3.4.1. Technical terms in GNSS

Latitude: It is an angular measurement made from the center of the Earth to north or south of the equator. It comprises the north/south component of the latitude/longitude coordinate system, which is used in GNSS data collection.

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.

Think of latitude like a horizontal line in a city. If the Burger restaurant is located at 30 degrees north, you can visualize it as being aligned with other places that have the same north/south position on the map.

Longitude: It is an angular measurement made from the center of the Earth to the east or west of the Greenwich meridian. It comprises the east/west component of the latitude/longitude coordinate system, which is used in GNSS data collection.

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.

You can think of longitude as the vertical lines on a map, similar to the lines on a checkerboard. If a car is at 45 degrees west longitude, it’s directly in line with other locations that also share that same westward position.

Datum: The GNSS receivers are designed to collect positions relative to the WGS-84 datum, however the user has the option of defining the datum in which the data will be collected.

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.

Imagine you’re working on a giant puzzle. The datum is like the puzzle's border. If you have the wrong border, even if the pieces fit together, they won't represent the actual picture.

Ellipsoid: An ellipsoid is the 3-D shape that is used as the basis for mathematical modelling of Earth's surface. The ellipsoid is defined by the lengths of minor axes (polar axis) and major axes (equatorial axis).

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.

You can visualize an ellipsoid like a football. It’s not perfectly round like a basketball; instead, it’s slightly flattened at the poles and bulges in the middle, resembling how the Earth is shaped.

Geoid: A mathematical surface of constant gravitational potential that approximates the sea level, or the equipotential surface of the Earth's gravity field which best fits, in a least squares sense, global mean sea level.

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.

You can think of the geoid as an invisible, undulating sea level that reflects the uneven distribution of mass within the Earth. Imagine placing a large, soft rubber sheet over a bumpy landscape; the bumpy areas represent the uneven gravitational forces affecting the shape of the geoid.

Satellite constellation: The arrangement in space of a set of satellites.

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.

Think of a satellite constellation like a group of friends using their phones to coordinate a meet-up. Each friend (satellite) needs to be in contact with one another and the host (receiver) to ensure everyone knows the exact time and place for the gathering.

NAVSTAR (NAVigation Satellite Timing And Ranging) System): The formal name given to the United States NAVigation Satellite Timing And Ranging (NAVSTAR) System, which comprises of GPS satellites, monitoring stations, and master control station.

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.

You can liken the NAVSTAR system to a school system where the satellites are the teachers providing information (like lessons) to the students (the GPS devices), while the control station works as the principal ensuring that everything runs smoothly.

Ephemeris: The current satellite position predictions that are transmitted from a GNSS satellite in the NAVDATA message.

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.

Think of ephemeris as the GPS version of a flight schedule. Just as airlines update schedules to reflect the real-time position of planes, satellites continuously send updates about their positions to ensure accurate navigation.

Rover (GNSS) Receiver: Any mobile GNSS receiver and data collector used for determining location in the field. A roving GNSS position can be differentially corrected relative to a stationary base GNSS receiver.

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.

Imagine a chef using a spoon (the rover) to taste a dish while following a recipe (the satellites). The chef can adjust flavors based on what the recipe suggests and ensures every ingredient is perfectly balanced.

L1 Frequency: The primary L-band carrier used by GNSS satellites to transmit satellite data in frequency 1575.42 MHz. It is modulated by C/A code, P-code and a 50 bit/second navigation message. L2 Frequency: The secondary L-band carrier used by GPS satellites to transmit satellite data in frequency 1227.6 MHz. It is modulated by P-code and a 50 bit/second navigation message. L5 Frequency: The L5 signal operates at 1176.45 MHz which is planned to be used to improve accuracy for civilian use, such as aircraft precision approach guidance.

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.

Think of these frequencies as different lanes on a multi-lane highway. Each lane allows for a different type of vehicle or data to travel without interference, helping everything run smoothly and efficiently.

Epoch: The measurement interval of a GNSS receiver.

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.

You can think of an epoch as a snapshot on your phone. Just as you capture a moment in time to remember it later, a GNSS receiver captures its location at specific epochs to track movement accurately.

Dual-frequency (GNSS) Receiver: A type of GNSS receiver that uses both L1 and L2 signals from satellites. A dual-frequency GNSS receiver can compute more precise position fixes over longer distances and under more adverse conditions by compensating for ionospheric delays.

