Satellite Geodesy part 2

What kind of satellite positions are required for nearly all tasks in satellite geodesy?

Precise, time-dependent satellite positions in a suitable reference frame.

What station coordinates can precise satellite orbits help determine?

Geocentric coordinates of observation stations.

What gravity-related products can precise satellite orbits help determine?

Terrestrial gravity-field parameters and a precise, high-resolution geoid.

What navigation and Earth-orientation applications require precise satellite orbits?

Real-time vehicle navigation and determination of Earth’s orientation parameters in space.

What must users of satellite methods understand to assess orbit-determination requirements?

Satellite orbital motion and its major perturbations.

Who developed the three laws of planetary motion, and whose observations did he mainly use?

Johannes Kepler; observations collected by Tycho Brahe.

When were Kepler’s laws published?

1609 and 1619.

Whose law of gravitation later explained Kepler’s laws?

Isaac Newton’s law of gravitation.

According to the reviewer, in what year was Isaac Newton born?

1643.

What worldview did Kepler’s laws challenge?

The geocentric theory, which placed Earth at the centre of the universe.

Which two pioneers, together with Kepler, helped establish a Sun-centred cosmology?

Nicolaus Copernicus and Galileo Galilei.

What does heliocentric mean?

Sun-centred.

What is Kepler’s first law?

Each planet moves in an elliptical orbit with the Sun at one focus.

What is Kepler’s second law?

A line joining the Sun and a planet sweeps out equal areas in equal times.

What is Kepler’s third law?

The square of the orbital period is proportional to the cube of the semi-major axis: T² ∝ a³.

How does orbital period depend on the semi-major axis?

T ∝ a^(3/2).

What is an ellipse?

A closed plane curve for which the sum of the distances from any point to two fixed focal points is constant.

What equation expresses the defining distance property of an ellipse?

r₁ + r₂ = 2a.

What does a represent in the ellipse equation r₁ + r₂ = 2a?

The semi-major axis.

If an ellipse’s focal points are separated by 2c, what does c represent?

Half the distance between the focal points.

What closed orbital shapes are possible under the attractive inverse-square force described in the reviewer?

A circle or an ellipse.

What mathematical expression represents Kepler’s second law?

dA/dt = constant.

What mathematical expression represents Kepler’s third law?

T²/a³ = constant.

How is the constant in Kepler’s third law expressed using the Sun’s mass?

T²/a³ = 4π²/(GM), where G is the universal gravitational constant and M is the Sun’s mass.

In the idealized explanation of satellite motion, what is a satellite?

A projectile acted on only by gravity.

Who theorized that a projectile launched with sufficient speed could orbit Earth?

Isaac Newton.

What imaginary location illustrates Newton’s explanation of orbital motion?

Newton’s Mountain, high above the influence of air drag.

In Newton’s orbital-motion example, in what direction is the projectile launched, and in what direction does gravity pull it?

It is launched tangent to Earth’s surface; gravity pulls it downwards toward Earth.

What happens if the projectile’s launch speed is too small?

It eventually falls to Earth.

What are small deviations from ideal two-body orbital motion called?

Orbit perturbations.

What features of Earth’s mass distribution cause orbital perturbations?

Earth’s nonspherical shape and inhomogeneous mass distribution.

What external celestial bodies cause orbital perturbations?

The Sun, Moon, and planets.

What tidal effects cause orbital perturbations?

Earth tides and ocean tides.

What atmospheric force causes orbital perturbations?

Atmospheric drag.

What radiation-related forces cause orbital perturbations?

Direct solar radiation pressure and Earth-reflected solar radiation pressure.

What does orbit determination essentially determine?

Orbital parameters and satellite clock biases.

What is the first step in the two-step orbit-determination procedure described in the reviewer?

Fit a Keplerian ellipse to the observations.

What is the second step in the orbit-determination procedure?

Use the fitted ellipse as a reference and improve the orbit by accounting for perturbing accelerations.

What stations supply observations for GNSS satellite orbit determination?

Monitor stations of the respective control segment.

How do global monitoring networks compare with regional networks for GNSS orbit determination?

Global networks provide higher orbit accuracy and reliability.

How is the GNSS orbital system tied to terrestrial reference frames?

Through colocation of GNSS receivers with instruments such as VLBI and SLR trackers.

What two station-distribution approaches are described for GNSS orbit determination?

Stations regularly distributed around the globe, or network sites surrounded by clusters of additional points.

Why are additional points clustered around GNSS network sites?

