Radio Waves, Radar & SSR

C. 16

Radar range varies as the fourth root of transmitted power, so doubling range needs 2^4 = 16 times the power.

A. 1030 MHz; 1090 MHz

Interrogation 1030 MHz up, reply 1090 MHz down - UHF, secondary radar.

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B. pulse recurrence frequency

The maximum unambiguous range of a pulse radar is set by the pulse recurrence frequency (range = c / 2 PRF); a higher PRF means a shorter maximum range.

C. 5649KHz

VHF range at FL350 is only about 230 NM, so 750 NM needs HF. At night the D layer disappears and lower HF frequencies refract best, so the lower frequency 5649 kHz is used (17286 kHz is the daytime frequency).

B. 3 nm

Azimuth resolution = range x beam width (radians) = 60 NM x 0.0524 = about 3.1 NM; the pulse length affects range resolution, not azimuth.

C. a target that responds - an aircraft or a ground transponder

Secondary radar works by interrogation and reply. In SSR the aircraft replies; in DME the ground beacon does.

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B. 7700, 7500, 7600

Emergency 7700, unlawful interference 7500, radio communication failure 7600.

D. 125 MHz

Frequency = 300 / wavelength (m) = 300 / 2.4 = 125 MHz.

B. 4096

Mode A uses four octal digits: 8 x 8 x 8 x 8 = 4096 codes.

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C. it gives the aircraft's identity and pressure altitude

Primary radar detects reflections from anything, cooperating or not, but it cannot tell who the target is or its altitude - that is what SSR adds.

B. +/- 50 ft

Mode C reports pressure altitude (1013.25) in 100 ft steps, so the displayed figure is within +/- 50 ft of the encoder value.

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B. 2000

ICAO procedures: 2000 when no instruction has been received from ATC (7000 is used as the VFR conspicuity code in some regions).

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A. 7600

SSR codes: 7700 emergency, 7600 radio communication failure, 7500 unlawful interference.

A. the frequency is increased (and at night, when the layer is higher)

A higher frequency is refracted less, so it returns further away; at night the reflecting layer is higher, which also lengthens the skip.

B. Pulse width

The minimum range of a pulse radar is set by the pulse length (the receiver cannot listen while transmitting).

C. the frequency shift of its own signals reflected from the ground

Beams are aimed at the ground ahead/behind and sideways; the Doppler shift of the echoes gives speed along and across the aircraft axis - groundspeed and drift.

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B. the searchlight principle

The narrow beam rotates like a searchlight; the bearing is the direction the aerial points when the echo returns.

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C. 4

For one-way radio communication the signal strength falls with the square of distance, so doubling the range needs four times the power.

C. pulse recurrence rate

Maximum unambiguous range = c / (2 x PRF), so it is limited by the pulse recurrence rate.

A. 6 cm

Wavelength = 300 / f(MHz) metres = 300 / 5035 = 0.06 m = 6 cm (an MLS frequency).

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D. it shows weather returns

SSR receives transponder replies (one-way path, so far less power) and therefore sees no weather or ground clutter - which is also why it cannot show weather.

B. it uses just twice as many codes as Modes A and C

Mode S gives every aircraft a unique 24-bit address (about 16.7 million), not "twice" the 4096 Mode A codes; it interrogates selectively, is compatible with A/C and carries data link.

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A. 80 NM

1.23 (sqrt 2500 + sqrt 196) = 1.23 (50 + 14) = 79 NM.

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B. the pulse (echo-timing) technique

Range = speed of light x time to the echo / 2; bearing comes from the aerial direction (searchlight principle).

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D. 125 km, 1350 NM

E layer about 125 km; with the wave grazing the horizon, the longest single hop is about 1350 NM (2500 km). The F layer, being higher, gives about 2200 NM.

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A. +/- 200 ft in RVSM airspace (+/- 300 ft elsewhere)

PANS-ATM 8.5.5: tolerance +/- 200 ft (60 m) in RVSM airspace, +/- 300 ft (90 m) elsewhere; outside it the pilot checks the setting and, if it persists, is asked to stop Mode C.

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C. indicate the mode of interrogation

The spacing between P1 and P3 tells the transponder which mode is wanted (8 us Mode A, 21 us Mode C). Side-lobe suppression uses P2.

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A. 10 GHz

f = c / wavelength = 300,000,000 / 0.03 = 10,000,000,000 Hz = 10 GHz (SHF - weather radar region).

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B. frequency

If the frequencies differ, the phase relationship keeps changing and the comparison means nothing. Amplitude does not matter.

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B. lower than transmitted

Doppler effect: relative motion apart lowers the received frequency, together raises it. Doppler navigation measures this shift.

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B. Number of pulses per second

Pulse recurrence rate (PRR or PRF) is the number of pulses transmitted per second.

