DGCA CPL / ATPL Study Notes • Radio Navigation • Ch 18

🌠 Chapter 18: GNSS
Global Navigation Satellite System — GPS, GLONASS, Galileo & Augmentation

📋 Contents

1. Introduction & Systems 2. Satellite Orbits & WGS84 3. GPS — Three Segments 4. Principle of Operation 5. GPS Errors 6. System Accuracy & RAIM 7. Differential GPS (DGPS) 8. SBAS — EGNOS/WAAS 9. GNSS Systems Comparison 10. GNSS Summary 11. Practice Questions (26 Q)

1. Introduction & Systems

GNSS provides global navigation with precision measured in metres. Two fully operational systems exist; more are under development. GNSS is intended to eventually replace all terrestrial radio navigation aids.

Fig 18.1: GNSS overview — GPS, GLONASS, Galileo, Beidou constellations

📡 Current & Planned GNSS Systems

System	Operator	Status
NAVSTAR/GPS	USA (DoD)	Fully operational; ICAO standard
GLONASS	Russia	Fully operational
Galileo	European Union	Under development (planned full ops ~2020+)
Beidou/Compass	China	Under development (planned ~2020)

⚡ Why Galileo? European Sovereignty Europe developed Galileo because access to full GPS/GLONASS capability is outside European control. A European system provides independent navigation capability not subject to US or Russian military decisions.

2. Satellite Orbits & WGS84

Fig 18.2: GPS satellite constellation — 24 SVs in 6 orbital planes, each inclined 55° to equator

📡 Kepler's Laws Applied to GNSS

Satellites (Space Vehicles, SVs) calculate their own position using Keplerian laws
Each SV's computed orbital position = ephemeris
Orbits affected by: sun/moon/planetary gravity + solar radiation → ephemeris errors

📡 WGS84 — World Geodetic Survey 1984

GPS uses WGS84 as its earth model (ellipsoid) — ICAO standard for aeronautical positions
GLONASS uses PZ90; Galileo uses ETRS89 — all mathematically interconvertible
WGS84 maximum vertical discrepancy from mean sea level: ~50 m
Therefore: GPS altitude cannot be used in isolation for vertical navigation (except medium/high cruise)

3. GPS — Three Segments

Fig 18.3: Three GPS segments — Space, Control and User

📡 Space Segment

Parameter	GPS (NAVSTAR)
Number of SVs	24 (21 operational + 3 active spares)
Orbital planes	6 planes, 4 SVs each
Orbital height	20,180 km (10,898 NM)
Orbital inclination	55° to equator
Orbital period	11 h 56 min
Frequencies	L1: 1575.42 MHz (civil C/A code); L2: 1227.6 MHz (military P code)
Clock	Atomic clocks (rubidium + caesium)

📡 Control Segment

Master Control Station: Colorado Springs, USA
Back-up Control Station: Onizuka AFB, California
5 Monitoring Stations: Colorado Springs, Hawaii, Ascension Island, Diego Garcia, Kwajalein
Monitor stations check SV position and clock every 12 hours minimum
Send corrections to SVs: position (ephemeris) and clock errors
Clock errors: cannot adjust atomic clocks → error is included in broadcast data

📡 User Segment

All GPS receivers using the space segment. Three types:

Sequential: 1–2 channels; scans SVs sequentially; less accurate/slower
Multiplex: several channels; rapidly cycles through multiple SVs
All-in-view (parallel): dedicated channel per SV; tracks all visible SVs simultaneously; selects best 4 for fix; most accurate

4. Principle of Operation

Fig 18.13: Pseudo-random code time measurement — receiver generates identical code; time offset = range

📡 GPS Navigation Message

Each frame: 5 sub-frames × 6 seconds = 30 seconds total
Sub-frame 1: SV clock error correction
Sub-frames 2&3: SV ephemeris data
Sub-frame 4: ionospheric model, GPS time/UTC correlation
Sub-frame 5: constellation almanac data (25 frames = 12.5 minutes for full almanac download)
Almanac: valid 4 hours to several months; usually downloaded hourly

📡 Position Determination — Pseudo-Range

SV broadcasts pseudo-random noise (PRN) code on L1 (C/A) or L2 (P)
Receiver generates identical code and compares phase shift → measures time delay
Time delay × speed of light = pseudo-range (not yet corrected for receiver clock error)
Each pseudo-range = position on surface of sphere with radius >10,900 NM
2 SVs: circle (intersection of 2 spheres)
3 SVs: 2 position possibilities (one in space, one on earth) → 3D fix possible but less accurate
4 SVs: 4 simultaneous equations, 4 unknowns (X, Y, Z, T) → true 3D fix + accurate time

Fig 18.14–18.17: Progressive fix determination — 2 SVs circle, 3 SVs two points, 4 SVs unique 3D+time fix

⚡ Why 4 SVs? Receiver Clock Error Receiver clock (crystal oscillator) is less accurate than SV atomic clocks → pseudo-ranges contain receiver clock error. A 4th SV provides a 4th equation to solve for the 4th unknown (time error T) → eliminates receiver clock error from the fix. Result: accurate 3D position AND an accurate time reference (4D fix).

