Satellite Ranging
Satellite ranging in Global Navigation Satellite Systems (GNSS) refers to the process of determining the distance between a GNSS receiver and a satellite in orbit. This distance is known as a “range” and is typically measured in meters. The receiver uses the signals transmitted by the satellite to determine the range, which can then be used to calculate the receiver’s position. To do this, the receiver measures the time it takes for the signal to travel from the satellite to the receiver and then applies the speed of light to calculate the distance. This process is known as “time of flight” measurement. Other methods also exist like phase-based ranging, it is more precise but require more processing power.
This information is then combined with the position of the satellite, as provided by the satellite’s onboard navigation system, to calculate the receiver’s position. GNSS systems such as GPS, GLONASS, Galileo, BeiDou and QZSS, uses this mechanism to calculate the position of the receiver.
Resection
Resection is a process used in GPS and GNSS (Global Navigation Satellite Systems) to determine the location of a receiver on the earth’s surface. It is a mathematical process that uses the known positions of at least three GPS satellites to calculate the position of the receiver.
The working principle of resection involves the receiver receiving signals from at least three GPS satellites. The receiver then uses the time delay of the signals from each satellite to determine the distance from the receiver to each satellite. This is known as the pseudorange. The receiver then uses the known positions of the satellites and the pseudoranges to calculate the position of the receiver using trigonometry and algebraic equations.
The accuracy of resection is dependent on the number of satellites used and the quality of the pseudoranges. The more satellites used and the better the pseudoranges, the more accurate the position of the receiver will be.
Resection is used in a variety of applications, such as surveying, navigation, and mapping. It is also used in emergency response, search and rescue, and other critical operations where accurate location information is needed.
Overall, resection is an important process in GPS and GNSS, allowing for the determination of a receiver’s location on the earth’s surface, making it a vital tool for navigation, mapping, and other critical operations.
Error Sources Atmospheric
1. Ionosphere and Troposphere Delays:
The ionosphere and troposphere can cause signal delays which can affect the accuracy of the GPS measurements. The ionosphere is a layer of charged particles in the upper atmosphere that can cause signal delays, while the troposphere is the lower atmospheric layer that can also cause signal delays due to its temperature and pressure variations.
2. Multipath:
Multipath occurs when the GPS signals reflect off of objects such as buildings or other structures before reaching the receiver. This can cause confusion for the receiver, resulting in errors in the measurement.
3. Satellite Clock Errors:
The GPS satellites are equipped with atomic clocks that are used to synchronize the signals sent to the receiver. However, these clocks are not perfect and can experience errors which can affect the accuracy of the measurements.
4. Receiver Noise:
The receiver itself can be a source of errors due to noise or other electronic interference. This can cause the receiver to misinterpret the signals or make errors in the measurement.
5. Ephemeris Errors:
The ephemeris is the data that is used to calculate the position of the satellites. If this data is incorrect, it can cause errors in the measurements.
6. Orbital errors:
The satellite orbits are calculated based on mathematical models, but the orbits can change over time due to various factors, such as solar radiation pressure, and cause errors in the measurements.
Ionospheric Errors
The ionosphere is a layer of charged particles in the Earth’s upper atmosphere that can cause errors in GPS signals as they pass through it. The ionosphere is constantly changing, which means that the errors caused by it can be unpredictable.
One way that ionospheric errors affect GPS signals is by delaying the signal. When the signal passes through the ionosphere, it can be slowed down by the charged particles, causing the signal to arrive at the receiver later than expected. This delay can cause the receiver to calculate a location that is further away from the true location.
Another way that ionospheric errors affect GPS signals is by causing the signal to be refracted or bent. This can cause the signal to be received from a different direction than expected, which can cause the receiver to calculate a location that is in the wrong direction.
To minimize ionospheric errors in GPS, the system uses a technique called dual-frequency measurement. This involves measuring the signals at two different frequencies, which can help to correct the effects of the ionosphere. Additionally, the GPS system uses ionosphere models and correction algorithms to account for the effects of the ionosphere on the signals.
Overall, the ionosphere can cause significant errors in GPS and GNSS systems, but the system uses various techniques to minimize these errors and provide accurate location information.
Multipath
Multipath refers to the phenomenon where GPS signals are reflected off of surfaces such as buildings, bridges, and other man-made structures before reaching the receiver. This causes the receiver to receive multiple copies of the same signal, leading to errors in position determination.
Multipath can cause several issues in GPS and GNSS systems, including:
- Signal delay: When multipath signals reach the receiver, they are delayed by the time it takes for the signal to travel to the reflecting surface and back. This delay can cause errors in the receiver’s position calculation.
