Chapter 2 - Primary Radar Flashcards
What are the three things which must be displayed by a WXR?
- Range
- Bearing
- Signal intensity
How is bearing measurement achieved in WXR?
Ensuring that the timebase line is in the same direction as the WXR scanner
What are the typical angles of a WXR scanner?
± 60° to ±90°
What happens if a WXR beam is too wide?
- Several targets ahead would tend to merge together and be less distinct
- Improve target resolution
How are targets indicated on the CRT display?
Signal strength within the returns are identified by larger returns creating greater bright up and therefore a more intense display
Most intense displays depicting the most dangerous cells
What is an ISO echo contour?
Contouring a bright area, so that areas of turbulence within dangerous cells are shown as ‘holes’
Returns above a predetermined value are inverted in the receiver to give, in effect, very low output to the display
What is the operator selecting with range selection of WRX?
The displayed range and not the transmitter/receiver range
Alter the length of time it takes for the timebase to sweep up the CRT
Adequate timebase ‘rest time’ between each sweep
What are range markers on WXR?
Series of semi circles appearing at different ranges from the transmitter source on the CRT
What is the manual adjustment of the WXR in the vertical plane?
±15°
What functionality does tilting of the WXR antenna provide?
- Tilt up - remove ground returns and estimate height of targets
- ‘Look down’ - creates the necessary ground returns used in mapping mode
What might be an issue where the frequency of a WRX is too high?
Most of the energy will be reflected or scattered, leaving little to continue through the cloud to greater ranges
What are the frequencies of most WXR systems?
X band - 7 GHz to 12.5 GHz
What are the frequencies of some WXR systems?
C band - 3.5 GHz to 7 GHz
What effect does the frequency of WXR have on the beam width?
Higher frequency will give a narrow beam for the same antenna when used with a lower frequency
How will pulse width affect target definition?
A shorter pulse would ensure that the leading edge of one pulse isn’t reflecting off of target 2 while the trailing edge is still reflecting off of target 1
What does Sensitivity Time Control (STC) have the effect of?
Making a video signal on the CRT independent of the range of the target
What would happen without STC?
- Small harmless targets will clutter the screen
- Reducing the gain could result in dangerous far away cells not being amplified enough to bright up the display
How does STC work?
Gain of the IF stage is reduced for close targets, but allowed to a max pre-set value for more distant targets
What problems may be encountered where noise derived AGC is used in WRX systems?
If the noise levels in the Rx are higher than normal, AGC might reduce overall gain to such a level that targets will not contour properly
What is disarmed automatically when ‘map’ is selected?
STC
Earth’s surface provides excellent radar returns and so can swamp the receiver
Why is a cosecant beam used in WXR?
Creates equal amplitudes of returns from all ground targets regardless of wherever it appears in the beam
Cosecant beam throws more energy into the upper portion of the beam which has the farthest journey
How does line of sight stabilisation differ from older stabilisation systems?
Only pitch is corrected, and the pitch tilt motor is fed error signals from pitch vertical reference and roll vertical reference systems
What is important about the slots in a flat plate antenna with regards to the signal they transmit?
The slots radiate in phase, achieved by judicious slot positioning
What is the principle of operation of a flat antenna?
Slots will radiate power which reinforce each other in the direction of propagation
In other directions, the beams are not in phase and some cancellation takes place (total cancellation at 90°)
Very narrow beam is produced with minimal sidelobes
What is a Bendix system?
A grid permanently affixed to the upper part of the dish with the reflector elements aligned vertically
More energy is placed at the forward portion of the beam
What is the position of the radiator assembly during the different modes of operation?
Horizontal during WXR modes
Vertical for map modes
What polarisation produces less returns from the sea?
Horizontally polarised
How does the Bendix version of antenna achieve rotation of the polarisation plane of transmitted signal?
Ferrite device employed as a gyrator
What are the required properties of a radome?
- Must pass at least 90% of the transmitted energy
- Must possess high structural strength
- Must resist spark discharges and lightning strikes
What will the presence of water within the honeycomb nose cone cause?
Excessive attenuation, seen as ‘poor returns’ or ‘poor range’
What is the desired thickness of the radome?
Quarter wavelength at the system frequency
How is the L band frequency of a crystal oscillator converted to X and C band frequencies?
Varactor multiplexer
What is jittering with regards to WXR?
