Vehicle Reflection

Radar cross section (RCS) is a measure of an objects' reflectivity, reflected energy divided by incident energy. Most objects scatter most of the incident energy, reflecting only a small fraction back to the radar. Large objects usually, but not always, have large reflections and a longer radar detection range.

The RCS dimension is in area, most commonly in square meters (m²), or decibels with respect to 1 square meter (dBsm). Actual object area and RCS area use the same dimension, but are different concepts. RCS measures energy reflected based on incident energy and accounts for size, shape, directivity and reflectivity.

Shape
Flat surfaces reflect energy better than contoured shapes. Edges and cavities with diameters on the order of wavelength tend to be good reflectors. Shape can be as important as, or even more important than, physical size.

Some extreme examples include stealth aircraft such as the F-117 light bomber and B-2 bomber, reported to have RCS's on the order of the size of insects. The St. Louis Gateway Arch, a stainless steel 3 sided catenary curve structure 630 feet tall with a 630 foot base, is almost completely invisible to radar.

Reflectivity
All metals and alloys are excellent reflectors at all frequencies. Metal screens with gap spacing less than a quarter wavelength are as reflective as a solid surface. At microwave frequencies carbon composites, fiberglass, and plastics are more transparent than reflective.

Virtually everything is reflective, the degree of reflectivity varies with material and frequency.

Frequency / Polarization
RCS depends on frequency and polarization. All electromagnetic waves have a polarization which is the orientation of the transmitted signal electric field, E-field. Common polarizations include vertical, horizontal, right hand circular, and left hand circular. Other polarizations include linear angled and elliptical. Police radars tend to use vertical or circular polarization.

Different frequencies or polarizations will produce different RCS patterns for the same object. Even ground reflections vary with frequency and polarization. In general the higher the frequency the larger the RCS.

Angle
Most objects, spheres are an exception, reflect different amounts of energy for different angles. Many times when RCS is stated it will be an average of all important angles. Most automobiles have a larger RCS from the rear than the front.

RCS examples
For a class of objects the larger the object the larger the RCS. Objects that do the same tasks tend to be the same shape and made up of the same materials. For physical size vehicles have relatively large RCS's compared to aircraft and watercraft.

At microwave frequencies average RCS can vary from 0.00001 m² for an insect to over 200 m² for a for a pickup truck. Depending on frequency a man measures about 1 - 4 m². Weather, rain, sleet, hail, snow, fog, and clouds vary from about 100 - 10,000 m², and are extremely frequency and polarization dependent. Ground, clutter, varies from about 1,000 - 100,000 m².

Typical automobile radar cross sections vary from about 10 to 100 or more square meters. Trucks are on the order of hundreds of square meters.

Source: System Considerations for the Design of Radar Braking Sensors, IEEE Transactions on Vehicular Technology, vol VT-26, no. 2, page 151-160, May 1977.

The Corvette has a low RCS because a fiberglass body is much less reflective then a metal body. The Mustang convertible also has a low RCS in large part due to not having a metal roof. A vehicle's RCS is influenced by all the reflectors on the vehicle.

A radar detector increases a vehicle RCS because the detector antenna is a good radar reflector. Some police jammer antennas are reported to increase radar detection range 10% to 30%.

Some electrically heated windshields have metal film coatings reflective to microwave signals. These windshields will increase a vehicle's RCS dramatically, not to mention prevent the use of a radar or radar detector from behind the windshield.

For motorist with a vehicle that has a small RCS the good news is the vehicle is harder to track, the bad news is the vehicle may be mistaken for a more distant vehicle. Motorcycles are especially vulnerable because the RCS is so small compared to cars and trucks, on the order of 2 to 8 times smaller than a car, 13 times smaller than a pickup truck, and 27 times smaller than an RV.

A vehicle with a large RCS could have the same echo signal power at a greater range than a close small vehicle. The range a distant large vehicle has the same signal return power or greater is a function of both vehicles' RCS and the range of the close small vehicle.

Vehicle Ranges for Equal Echo Power

Close Smaller Vehicle 1 R1 = Radar Range to Vehicle 1 rcs1 = Vehicle 1 Radar Cross Section

Distant Larger Vehicle 2 R2 = Radar Range to Vehicle 2 rcs2 = Vehicle 2 Radar Cross Section

In the range equation everything cancels out except for range and the RCS's. The result shows the range difference is a function of range of close vehicle, and the vehicles' RCS ratio.

The illustration below graphs the range difference (Rd) between 2 vehicles when the received signals from both vehicles are equal. The difference is a function of close small vehicle 1 range (R1) from the radar and the ratio of Vehicle 2 RCS / Vehicle 1 RCS.

Figure 3.6-2 -- Distance Between Vehicles when Echoes Equal
Both range scales (R₁ and R_d) in the same dimensions (feet, yards, meters, etc.).

A radar 500 feet (R₁) from a small car with an RCS of 30 m² receives the same signal echo strength from a full size car with an RCS of 120 m² that is 210 feet behind the small car. The RCS ratio is 120/30 which equals 4.

Two or more vehicles traveling close to each other and at the same speed may appear to the radar as a single extended target, with an RCS greater than the sum of all target RCS's. Microwave traffic radar uses time and frequency to resolve targets, multiple targets in the beam at the same time and at or near the same speed produce an echo return signal that is the vector sum of both target vehicles.