Radar & GPS

062 Radio NavigationChapter 5

Primary radar principles

Formula — Radar range equations

Distance = Speed × Time
Max Range = c × PRI × ½
Min Range = c × Pulse Length × ½  (or Pulse Width × ½)
PRI = 1 / PRF

Key fact

Radio waves travel at 3 × 10⁸ m/s or 300 m/μs. Radar mile = 12.36 m/μs (time for 1 nm out and back). Pulse length = duration in μs; pulse width = length in metres. PRI = interval from start of one pulse to the next; PRF = pulses per second.

Radar cycle

Key fact

Pulse transmitted for pulse length duration → transmitter off, receiver on (recovery/listening phase) → echoes return → at PRI, switches back to TX. Radar spends most of its time listening. Pulse length must be less than PRI.

Parameter	Determines
PRI (long)	Max range — longer recovery for distant returns
Pulse length (short)	Min range — close target may return before RX enabled

Key fact

To double radar range requires 16× transmitter power (4× each way). Normal transmitter needs 4× power to double one-way range. Large pulse width merges close targets (poor radial resolution); short pulse width improves resolution but reduces illumination time. Azimuth resolution improved by narrow beam — needs higher frequency and larger antenna.

Ground radar types

Radar	Usage	Range	Wavelength
Long range	Airways	250 nm	50 cm
Terminal	DEP/ARR	75 nm	25 cm
ASR + SRA	Vectoring	25 nm	10 cm
PAR	Precision approach	10 nm	3 cm (scanning)
SMR	Surface movement	2.5 nm	3 cm

Key fact

Longer wavelength = better penetration through cloud/precipitation — suited for ATC radar. Shorter wavelength = greater attenuation — suited for weather radar. Parabolic = old type; phased array = new type (less power, arrays powered individually). MTI displays only moving targets (Doppler shift) — filters fixed clutter. SRA uses ASR, azimuth only, normally terminated 2 nm from touchdown. PAR provides azimuth and glideslope — both normally military/emergency civil.

Secondary radar (SSR)

Key fact

Ground TX 1030 MHz; aircraft replies 1090 MHz (60 MHz separation). Primary radar identifies target; SSR interrogates for additional data. 4⁴ = 4,096 squawk combinations. Squitters — aircraft transmit every second for TCAS even without interrogation.

Mode	Pulse 1–3 interval	Information
A	8 μs	Transponder code
C	21 μs	Pressure altitude (100 ft increments, 1013 hPa)

Key fact

Reply: 2 framing pulses 20.3 μs apart with information pulses between (pulse position modulation). IDENT transmits for 20 seconds after second framing pulse.

Special codes

Code	Meaning
7700	General emergency
7600	Radio failure
7500	Unlawful interference
2000	Entering airspace from non-SSR region
7000	General conspicuity

SSR disadvantages and advantages

Disadvantage	Advantage
Garbling (replies overlap within 1.7 nm)	Long range
Fruiting (wrong reply from other stations)	No clutter
Antenna shielding in turns	Reply gives range, bearing, height, speed
Ghost targets (terrain reflection)	Less power required

Key fact

Side-lobe suppression: P2 transmitted omnidirectionally between P1 and P3. If P2 amplitude greater than P1 → side lobe, no reply. If P1 greater than P2 → genuine interrogation, reply. Mode C accuracy: up to 50 ft rounding error; EASA max discrepancy 300 ft between reported level and readout.

Mode S

Key fact

Eliminates fruiting and garbling. Datalink for air-air/ground-air. TCAS enhancements. Height accuracy ± 25 ft. IC = II + SI (15 interrogator identifiers × 63 surveillance identifiers). All-call (⅓ of time) discovers new aircraft; roll-call (⅔) for selective interrogation. Aircraft identified by 24-bit ICAO address, aircraft identification, and squawk. Enhanced surveillance adds DAPs: groundspeed, track angle, turn rate, roll, vertical rate, mag heading, IAS, Mach, true track.

Airborne weather radar (AWR)

Key fact

9.75 GHz (SHF), wavelength 3 cm, beam width 3.5° to 5°. Phased array antenna — narrow beam without large antenna or high power; fewer side lobes. Detects reflection from water droplets. Compromise wavelength — ideal ½ droplet size would cause excessive attenuation.

Colour	Meaning
Green / Yellow / Red / Magenta	Increasing intensity; magenta = turbulence

Key fact

STAB ON — antenna stabilised in pitch/roll by gyros. Without STAB, radar may overscan in climb. Gain amplifies input (not volume). AUTO gain in WX mode allows STC (Sensitivity Time Control) — applies less gain to closer returns (to 60 nm) for uniform picture. Tilt ± 15° — too low causes ground return mixing; too high causes overscan. WX/TURB mode activates Doppler — high PRF reduces range to ~50 nm.

Formula — Cloud height (1-in-60 rule)

Height = [tilt − beam width/2] × range × 100 ft

GNSS / GPS

Key fact

24 satellites (21 operational + 3 spares), 6 orbital planes, max 4 per plane, orbit 20,200 km, period 12 hrs, inclination 55°, model WGS-84. Satellites transmit L1 (1,575.42 MHz) civil + military and L2 (1,227.6 MHz) military only. C/A code = civil; P code = military.

Satellites	Fix type
3	2D fix
4	3D fix + receiver clock correction
5	3D with RAIM (exclude faulty SAT)
6	3D with RAIM (no exclusion)

Key fact

Pseudorange includes receiver clock error — minimised by 4th satellite. Almanac (all satellite positions) updated every 24 hrs; Ephemeris (individual satellite) every 2 hrs. Mask angle typically 5° or 7.5°. RAIM monitors satellite integrity. Selective availability degraded GPS by dithering signals. Aerial installed on top of fuselage above COG.

Augmentation systems

System	Coverage	Notes
GBAS/LAAS/DGPS	30 km / 15 nm	VHF band; nearer to reference = more accurate; does not account for multipath
SBAS/WAAS/EGNOS	Wide area	Same frequency as GNSS; geostationary satellites relay corrections; integrity update within 6 s
ABAS	Onboard	Includes AAIM — does not use SAT-provided altitude

PBN / RNAV

Key fact

RNP includes on-board performance monitoring and alerting; RNAV does not. RNP 1 / P-RNAV: ± 1 nm 95%. RNP 5 / B-RNAV: ± 5 nm 95%. RNAV 10 requires 2 long-range navigation systems (INS, IRS/FMS, or GNSS). Errors: PDE (path definition), FTE (flight technical), NSE (navigation system), TSE (total system).

Exam trap

Transponder in aircraft, interrogator on ground (DME is the reverse). PSR shows only range and bearing but is more accurate for position; SSR consumes less power. False alley on AWR — attenuation behind a TS masks clouds beyond, creating a clear corridor illusion. GPS altitude information is barometric altitude when augmented.