Chapter 3
Radars and the Electromagnetic Spectrum
Introduction
The range of all electromagnetic waves is called the electromagnetic spectrum. In the electromagnetic spectrum, long wavelengths correspond to the radio frequency spectrum, intermediate wavelengths to millimeter and infrared radiation, short wavelengths to visible and ultraviolet light, and extremely short wavelengths to x-rays and gamma rays. Figure 3-1 is a graphical depiction of the electromagnetic spectrum.
In the electromagnetic spectrum, there is generally no basic bounds on radar frequencies. Any electromagnetic device that detects and locates a target by radiating electromagnetic energy and uses the echos scattered from a target can be classified as a radar, regardless of its frequency. Radars have operated at frequencies from a few megahertz to the ultraviolet region of the electromagnetic spectrum. The fundamental principles of radars are the same at any frequency; however, the technical implementation is widely different. Most radars, in practice, operate between 400 MHz to 36 GHz; however, there are some notable exceptions.
The optical and radio portions of the electromagnetic spectrum occupy positions coincidental with two important transparent bands in the Earth's atmosphere and ionosphere. These transparent bands are commonly referred to as the optical and radio windows, and are depicted in Figure 3. The optical and radio windows are important because they allow these electromagnetic waves to pass through the atmosphere and be received on the Earth's surface.(1)
Radars operate in radio frequency bands normally designated for radar operations to ensure compatible operations with other radar systems. These frequency bands are sometimes identified by a letter band and are used by many radar engineers and others involved with radar matters. These letters generally refer to older military designations of the different frequency bands in which these radars operate. They relate back to the early development of radar in World War II when the letter designators were used for purposes of secrecy and after the requirement for secrecy was no longer needed, these letter band designators remained.
Radar Frequency Selection
The best frequency to use for a radar depends upon its application. Like most other radio design decisions, the choice of frequency usually involves tradeoffs among several factors such as physical size, transmitted power, antenna beamwidth, and atmospheric attenuation.
Physical Size. The dimensions of the hardware used to generate and transmit radio frequency power are, in general, proportional to wavelength. At lower frequencies where wavelengths are longer, the hardware is usually large and heavy. At the higher frequencies where the wavelengths are shorter, radars can be housed in smaller packages and operate in more limited spaces with correspondingly less weight.
Transmitted Power. The choice of frequency (wavelength) indirectly influences the ability of radar to transmit large amounts of power because of its impact on hardware size. The levels of power that can be reasonably handled by a radar transmitter are largely limited by voltage gradients and heat dissipation requirements--the larger, heavier radars operating at wavelengths on the order of meters can transmit megawatts of average power, whereas millimeter-wave radars may be limited to only a few hundred watts of average power.
Beamwidth. The narrower the beam, the greater the transmitted power that is concentrated in a particular direction at any one time, and the finer the angular resolution. The width of the radar's antenna beam is directly proportional to the ratio of the wavelength to the width of the antenna. At low frequencies, large antennas must generally be used to achieve acceptably narrow beams. At higher frequencies, small antennas will suffice.
Atmospheric Attenuation. In passing through the atmosphere, radio waves may be attenuated by two basic mechanisms: absorption and scattering. The absorption is mainly due to oxygen and water vapor. The scattering is due almost entirely to liquid hydrometeors. Both absorption and scattering increase with frequency. Below about 100 MHz, atmospheric attenuation is negligible. Above about 10 GHz, it becomes increasingly important.
Figure 3-2 below is intended to show how, at about 10 GHz, absorption, scattering and refraction by atmospheric gases and liquid hydrometeors (rain, fog, sleet, and snow) become important limiting factors for electromagnetic wave propagation.
Radar Design Considerations
From the preceding discussion, it is apparent that the selection of the operating radio frequency band is influenced by several factors such as the radar's intended function, its operational environment, the physical constraints of the radar's operating platform, and cost. To illustrate this, the following table depicts the relationship among the radar design requirements and spectrum issues.
| TABLE 4
Spectrum Issues Versus Radar Design Requirements | |||
| Radar Design Requirements | Impact on Spectrum Characteristics | ||
| Frequency | Bandwidth | Power | |
| Mission: surveillance, tracking, etc. | Primary | Primary | Secondary |
| Target Recognition: aircraft, ship, clouds, etc. | Primary | Primary | Secondary |
| Antenna: shape, size | Primary | Primary | Secondary |
| Mobility: size, weight, etc. | Primary | Secondary | Secondary |
| Environment: land, sea, air, etc. | Primary | Secondary | Secondary |
| Mutual Interference | Primary | Primary | Secondary |
| Spectrum Regulations | Secondary | Secondary | Secondary |
| Note: In varying degrees, the above radar design requirements will influence frequency selection, bandwidth, and power. However, some factors have more influence than others on the spectrum elements. The primary and secondary designations are relative indications of this influence on the radar design. Physical constraints are shown to outweigh regulations since they are less amenable to modification (i.e., the mission and the laws of nature can not be change). | |||
The characteristics of the specific electromagnetic spectrum band selected for radar development have a significant impact on the information provided to the user. Radar systems in low bands provide the ability to detect targets at long distances and track space assets. On the other hand, high-band systems have only limited ability for search functions, but can track objects with very high precision, potentially forming an actual image of the object to assist in classification and discrimination. For example, the 8-12 GHz missile defense radar requires cuing by low-band radar systems to focus on a specific search area. Because of the relationships between frequency bands and capabilities, DOD and the Navy specifically, will continue to retain radar systems that operate throughout the spectrum.(2)
Ground-Based Applications. Ground-based radars operate in most allocated frequency bands. At one end are the long-range multi-megawatt surveillance radars. Unconstrained by size limitations, they can be constructed large enough to provide acceptable high angular resolution while operating at relatively low frequencies. Over-the-horizon radars can operate in the HF band (3-30 MHz) where the ionosphere is ideally reflective. Space surveillance and early warning radars operate in the very high frequency (VHF) and ultrahigh frequency (UHF) bands where atmospheric attenuation and natural noise are nearly negligible. These bands, however, are crowded with communications signals, so their use by radars is restricted to special applications and geographic areas. Where such long ranges are not required and some atmospheric attenuation and noise is tolerable, ground-based radars may be reduced in size by moving up to higher frequencies.
