bonus points for a nice pixellated image to accompany the post. You can read Arjen’s post with the same title here.

I’m not interested in another Swami theory, so please …

Hold your horses, we’re talking about this wikipedia page not this one; remote sensing is the small or large-scale acquisition of information of an object or phenomenon, by the use of either recording or real-time sensing device(s) that are wireless, or not in physical or intimate contact with the object (such as by way of aircraft, spacecraft, satellite, buoy, or ship).

So you’re taking pictures while not actually being there?

Kind of.  Though they’re nothing like the pictures you take with your pocket camera. On the other hand they might be very much like those pictures.

What do you mean?

First, of all, in the case of earth observation remote sensing, the sensor platform is often a satellite. That means that the sensor is at a huge distance from the object of interest compared to you taking a picture of your grandma. Sometimes the latter is referred to as ‘Close Sensing‘ though that is not a common term. It’s used for hand held sensing devices.

Ok, like, when I want to take a city view picture, I climb the highest tower or peak in the area to get a good overview.

Exactly. But you’ll also notice that in those pictures it’s not easy to distinguish far away features like faces or distant bill boards. That’s because the spatial resolution of your camera is only so many megapixel. Likewise the spatial resolution of a satellite sensor determines the size of the smallest object still recognizable in the image (notice how I sneakily swift from your camera picture to a satellite image?). Some satellites have a spatial resolution of 1km which doesn’t make them very suitable for e.g. mapping small rivers.

I see, so to map small rivers they need higher resolution sensors which means more data which means smaller areas being observed.

Yes. Moreover, when you take a picture at night, you see nothing provided you don’t use any artificial lighting. The same applies to the visible range of the electromagnetic spectrum of optical satellite sensors.

The visible what?

The electromagnetic spectrum is the range of possible wavelengths (or frequencies) of electromagnetic radiation. Electromagnetic radiation is what we measure with our sensors (your camera and my satellite sensor). The visible range is generally referred to as light or the things we see with our eyes. Your camera usually only has three channels or bands: red, green and blue. Satellite sensors often have a whole bunch of bands including infrared and ultraviolet. The number of bands is usually called the spectral resolution. The sensitivity of the sensor of measuring radiation intensity is usually called radiometric resolution. Your camera would call this colour depth (an 16bit camera can see more colours than an 8bit one).

Aha, so the more bands and the higher the radiometric resolution the more you can see?

True. Conveniently, objects on the earth all have their own typical spectral signature which are collected in spectral libraries. That makes it possible to identify and classify remotely observed objects into thematic maps.

And what about this optical thing in sensors you talked about before?

In remote sensing there are basically two types of sensors: active and passive. The most commonly known and easiest to interpret are active sensors which measure detect natural radiation that is emitted or reflected by the object or surrounding area being observed. Reflected sunlight is the most common source of radiation measured by passive sensors (wikipedia). All optical sensors are passive. Active sensors have their own source of illumination. That makes them independent of sunlight which is very handy because you can also measure at night. The most common active sensing method for earth observation is with imaging radar, more specifically Synthetic Aperture Radar.

So with radar satellites you can ALWAYS make pictures, that’s awesome!

That is awesome. Moreover because radar waves are long (paradoxically, they are called microwaves) they can see through clouds. Those two features make imaging radar satellites very suitable for monitoring purposes. Like monitoring tropical forests. In case of monitoring one more aspect is important which is temporal resolution. That is the frequency of flyovers of satellites.

Radar can see through clouds? That’s BS, I know a website which measures clouds with radar! Moreover, that temporal resolution stuff also puzzles me because on that same website there is continuous near real time information.

Very smart. I agree that it must be confusing me telling you that radar van see through clouds and all the weather guys watching their cloud monitoring radars. The answer is easy: radar bands which detects clouds have a shorter wavelength than radar which can see through clouds (typical bands are X,C,L and P band for imaging radar and K band for cloud radar). The near real time information is provided by the fact that lots of weather satellites are geostationary. That means they are constantly watching one part of the earth. Their temporal resolution is thus infinite.

Great, then lets only build imaging radar satellites from now on and put them in geostationary orbit to infinitely monitor everything!

Yes let’s do that. You pay. Honestly, imaging radar is nice, very nice. Because it has an active sensor you can even do more cool things like interferometry and polarimetry but we’ll not go into that now. The ‘downside’ is that radar waves are most suitable for measuring geophysical characteristics of objects. If you want to know about the biochemical characteristics, you’ll have to use shorter wave lengths like those in passive optical satellites. Another disadvantage of radar images is that it they more difficult to interpret and classify than optical images. An example is the foreshortening effect of high objects like mountains caused in radar images by radar parallax (hey, where did I hear that term before?).

OK, so now we have all this fancy imagery. What’s next, what to do with it?

Rondonia, Brazil, 2006

Rondonia, Brazil, 1995

That is a very sensible question. Every time one of the major space agencies (NASA, ESA or JAXA) launches a new remote sensing satellite, they setup downstream programs offering opportunities to commercial businesses and / or research institutes to come up with useful applications. Examples of these applications can be: Water Quality Mapping and Monitoring, Drought Monitoring, Tropical Forest Mapping and Monitoring.

One of the most commonly known uses of remotely sensed satellite imagery are Google Earth and Google Maps (I find it quite ridiculous to link to any Google site. If you can’t find a Google site, it’s time to see a doctor). In Google Earth you can very nicely see the concept of spatial resolution when zooming in into a place. Moreover, you can see the downside of optical sensors in permanently clouded areas of the globe (e.g. the rain forest on Indonesian Papua).

I think I get the major picture but then how about those movies where the CIA tracks the bad guys running across roof tops in real time using satellites?

You should now be able to understand that that is still far from possible;
1. The resolution (spatial, radiometric) they usually show in such scenes is amazingly high (face recognition anyone?). While the military has some quite advanced high resolution sensors, it’s (still) impossible to stream the amount of data in real time for that kind of resolution from a satellite to a ground station.
2. Look outside right now. What do you see? That’s right, clouds. And we just learned that optical sensors can’t see through clouds making these scenes quite ridiculous.

So it’s actually not always true what happens in the movies?

This topic was supposed to be about remote sensing!