Remote sensing is the acquisition of information without physically contacting the object being sensed. Remote sensors are sensors that collect data by detecting the energy that is reflected from the Earth. These sensors can be on a satellite similar to Landsat satellites or can be mounted to the bottom of an aircraft or unmanned vehicle or even ground based vehicles like tractors.
Remote sensing is not new technology and has actually been in use for many years (i.e. since the invention of the telescope). The term remote sensing was first introduced to describe the imagery collected by Americas first spy satellite program; CORONA, 1959-1972.
Generally speaking, there are two types of remote sensors: (1) active sensors and (2) passive sensors. Both forms of remote sensing sense energy in the form of the electromagnetic spectrum.
Active sensors are designed to actively create a signal or stimulis in hardware which is propagated to the Earth. The sensor detects the response that is reflected from the Earth and processes the reflected signal to extract useful information. Two common active sensors in remote sensing are LIDAR and SAR as discussed further below.
Passive sensors do not create and radiate a signal or stimulus to the Earth. Passive sensors detect reflected or radiated energy from natural sources like the sun. Electro-optical systems (EO) like the Landsat satellite or the EZ Health multispectral cameras are examples of passive sensors.
EXAMPLES OF ACTIVE SENSORS
A commonly used active remote sensor in the geospatial community is a LIDAR (Light Detection and Ranging) sensor. It directs a laser beam onto the surface of the Earth and measures the time of flight to the ground. The time of flight data is used to produce a three dimensional topographic map that accurately reflects the terrain and height variations.
Other active sensors include microwave remote sensing sensors. Synthetic Aperture Radar (SAR) is an example used on satellites, military aircraft, and UAV’s. It is a microwave imaging system that emits microwave pulses and post processes the reflected pulses to form images. SAR images can be produced through clouds, rain, and snow, day or night.
EXAMPLES OF PASSIVE SENSORS
The EZ Heath Multispectral UAV payload is an example of a passive remote sensor. The sunlight acts as the emitting source and the EZ Health cameras detect the light reflected off the crop as the UAV is flying over.
The EZ Health sensor is sensitive across four independent spectral bands, Red,Green, Blue (RGB), and NIR. In the EZ Health system post-processing software spectrally aligns the RGB and NIR imagery and stitches the aligned imagery into 2D RGB, NIR and NDVI mosaics.
Landsat satellites are another example of passive remote sensing systems. The satellites have a combination of spectral sensors aboard that are specific for Earth observation. Landsat 7 and Landsat 8 satellites are the only that remain active today.
Landsat 7 is orbiting Earth at roughly 705 km and completes an orbit every 16 days. The main sensor onboard is the Enhanced Thematic Mapper Plus (ETM+). The ETM+ has 8 spectral bands with the visible bands (Red, Green, Blue), NIR, and MIR bands having a spatial resolution of 30 meters by 30 meters. Band 8 is a panchromatic band with a spatial resolution of 15 meters and band 6 is thermal infrared with a spatial resolution of 60 meters. Most commercially available satellite imagery like google maps/google Earth are based on enhanced Landsat 7 imagery.
Landsat 8 joined Landsat 7 in the same orbit to produce increased coverage. Landsat 8 carries two sensor payloads the Operational Land Imager (OLI) and a Thermal Infrared Sensor (TIRS).