Maximum Unambiguous Range:

The maximum range at witch a target can be located so as to guaratee that the leading edge of the recieved backscatter from that target is receivd before transmission begins for the

next pulse. This range is called maximum unambiguous range or the first range ambiguity.

The pulse repetition time (PRT) of the radar is important when determining the maximum range because target return-times that exceed the PRT of the radar system appear at incorrect locations (ranges) on the radar screen.

Returns that appear at these incorrect ranges are referred as ambiguous returns or second-sweep echoes.

The pulse-repetition frequency (PRF) determines this maximum unambiguous range of a given radar before ambiguities start to occur

The slant range is the length of the skywave path between target and radar not the distance as measured along the Earth's surface (the so called Down Range).

Minimal Measuring Range:

The minimal measuring range Rmin (“blind range”) is the minimum distance which the target must have to be detect.

The transmitting time τ and the recovery time trecovery should are as short as possible, if targets shall be detected in the local area

During the transmitting time the radar cannot receive: the radar receiver is switched off using an

electronic switch, called duplexer.

radars with longer pulse width suffer a relatively large minimum range, notably pulse compression radars, which can use pulse lengths of the order of tens or even hundreds of microseconds. Targets at ranges closer than this minimum are said to be eclipsed.

Range Resolution:

Resolution is usually divided into two categories; range resolution and bearing resolution.

The target resolution of a radar is its ability to distinguish between targets that are very close in either range or bearing

Range resolution is the ability of a radar system to distinguish between two or more targets on the same bearing but at different ranges.

The Pulse Repetition Frequency (PRF) of the radar system is the

number of pulses that are transmitted per second.

The time between the beginning of one pulse and the start of the next pulse is called

pulse-repetition time (prt)

In order to obtain an unambiguous measurement of target range, the interval between radar pulses must be greater than the time required for a single pulse to propagate to a target at a given range and back.

The time that an antenna beam spends on a target is called dwell time TD. The dwell time of a 2D–search radar depends predominantly on

the antennas horizontally beam width ΘAZ and the turn speed n of the antenna (rotations per minute).

The value of hits per scan m says how many echo signals per single target during every antenna swing are received. The hit number stands e.g. for a search radar with a rotating antenna for the number of the received echo pulses of a single target per antenna turn.

Free-Space Path Loss (FSPL)

In telecommunication, free-space path loss (FSPL) is the loss in signal strength of an electromagnetic wave that would result from a line-of-sight path through free space, with no obstacles nearby to cause reflection or diffraction. The FSPL appers in vacuum under ideally conditions, e.g. a radio communication between satellites. It is a criterion for the derivation of the radar equation too.

Isotropic Radiation

Some antenna sources radiate energy equally in all directions. Radiation of this type is known as isotropic radiation.

All other antennae have a gain opposite the isotropic radiator. This is caused either by reflections or by the geometric extension of the antenna.

Free-Space Path Loss (FSPL)

If high-frequency energy is emitted by an isotropic radiator, than the energy propagate uniformly in all directions. Areas with the same power density therefore form spheres ( A= 4 π R² ) around the radiator. The same amount of energy spreads out on an incremented spherical surface at an incremented spherical radius. That means: the power density on the surface of a sphere is inversily proportional to the radius of the sphere.

The expression for FSPL actually encapsulates two effects. Free-space power loss is proportional to the square of the distance between the transmitter and receiver, and also proportional to the square of the frequency of the radio signal.

The second effect is that of the receiving antenna's aperture, which describes how well an antenna can pick up power from an incoming electromagnetic wave.

Radar Cross Section The size and ability of a target to reflect radar energy can be summarized into a single

term, σ, known as the radar cross-section, which has units of m². If absolutely all of the incident radar energy on the target were reflected equally in all directions, then the radar cross section would be equal to the target's cross-sectional area as seen by the transmitter. In practice, some energy is absorbed and the reflected energy is not distributed equally in all directions. Therefore, the radar cross-section is quite difficult to estimate and is normally determined by measurement.

Waves and Frequency Ranges

The higher the frequency of a radar system, the more it is affected by weather conditions such as rain or clouds. But the higher the transmitted frequency, the better is the accuracy of the radar system.

Different RADARS

Monostatic pulse radar sets

pulse compression radars

short range radar

Height-finding radar systems

Weapons-control radar

Search radar

continuous-wave radar

a 2D–search radar

TECHNICAL WORDS OF RADARS

returning echoes,

Dwell Time

Hits per Scan

antenna's aperture, azimuth angle

stealth technology

Peak- and Average Power

The amount of energy in this waveform is important because maximum range is directly related to transmitter output power. The more energy the radar system transmits, the greater the target detection range will be.

The energy content of the pulse is equal to the peak (maximum) power level of the pulse multiplied by the pulse width.

Peak power must be calculated more often than average power. This is because most measurement instruments measure average power directly. Transposing the upper equation gives us a common way for calculating peak power/average power.

Duty cycle is the fraction of time that a system is in an “active” state. Duty cycle is the proportion of time during which a component, device, or system is operated. Suppose a transmitter operates for 1 microsecond, and is shut off for 99 microseconds, then is run for 1 microsecond again, and so on. The transmitter runs for one out of 100 microseconds, or 1/100 of the time, and its duty cycle is therefore 1/100, or 1 percent. The duty cycle is used to calculate both the peak power and average power of a radar system.