Question

We have seen that both Lidar and Radar can be used to map the topography (height) of the surface. Point out a difference in the way each accomplishes this, and explain how this creates limitations or advantages of one method over another for any given objective.

Answer 1

The LiDAR technology functions by emitting rapid laser signal pulses (sometimes up to 150,000 pulses per second) which are are reflected from the obstacle i.e. surface. The sensor mounted on the instrument measures the time difference between a pulse being emitted and reflected pulse returning to the system. Current state-of-the-art LiDAR largely use lasers of two wavelengths: 905 nm and 1550 nm.

Radar follows a similar principle of measuring distance of obstacle from measuring time delay between emitted and reflected signal, but it emits radio waves instead of laser as signal. Radio waves have wavelength ranging from 1 mm to 10000 km.

Due to the vast difference in wavelength used in the two technologies, LiDAR and RADAR has certain strengths and weaknesses. LiDAR has much higher resolution and accuracy in detection; it can identify the details of a few centimeters at more than 100 meters and exact size and shape of target objects. RADAR on the other hand cannot detect such small targets. It can even erronously detect two or more smaller targets that are situated nearby as one large target. Hence, much smaller wavelength gives LiDAR an advantage over RADAR in terms of resolution and accuracy.

On the other hand, smaller wavelength causes LiDAR signal to have much higher attenuation compared to RADAR. Adverse conditions like snow, fog, rain, and airborne dust causes LiDAR performance to significantly degrade. RADAR performance is not degraded due to such conditions since it has much less attenuation. For the same reason it can detect reasonably well over a much longer distance compared to LiDAR, (though with less accuracy in details).

The framework required for producing and detecting laser pulses is expensive compared to that of radio wave, hence RADAR is less expensive compared to LiDAR.