⚡ 3D Fix with Altitude Input Some receivers can produce a 3D fix using only 3 SVs + barometric altitude input. The altitude simulates a 4th SV at the earth's centre. Less accurate than a true 4-SV fix.

5. GPS Errors

Fig 18.18: PDOP — good geometry (SVs spread out) vs poor geometry (SVs clustered) — good geometry gives smaller fix uncertainty

⚠ GPS Error Sources (All at 95% level)

Error Source	Max Error	Detail
Ephemeris error	2.5 m	SV position error from gravitational & solar radiation effects; corrected every 12 h
SV clock error	1.5 m	Atomic clock drift; corrected every 12 h; included in broadcast
Ionospheric delay	Most significant	Ionospheric particles slow radio energy → over-reads range. Model in receiver corrects most. Worse: higher ionization, lower elevation SVs, single frequency. Dual-frequency (L1+L2) eliminates ~99% of error.
Tropospheric delay	Small	Refraction in lower atmosphere; modelled; worse at low elevation angles
Multipath	Variable	Signal reflected from buildings/terrain arrives late → longer apparent range
PDOP (geometry)	Multiplicative	Poor SV geometry multiplies all other errors. Best: 1 SV overhead + 3 near horizon 120° apart
Receiver clock	Eliminated by 4th SV	Crystal oscillator; corrected by solving 4 simultaneous equations
Selective Availability (SA)	Now 0	Deliberate USAF degradation of civil signal; cancelled in 2000; no longer an error source
Aircraft manoeuvre	Temporary	Wing/fuselage may shadow an SV → temporary accuracy degradation until another SV selected

📡 DOP Types

DOP	Meaning
HDOP	Horizontal Dilution of Precision (X, Y errors)
VDOP	Vertical Dilution of Precision (Z errors)
PDOP	Position DOP = combination of HDOP + VDOP
TDOP	Time DOP (timing errors)
GDOP	Geometric DOP = PDOP + TDOP combined

6. System Accuracy & RAIM

✓ ICAO GPS Accuracy Specification (SPS — Standard Positioning Service)

Parameter	Accuracy (95%)
Horizontal	±13 m
Vertical	±22 m
Time	40 nanoseconds

📡 RAIM — Receiver Autonomous Integrity Monitoring

With only 4 SVs providing 3D fix, there is NO way to detect if one SV is providing bad data
RAIM uses additional SVs to cross-check fix consistency
Minimum SVs for fault detection: 5
Minimum SVs for fault exclusion (remove bad SV): 6
ICAO integrity requirement: 2 seconds warning for precision approach; 8 seconds for non-precision
If insufficient SVs for RAIM: GPS should not be used for primary navigation (flag generated)

7. Differential GPS (DGPS)

Fig 18.19: LAAS (GBAS) — precisely surveyed ground reference station detects SV errors; transmits corrections to aircraft via VHF

📡 DGPS Concept

A precisely surveyed ground reference station measures its GPS position continuously. Since its true position is known, any GPS error = computed position − known position. Corrections are transmitted to aircraft to improve their GPS position accuracy.

Eliminates common errors: ionospheric/tropospheric delay (virtually identical paths for ground + aircraft when close), ephemeris, SV clock
Two implementations: LAAS (local area) and SBAS (wide area)

📡 LAAS — Local Area Augmentation System (GBAS)

Precisely surveyed site on the aerodrome
GPS receiver computes position → compares with known position → generates corrections
Corrections transmitted to aircraft via dedicated VHF data link
A pseudolite (pseudo-satellite) provides range to runway threshold
When aircraft close to LAAS: ionospheric & tropospheric paths virtually identical → errors effectively eliminated
Potential: CAT IIIC operations

8. SBAS — Satellite Based Augmentation System

Fig 18.20: EGNOS segments — reference stations, regional control stations, master control, geostationary SVs

📡 SBAS (Wide Area DGPS)