Varying of PRF to ensure WX returns are received and unwanted signals from other WXRs can be ignored
How is a turbulent WX target identified?
Wide variation in its internal droplet velocities
What is percentage of droplet velocities outside the threshold which would discern when a target is can be considered turbulent?
68%
What was the first method of producing very short wavelength radiation?
Cavity magnetron
What are the two outputs of a doppler system?
- Drift angle
- Ground speed
What is computed when measuring velocity with a doppler system?
Relative velocity between the transmitter and the target
What is the formula for the difference frequency in a doppler system?
FD=(2Vrftx)/c
Where Vr is component velocity along the sighting line of the beam, ftx is the transmitted frequency and c is the velocity of em waves
What are most doppler angles?
60° to 70°
What are the frequencies used for doppler?
8.8 GHz and 13.3 GHz
In a single beam Doppler system, what will variations of pitch give?
Ground speed errors
What three types of transmission are used in airborne doppler?
- CW
- FMCW
- Pulsed
What is the major complication of CW doppler systems?
Cross coupling between transmitting and receiving sections of the aerial array
How can the cross coupling effect be reduced in a CW doppler?
Using two aerials, separated by the insertion of a metal baffle plate between the two arrays
Which type of transmission is immune to the effects of cross coupling?
FMCW
How can cross coupling be avoided in FMCW?
Using the third harmonic modulation frequency component
Amplitudes of the unwanted nearby echo signals are very small, while the output from ground returns (at greater range) can be kept high
In FMCW doppler, what is a ‘height hole’?
Aircraft is at such a height that the echo is in modulation phase with the transmitted signal
Gives the same response as a signal at zero range and so is ignored
How can the ‘height hole’ effect be reduced in FMCW doppler?
Wobbulating the modulation frequency between 340 kHz and 460 kHz at a rate of 10 times per minute
What is meant by beam shape and doppler spectrum?
Doppler calculates ground speed from difference frequency, which is calculated using the cosine of an angle
This assumes the beam has no width
Width of beam is actually about 4° therefore the angle is somewhere between 58° and 62°
The echo contains a spectrum of doppler frequencies, and a frequency tracker therefore measures the midpoint of the spectrum to obtain ground speed
What is sea bias?
More echo energy is reflected from the rear edge of a forward beam than the front edge
This isn’t normally a problem over land where surface irregularities mask the variations
The effect is pronounced over a calm sea and this has the effect of moving the mid frequency point and giving rise to an error in ground speed movement
What does the Low Range Radio Altimeter measure?
Vertical distance from the aircraft to the terrain below
What are the limits of measurement of the LRRA?
20.75 feet to 2500 feet
Why is it important that the LRRA can measure beyond 2500 feet, even if it cannot indicate it?
Necessary for the GPWS system
Where can the outputs of the LRRA be fed?
- ‘Rising runway’ indicator on the ADI
- Decision height giving a visual and aural warning
- Altitude trips are provided to the AFCS
What frequencies are used for radio altimeters?
C band - 4.3 GHz
What is the antenna requirement where CW is used for LRRA?
Two antenna per system
How are the problems overcome of using coax cable with a high frequency system in LRRA?
- Use of very short runs between antennae and T/R
- Thicker grade of coax with a large cross sectional area
What is the principle of operation of the LRRA?
The RA modulates a CW at a fixed rate
Transmitted signal will be sent at an instantaneous frequency other than the basic 4.3 GHz due to the FM being applied
The signal is swinging alternately approximately 50 MHz above and below the centre frequency
This signal travels to the ground and is reflected back after a measurable time delay so that two frequencies exist at the aircraft, the current transmitted signal frequency and the returned pulse
The difference frequency can be used to calculate the aircraft height
What is wobbulating used for in LRRA?
Deliberately varying the transmitted frequency to overcome situations whereby the time taken for the returned pulse matches the time it takes for the frequency to return to the original frequency
Why don’t LRRA suffer from the standard fact of radar emissions signal strength varying by the fourth power of range?
Only varies with the square of range, as the effective beam of the RAs illuminate more ground as the height increases
What are second echoes?
Echos which have carried out two return trips due to a second reflection from the aircraft underside
How is AID calculated?
Rx ant to ground + Tx ant to ground + (co-ax length/0.67)
What should the VSWR be in a horn antenna of LRRA?
1.4:1
What is the beam width of a typical horn antennae?
20° to 40° from the vertical
