Satellite Orbit

Space Segment → Control Segment → User Segment

A Global Navigation Satellite System (GNSS) is generally composed of three major segments

:space segment

consists of the satellites that make up a GNSS constellation.

space segment

receive and store data transmitted by the control segment, maintain accurate time using atomic frequency standards, and transmit information and signals to users.

control segment

is responsible for steering and managing the entire GNSS system.

control segment

responsibilities include deploying and maintaining satellites, tracking satellite movements, monitoring satellite clocks and orbits, and uploading data messages to the satellites.

Schriever Air Force Base, Colorado

The Master Control Station for GPS is located at

user segment

consists of the people, equipment, and applications that use GNSS signals.

pseudorange

A receiver can determine its position by measuring the ___ to satellites.

X, Y, and Z;

clock bias

The receiver must determine its three-dimensional position, represented by ___, while also accounting for the receiver's ___.

pseudorange

is an apparent distance between a receiver and satellite based primarily on the measured travel time of the satellite signal.

pseudorange

measured value contains errors, including the effect of receiver clock offset and other signal propagation effects.

hyperbolic positioning

Pseudorange positioning using between-satellite differenced pseudoranges is also called

Differencing

measurements can reduce or eliminate common errors.

Doppler principle of radio signals

Velocity can be determined using the

Doppler effect

occurs when there is relative motion between the transmitter and receiver, producing an apparent change in the observed frequency.

Attitude

refers to the orientation of a specific body frame attached to a land vehicle, ship, or aircraft with respect to a reference frame.

1. North axis
2. East axis
3. Up axis

A commonly used reference is a local-level frame, represented by the

:Roll → Pitch → Yaw

three main components of attitude

Roll

describes rotation about the vehicle's longitudinal axis.

Pitch

describes rotation about the vehicle's lateral axis.

Yaw

describes rotation about the vehicle's vertical axis.

Code pseudorange

is obtained from the measurement of the travel time of the satellite's ranging code.

Phase pseudorange

more commonly discussed as carrier-phase measurement, uses the phase of the carrier signal and provides much higher precision than ordinary code measurements.

Absolute positioning

determines the position of a receiver directly with respect to the GNSS reference system.

Relative positioning

determines the position of one receiver relative to another receiver or reference station.

Static positioning

involves a receiver that remains stationary during the observation period.

Kinematic positioning

involves a receiver that is moving while observations are being collected.

Real-time processing

means GNSS observations are processed while the survey or navigation activity is taking place.

Postprocessing

means the observations are collected first and processed later.

geocentric coordinate system

uses the center of mass of the Earth as its origin.

conventional celestial reference system

is used to describe positions and directions in relation to the celestial sphere.

Celestial Reference Frame (CRF)

provides a reference for describing celestial directions.

International Celestial Reference System (ICRS)

is realized through the work of the International Earth Rotation and Reference Systems Service (IERS).

conventional terrestrial reference system

is used to describe positions relative to the Earth.

International Terrestrial Reference Frame (ITRF)

is a realization of the terrestrial reference system.

1. VLBI → Very Long Baseline Interferometry
2. LLR → Lunar Laser Ranging
3. SLR → Satellite Laser Ranging
4. GNSS → Global Navigation Satellite Systems

The ITRF uses observations from several space-geodetic techniques, including

:Celestial reference system

reference relative to celestial space

Terrestrial reference system

reference fixed to the Earth

Universal Time (UT)

is related to the rotation of the Earth.

Universal Time (UT)

in relation to the Greenwich hour angle (GHA) and a fictitious Sun moving uniformly around the equatorial plane.

Sidereal Time (ST)

defined using the hour angle (HA) of the vernal equinox.

Sidereal Time (ST)

is related to the rotation of the Earth relative to the stars rather than the apparent position of the Sun.

solar time and sidereal time

are associated with Earth's rotation and are therefore not perfectly uniform.

Dynamic time

is derived from the motions of planets and other bodies in the solar system.

Atomic Time

is based on the highly stable oscillations of atomic clocks.

1. TAI → International Atomic Time
2. UTC → Coordinated Universal Time

Important atomic time systems include

:satellite orbit

is the path followed by a satellite around the Earth under the influence of gravity and other forces.

Keplerian motion

describes ideal satellite motion under the assumption that the dominant force acting on the satellite is the Earth's gravitational attraction.

ellipse

The idealized orbit is an

1. Semi-major axis (a)
2. Eccentricity (e)
3. Inclination (i)
4. Right Ascension of the Ascending Node (Ω)
5. Argument of Perigee (ω)
6. True Anomaly (ν)

6 Keplerian orbital elements

semi-major axis

represented by a, describes the size of the orbit.

eccentricity

represented by e, describes the shape of the orbit.

0

e for a circular orbit

elongated

As eccentricity increases, the orbit becomes more

inclination

represented by i, describes the angle between the satellite's orbital plane and the reference plane, normally the Earth's equatorial plane in geocentric orbital descriptions.

Right Ascension of the Ascending Node

represented by Ω (capital omega), describes the orientation of the ascending node relative to a reference direction.

ascending node

is the point where the satellite crosses the reference plane while moving from the southern side toward the northern side.

argument of perigee

represented by ω (lowercase omega), describes the position of perigee within the orbital plane relative to the ascending node.

Perigee

is the point where the satellite is closest to Earth.

true anomaly

represented by ν (nu), describes the satellite's current position along its orbit relative to perigee.

ephemeris

contains information describing the position and/or orbit of a satellite as a function of time.

Almanac data

provide relatively general information about the orbit and satellite clock corrections.

Almanac data

Their main advantage is that they provide a general picture of the satellite constellation and can help receivers know which satellites should be available.

Broadcast ephemerides

are more accurate orbital and clock information transmitted by the GNSS satellites themselves.

Precise ephemerides

provide highly accurate satellite orbit information, they are provided by the International GNSS Service (IGS) and are freely available.

1. Almanac → km
2. Broadcast → m
3. Precise → cm

ephemerides accura