15+Times Asked
~80Total Marks
6Topics
HighPriority
1 Three Segments
Space Segment
- 24+ satellites, 6 orbital planes, ~20,200 km altitude, 12-hour orbit.
- Broadcasts on L1 (1575.42 MHz) and L2 (1227.60 MHz).
Control Segment
- 1 Master Control Station (Colorado Springs) + 5 Monitor Stations + 3 Ground Antennas.
- Tracks satellites, computes orbits, uploads corrections.
User Segment
- GPS receivers: handheld, survey-grade, vehicle-mounted.
2 Trilateration
- Distance = Speed of light × Travel time.
- 1 satellite = sphere. 2 = circle. 3 = two points. 4th = unique point + clock correction.
- Minimum 4 satellites: 3 for X,Y,Z + 1 for clock bias.
3 DGPS and RTK
- Base station at known location computes error, sends correction to rover.
- Real-time DGPS: Corrections via radio/cellular. Accuracy 1–3 m.
- RTK: Carrier phase. Centimetre-level accuracy (1–2 cm).
4 GPS Error Sources
| Error | Magnitude | Correction |
|---|---|---|
| Ionospheric delay | 5–15 m | Dual-frequency, models |
| Tropospheric delay | 2–5 m | Atmospheric models |
| Multipath | 1–5 m | Ground plane antenna |
| Clock error | 1–3 m | 4th satellite |
| Ephemeris error | 1–2 m | Control segment updates |
| GDOP | Variable | Good satellite geometry |
| Selective Availability | ~100 m | Turned off May 2000 |
5 GDOP
- Low GDOP (1–2) = satellites spread = accurate. High (>6) = clustered = poor.
- Types: PDOP (position), HDOP (horizontal), VDOP (vertical), TDOP (time).
6 GPS-GIS Integration
- Plan survey (accuracy, coordinate system).
- Collect GPS data with attributes.
- Differential correction.
- Transfer and coordinate transformation.
- Import into GIS, overlay, analyse.
Past Exam Questions
+Explain the components of GPS (three segments). (2071–2082, 4–8 marks)
Space: 24+ sats, 6 planes, 20,200 km. Control: MCS + monitors + antennas. User: receivers. Draw signal flow diagram.
+How does GPS work? Explain trilateration. (2074, 2075, 2079)
Distance = c × dt. Sphere intersection. Draw diagram with 4 satellites and receiver.
+Explain DGPS and how it improves accuracy. (2073, 2074, 2076, 2080, 2081)
Base station computes error, sends correction. Standalone 5–10 m → DGPS 1–3 m → RTK 1–2 cm.
+Sources of errors in GPS and their corrections. (2072, 2075, 2078, 2079, 2081, 2082)
Use error table. Seven sources with magnitude and correction for each.
+What is GDOP? Why is satellite geometry important? (2079 Chaitra)
GDOP quantifies geometry effect on accuracy. Low = good, High = poor. Types: PDOP, HDOP, VDOP, TDOP.
+Steps of integrating GPS with GIS. (2071, 2072)
Plan, collect, correct, transfer, transform, import, overlay, analyse.