Lecture 7 - Satellite Positioning Flashcards
What does GNSS stand for?
Global Navigation Satellite System
Summarize GNSS.
Comprises the satellites, collectively known as a constellation, which broadcasts signals to both the control segment and the users
Satellites are distributed between a number of medium Earth orbits inclined at roughly 60 degrees to the equator with around 2 orbits per day
Compared to geostationary orbits, these orbits give better signal geometry for positioning and better coverage in polar regions
Explain geometric function of satellites.
Used as high orbiting targets, which are visible over large distances. The satellites may be regarded as ‘fixed’ control points within large-scale or global 3D networks. If the satellites are observed simultaneously from different ground stations as targets at high altitude –> purely geometric consideration leads to the geometrical method of satellite geodesy
Explain dynamic function of satellites.
Satellites can be considered to be a probe or a sensor in the gravity field of Earth. The orbital motion, and the variation of the parameters describing the orbit are observed in order to draw conclusions about the forces acting. Of particular interest is the relation between the features of the terrestrial gravity field and the resulting deviations of the true satellite orbit from an undisturbed Keplerian motion. The essential value of the satellite is that it is a moving body within Earth’s gravity field. This view leads to the dynamical method of satellite geodesy.
What are the tasks related to satellite orbit?
Orbit determination
Orbit correction
The advanced orbit perturbation theory
Briefly explain different gravitational forces exerted on an artificial Earth’s satellite.
The central force: the gravitational force of Earth’s mass center: earth as a sphere with the uniformly distributed mass of the Earth and the Non-spherical gravitational force as earth is an ellipsoid
The perturbation force: From the Sun; from the moon; the atmospheric drag; Solar radiation pressure; Earth’s tides.
Outline the general idea how to model the effect of different forces in terms of
determining satellite orbit.
Kepler Laws
- every orbit is an ellipse with 2 foci
- a line joining a planet and the Sun sweeps out equal areas during equal intervals of time.
- P^2 = a^3
What is the normal orbit of a satellite?
- The satellites move in their orbital planes that are fixed in space.
- The actual path of a moving satellite in the orbital plane is mathematically an ellipse.
- One focal point of the orbital ellipse is at the center of the Earth
- Conditions: The earth as a point mass; the mass of a satellite is negligible; the motion of a satellite is in a vacuum (no atmospheric drag); no sun, moon, or other celestial body exerts a gravitational attraction on a satellite
List the six orbital parameters and briefly explain their functions.
Right ascension of the ascending node: decide the place of the orbital plane.
Inclination of orbital plane: decide the place of the orbital plane.
Argument of perigee: direction of the ellipse.
Semi-major axis of orbital ellipse: size of ellipse.
Numerical eccentricity of ellipse: shape of ellipse
Epoch of perigee passage or the true anomaly of the satellite counted from the perigee (not a constant): indicates the position of the satellite in the orbit plane or measures the true anomaly of the satellite counted from the perigee, a function of time, not a constant.
What is the Earth’s asphericity?
The earth is not a perfect sphere, as its rotation causes it to be slightly flattened at the poles. This oblateness can be modeled as an extra band of material encircling the equator. The extra material exerts torque on an orbiting satellite, causing the orbital planes to slowly precess in space rather than being fixed in an inertial frame. The oblatenness torque will also cause the line of the apsides to rotate slowly in the orbital plane itself
What is orbital perturbation?
The departure from Keplerian orbit.
Main causes: asphericity of Earth’s gravitational potential; lunar and solar gravitational effects; atmospheric drag <1000km altitude
Describe GNSS control.
Consists of a network of monitor stations, one or more control stations, and a number of uplink stations.
The monitor stations obtain ranging measurements from the satellites and send these to the control stations
The monitor stations are at precisely surveyed locations and have synchronized clocks, enabling their ranging measurements to be used to determine the satellite orbits and calibrate the satellite clocks
Calculate the navigation data message for each satellite and determine whether any maneuvers must be performed, and then transmit to the pace segment by the uplink stations. Most satellite maneuvers are small, infrequent corrections, known as station keeping, which are used to maintain their satellites in their correct orbits.
