Radar Principles Flashcards
What is Radar
Radio detection and ranging
Explain the principle of operation of PSR, including:
(a) the frequency bands/wavelengths typically used
The basic operation of the primary surveillance radar is by using a transceiver (a combined receiver and transmitter unit) to send out a pulse/signal. This pulse then reflects off an object and the receiver ‘picks up’ this reflection. This reflected signal gives position information to ATC.
(A) 1240-1400 MHZ (UHF band range)
Explain the principle of operation of PSR, including:
Principles of radar ranging and direction
Radar Ranging
Because of the short wavelength (10-100cm) the signals are able to be reflected off relatively small objects, such as aircraft. Narrow pulses or ‘beams’ can be used, and these frequencies are also free from any atmospheric noise/static and scattering.
Direction
To be able to transmit this signal in all directions, the radar spins around at approximately 2-10 rotations per minute. Distance of an object from PSR can be calculated by using distance = speed x time. (Radio wave travels at 300,000,000 m/s)
Explain the factors affecting the operational range of PSR.
Power - The power of the transmitter will determine the reflection strength. Lower power = lower reflection strength (and vice versa).
Range - If an object is further away from the PSR unit, the reflection strength will be much lower, meaning closer objects will experience greater signal strength.
Shape/size - The size, shape, surface and even orientation of an abject will have an impact on the resulting reflected signal strength.
State the maximum range of PSR in New Zealand.
The PSR unit uses UHF and SHF transmissions, meaning that the signal is ‘line of sight
80NM
Explain the factors which effect the minimum and maximum range of a primary radar system. (Pulse recurrence frequency & Pulse Width & Atmospheric Condition)
PRF - rate at which pulse is transmitted, a limiting factor because each pulse must have enough time to be sent out, reflected and received before next pulse is transmitted. (Tennis ball thing)
Pulse width - the wider the pulse, the longer it has to travel before it can be reflected without running into itself. This creates a minimum distance for the transmitter ( S…….S……..S)
Atmospheric condition - due to shorter wavelength of UHF, frequency transmitted is susceptible to being absorbed and scattered by water droplets in atmosphere causing clutter on radar screen.
Describe the limitations in the operational use of PSR.
Blind Spots - water particles can cause blind spots and also clutter on the radar. Mountains and high terrain can also cause blind spots.
Clutter - caused by weather, but can also be caused by ground or sea return transmission known as surface clutter.
Return Strength - The strength of the reflected transition is very dependent on the reflectivity of the target and also the signal strength of the PSR unit
Explain the principle of operation of SSR.
SSR transmits a signal thats received by aircraft transponder which sends a signal back with specific info of position and altitude.
Distinguish between primary surveillance radar (PSR) and secondary surveillance radar (SSR).
the signal strength required for the SSR compared to the PSR is much less. Due to it not relying on its own reflection.
Also doesn’t rely on a reflected signal
Describe the advantages and disadvantages of SSR over PSR.
No unwanted clutter - Because SSR isn’t reliant on reflections, clutter is avoided
.
Smaller transmitter required - It doesn’t need the reflection signal because the transponder will send the reply. Therefore, less power is required meaning the transmitter can be smaller.
Longer operating range - Again, the signal only needs to reach the aircraft, meaning the range of the unit is much greater
Identity and altitude known - The transponder allows ATC to see the aircraft’s identity and altitude when mode C is activated.
Max operating range of SSR
256NM
Explain the factors affecting the operational range of SSR.
SSR can be subject to terrain shielding like PSR
How does transponder interrogation work?
The transponder receives a 1030MHz signal from the SSR. This is known as the ‘interrogation’ signal and it is made up of 2 pulses, which are transmitted 8 microseconds apart.
After the transponder is interrogated, it will then send a reply to the SSR unit. The reply consists of anything between 2 - 14 pulses, taking 20.3 microseconds at 1090MHz.
Explain the principle of operation of airborne weather radar. (Radar/antenna, control panel, display unit)
Radar/antenna - mounted in the nose or wing of the aircraft to maintain level with the horizon while the aircraft rolls.
Acts like a PSR transmitting a signal then switching to receiver mode. Distance and intensity of weather shown on display is determined by strength of return signal and time taken.
Control Panel - allows pilot to adjust what’s being displayed on display unit. Vary from range, gain, tilt, clutter functions.
Display unit - the screen that displays information.
How does Doppler effect work with weather radar?
Imagine a Car.
the frequency will vary of the reflected wave depending on the velocity of the object. When the car is moving away from the transmitter, the frequency reduces. If the car is moving toward the transmitter, the frequency increases.
