Rep5- Satellite Navigation

1) Explain the principle of triangulation!

Short: Determine position by intersecting distance spheres from multiple known points (satellites).
Long: A receiver measures signal travel times to at least four satellites, converts them to distances (pseudo-ranges), and computes the unique 3D position (lat, lon, alt) plus clock bias where the corresponding spheres intersect. Line-of-sight and a common precise time base are essential.

2) Address 3 components of NAVSTAR GPS!

Short: Space segment, control segment, user segment.
Long: The space segment is the satellite constellation (SVs) in six 55° inclined circular orbits (~20,183 km, ~12 h). The control segment (MCS + monitor stations) determines precise orbits/clocks and uploads parameters. The user segment comprises receivers/antennas that process L-band signals to output PVT.

3) What are the 4 tasks of the control segment?

Short: Orbit determination, clock synchronization, compute corrections, upload to satellites.
Long: It (1) tracks satellites to estimate precise ephemerides, (2) estimates/synchronizes satellite clock biases, (3) computes navigation and correction parameters (including health/integrity), and (4) uplinks these via S-band to the satellites; it also performs general monitoring/access control.

4) What are the differences of the GPS L1 and L2 carrier signals?

Short: Frequencies and codes: L1=1575.42 MHz (C/A + P via quadrature), L2=1227.60 MHz (P/Y).
Long: Both derive from 10.23 MHz. L1 carries the civil C/A code and the P(Y) code (phase-quadrature). L2 carries P(Y) for dual-frequency users (PPS). Dual-frequency L1/L2 enables ionospheric delay mitigation.

5) What purpose has the C/A Code in GPS?

Short: Fast acquisition and coarse ranging for SPS (civil) users.
Long: The C/A (Coarse/Acquisition) PRN repeats every millisecond, allowing quick lock and robust identification of each satellite on L1; it provides meter-level pseudo-range accuracy suitable for standard positioning.

6) What is the influence of the receiver clock error with respect to the GPS position?

Short: It adds a common range bias; you need a 4th satellite to solve for time.
Long: Receiver time offset appears as the same pseudo-range error to all satellites (~0.3 m per ns). The position solution simultaneously estimates x, y, z and the receiver clock bias; hence ≥4 satellites are required.

7) What is the difference between slant distance and pseudo-range?

Short: Slant distance = true geometric range; pseudo-range = measured range including errors.
Long: Slant distance is the ideal Euclidean distance satellite–receiver. Pseudo-range includes clock biases and propagation effects (ionosphere/troposphere, multipath, noise), so it must be corrected/solved out.

8) What is S/A? What is the purpose of S/A?

Short: Selective Availability; intentional SPS accuracy degradation (historical).
Long: S/A introduced controlled ephemeris/clock dithers to worsen civil accuracy (~100 m historically), reserving best performance for authorized PPS users. (It was later turned off, but conceptually that was its purpose.)

9) Address 3 errors of the space segment!

Short: Orbit perturbations, Earth/Moon/Sun gravitational effects, satellite clock bias.
Long: Space-segment errors arise from imperfect ephemerides (e.g., J2 flattening, third-body forces, solar radiation pressure) and residual satellite clock instabilities that map into range errors.

10) Address 3 errors of the user segment!

Short: Multipath, ionospheric delay, tropospheric delay.
Long: User-side errors include multipath reflections (meters), ionospheric group delay (day 20–30 m; dual-frequency mitigates), tropospheric delay (2–20 m, modelable), plus receiver clock/thermal noise contributions.

11) What is the DOP?

Short: Dilution of Precision—geometry factor scaling measurement noise into position/time error.
Long: DOP metrics (GDOP/PDOP/HDOP/VDOP/TDOP) quantify satellite geometry quality; broader angular spread lowers DOP (better). Position/time variances increase proportionally with the relevant DOP.

12) What is the correlation between DOP and GPS position accuracy?

