GNSSRadio Navigation — DGCA CPL practice questions
Question 1 of 25
NAVSTAR/GPS operates in the ___ band; receiver determines position by ___:
All 25 questions — GNSS
Radio Navigation · DGCA CPL. The correct option is marked on each.
Q1. NAVSTAR/GPS operates in the ___ band; receiver determines position by ___:
- A.UHF, range position lines✓
- B.UHF, secondary radar principles
- C.SHF, secondary radar principles
- D.SHF, range position lines
Why: GPS L1 (1575 MHz) and L2 (1227 MHz) are in the UHF band (300 MHz–3 GHz). Position determined by pseudo-range position lines (spheres of position from each SV).
Q2. The GPS control segment comprises:
- A.space segment, user segment and ground segment
- B.ground segment and INMARSAT geostationary satellites
- C.master control station, back-up control station and five monitoring stations✓
- D.master + back-up control stations, five monitoring stations and INMARSAT
Why: GPS control segment: Master Control Station + Back-up Control Station + 5 monitoring stations .
Q3. Orbital height and inclination of GPS constellation:
- A.20,180 km, 65°
- B.20,180 km, 55°✓
- C.19,099 km, 65°
- D.19,099 km, 55°
Why: GPS: orbital height 20,180 km , inclination 55° to equator.
Q4. Earth model used for NAVSTAR/GPS:
- A.WGS90
- B.PZ90
- C.WGS84✓
- D.PZ84
Why: GPS uses WGS84 (World Geodetic Survey 1984) — the ICAO standard for aeronautical positions.
Q5. Minimum satellites required for a 3D fix:
- A.3
- B.4✓
- C.5
- D.6
Why: 4 SVs are required for a true 3D fix. The 4th SV provides the equation needed to eliminate receiver clock error (the 4th unknown: X, Y, Z, T).
Q6. GPS operational constellation comprises how many satellites:
- A.12
- B.21
- C.24✓
- D.30
Why: GPS operational constellation: 24 SVs (21 active + 3 spares) in 6 orbital planes, 4 per plane.
Q7. Most accurate fixing information from:
- A.four satellites clustered on one side of the sky
- B.four satellites equally spaced in azimuth at same elevation
- C.one satellite overhead, three near horizon 120° apart
- D.four satellites at high elevation angles✓
Why: Best PDOP geometry: one SV directly overhead + three SVs close to the horizon spaced 120° apart . This maximizes the angle of cut between position spheres.
Q8. Most significant GPS error source (single frequency):
- A.PDOP
- B.receiver clock
- C.ionospheric propagation✓
- D.ephemeris
Why: Ionospheric propagation delay is the most significant GPS error for single-frequency (L1-only) receivers. It causes the signal to slow down, over-estimating range.
Q9. Frequency available to civil (non-authorized) GPS users:
- A.1227.6 MHz
- B.1575.42 MHz✓
- C.1602 MHz
- D.1246 MHz
Why: Civil GPS users have access to L1: 1575.42 MHz (C/A code). L2: 1227.6 MHz is military (P code). 1602 MHz is GLONASS.
Q10. PRN codes are used to:
- A.identify the satellites✓
- B.pass the almanac data
- C.pass navigation and system data
- D.pass ephemeris and time information
Why: PRN (Pseudo-Random Noise) codes are used to identify the satellites and measure signal travel time (pseudo-range). Navigation data (almanac, ephemeris, clock corrections) is modulated on top of the PRN codes.
Q11. Minimum satellites for Receiver Autonomous Integrity Monitoring (RAIM):
- A.3
- B.4
- C.5✓
- D.6
Why: RAIM requires a minimum of 5 SVs for fault detection. 6 SVs are needed for fault exclusion (identifying and removing the faulty SV).
Q12. Time to download the full GPS almanac:
- A.2.5 minutes
- B.12.5 minutes✓
- C.25 minutes
- D.15 minutes
Why: Full almanac = 25 frames × 30 seconds per frame = 12.5 minutes . Sub-frame 5 carries 1 page of almanac per frame; 25 frames complete the constellation almanac.
Q13. LAAS and WAAS remove errors caused by:
- A.propagation, selective availability, satellite ephemeris and clock
- B.selective availability, satellite ephemeris and clock✓
- C.PDOP, selective availability and propagation
- D.receiver clock, PDOP, satellite ephemeris and clock
Why: DGPS removes: propagation delay, ephemeris, and SV clock errors . SA (selective availability) was cancelled in 2000. PDOP and receiver clock are not corrected by DGPS.
