DEFINITION
The acronym 'RADAR' was introduced in the mid-twentieth century and stands for RAdio Detection And Ranging. In terrestrial applications, a radar system may be located on the ground (fixed), in a terrestrial vehicle, a marine or naval vehicle or an aircraft.
More recently radar has been used to detect and track objects in space. This is usually from a fixed radar on the ground. It has also been used on space platforms, such as to assist in docking operations, or to image terrestrial activities and monitor the terrestrial environment.
This document will introduce the basic concepts of radar.
INTRODUCTION
A radar system consists of an antenna, a transmitter, a receiver, a controller and a display. Radar systems can be classified in a number of ways. One way is the classification of monostatic and bistatic. These will
Monostatic Radar
The receiver is characterised by a noise temperature T that limits the receiver sensitivity.
The antenna is characterised by a gain Gt for transmission purposes and an effective collecting area Ae for receive purposes.
The designated target is located at a range r, and has an effective radar cross section (area) or RCS of σ . This is likely to be dependent on the frequency f of the radar.
Bistatic Radar
A bistatic radar is one where the receiver and transmitter are physically separated and so each has its own antenna. It is usually necessary to have some means of communication between the transmitter and receiver to synchronise them.
The antennae in a bistatic radar are separate and thus must be characterised separately.
THE RADAR EQUATION
The radar equation for a monostatic radar may be derived using the figure below:
The equation highlighted in the above diagram is the power received by the antenna (and thus present at the antenna output terminals). This equation does not include the receiver bandwidth. However the receiver band width must wide enough to contain all the information transmitted to the target. In the case of a pulse transmitter this bandwidth B ~ 1 / δτ where δτ is the width (in time/secs) of the transmitted pulse.
Using this bandwidth we can then calculate the noise present at the antenna terminals from the external environment/ Or, in the case of noise generated in the receiver itself, the noise referred back to the antenna terminals. Both the external and internal noise is defined by a noise temperature Tn = Text + Tint. The noise power (in watts) is then given by Pn = k * Tn * B where B is the receiver bandwidth in Hz, and k = 1.38 x 10-23 is Boltzmann's constant. The receiver signal to noise ratio is then given by SNR = Pr / Pn. This SNR is a pure number sometimes expressed in decibels as SNR(dB) = 10 log10(SNR).
The Radar Equation is often transposed to give the range of the radar for a specified SNR.
The situation for a bistatic radar is shown in the figure below:
If the transmitter, target and receiver all lie in the same plane (ie a 2D situation) then:
r22 = h2 + (s - x1)2
In general the three locations T, O and R will not lie in the same plane and we will have a 3D situation where the y coordinate of the target (O) will be a distance y from the line joining T and R. The distances r1 and r2 will then be given by:
r22 = h2 + (s - x1)2 + y2
The signal power Pr received by the bistatic radar is the signal flux density at the receiver times the receive antenna effective collecting area Ae. Thus:
The bistatic radar, like the monostatic radar also shows an essentially range to the fourth power dependence for the returned signal. Because of this, most space radars are limited to low Earth orbit. They do however, have the advantage of being able to work both in daylight and in adverse weather conditions, although the latter will affect radar performance for frequencies above S-band (~ 2 GHz).
RADAR CROSS SECTION
If the size of the radar target is a lot less than the wavelength of the radar then the effective radar cross-section σe of the object/target is its optical cross-section σo times its albedo α which is a function of its reflectivity the radar frequency and the phase angle between the transmitter and receiver. The latter determines what fraction of the target the transmitter illuminated that the receiver can see. For a monostatuc radar this should 100% as the transmitter and receiver are at the same location.
For a sphere the optical cross-section is π r2 where r is the radius of the sphere.
If the wavelength of the radar is around the same size or greater than the size of the target we must refer to the graph below:
As the target size becomes smaller we progress from the optical scattering regime to the Mie scattering regime where the target cross-section oscillates over a factor of ten. For even smaller targets the cross-section with inverse fourth power of the target size and quickly vanishes to the radar.
RADAR DISPLAYS
When radar was first developed the displays were quite simple. What was known as an 'A' display was simply an oscilloscope that displayed a horizontal line (X-axis) where the distance along the line was proportional to the time since the start of the line/trace and thus were a measure of distance to the target. Deviations in the Y-axis indicated a radar reflection from a target with the vertical extent of the deflection being proportional to the intensity of the reflection.
Different types of display were further developed and labelled B, C, D, E, F, G, ..... P(PPI) where the PPI or Plan Position Indicator was the closest to a map of surrounding targets. This is still one of the primary displays used today. The other displays were used to display a variety of target parameters such as azimuth, elevation, deviation from a bearing or an approach glide slope, et al. Some of these are indicated below.
With computers now readily available it is possible to produce any type of display very readily and to overlay the display with various types of additional information. Modern radars are not limited to the historical types of displays. The PPI form is still the underlying display type used. Various radar displays are shown below.
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PRIMAY AND SECONDARY RADAR
Another division of radar is between primary and secondary radar. Primary radar does not require any cooperation from the target. A pulse is sent out from the radar transmitter, is reflected from the surface of the target and then appears on the radar display. A secondary radar sends out an interrogating signal to a transponder on an aircraft at a frequency of 1030 MHz and the aircraft transponder (figure below) responds with a code which is set by the pilot, and also an altitude value, on a frequency of 1090 MHz.
These codes can be displayed on the radar display. The air traffic controller can ask the pilot, by radio, to change the code that the transponder relies with. This is done by the four knobs on the front of the transponder. These set a four digit numeric code. The value '1200' shown in the figure means that the aircraft is flying under Visual Flight Rules (VFR).
If the radar controller has difficulty in identifying a particular aircraft on the display, they can either ask the pilot to change the code. The phrase used is "Apache 01, squawk code 1500". If the situation is more critical the radar controller can request the pilot to "Squawk IDENT". This is a request to the pilot to press the 'IDENT' button to the left of the code knobs. This causes the transponder to send a code that highlights the aircraft position on the radar display with a higher than usual intensity image. There is then no doubt where the aircraft lies on the display.
Different countries use different transponder codes for different purposes. However there are 3 international codes used in emergencies:
| 7500 | Aircraft Hijacking |
| 7600 | Radio failure - Lost communications |
| 7700 | Emergency |
The figure below shows an ATC (Air Traffic Control) radar that has both a primary and secondary radar. The secondary radar antenna is on top of the primary radar antenna.
The primary radar typically operates at S-band between 2700 and 2900 MHz.
The advantage of using secondary radar. as well as identifying the aircraft is that it functions at much greater range. The reason for this is that the forward path (to the aircraft) and the reverse path (back to the radar) are both governed by inverse square law dependencies, rather than the inverse fourth power dependency of the primary radar. The primary ATC radars can typically see aircraft out to 100 km, whereas the secondary radar could be used out to 1000 km.
The disadvantage of the secondary radar is that it can be turned off, for instance if a hi-jacker wishes to minimise evidence of the aircraft flight (as has happened). It should be noted however, that most large aircraft also carry a system know as ADS-B (Automatic Dependent Surveillance-Broadcast) that does not require a ground radar system. It will automatically transmit a signal giving the aircraft details such as identity, position and altitude. These operate at 1090 or 978 MHz. These beacons cannot be turned off by the pilot. Signals from these beacons can allow a ground receive station to construct a map of aircraft positions without the need for a radar. Of course only aircraft fitted with ADS-B will be displayed.
REFERENCES
Australian Space Academy