Short: Accuracy ≈ measurement error × DOP; higher DOP → worse accuracy.
Long: For a given pseudo-range standard deviation, the estimator scales errors by the geometry matrix; the resulting covariance is inflated by DOP. As a rule, GDOP > ~5 indicates unfavorable geometry and degraded solutions.

13) How DGPS is working? What is the fundamental idea?

Short: Reference station computes corrections and broadcasts them to nearby users.
Long: A fixed station with known coordinates derives satellite-specific corrections (or raw/position corrections) from its measurement residuals and transmits them (e.g., RTCM). Users apply them to reduce common-mode errors and achieve <10 m (95%) or better.

14) Which 5 GPS errors can be reduced due to the DGPS concept?

Short: S/A, ionosphere, troposphere, satellite orbits, satellite clocks.
Long: DGPS chiefly cancels spatially correlated errors between the reference and user: intentional S/A dithers (historical), ephemeris and satellite clock biases, and atmospheric (iono/tropo) delays; receiver/multipath remain.

15) What is integrity monitoring for?

Short: To warn users in time when navigation data are unsafe.
Long: Integrity means timely detection and annunciation of faults relative to defined alarm limits and time-to-alert (e.g., HAL 0.3 nm, TA 10 s for NPA), enabling the user to stop relying on the solution.

16) What is the meaning of RAIM?

Short: Receiver Autonomous Integrity Monitoring.
Long: An onboard, geometry-based method that uses redundant pseudo-ranges to detect (and with more redundancy, isolate) faulty measurements/satellites without external aid; typically needs ≥5 sats for detection, ≥6 for isolation.

17) Address 3 common RAIM approaches! What are their differences?

Short: PCM (position comparison), RCM (range residuals), Parity Space (conditional tests).
Long: PCM computes multiple position solutions from subsets and compares them statistically; RCM forms pseudo-range residuals after a common solution and tests for outliers; Parity Space transforms the overdetermined system and applies statistical tests to parity/conditional equations.

18) What is the meaning of GIC?

Short: GNSS Integrity Channel.
Long: A networked reference-station service that monitors satellites, compares measurements to thresholds, and disseminates satellite status/integrity information to users via terrestrial or satellite links.

19) What is the advantage of GIC with respect to RAIM?

Short: Lower redundancy need and geometry independence.
Long: Because reference-station positions are precisely known, only receiver clock terms must be estimated; integrity determination is less dependent on user-side satellite geometry and requires fewer redundant measurements than RAIM

20) What are augmentation systems for?

Short: To enhance GNSS accuracy, continuity, and integrity.
Long: Augmentation supplies corrections (wide- or local-area) and integrity data via space-based (SBAS) or ground-based (GBAS) infrastructures so safety-critical operations (e.g., approaches) can meet performance requirements.

21) Address 5 augmentation systems!

Short: WAAS, EGNOS, MSAS, LAAS/GBAS, EUROFIX.
Long: WAAS (US SBAS), EGNOS (EU SBAS), MSAS (Japan SBAS) provide GEO-broadcast corrections/integrity; LAAS/GBAS is airport-local ground-based augmentation for CAT I/II/III; EUROFIX overlays GNSS corrections on LORAN-C/Chayka.

22) Address 4 basic components of an augmentation system!

Short: Reference stations, master/processing center, uplink/comm links, broadcast transmitters (GEO or local).
Long: A typical architecture has (1) widely distributed reference stations, (2) master/control processing centers estimating corrections/integrity, (3) uplink or terrestrial distribution networks, and (4) transmitters—GEO satellites for SBAS or local VHF/UHF/pseudolites for GBAS.

23) How the dissimilarity of EUROFIX is gained?

Short: By combining satellite navigation with terrestrial LORAN-C/Chayka.
Long: EUROFIX achieves dissimilar redundancy through two fundamentally different technologies—GNSS and low-frequency terrestrial radionavigation—modulating GNSS corrections/integrity onto LORAN-C/Chayka signals, improving integrity and continuity even during short GNSS outages.