Heinrich Hertz

First to demonstrate basic radar priciples between 1885-1888.

S.G. Marconi*

1) Transmitter

2) Receiver

3) Antenna

4) Display

How is Range determined?

If a target is in motion, there will be a shift in the frequency of the echo and that frequency shift will be proportional to the velocity of the target.

1) Electric Field (E)

2) Magnetic Field (H)

For an electric field to flow, an electrical field must exist and when an electrical current flows it produces a magnetic field.

What does 'If the electric field changes with time, the relationship with the magnetic field also changes with time' mean?

What are some things to know about the nature of radio waves?

-They are sinusoidal electric & magnetic waves.

-The relationship between the E & H waves causes electromagnetic waves.

-When a charge accelerates an electron it radiates electromagnetic energy.

-Electrons are in constant motion due to thermal agitation.

-Difference between radio waves, visible light, and radiant heat is the wavelengths of the radiation.

1) How does a RADAR produce waves?

2) Where is the antenna in a RADAR system?

1) It produces waves of electromagnetic radiation by exciting a tuned circuit with a strong electric current.

2) Most often, the antenna is located internally within the system. (Most simple antenna is the Dipole)

An alternating voltage is applied at the center of the thin conductor and at the same time, + and - charges are alternately building up on the plates.  The majority of the energy each field contains returns to the antenna with every oscillation, but a portion 'escapes' and the fields propoagate from the dipole as 'ripples in a pond'.

What are the 8 characteristics of Radio Waves?

Acronym:

Sam Dropped Pigskin Interceptions While Practicing Peewee Football

They are:

1) Speed     2) Direction     3) Polarization

4) Intensity 5) Wavelength 6) Period    

7) Phase     8) Frequency

Define the term, SPEED, as it relates to RADAR?

Define the term, DIRECTION, as it relates to RADAR?

Define the term, POLARIZATION, as it relates to RADAR?

Polarization is used to express the orientation of the wave's fields and is thought to be the direction of the electrical field. When the electrical field is vertical, the radio wave is said to be vertically polarized and vice-versa.

Why is POLARIZATION important?

Define the term, INTENSITY, as it relates to RADAR?

What is Power?

What is Power Density

Power is the rate of flow of energy.

Power Density is the power of a radiated wave.

(The power of the received signal then is the power density times the area of the antenna.)

Define the term, WAVELENGTH, as it relates to RADAR?

Wavelengths are the distance between successive crests in a propogated wave.  Represented by lambda (λ) and is expressed in the metric system (m, cm, mm)

Define the term, FREQUENCY, as it relates to RADAR?

Frequency is the number of cycles a radiated wave goes through per second.  Expressed in Hertz and represented (f).  

•  f = c/λ in m/s. 

Define the term, PERIOD, as it relates to RADAR?

Period is the length of time a wave or signal takes to complete on cycle. Represented by (T). 

• T = 1/f

Define the term, PHASE, as it relates to RADAR?

Phase is the degree to which the individual cycles of a wave or signal coincide with those of a reference of the same frequency.

• Generally expressed in degrees with 360° corresponding to a complete cycle.
• Referenced to the point in a cycle where amplitude strength passes through the 0 point.

If a target is within line of sight of a radar, does it mean it will be detected? YES/NO and WHY?

No, it does not. The echoes must be strong enough to be detected above the background of electrical noise that always exists from:

1) the output of the receiver itself

2) Simultaneously received echoes from the ground (ground clutter).

They are inversely proportional to one another. 

(1/R⁴)

• As the target gets closer the echoes grow stronger.

What are the 8 things that target echo strength depend on?

Acronym:

Power Loves Sailors Rigging Rowboats Near Whiskey Bars

1) Power of the transmitted waves.

2) How Long the power is transmitted.

3) Size of the antenna.

4) Reflecting characteristics of the target.

5) Length of time the target is in the Radio beam during the search scan pattern.

6) Number of search scans in which the target appears.

7) Wavelength of the radio waves.

8) Strength of the Background noise or clutter.

Power density is equal to the radiated power divided by the surface area of an imaginary sphere at a distance R from the radar.

Power Density = P/4ΠR²

The portion of the energy reradiated is dependant upon the target's what?

Radar Cross Section

Total amount of energy sent to the radar receiver for processing is a function of what?

Hint: Energy's life cycle from TX-RX

-How much was sent out?

-How much reached the target?

-How much was reflected back from the taget?

-How much reached the receiving antenna?

-How much was "absorbed" by the receiving antenna.

The receiver generates the radio-frequency carrier wave for the duration of the timing pulse received by the modulator.  It amplifies, modulates, and sends the carrier wave & signal.

What does the Receiver Protection Device do, with respect to the Radar Block Diagram?

The Intermediate Frequency (IF) is the difference between the echo's original frequency and the local oscillator frequency. 

• *The amplified, filtered IF is sent to a demodulator or second detector where the signal is extracted from the carrier.

Name two ideas to know for the exam regarding radar cross section?

1) Some aircraft have radar cross sections that are much greater that the geometrical cross section. (Best for directivity)

2) Some aircraft have large geometrical cross sections with relatively small radar cross sections. (Best for low directivity).

What is the main part of the energy radiated called?

What are the weaker regions called?

Mainlobes

Sidelobes

-Ground Clutter

-Rain, hail, or snow

-Transmissions from other radars

-Jamming

-Ever present electrical noise in the radar receiver. (Noise = electrical energy of random amplitude & frequency.  The majority comes from the receiver itself.)

*Ultimately depends on the strength of the echo returns to the strength of the background noise or Signal - noise ration (S-MIN).

What is Average Power?

What is Noise?

What is Time on Target?

What is Radar Cross Section? 

With Respect to RADAR

• Increasing power of a transmitter increases the detection range by 1/4 root of that power.
• Decreasing average level of background noise has the same affect as increasing the average power by the same factor.
• If we slow down the scan in order to double the ToT, it is like doubling the power.
• The larger the RCS of the target the greater the detection range.