Q14. Most accurate satellite fixing information from:
- A.NAVSTAR/GPS & GLONASS✓
- B.TRANSIT & NAVSTAR/GPS
- C.COSPAS/SARSAT & GLONASS
- D.NAVSTAR/GPS alone
Why: Combining GPS and GLONASS provides more SVs in view → better geometry (PDOP) → improved accuracy and integrity monitoring.
Q15. A LAAS requires:
- A.accurately surveyed site + INMARSAT link to pass X,Y,Z corrections
- B.accurately surveyed site + INMARSAT link to pass range corrections
- C.accurately surveyed site + pseudolite to pass range corrections✓
- D.accurately surveyed site + pseudolite to pass X,Y,Z corrections
Why: LAAS: precisely surveyed aerodrome site + corrections transmitted via VHF data link . A pseudolite provides runway threshold ranging. Corrections are pseudo-range corrections (not X,Y,Z).
Q16. GPS position errors include:
- A.selective availability, sky wave interference, PDOP
- B.propagation, selective availability, ephemeris✓
- C.PDOP, static interference, instrument
- D.ephemeris, PDOP, siting
Why: GPS errors: ionospheric/tropospheric propagation, SA (historically), ephemeris, SV clock, PDOP, multipath, receiver clock . Sky wave, static interference, and siting are ground-based navaid errors.
Q17. PRN codes are used to determine range by:
- A.measuring phase difference between received and generated code✓
- B.measuring Doppler shift
- C.measuring signal amplitude
- D.comparing carrier phase
Why: The receiver generates an identical PRN code and aligns it with the received code. The time offset (phase difference) between the two codes = signal travel time = pseudo-range.
Q18. GPS navigation data is transmitted at:
- A.50 Hz
- B.1 kHz✓
- C.50 bps
- D.1575 MHz
Why: GPS navigation data (ephemeris, almanac, clock corrections) is transmitted at 50 bits per second (50 Hz) modulated onto the PRN codes.
Q19. Geostationary satellites used by SBAS have an orbital altitude of:
- A.20,180 km
- B.19,099 km✓
- C.35,800 km
- D.23,222 km
Why: Geostationary satellites orbit at 35,800 km (geosynchronous orbit) in the equatorial plane with a 24-hour period. Not 20,180 km (GPS) or 19,099 km (GLONASS).
Q20. The initial range calculation is called pseudo-range because it is not corrected for:
- A.receiver clock errors✓
- B.receiver and satellite clock errors
- C.receiver & satellite clock errors + propagation errors
- D.receiver & satellite clock errors + ephemeris errors
Why: Pseudo-range is initially uncorrected for receiver clock error . The receiver's crystal oscillator has deliberate offset error; this is solved by the 4th SV equation. SV clock error is included in the broadcast and corrected before computing pseudo-range.
Q21. Navigation and system data message transmitted through:
- A.50 Hz modulation✓
- B.C/A and P PRN codes
- C.C/A code only
- D.P code only
Why: Navigation data is transmitted at 50 bits per second modulated on both L1 C/A and L2 P codes. The data frame structure carries ephemeris, almanac, ionospheric model, and clock corrections.
Q22. An all-in-view receiver:
- A.informs operator all required satellites are available
- B.checks all SVs in view and selects 4 with best geometry✓
- C.requires 5 SVs for a 4D fix
- D.uses all visible SVs for fixing
Why: An all-in-view receiver tracks all visible SVs simultaneously (dedicated channel per SV) and selects the 4 with best geometry (lowest PDOP) for the navigation fix.
Q23. When using GNSS for a non-precision approach, MDA is determined using:
- A.barometric altitude✓
- B.GPS altitude
- C.radio altimeter height
- D.either barometric or radio altimeter
Why: GPS vertical accuracy is insufficient for MDA determination due to the WGS84/MSL discrepancy (up to 50 m). Barometric altitude must be used for MDA on non-precision approaches.
Q24. If an aircraft manoeuvre shadows a satellite being used for fixing:
- A.accuracy unaffected
- B.accuracy temporarily downgraded✓
- C.fix lost permanently
- D.RAIM activates automatically
Why: Shadowing a SV temporarily removes it from the fix solution. The receiver may reselect from remaining visible SVs. Accuracy is temporarily downgraded until the SV is reacquired or another selected.
Q25. The maximum discrepancy between WGS84 ellipsoid and mean sea level is approximately:
- A.5 m
- B.50 m
- C.500 m✓
- D.0.5 m
Why: The maximum discrepancy between the WGS84 ellipsoid and mean sea level (geoid) is approximately 50 m . This is why GPS altitude alone is insufficient for precision vertical navigation.