Science - GATE GE (Geomatics Engineering)
The Science domain tests physical geodesy, Earth observation, atmospheric propagation, and geophysical interpretation. Expect gravity corrections, geoid relationships, GNSS delay models, and InSAR displacement sensitivity.
Core rules and concepts
- The free-air correction is about 0.3086 mGal per metre of elevation; gravity decreases with height.
- The Bouguer correction is approximately 0.0419 ρ h mGal, with ρ in g/cm³ and h in metres.
- Ellipsoidal height h, orthometric height H, and geoid undulation N satisfy h = H + N.
- Deflection of vertical has two components, ξ (north-south) and η (east-west).
- The ionosphere is dispersive, so dual-frequency GNSS can remove first-order ionospheric delay; the troposphere is non-dispersive and is modelled.
- InSAR phase is wrapped; interferograms measure line-of-sight displacement, not horizontal motion alone.
Worked example
A gravity station sits 100 m above the reference surface. The free-air correction is 0.3086 × 100 = 30.86 mGal. The reasoning is that the observed gravity must be reduced to the geoid; the free-air term compensates for the station being farther from Earth's centre. The sign must be applied consistently with the reduction convention.
Traps that cost marks
- Reversing the sign of gravity corrections.
- Using h = H + N when the relationship is h = H + N but confusing which height is which.
- Ignoring ionospheric delay in single-frequency GNSS.
- Treating wrapped InSAR phase as an absolute displacement without unwrapping.
- Mixing free-air and Bouguer corrections without accounting for terrain density.