Why don't commercial airplanes carry Earth-observing instruments? Governments and private companies spend hundreds of millions of dollars to send Earth-observing satellites to capture imagery or scan the terrain with lasers or radars.

Meanwhile, commercial planes are constantly flying around the world, and even if they do not offer global coverage, they could still carry Earth-observing instrumentation with almost no additional cost. I'm sure the data they could provide would have many scientific and commercial applications. In some cases they could be even better than satellite observations due to much shorter revisit times, less atmospheric influence and shorter round-trip times for laser and radar signals.

For example, some satellites like IceSat and IceSat-2 were designed to repeat laser altimetry observations over the same ground tracks over and over. With much interest in Greenland, the icefields in Yukon/Alaska and Antarctica. Again, in this case, commercial flight could provide very good coverage of the first two areas of interest at a fraction of the cost.

I'm a glaciologist working on the Patagonian icefields, where the persistent cloud cover renders most of the images captured by Earth-observing satellite useless. I'm evaluating the possibility of contacting authorities and airlines to collaborate in a project to put instrumentation on the planes that overfly the icefields several times a day. However, I'm sure that I'm not the first with this idea of putting instruments on commercial planes. Thus, I'm surprised that no commercial airplanes carry Earth-observing instruments.

Therefore my question:

Is there any insurmountable technical or legal limitation to equipping commercial airplanes with Earth-observing instruments?
They do! Well, at least some of them. There is for example a project called TAMDAR (Tropospheric Airborne Meteorological Data Reporting) that e.g. Icelandair is a part of. There is a document published by the Icelandic Meteorological Office about it.

Another example is MOZAIC/IAGOS: Atmospheric Research Using Commercial Airliners.