3.4.1 - Technical terms in GNSS

Latitude is measured in degrees, with the equator at 0 degrees, the North Pole at 90 degrees north, and the South Pole at 90 degrees south. For example, if you are at 30 degrees north latitude, you are located 30 degrees north of the equator. In GNSS, latitude is crucial for determining your exact position on the Earth's surface, especially when combined with longitude.

Longitude is likewise measured in degrees, where the Prime Meridian (0 degrees longitude) runs through Greenwich, England. It ranges from 0 degrees at the Prime Meridian to 180 degrees east or west. Knowing your longitude is essential in GNSS as it helps pinpoint your location in relation to other places on the globe.

A datum is a framework that allows for the accurate definition of locations on the Earth's surface. The WGS-84 datum is a global standard, meaning that when GNSS receivers use this reference system, locations can be recognized universally across various devices and maps. This is significant because different datums can yield different coordinates for the same physical location due to variations in projection methods.

An ellipsoid is a mathematically simplified shape of the Earth that provides a useful representation of its surface. Unlike a perfect sphere, an ellipsoid accounts for the bulging at the equator due to Earth’s rotation, making it a more accurate model for tasks involving distances and areas across the surface. The major axis runs through the equator, and the minor axis runs through the poles.

The geoid represents how the Earth’s gravity affects the surface, effectively showing what sea level would be if it were influenced only by gravity and rotation—without the additional effects of tides, winds, and other factors. It provides a reference for understanding elevation and depth when working with GNSS.

A satellite constellation refers to a group of satellites that work together to provide coverage over a specific area on Earth. For GPS, a minimum of four satellites needs to be visible at any point for accurate positioning. These satellites are strategically communicated with each other and the GNSS receivers to ensure data accuracy and availability.

NAVSTAR is the technical name used for the system of satellites that make up GPS. This system includes the satellites themselves, various ground stations that help monitor their function, and a master control station that oversees the entire operation of the GPS network. It is essential for ensuring that the data returned to users is accurate and reliable.

Ephemeris data contains the precise positions of the satellites at specific times and helps the GNSS receiver calculate the location and timing for accurate positioning. This data is frequently updated to reflect the dynamic nature of satellite motion.

A rover receiver is the device that users carry to determine their real-time location using GNSS signals. By communicating with stationary base receivers, these rovers can provide more accurate positioning by correcting data transmitted from satellites for any errors detected.

These frequencies represent the different bands utilized by GNSS satellites for transmitting various types of data. The primary L1 band is utilized extensively for civilian applications, while L2 serves supplemental purposes usually for military usage. L5 aims to enhance accuracy for critical applications, particularly in aviation.

An epoch refers to a specific time frame or measurement snapshot captured by the GNSS receiver. This concept is critical because it allows the device to calculate and correct for any movement or changes in its position at different intervals, ensuring that updates remain consistent and accurate.

A dual-frequency GNSS receiver leverages signals from both L1 and L2 bands to enhance positional accuracy, particularly in challenging atmospheric conditions. This dual reception helps to reduce interference and errors caused by variability in the ionosphere's influence on signals.

These advanced receivers can connect to multiple satellite systems (like GPS, Galileo, etc.) at different frequency bands, greatly improving their positioning capabilities and accuracy, even in obstructive environments. This multi-constellation approach allows for redundancy and increased reliability.

A data message contains critical information about satellite positions and health status. The GNSS receivers utilize this data to ascertain the reliability of their positional calculations and correct them as needed, ensuring accuracy.

PRN codes are unique identifiers for each satellite, allowing receivers to distinguish between signals from different satellites. This uniqueness ensures accuracy in position calculations, as each satellite's signal can be correctly assigned and averaged.

Carrier phase measurements have a higher accuracy level than pseudo-ranges provided by standard GNSS signals, allowing for precise positioning. By measuring the phase differences, receivers can achieve measurements with an accuracy of several millimeters.

The C/A code serves as the primary signal for civilian GPS users to obtain initial positioning. This code helps GNSS receivers to calculate the distances effectively, allowing them to establish their locations through trilateration.

