What's Up There?

So when I talk about using "satellite imagery" to measure trees, what do I mean?

It turns out that there a bewildering number of national, international and commercial space programmes that are pointing a variety of cameras and sensors at the Earth. To make some sense of this, I tend to put them into three broad categories.

1. Passive sensors are a bit like eyes. They let the sun make the light, and then they detect the light that reflects from the surface of the Earth and its atmosphere. Whilst our eyes are sensitive to three wavelengths of light (which we see as red, green, and blue), many of the passive satellites in orbit record can detect separate images for each of ten or even hundreds of different wavelengths (these are known as bands).
2. Lidar sensors have on board lasers. They create short pulses of light of a specific wavelength, then wait for the pulses to bounce back from the Earth. A bit like sonar, by timing how long it takes for the pulse to come back, lidar can provide height information, which is particularly useful when it comes to measuring tree height and forest structure. The only downside is that it takes such a huge amount of energy to run the laser, that only small sample footprints can be measured.
3. Radar satellites also create their own energy, and they operate at much longer wavelengths- in the realm of centimetres rather than nanometres. The advantages of this are that they can afford to obtain full coverage, and they see straight through clouds which get in the way of passive and lidar sensors.

Every satellite has its own advantages- some have very high resolution, some have open data access policies, some obtain regular images (short repeat cycles) and some have richer information. Over the last couple of weeks, I've made a shortlist of the ones that might come in useful for detecting tropical forest changes over the next few years, and put them together in a poster.