1960 Weather Radar Image of Hurricane Abby. Image Credit: NOAA
We have all heard of radar, but do we understand how it was developed and how it is used. Radar is an object-detection system. It utilizes radio waves to determine the range, altitude, direction, or speed of any given object. Radar works because the antenna transmits pulses of radio waves or microwaves which bounce off objects in their path. The object returns a tiny part of the wave’s energy to a receiver normally located close to the transmitter. The modern uses of radar are highly diverse, including air traffic control, radar astronomy, air-defense systems, antimissile systems; marine radars to locate landmarks and other ships; aircraft anticollision systems; ocean surveillance systems, outer space surveillance and rendezvous systems; meteorological precipitation monitoring; altimetry and flight control systems; guided missile target locating systems; and ground-penetrating radar for geological observations. High tech radar systems are associated with digital signal processing and are capable of extracting objects from very high noise levels.
We think of radar as a fairly modern invention, but that is not necessarily true. In1886, Heinrich Hertz showed that radio waves could be reflected from solid objects. In 1895, Alexander Popov developed an apparatus using a coherer tube for detecting distant lightning strikes. He then added a spark-gap transmitter. While testing this in communicating between two ships in the Baltic Sea, he noted an interference beat caused by the passage of a third vessel. He suggested that report this phenomenon might be used for detecting objects. Christian Huelsmeyer was the first to use radio waves to detect ”the presence of distant metallic objects” in 1904. His device was able to detect ships in a dense fog, but could not determine distance. After some revision, he was able to find a way to determine distance as well. In 1917 Nikola Tesla outlined a concept for a primitive version of modern radar units The practical development of radar was really pushed during WWII to help locate air, ground and sea targets.
A radar system has a transmitter that emits radio waves in predetermined directions. When these come into contact with an object they are reflected or scattered in many directions. Radar signals are reflected best by materials that conduct electricity(most metals, seawater, wet land). The receiver end of a radar unit detects radio waves that are reflected. If an object is moving, there is a slight change in the frequency of the radio waves, caused by the Doppler effect. Reflected radar signals are very weak in most cases and must be strengthened by electronic amplifiers. Since most mediums barely absorb radio waves, radar sets are able to detect objects at relatively long ranges, even across interplanetary space. Radio frequencies that are absorbed or scattered by water vapor, raindrops, or atmospheric gases (mainly oxygen) are not used in radar units unless their detection is necessary as in assessing the atmospheric content of other celestial bodies.
Distance can be determined using radar by transmit a short pulse of electromagnetic radiation, then measure the time it takes for the reflection to return. The calculation needed is: Distance equals one-half the product of the round trip time and the speed of the signal. Through the use of a duplexer, the radar switches between transmitting and receiving at a predetermined rate. A similar effect imposes a maximum range as well. In order to maximize range, longer times between pulses should be used, referred to as a pulse repetition time, or its reciprocal, pulse repetition frequency. This makes it difficult to combine both good short range and good long range in a single radar. The newest radars fire two pulses during one cell, one for short range and a separate signal for longer ranges. In radar, distance is measured as a function of time. A radar mile is the time it takes for a radar pulse to travel one nautical mile, reflect off a target, and return. A nautical mile is defined as 1,852 m, so dividing this distance by the speed of light (299,792,458 m/s), then multiplying the result by 2 yields a result of 12.36 microseconds. If this amount of time changes for a specific object, you can tell if it is approaching a location or moving away.
This is a very basic description of radar. Each application has various features used and would take volumes to explain. The NASA link below will take you to a page where you can link out ot many different types of radar applications.
Here are some articles about radar written for Universe Today.
Amazing Radar Image from Space Highlights Costa Concordia Catastrophe
Deep Space Radar Unveils Rotating Asteroid 2010 JL33
If you’d like more info on the different uses of radar, check out these articles:
Doppler Radar National Mosaic – NOAA’s National Weather Service
NASA – NASA Radar Finds Ice Deposits at Moon’s North Pole
We’ve also recorded an entire episode of Astronomy Cast all about Radar. Listen here, Episode 233: Radar.
Source: NASA
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