1Why is a stable framework needed in geodesy?
Because Earth is not rigid; rotation, tides, and plate tectonics affect time and coordinates
2What are the four basic units used in geodesy?
Time, length, angle, mass
3What is a reference frame?
A realization of a reference system, represented by control points on the ground or satellites
4Where are international standards for units maintained?
Bureau International des Poids et Mésures (BIPM), France
5How is the second defined?
By 9,192,631,770 radiation periods of the cesium-133 atom transition (since 1967)
6What was the older definition of a second?
1/86,000 of a mean solar day
7What are the three main geodetic time systems?
UT1 (Earth’s rotation), TAI (atomic time), UTC (compromise using leap seconds)
8Why is time important in satellite positioning?
Distance = speed of light × signal travel time (c × ∆t)
9How was the meter originally defined in 1791?
1/10,000,000 of the distance from the equator to the North Pole along a great circle
10How is the meter defined today?
The length of the path traveled by light in vacuum in 1/299,792,458 of a second
11What is the uncertainty in the modern meter definition?
About 0.1 nm (10⁻¹⁰ m)
12What is the kilogram originally based on?
The mass of one liter of water (1795), later the Kilogramme des Archives (1799)
13What is the kilogram defined by today?
The Planck constant (2019)
14What is a radian?
An angle subtended by an arc equal in length to the circle’s radius
15How many degrees are in a full rotation?
360° = 2π radians
16What is 1° equal to?
60 arcminutes (′), and 1′ = 60 arcseconds (″)
17Which unit is natural for calculus and math?
Radians
18Which unit is best for maps and visualization?
Degrees
19What are the two main angle notations?
Decimal Degrees (DD) and Degrees-Minutes-Seconds (DMS)
20Why is conversion between DMS and DD necessary?
Old data are often in DMS, while computers require DD
21What is the gravitational constant (G)?
(6.67428 ± 0.00067) × 10⁻¹¹ m³ kg⁻¹ s⁻²
22What are uses of G in geodesy?
Orbital mechanics, gravity strength, geoid modeling
23What is a celestial reference system?
A space-fixed system, equatorial-based, centered at Earth, defining celestial bodies with spherical coordinates
24What is a Cartesian coordinate system?
A 3D straight-line system (X, Y, Z) used for satellite orbits and global calculations
25What is a curvilinear coordinate system?
Coordinates (φ, λ, h) relative to an ellipsoid, intuitive for mapping/navigation
26What is a terrestrial reference system (ECEF)?
A geocentric system with origin at Earth’s center of mass, including atmosphere and ocean