Shipborne Applications. Physical size becomes a limiting factor aboard ships for many applications including radar systems. While at the same time, the ship's requirement to operate in various types of weather conditions puts a constraint on the upper portions of the radio frequency spectrum that may be used. This limit is eased somewhat where extremely long ranges are not required. Higher frequencies are usually employed when operating against surface targets and targets at low elevation angles, such as sea-skimming missiles.
The radar return received directly from targets at very low elevation angles are very nearly canceled by the return from the same target reflected off the water (multipath propagation). This cancellation is due to a 180 phase reversal occurring when the return is reflected. As the elevation angle increases, this multipath propagation problem decreases. This multipath problem decreases at higher frequencies. For this reason, the short wavelengths of the 2.7-10.5 GHz band are widely used for surface search, detection of low flying targets, and piloting of ships.(3)
Airborne Applications. In an aircraft, the housing limitations of radars are more severe than on ships. Frequencies in the 400-1000 MHz and 2000-4000 GHz bands are the lowest frequency bands where aircraft radars operate. These bands provide the long detection ranges required by military airborne early warning aircraft and their antennas are very large to provide the desired angular resolution. Above these bands, the next application are radar altimeters using the 4200-4400 MHz band.(4)
Airborne weather radars, which require greater directivity, operate in 5.2-5.9 GHz and 8.5-11 GHz bands. The choice of these two bands for airborne weather radars indicates a dual trade-off of functional requirements: one is between storm penetration/scattering and the other is storm penetration/equipment size. If there is too much scattering, the radar will not penetrate deeply enough into the storm to see its full extent. However, if too little energy is scattered back to the radar, the storm will not be visible on the radar scope. The larger aircraft use 5.2-5.9 GHz band weather radars, even though they are larger and heavier, because of the better storm penetration capabilities and longer range performance. The majority of smaller aircraft employ lighter weight 8.5-11 GHz band weather radars that provide adequate performance.
The majority of military fighter aircraft have attack and reconnaissance radars operating in the 8.5-11 GHz and 13-18 GHz bands with a large number operating in the upper portions in the 8.5-11 GHz band. This upper portion of the 8.5-11 GHz band is very attractive because of its relatively low atmospheric attenuation and availability of narrow beamwidths.
Frequencies above the 8.5-11 GHz and 13-18 GHz bands are used where limited range is not a problem and angular resolution and small size are desirable. Aircraft radars operating in the 31-36 GHz band are used for ground search functions and for terrain following and terrain avoidance.
Spaceborne Applications. In a space platform, the power and space limitations for radar systems are most severe. Considering the limited power available on space platforms and the availability of small radar components make radars operating in the UHF and above frequency ranges a better choice. For spaceborne surveillance radars, the frequencies from 1--18 GHz would be good choices. Frequencies from 1-11 GHz are frequency bands where SAR imaging radars operate. Spaceborne altimeters generally use frequencies in the 2-18 GHz band while scatterometers generally use frequencies between 5-36 GHz. Frequencies around the 13-18 GHz band is where precipitation radars generally operate.
1. Source for these transparent bands came from John D. Kraus. See John D. Kraus, Radio Astronomy, 2nd ed., 1986, at 1-4. These transparent bands are the result of the resistance (opacity) of Earth's gases to certain electromagnetic radiation. The opacity of air to visible light is very small; hence, we have no difficulty seeing each other. The water vapor contained in the air is opaque to various wavelengths in the infrared frequency ranges.
2. Department of Navy email comments from Mr Bruce Swearingen, Director, Naval Electromagnetic Spectrum Center, at 2. (See email from swearinb@navemscen.navy.mil, March 24, 2000 on file at NTIA).
3. For a discussion of ground echoes, see Richard K. Moore, Ground Echo, Radar Handbook, at 12.3 (Merrill I. Skolnik, 2nd ed. 1990).
4. Radar altimeter use of the 4200-4400 MHz band allows for conveniently small equipment packages because the band permits good cloud penetration, require modest amounts of power, and do not require highly directional antennas for satisfy altimeter requirements.