08 Basic Radar Theory Flashcards
What frequency do radars operate in (basic radar theory)?
5 to 15 GHz
What determines the Max Unambiguous Range?
Pulse Repetition Frequency (PRF)
What factors effect the range of a radar?
- Transmitter Power: Range is proportional to Power
- PRF: Range is inversely proportional to the PRF
- Dish RPM: Range is inversely proportional to the RPM
What happens to the width of a radar beam as the dish size increases?
As dish size increases, the beam becomes narrower
What is the equation for the Range of a target from the Radar?
Range(m) = (Time of travel of pulse x C) / 2
- Time is measured from the leading edge of the sent pulse to the leading edge of the return pulse
What is the equation for the Maximum Unambiguous Range of a Radar?
Range(m) = C / (2 x PRF)
What is the equation for the maximum theoretical range of a Radar?
Range (nm) = 1.23 x (sqrt(Tx) + sqrt(Hx))
- Tx is the height of the antenna
- Hx is the height of the object/AC
What is the Pulse Recurrence Interval for a Radar?
The time interval between the start of one pulse and the start of the next
What is the Pulse Repetition Frequency for a Radar?
The number of pulses in one second
What is the Pulse Transit time for a Radar?
The time interval between the sending of the pulse, it reaching the target and returning to the Radar
For a Radar display, what does a Moving Target indicator do differently from a standard display?
- Only displays moving targets on the indicator
- Stationary objects (such as buildings) are removed
Describe atmospheric attenuation of a Radar
- Water in the atmosphere (rain, vapour etc) degrade Radar signals
- Energy from the radar signal is absorbed by the water and converted to heat energy
- More pronounced over longer distances and at higher frequencies
Describe the errors associated with Radars
Super-refraction (Ducting):
- Radar signal is bent towards the Earth’s surface
- Usually caused by strong, low level inversion
- Increases Radar range
Sub-refraction:
- Radar signal is bent away from the Earth’s surface
- Caused by warm, moist air moving over a cool surface
- Decreases Radar range
What are the different types of ATC radar and is their;
- Range
- PRF
- Antenna RPM
Area (Terminal Area) Radar:
- Long Range
- Low PRF
- Low RPM
Surveillance Radar:
- Shorter Range
- Higher PRF
- Higher RPM
Ground Radar:
- Very short range
- Very high PRF
- Very High RPM
What is the approximate range for an ATC Long Range Surveillance Radar?
200 to 300nm
What is the Pulse Recurrence Rate for a Radar?
The number of pulses per second
The same as the PRF
What is the equation for the minimum range of a radar?
Minimum range (m) = (C x Pulse Length) / 2
In order to double the effective range of a Primary Radar the power output must be increased by a factor of what?
16
What determines a Primary Radars ability to discriminate between targets in:
- Azimuth
- Depth
- Azimuth: Beam width
- Depth: Pulse length
Compared to a Parabolic Reflector, what is one of the advantages of a Flat Plate or ‘Phase Array’ antenna?
- Generates less side lobes than a Parabolic Reflector
- Wastes less energy
Describe the phenomenon known as Fading?
- Caused by sky waves and ground waves interfering with each other
- Can occur at night
- When sky waves, refracted from the ionosphere, interfere with ground waves
- When ground waves are reflected up into sky waves by obstacles on the ground
What can increase the Skip Distance of HF transmissions?
- Higher frequency transmission: allow the signal to penetrate the ionosphere to a higher level and so will increase the skip distance
- Higher position of the reflecting ionosphere: higher level reflection will increases the skip distance
