Question 2 of 6: Radar remote sensing

Question 2: Radar remote sensing (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

a) What is RADAR, and its advantages/disadvantages [15 marks]. RADAR (RAdio Detection And Ranging) is an active microwave remote-sensing system: it transmits its own pulses of microwave energy toward the ground and records the amplitude, phase, and two-way travel time of the backscattered return, rather than passively recording reflected or emitted sunlight/heat as optical and thermal sensors do. Because the sensor supplies its own illumination, radar imaging is entirely independent of solar illumination and largely independent of atmospheric condition.

Advantages. (1) All-weather, day/night operation — microwave wavelengths pass through cloud, haze, smoke, and most rainfall largely unattenuated (the same atmospheric-window logic as Q1b), so radar is the only imaging option for persistently cloudy regions (e.g. tropical, monsoon, and Canadian Arctic/coastal environments) and for operational monitoring that cannot wait for a clear-sky pass. (2) Sensitivity to surface roughness, dielectric constant (strongly controlled by moisture content), and geometric structure rather than to colour/spectral reflectance, giving radar unique discriminating power for soil moisture, sea state, sea ice type/age, and flood-extent mapping (open water is a very strong specular reflector and appears dark). (3) Limited penetration into dry, low-loss media (dry sand, dry snow, vegetation canopy) at longer wavelengths (L-band), useful for sub-canopy or shallow subsurface structure. (4) The side-looking oblique geometry enhances subtle topographic and structural expression (faults, lineaments, drainage) through shadowing, which is valuable for geological structure mapping. (5) Coherent phase information enables interferometric SAR (InSAR) for millimetre-precision ground-deformation monitoring and DEM generation, something no optical sensor can do. Example applications: RADARSAT-2/Constellation for Canadian Arctic sea-ice charting and ship detection; InSAR for volcanic and permafrost-subsidence monitoring; SAR flood mapping through persistent monsoon cloud cover; agricultural crop-type/soil-moisture monitoring via polarimetric decomposition.

Disadvantages. (1) The oblique side-looking geometry produces radar-specific geometric distortions absent from nadir optical imagery — foreshortening and layover on slopes facing the sensor, and radar shadow on slopes facing away — which are severe in mountainous or urban terrain and can render parts of a scene uninterpretable. (2) Coherent-imaging speckle noise (constructive/destructive interference of many sub-resolution scatterers) gives radar imagery a grainy appearance that must be reduced by multilooking or filtering at the cost of resolution. (3) Backscatter intensity is a complex, non-intuitive function of roughness, moisture, dielectric properties, and incidence angle rather than a familiar "colour," so radar interpretation demands specialist training that optical photointerpretation does not. (4) Real-aperture systems (unlike SAR) suffer resolution that degrades linearly with range (Q2b), and even SAR systems are typically costlier to build, launch, and process than equivalent optical systems. (5) Strong, non-Lambertian specular returns from smooth surfaces (calm water, roads) and corner-reflector effects from man-made structures can locally saturate or bias the recorded backscatter.

b) Required wavelength for a real-aperture azimuth resolution of 4 m [5 marks].

Given. Real-aperture (non-SAR) RADAR system; antenna length L = 5 m; platform height h = 1 km above the ground; depression angle γ = 65° from the horizontal to the target; required along-track (azimuth) ground resolution R_a = 4 m (the minimum separation at which two objects are just resolved).

Find. The radar wavelength λ that, in principle, gives exactly this azimuth resolution at the stated geometry.

Approach. For a real (physical) aperture, the azimuth beamwidth is diffraction-limited to β ≈ λ/L radians, so the along-track ground resolution at slant range R is R_a = Rβ = Rλ/L; first find the slant range R from the height and depression angle, then solve for λ.

1. Convert height/depression angle to slant range. The platform stands a vertical height h above the ground and looks down at depression angle γ from the horizontal, so the slant range is $$R = \dfrac{h}{\sin\gamma}$$ Substituting h = 1000 m and γ = 65°: $$R = \dfrac{1000}{\sin 65^\circ} = \dfrac{1000}{0.9063} \approx 1103\ \text{m}$$
2. Solve the diffraction-limited azimuth-resolution relation for λ. Starting from R_a = Rλ/L, $$\lambda = \dfrac{R_a\,L}{R}$$ Substituting R_a = 4 m, L = 5 m, R ≈ 1103 m: $$\lambda = \dfrac{4 \times 5}{1103} \approx 0.0181\ \text{m} = \boxed{1.81\ \text{cm}\ (\approx 18\ \text{mm})}$$