Radar Basics
A radar—short for RAdio Detection And Ranging—establishes the direction and range, relative to an antenna, of objects that reflect radio waves, and displays this information on a screen that gives a two-dimensional, top-down representation of the airspace around the antenna. To do this, the electronics of a traditional radar emit a continual series of pulses of radio energy through an antenna as it rotates around its vertical axis to scan the sky in azimuth. To establish direction, the electronics note the orientation of the antenna at the time when an object is detected. The reason for a series of pulses is to establish range: the time it takes for the pulse to reach the object and return to the antenna indicates the range, with reflections from farther objects taking more time to return to the antenna. Under optimal conditions, a radar creates one blip per scan for each object it detects, allowing the operator to track moving objects by connecting the blips. On Cold War era radar displays, phosphor coatings were chosen because of their half life. This half life was on the order of more than ten seconds, or more than one scan of the radar; this allowed the returns from any one target to "add up" to a bigger blip, making them easier to for the operator to recognize.
Within these parameters, several factors determined the capabilities of Cold-War era radar system, and in turn their vulnerability to blip/scan spoofing. One key characteristic in all radars is the pulse repetition frequency (PRF). This determines the maximum effective range. The time between pulses must be long enough so that a single pulse can reach the maximum range of the system and then return before the next pulse begins. For instance, a radar designed to have a range of 300 km needs to wait 2 milliseconds for a pulse to travel 300 km to the maximum range and back at the speed of light (300000 km/s). This means that such a radar can send out at most 500 pulses per second. If the radar sent out 1000 pulses per second, it would be impossible to determine whether a particular reflection came from an object at 150 km from the pulse just sent out, or an object at 300 km reflecting the previous pulse. On the other hand, a 150 km radar requires only 1 milliseconds; this makes a PRF of 1000 possible, increasing the radar's blip/scan ratio and decreasing its vulnerability to spoofing.
Intertwined with the PRF is the length of the pulse, or duty cycle. This determines the minimum range of the system. Longer pulses mean that more energy has the potential to be reflected by the object. However, the radar system cannot detect reflections while it is sending a pulse. To have a 30 km minimum range, for instance, a radar can have pulses no longer than 0.1 ms in duration. For an early warning radar the minimum range is generally not important, so longer pulses are used to maximize the returns, but the duty cycle was nonetheless an important design consideration.
With the development of high-altitude aircraft, early-detection radar antennae were shaped to produce a beam with a vertical cross-section of 30 to 40 degrees. This was large enough to scan the entire sky from the horizon to high altitudes.
Beamwidth, the horizontal span of the beam measured in degrees of angle, determines the balance between precision and accuracy of a radar. The narrower the beam, the more precise the measurement of direction is. However, the narrower the beam, the less time it spends painting an object as the antenna spins, meaning that it may not make reliable detections.
Intertwined with beamwidth is the speed of the antenna’s rotation, because it also determines the amount of time that a spinning radar will spend painting a given object on every scan. Take for example a radar with a beamwidth of one degree and an antenna that rotates once every ten seconds, or 36 degrees per second. An aircraft will be painted by the beam for only 1/36th of a second as the one-degree beam sweeps over it. If the above radar has a PRF of 500, an aircraft will be painted with 14 pulses per scan at most.
Moreover, Cold-War radar systems were far from perfect. The system created a visible blip on the operator’s display if and only if it received enough returns with enough energy to rise above the background noise of the system. Atmospheric conditions, electronic interference from internal components, and other factors sometimes created false returns known as “clutter,” concealed real returns, or made blips difficult for the operator to interpret correctly.
These design characteristics and susceptibility to glitches combine to determine a radar's blip/scan.
Read more about this topic: Blip-to-scan Ratio
Famous quotes containing the word radar:
“So I begin to understand why my mother’s radar is so sensitive to criticism. She still treads the well-worn ruts of her youth, when her impression of mother was of a woman hard to please, frequently negative, and rarely satisfied with anyone—least of all herself.”
—Melinda M. Marshall (20th century)