Wide-area network of precisely surveyed reference stations across a region
Each reference station determines GPS errors for its area
Corrections sent to Master Control Station → uplinked to geostationary satellites
Geostationary SVs broadcast corrections to all users in region
Geostationary SV: orbit period 24 hours; altitude 35,800 km; equatorial orbit

Fig: SBAS augmentation — geostationary SV broadcasts corrections; also acts as additional ranging source

📡 SBAS Implementations

System	Region	Name
WAAS	USA	Wide Area Augmentation System
EGNOS	Europe	European Geostationary Navigation Overlay Service
MSAS	Japan	MTSAT Satellite Augmentation System
GAGAN	India	GPS-Aided Geo Augmented Navigation

9. GNSS Systems Comparison

Fig 18.6: GPS vs GLONASS vs Galileo — orbits, heights, inclinations, frequencies, earth models compared

📡 GPS vs GLONASS vs Galileo

Parameter	GPS (NAVSTAR)	GLONASS	Galileo
Operator	USA	Russia	EU
No. of SVs	24	24	30
Orbital planes	6	3	3
Orbital height	20,180 km (10,898 NM)	19,099 km	23,222 km
Inclination	55°	65°	56°
Orbit period	11 h 56 min	11 h 15 min	14 h 8 min
Civil frequency	L1: 1575.42 MHz	L1: 1600 MHz	E1: 1559–1591 MHz
Military freq	L2: 1227.6 MHz	L2: 1250 MHz	E6: 1260–1300 MHz
Earth model	WGS84	PZ90	ETRS89

⚡ Combined GPS + GLONASS Combining both systems: improved accuracy + enhanced integrity monitoring (more SVs available). However: different earth models (WGS84 vs PZ90) must be converted — software handles this automatically.

10. GNSS Summary

Parameter	Detail
GPS constellation	24 SVs, 6 planes, 20,180 km, 55°, 11h 56m
GPS frequencies	L1 1575.42 MHz (civil C/A); L2 1227.6 MHz (military P)
Earth model	WGS84 (ICAO standard)
Control segment	Master + Backup control + 5 monitoring stations
Fix requirement	4 SVs for 3D fix + time (4D). 3 SVs + altitude input for 3D only
RAIM detection	5 SVs minimum
RAIM exclusion	6 SVs minimum
Almanac download	12.5 minutes (25 frames × 30 s)
GPS accuracy (ICAO)	Horizontal ±13 m; Vertical ±22 m; Time 40 ns (all 95%)
Biggest GPS error	Ionospheric propagation delay (most significant in single-frequency)
SA	Selective Availability — cancelled May 2000; no longer active
LAAS/GBAS	Local area DGPS; VHF data link; pseudolite; potential CAT IIIC
SBAS	WAAS (USA), EGNOS (Europe), MSAS (Japan); corrections via geostationary SV at 35,800 km
GPS vertical	Not usable for MDA in non-precision approaches; use barometric altimetry

11. Practice Questions

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

Answer: (a)
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

Answer: (c)
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°

Answer: (b)
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

Answer: (c)
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

Answer: (b)
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

Answer: (c)
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

Answer: (d)
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

Answer: (c)
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

Answer: (b)
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

Answer: (a)
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

Answer: (c)
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

Answer: (b)
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

Answer: (b)
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

Answer: (a)
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

Answer: (c)
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

Answer: (b)
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. EGNOS is:

(a) proposed European satellite navigation system

(b) a LAAS

(c) a WAAS

(d) system to remove geoid-ellipsoid differences

Answer: (c)
EGNOS (European Geostationary Navigation Overlay Service) is Europe's SBAS/WAAS — wide area DGPS using geostationary satellites to broadcast corrections.

Q18. 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

Answer: (a)
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.

Q19. GPS navigation data is transmitted at:

(a) 50 Hz

(b) 1 kHz

(c) 50 bps

(d) 1575 MHz

Answer: (b)
GPS navigation data (ephemeris, almanac, clock corrections) is transmitted at 50 bits per second (50 Hz) modulated onto the PRN codes.

Q20. 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

Answer: (b)
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).

Q21. 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

Answer: (a)
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.

Q22. 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

Answer: (a)
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.

Q23. 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

Answer: (b)
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.

Q24. 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

Answer: (a)
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.

Q25. 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

Answer: (b)
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.

Q26. 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

Answer: (c)
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.

DGCA CPL/ATPL Radio Navigation Study Notes
Chapter 18 — GNSS (Global Navigation Satellite System)
Capt Pankaj Pahil | www.ghostaviator.com
For personal study use only.

🌠 Chapter 18: GNSSGlobal Navigation Satellite System — GPS, GLONASS, Galileo & Augmentation