Active vs passive sensors

Following a lecture on active vs passive sensors, I was intrigued as to their differences and applications. A useful analogy I discovered was thinking of a camera with its flash on and thus providing a light source (active) and without the flash (passive).

Passive sensors

A key advantage of passive remote sensing that we used in our practicals was from Landsat, of which multispectral images could be produced to provide even more information than could be inferred through the naked eye. Hyperspectral measurements can be made using passive sensors, such as Sentinel 5P which is a multispectral radiometer launched in 2017 (ESA, 2018). This comprehensive Sentinel mission observes air and atmosphere quality and ozone levels, amongst more. The advanced multispectral imaging spectrometer (Tropomi) detects the unique signatures of atmospheric gases and a wide range of pollutants in the electromagnetic spectrum, demonstrated here through assessing the ozone layer. A key focus of the Copernicus satellite is its high spatio-temporal coverage which is used to forecast air quality (ESA, 2018).

Active sensors

Whereas, active remote sensing is advantageous through its lack of temporal and meteorological restrictions; an active sensor is not hindered by clouds and other aspects of atmospheric interference and can obtain images without depending on an external light source. Examples include LiDAR, of which was key in calculating elevations. LiDAR was particularly important and something which caught my attention was in 2018 where the full extent of ancient Maya ruins were discovered in Guatemala. This revelation showed researchers that Central American civilisations 1,200 years ago were comparable to ancient Greece than to the underdeveloped and sparse cities previously imagined through ground-based observation (Clynes, 2018). Moreover, LiDAR technology has been used not just for unearthing the past, but for safeguarding the future. McCoy (2018) in a report argued that LiDAR’s high-resolution images can be inputted to create 3-D virtual reality models of sites of cultural interest to obtain some level of preservation, if not physically but digitally. This report came out with concerns rising over enhanced sea level changes and the threat to low lying world cultural sites, such as the Easter Island statues.