GNSS, GPS, and Georeferencing Vocabulary

Vocabulary flashcards covering Global Navigation Satellite Systems, GPS architecture, ranging calculations, sources of positioning error, georeferencing, geometric transformations, and orthorectification.

Global Navigation Satellite System (GNSS)

A generic term describing any satellite constellation that provides locational services, including NAVSTAR GPS (USA), GLONASS (Russia), Galileo (EU), and BeiDou (China).

NAVSTAR GPS

The Global Positioning System maintained by the United States Department of Defense, using satellites to accurately measure 3-D coordinates anywhere on Earth.

GPS Space Segment

The constellation of 27 operating sun-synchronous satellites (24 required for full operational capability) arranged across 6 orbital planes with at least 4 satellite vehicles per plane, orbiting at an altitude of approximately $20,200\,m$ and completing one revolution every $12\,hours$.

GPS Control Segment

The system segment owned and operated by the U.S. Department of Defense that tracks satellites, monitors navigation messages, checks health, and broadcasts adjustments, managed from a Master Control Station at Falcon Air Force Base in Colorado Springs, CO, and five global monitoring stations.

GPS User Segment

The collection of GPS receivers and user applications, including target finding, aviation, vehicle tracking, marine navigation, surveying, emergency services, agriculture, recreation, and GIS data capture.

GPS Ranging Formula

The mathematical relationship used to calculate distance from a satellite: $D = v \times (t_2 - t_1)$, where $v$ is the speed of light ($186,282.397\,mile/s$), $t_1$ is transmission time, and $t_2$ is reception time.

Time Difference of Arrival (TDOA)

The difference between signal transmission time and reception time, expressed as $(t_2 - t_1)$, used to calculate range distance in satellite navigation.

Pseudorandom Code

A component of the GPS signal broadcast on the L1 frequency that uniquely identifies which satellite is transmitting the signal.

Ephemeris Data

The portion of a GPS satellite broadcast signal that informs the receiver of the expected orbital position of that satellite throughout the day.

Almanac Data

The part of the GPS signal containing current date, time, and health status for all satellites, essential for receivers to determine positions.

Trilateration

A mathematical technique used by GPS receivers to determine user position, speed, and elevation by calculating intersecting distance spheres from visible satellites.

Minimum Satellites for 3D Positioning

A minimum of $4$ satellites is required by a GPS receiver to calculate 3-D position coordinates $(X, Y, Z)$ and solve for clock timing error.

Differential GPS (DGPS)

A positioning method that uses a stationary reference receiver at a known position to calculate signal errors and transmit real-time differential corrections to roving receivers.

Ephemeris Error

A positioning error caused by the difference between a GPS satellite's predicted orbital location and its actual orbital location.

Dilution of Precision (DOP)

An indicator of receiver position measurement accuracy based on satellite constellation geometry, where lower values (between $1$ and $6$) indicate stronger geometry and higher precision.

Multipath Error

An error occurring when a GPS signal reflects off surrounding obstacles, such as buildings or terrain, before reaching the receiver antenna.

Georeferencing

The process of aligning an unreferenced spatial dataset with another dataset that already possesses spatial reference information.

Geometric Transformation

The process of converting a digital map or raster from one coordinate system to another using control points and mathematical transformation equations.

Control Points

Locations on a map or image with known spatial coordinates used to align and reference another dataset that spatially coincides with them.

Equiarea Transformation

A congruence transformation that allows rotation of an object while preserving both its original shape and size.

Similarity Transformation

A transformation method that allows rotation and scaling of an object while preserving its shape, but not its original size.

Affine Transformation

A first-order polynomial transformation that allows rotation, translation, skew, and differential scaling, preserving line parallelism while permitting angular distortion.

Projective Transformation

A geometric transformation method that allows both angular and length distortion, enabling a rectangular shape to transform into an irregular quadrilateral.

Polynomial Transformations

Transformation methods where second-order and third-order equations bend or curve raster datasets to match control points, requiring more links as polynomial degree increases.

Root Mean Square (RMS) Error

A statistical measure of spatial alignment accuracy computed as $\text{RMS Error} = \sqrt{\frac{\sum (\text{Residual})^2}{n}}$, where lower values signify closer agreement.

Orthorectification

A process for correcting aerial and satellite imagery to eliminate distortions caused by viewing angle, sensor geometry, and terrain displacement using elevation data.