C. i) Pulse length ii) PRF

Minimum range is set by pulse length, maximum unambiguous range by PRF.

B. 300 million m/sec

Radio waves travel at about 300 million metres per second.

C. F layer

The F layer (150-400 km) returns HF; the D layer mostly absorbs (by day) and disappears at night; the E layer weakens at night.

C. frequency increase, absorption increases

The correct answer is: frequency increase, absorption increases.

A. Aerobatic and display code countries

SSR code 7004 is the aerobatic and display code (in some countries).

A. 3000 M

Wavelength = 300 / f(MHz) = 300 / 0.1 = 3,000 m.

B. the electrical component of the signal is parallel to the aerial.

The plane of polarisation is that of the electric field, parallel to a vertical aerial.

B. it reflects from the larger water droplets (precipitation), not from small cloud droplets

A 3 cm wave is comparable to large raindrops and wet hail, so it shows the dangerous precipitation inside a Cb while ignoring harmless cloud.

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B. rain scatters the beams, giving returns from moving raindrops instead of the ground

The SHF beams are reflected (scattered) by heavy rain; echoes from falling, wind-driven drops carry their own Doppler shift and corrupt the ground-speed/drift solution.

B. electric wave perpendicular to magnetic wave

In a radio wave the electric and magnetic fields are at right angles to each other and to the direction of travel.

The question was recalled without its options; these options were written to it.

B. speed of light x time to echo / 2

The pulse goes out and back, so range = c x t / 2 (about 12.36 us per NM).

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A. 9375 MHz; 3.2 cm

AWR: 9375 MHz (SHF), wavelength 300/9375 = 0.032 m = 3.2 cm.

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B. LF

f = 300,000 / 1200 = 250 kHz - LF (30-300 kHz).

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A. the D layer

Without the sun the low, dense D layer de-ionises and vanishes, so MF/HF sky waves are no longer absorbed - night sky waves travel further. The E layer weakens and F1 and F2 merge into one F layer, higher up.

B. 143 NM

1.25 (sqrt 9000 + sqrt 400) = 1.25 (95 + 20) = 143 NM (1.23 gives 141).

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B. green, yellow, red (magenta strongest on some displays)

Green light precipitation, yellow moderate, red heavy (magenta extreme/turbulence). Avoid red cells by a wide margin.

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A. the heights of transmitter and receiver (line of sight) and the power

VHF travels by direct (space) wave, so range is set by line of sight: 1.23 (sqrt h1 + sqrt h2) NM, within the transmitter power.

B. the reflected echo; interrogation and transponder reply

Primary radar needs nothing on board: it detects its own pulse reflected from the target. Secondary radar transmits an interrogation and the aircraft transponder transmits a coded reply on another frequency (1030 MHz up, 1090 MHz down).

B. an identification pulse (SPI) is transmitted for about 20 s, 4.35 us after the last framing pulse

The special position identification pulse is added 4.35 us after F2 for about 20 seconds, making the return stand out.

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A. about 37,600 ft

300 = 1.23 (sqrt 2500 + sqrt h) = 1.23 (50 + sqrt h): sqrt h = 194, h = 37,600 ft.

A. 1030, 1090

The SSR ground interrogator transmits on 1030 MHz and the airborne transponder replies on 1090 MHz.

A. comparing successive returns and cancelling those that do not change (no Doppler shift)

MTI subtracts each sweep from the previous one; fixed echoes cancel, moving targets (with a phase/Doppler change) remain. PRF stagger removes blind speeds - it does not itself reject clutter.

D. directivity

A flat slotted array gives smaller side lobes, a narrower beam (better resolution) and needs less power for the same performance; directivity is a property of any radar dish, not an added advantage.

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C. pitch and roll

Stabilisation keeps the scan level with the horizon as the aircraft pitches and rolls; heading changes just rotate the picture.

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A. a narrow pulse width and a narrow beam width

Range resolution depends on pulse width; bearing (azimuth) resolution on beam width - both narrower is better.

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A. 170 NM

Range = c x PRI / 2 = 300,000 km/s x 0.0021 s / 2 = 315 km = 170 NM. (PRF 450 pps gives 333 km = 180 NM.)

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B. increases as frequency increases, so LF travels furthest

The higher the frequency, the more the surface wave loses energy into the ground; LF/MF (NDBs) have long surface-wave ranges, and the sea attenuates least.

A. the first sky wave to return to earth, at the critical angle - it defines the skip distance

The correct answer is: the first sky wave to return to earth, at the critical angle - it defines the skip distance.

B. 162 NM

A radar mile (out and back for 1 NM) takes 2 x 1852 m / 300,000 km/s = 12.36 microseconds. 2000 / 12.36 = about 162 NM.

B. the frequency of the carrier varies with the signal while its amplitude stays constant

FM varies the carrier frequency in step with the audio/data; AM varies the amplitude. FM is less affected by static noise.