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.

Consider a dual-frequency receiver like a well-equipped boat navigating through a storm. It uses two routes (Frequencies) to ensure it stays on course and accurately reaches its destination despite turbulent conditions.

Multi-constellation and Multi-frequency (GNSS) Receiver: Multi-frequency GNSS receivers are used for reliable positioning down to the centimeter level. Among various professional-grade GNSS receivers, there is variability in the satellite constellations and signals that a receiver can access.

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.

You can relate this technology to our ability to use multiple navigation apps on our phones at once to find the fastest route—if one app isn't accurate due to obstacles, another might provide a clearer path!

Data Message: A message included in the GNSS signal, which reports a satellite location, clock correction, and health. It also includes information on other satellites’ health and their approximate positions.

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.

Think of a data message similar to a status update from your friend. If your friend is in good health (satellite health) and gives you their current location, you’ll have a much clearer understanding of where to meet them later.

Pseudorandom Noise or Number (PRN): A signal that carries a code that appears to be randomly distributed, like noise, but can be exactly reproduced.

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.

Imagine a broadcasting station sending unique radio frequencies to different receivers. Just as each station has its own frequency (like a PRN), allowing listeners to distinguish who is broadcasting, GNSS uses PRNs to identify which satellite a signal is coming from.

Carrier Phase: The difference between the carrier signal generated by the internal oscillator of a roving GNSS receiver and the carrier signal emitted from a particular GNSS satellite.

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.

This concept is similar to tuning a musical instrument. When you perfectly tune an instrument, the phases of the sounds align harmoniously, just like the receiver aligns the signals from the satellites for precise measurements.

Coarse or Acquisition (C/A) Code: A pseudorandom noise (PRN) code modulated onto a L1 signal which helps the GNSS receiver to compute the distance from each satellite.

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.

Think of the C/A code as the opening instructions at a theme park; it guides visitors on how to find the best entry point and start their adventure smoothly, just like how this code helps receivers begin their process of finding positions.

P-Code: The precise code transmitted by the GNSS satellites. Each satellite has a unique code that is modulated onto both the L1 and L2 carrier.

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.

You can think of the P-Code like a secret passcode to a VIP area—it allows access to precisely tailored experiences as opposed to the general entry that everyone shares.

Selective Availability (SA): The artificial and deliberate degradation of GPS satellite signals by the United States Department of Defense.

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.

You can think of Selective Availability like a teacher intentionally giving some students harder math problems than others, restricting the easier versions for a more advanced group to maintain challenge and focus.

Signal-to-Noise Ratio (SNR): The signal strength of a satellite is a measure of the information content of the signal relative to the signal’s noise.

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.

Think of SNR like listening to music on a radio. If you have high signal strength (a clearly tuned station) and low noise (no static), you enjoy the music clearly. Conversely, if there's static (noise), the quality drops, making it difficult to enjoy the tunes.

Base Station: A base station is comprised of a GNSS antenna and GNSS receiver positioned at a known location specifically to collect data for differential correction.

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.

You can think of a base station as a traffic cop at a busy intersection—just as the cop monitors the flow of traffic and corrects it if there are any issues to ensure smooth travel, a base station corrects GNSS data.

VRS (Virtual Reference Station): A VRS system consists of GNSS hardware, software, and communication links. It uses data from a network of base stations to provide corrections to each rover that are more accurate than the corrections from a single base station.

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.

Imagine if you were trying to figure out the best route to a destination and instead of just one map, you received input from many different travelers simultaneously. This is how a VRS improves accuracy by integrating multiple sources of information.

Initialization refers to the procedure of telling a GNSS receiver where it is, when it is turned on for the first time.

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.

Think of initializing a GNSS receiver like entering your starting point into a GPS navigation app before a road trip. It ensures the application knows where you are located to give directions from that point onward.

Differential Correction: The process of correcting GNSS data collected on a rover with data collected simultaneously at a base station.

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.

Imagine a teacher providing extra hints to students who are taking a test based on common mistakes from previous test answers. This feedback helps students correct their misunderstandings while they work.

RTK (Real-Time Kinematic): A real-time differential GNSS method that uses carrier phase measurements for greater accuracy.