To facilitate ambiguity resolution and strengthen the orbital-parameter solution.

By what factor can station clusters strengthen the orbital-parameter solution, according to the reviewer?

A factor of three to five.

What characteristic commonly distinguishes satellite-orbit categories?

Orbital height.

What does LEO stand for, and what upper altitude does the reviewer give?

Low Earth Orbit; up to approximately 2,000 km.

Which gravity-field missions are listed as operating at about 400 km altitude?

CHAMP, GRACE, and GOCE.

Which remote-sensing satellites are listed in the reviewer’s 800–1,000 km altitude group?

SPOT, LANDSAT, and ERS.

Which altimeter satellites are listed in the reviewer’s 1,000–1,500 km altitude group?

TOPEX, ENVISAT, and JASON.

Which communication constellations use LEO?

Globalstar and Iridium.

What orbital-period range does the reviewer give for LEO satellites?

Approximately 90 minutes to 2 hours.

What launch-cost advantage do LEO satellites have?

Low launch costs.

Why can LEO satellites use low-power radio transmitters?

Their proximity provides sufficient signal strength for simple ground receivers.

What rapidly varying signal characteristic can support precise navigation using LEO satellites?

Doppler shift.

What two systems are given as examples of navigation using Doppler shift?

TRANSIT and DORIS.

How long are LEO satellites typically in sight, according to the reviewer?

Approximately 15–20 minutes.

Why are LEO satellites visible for only a short time?

Their orbital periods are short.

What can support continuous data transfer between LEO satellites and ground stations?

Relay satellites in higher orbits.

What does MEO stand for, and what altitude range does the reviewer give?

Medium Earth Orbit; approximately 5,000–20,000 km.

Does MEO require a single specific orbital path?

No; MEO supports different orbital paths and applications.

What application commonly uses MEO satellites?

Navigation.

What European navigation system is given as an example of MEO use?

Galileo.

How does Galileo provide coverage over large parts of the world simultaneously?

It uses a constellation of multiple satellites.

What visibility and communication advantages do MEO satellites have?

They remain in view for several hours and require less satellite swapping.

Why are MEO orbits relatively stable with respect to the atmosphere?

They are not affected by atmospheric drag in the reviewer’s description.

What disadvantages of MEO are listed in the answer key?

Low Doppler shift and more expensive launch costs.

What does GEO stand for, and what approximate altitude does the reviewer give?

Geostationary Orbit; approximately 36,000 km.

In what direction and above what part of Earth do geostationary satellites orbit?

West to east above the equator.

What is the orbital period of a geostationary satellite?

Approximately 23 hours, 56 minutes, and 4 seconds.

Why does a geostationary satellite appear stationary above a fixed location?

It orbits in the same direction and at the same angular rate as Earth’s rotation.

What more precise altitude and approximate speed are given for GEO satellites?

35,786 km altitude and approximately 3 km/s.

What two major applications use GEO satellites?

Telecommunications and weather monitoring.

Why can a ground antenna remain fixed when communicating with a GEO satellite?

The satellite appears to remain above the same location.

Why are GEO satellites useful for weather monitoring?

They continuously observe particular areas and track developing weather patterns.

What advantages of GEO are listed in the reviewer?

Stable orbits and relatively few satellites needed for broad coverage.

What disadvantages of GEO are listed in the reviewer?

High launch costs, limited geostationary orbital slots, and no coverage around the poles.

What is the general direction of travel in a polar orbit?

North to south, passing roughly over Earth’s poles.

Must a polar-orbiting satellite pass precisely over both poles?

No; the reviewer allows deviations of approximately 20–30 degrees.

What orbit category and altitude range does the reviewer associate with polar orbits?

LEO; approximately 200–1,000 km.

What does SSO stand for?

Sun-synchronous orbit.

What kind of orbit is a Sun-synchronous orbit?

A particular kind of near-polar orbit synchronized with the Sun.

What does Sun synchronization allow a satellite to maintain?

A consistent orbital orientation relative to the Sun and approximately the same local solar time at repeated passes.

Why are Sun-synchronous observations useful for comparing changes over time?

They observe locations at approximately the same local time, providing comparable illumination conditions.

Why are some Sun-synchronous satellites placed in dawn–dusk orbits?

To maintain prolonged sunlight exposure and avoid Earth’s shadow as much as the orbit allows.

What altitude range does the reviewer give for Sun-synchronous satellites?

Approximately 600–800 km.

What approximate speed does the reviewer give for an SSO satellite at 800 km altitude?

Approximately 7.5 km/s.