P-Code is designed for military applications, providing a higher level of accuracy for operations that require precise positioning. Unlike the C/A code, the P-code is more complex and intended for users that need detailed, secure navigation assistance.

Selective Availability was originally employed to limit the accuracy of civilian GPS signals for national security reasons. However, it was turned off in 2000. The intentional degradation would lead to error margins ranging from 30 to 100 meters for civilian users.

SNR describes the quality of the satellite signal received by GNSS devices. It influences how accurately the receiver can calculate its position—the higher the SNR, the less noise interferes with the signal, leading to better accuracy.

Base stations play a crucial role in improving the accuracy of GNSS readings. By having a fixed position, they can detect the errors in GNSS signals and assist in correcting the data received by roving receivers in real-time or during post-processing.

A VRS system enhances positional accuracy by aggregating data from multiple base stations instead of relying on a single reference point. It creates a virtual correction based on the information from the network, allowing users to enjoy higher precision in their location data.

Initialization sets the starting point for a GNSS receiver to determine its location accurately. Users provide the device with their approximate position and time, allowing it to align with satellites effectively and start delivering accurate data.

Differential correction is a technique where data from a stationary base station (with a known location) is used to enhance the accuracy of data collected by roving receivers. By comparing the observed errors, accurate corrections can be applied in real-time or post-processing.

RTK is a method of GNSS positioning that provides high accuracy (up to a centimeter) by utilizing carrier phase measurements in real-time. This technique is crucial for applications requiring precise positioning, such as in surveying and robotic guidance.

Accuracy refers to how close a measured value is to the actual or true value. In GNSS, the accuracy of location data is essential for many applications, meaning it must meet specific standards to be useful. It is different from precision, which relates to the consistency of results obtained from repeated measurements.

Precision reflects how consistently a GNSS receiver can produce similar results when measuring the same location multiple times. High precision implies that repeated measurements yield similar values, while low precision indicates greater variability.

Multipath errors occur when GNSS signals bounce off surfaces such as buildings or rocks before reaching the receiver. This reflected signal can lead to inaccurate positioning since it takes a longer and indirect path to the GNSS unit, altering the perceived location.

As GNSS signals travel through Earth’s atmosphere, they are delayed, which can affect accuracy. Atmospheric delays can be caused by different atmospheric conditions such as moisture and temperature variations, and GNSS systems typically correct for these delays to maintain accuracy.

Receiver clock errors arise because the internal clocks of GNSS receivers are less precise than the atomic clocks aboard satellites. Even small timing differences can lead to significant errors in position measurements, making clock synchronization critical for accuracy.

Orbital errors occur if the satellite's exact location differs from what is reported. These discrepancies can arise from gravitational influences, atmospheric drag, or other changes affecting a satellite's path, and can lead to errors in positioning measurements.

DOP is a measure based on the relative geometry of satellite positioning at any given time. A low DOP value indicates better positioning accuracy due to a good satellite configuration, while a high DOP value implies less accuracy due to satellites being in less optimal positions.

GDOP provides a comprehensive view of position accuracy based on satellite arrangement in three-dimensional space, indicating how precise the calculations can be for location data at any given moment. A lower GDOP value signifies higher accuracy.

PDOP focuses specifically on horizontal and vertical position accuracy based on satellite distribution. A low PDOP indicates higher accuracy, as it suggests that satellites are widely spaced, providing a clear line of sight for calculations.

HDOP measures the accuracy of position data in the horizontal plane, influencing how precise 2D locations can be determined. Lower HDOP values indicate better accuracy for horizontal positioning.

VDOP assesses how accurately vertical positions can be determined and is important for applications that require elevation data. A low VDOP indicates that signals received offer reliable vertical position accuracy.

Post-processing involves using sophisticated software after data collection to refine and improve the accuracy of the positioning results. By applying corrections based on known data from base stations, positions can be adjusted to meet higher accuracy standards.

SBAS includes a network of ground reference stations that relay correction information to GNSS satellites, effectively improving the accuracy of positional data across broader areas. Systems such as WAAS and EGNOS are examples of SBAS networks enhancing GNSS operations.