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.

Think of RTK navigation as a professional archer aiming for a bullseye. Just as the archer uses precision techniques to ensure each shot is accurate, RTK employs advanced measurements to make pinpointing locations extremely precise.

Accuracy: The degree of conformity with a standard or accepted value. Accuracy relates to the quality of the result, and is distinguished from precision which relates to the quality of the operation by which the result is obtained.

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.

Consider accuracy as hitting the correct target in darts (the bullseye), while precision would be how consistently your darts land in the same area, even if it’s not the target.

Precision: A measure of the repeatability or uniformity of a measurement.

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.

Think of precision like a sports player practicing shots. If the player consistently makes a basket from the same spot, they demonstrate precision, even if that spot is not the intended target (like being accurate).

Multipath Errors: Errors caused by reflected signals arriving at the GNSS receiver, as a result of nearby structures or other reflective surfaces.

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.

Imagine you're in a canyon shouting towards a friend. If their voice echoes back to you brightly, you might misjudge how far away they actually are due to the delay—the same principle applies to multipath errors with GNSS signals.

Atmospheric Delays: The satellite signal slows down as it passes through the atmosphere. The GNSS system uses a model that calculates an average amount of delay to correct for this type of error.

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.

You can think of atmospheric delays like a sprinter running through a thick crowd. Just as their progress is slowed down by people in the way, the satellite signals experience the same slowdown as they move through varying layers of the atmosphere.

Receiver Clock Errors: A receiver's built-in clock is not as accurate as the atomic clocks onboard the GNSS satellites, therefore, it may have very slight timing errors.

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.

Imagine a watch that runs a bit slower than time. If you're relying on it to meet a friend, you might arrive late or miss them because your estimate of time is incorrect—this is how receiver clock errors can impact GNSS accuracy.

Orbital Errors: Also known as ephemeris errors, these are inaccuracies of the satellite's reported location.

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.

You can relate orbital errors to a sports player missing their target because they were not aware that the goalposts shifted slightly after the game started. Just like that player, GNSS measurements can be thrown off if the satellite position isn't perfectly accurate.

Dilution of Precision (DOP): It is an indication of the quality of the results that can be expected from a GNSS point position.

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.

Think of DOP as a camera's focus. A clear shot with good depth of field (low DOP) results in sharper details, whereas a blurry image taken from the same spot but with misaligned focus (high DOP) can obscure important features.

Geometric Dilution of Precision (GDOP): It deals with the overall accuracy, 3D-coordinates and time.

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.

GDOP can be imagined as the degree of freedom you have while navigating through a crowded room; if you have lots of space and direct sight lines (lower GDOP), it’s easier to reach your destination than if you're boxed in by obstacles (higher GDOP).

Positional Dilution of Precision (PDOP): It deals with the position accuracy, and 3D-coordinates.

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.

Imagine trying to pinpoint your friend's location at a park. If your friend is far away (lower PDOP), it's easier to see them and point accurately. If they're close and surrounded by trees (higher PDOP), it’s much more challenging to accurately locate them.

Horizontal Dilution of Precision (HDOP): It deals with the horizontal accuracy, and 2D-coordinates.

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.

Think of HDOP like lining up a shot in basketball. When you are clear and spaced from other players (lower HDOP), you can shoot accurately to the hoop. If players crowd around you (higher HDOP), your ability to see the basket and aim accurately diminishes.

Vertical Dilution of Precision (VDOP): It deals with the vertical accuracy, and height.

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.

You can picture VDOP as a tree climber. When the climber has clear access to the full height of the tree (low VDOP), they can accurately gauge their elevation, whereas branches and leaves obscuring their view (high VDOP) make it hard to estimate how high they actually are.

Post-processing of Data: The processing of satellite data done after it has been collected in order to eliminate the errors.

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.

Imagine developing photographs after they are taken. Just like you might enhance brightness and clarity of an image in post-editing, GNSS data is refined post-collection to provide the clearest, most accurate location information.

SBAS (Satellite Based Augmentation System): This term refers to differential GNSS applied to a specific wide area, such as an entire continent.

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.

Think of SBAS as a nationwide weather service. Just as the service provides updates to everyone across the country to ensure safety and performance in travel or events, SBAS serves an entire region by enhancing the accuracy of GNSS data for users.

--