- Signal weakening: Multipath signals are often weaker than direct signals, causing the receiver to lose lock on the satellite and leading to a loss of positioning information.
- Signal interference: Multiple copies of the same signal can cause interference, leading to errors in the receiver’s position calculation.
To mitigate the effects of multipath, GNSS receivers use various techniques such as carrier phase measurement, multi-antenna arrays, and adaptive filtering to filter out or cancel out the multipath signals. Additionally, proper antenna placement and shielding can also help to reduce the effects of multipath.
Selective Availability
Selective availability (SA) is a feature in the Global Positioning System (GPS) that was implemented by the U.S. government to limit the accuracy of civilian GPS signals for national security reasons. SA works by introducing errors into the GPS signals, which makes the location information less accurate for civilian users.
The purpose of SA was to prevent hostile forces from using GPS to target weapons or navigate in enemy territory. However, it also had the unintended consequence of limiting the accuracy of civilian GPS signals, which hindered the use of GPS for navigation, mapping, and other civilian applications.
In 2000, the U.S. government turned off SA as part of a modernization effort to improve the accuracy of GPS signals for civilian users. Today, GPS signals are freely available to civilians and have an accuracy of around 5-10 meters. However, certain military and government users still have access to a higher accuracy signal known as Precise Positioning Service (PPS).
Antispoofing Error Rectification
Antispoofing error rectification in GPS and other Global Navigation Satellite Systems (GNSS) involves the use of various techniques to detect and prevent false or manipulated signals from being used to disrupt or deceive the system. This can include the use of cryptographic methods to authenticate signals, filtering algorithms to identify and reject anomalous signals, and multi-frequency measurements to improve accuracy and integrity. Additionally, GPS and GNSS systems may use advanced navigation algorithms and error correction techniques to minimize the impact of any spoofed signals that are able to bypass the system’s defenses. Overall, the goal of antispoofing error rectification is to ensure that the GPS and GNSS systems are able to provide accurate and reliable navigation information at all times, even in the presence of malicious interference.
Atmospheric and Ionospheric Models
Atmospheric and ionospheric models are important components in the working principles of GPS and other Global Navigation Satellite Systems (GNSS). These models are used to predict and correct for the effects of the Earth’s atmosphere and ionosphere on the signals received from satellites.
The atmosphere, which is made up of various gases and particles, can cause the signals from satellites to be delayed or refracted as they pass through it. This can cause errors in the GPS
receiver’s measurements of the satellite’s position, and can also affect the accuracy of the receiver’s clock.
To correct for these errors, atmospheric models are used to predict the amount of delay or refraction caused by the atmosphere at different locations and times. These predictions can then be used to correct the measurements made by the GPS receiver.
The ionosphere, which is a layer of the upper atmosphere that is ionized by solar radiation, can also cause errors in GPS measurements. This is because the ionosphere can bend or refract the signals from satellites, causing them to arrive at the receiver at different times than expected.
To correct for these errors, ionospheric models are used to predict the amount of refraction caused by the ionosphere at different locations and times. These predictions can then be used to correct the measurements made by the GPS receiver.
In summary, atmospheric and ionospheric models are an important component in the working principles of GPS and other GNSS systems, as they help to predict and correct for the effects of the Earth’s atmosphere and ionosphere on satellite signals, ensuring the accurate positioning and navigation provided by GPS.
Choke Ring
A choke ring is a type of antenna element that is used in some GPS receiver systems. It is also called a “choke ring antenna” or simply a “choke ring.”
The choke ring is a circular shaped antenna that surrounds the GPS receiver module and it is designed to reduce the amount of interference that can be caused by other electronic devices. The choke ring is typically made of a ferrite material, which acts as a “choke” to block or absorb any unwanted electromagnetic interference (EMI) that might be present in the area.
When the GPS receiver is in operation, it sends out a weak signal that is received by the GPS satellites in orbit. The satellites then send back a stronger signal, which is received by the GPS receiver. The choke ring helps to ensure that the GPS receiver is only able to receive signals from the GPS satellites, and not from any other electronic devices that might be nearby.
There are many different types of choke ring designs that can be used in GPS receiver systems, each with its own unique properties and performance characteristics. The choice of which choke ring to use will depend on the specific application and the type of interference that is expected to be present in the environment.
In summary, a Choke ring antenna is a device that helps GPS Receiver to filter out any unnecessary electromagnetic signals that might disturb the receiver from receiving an accurate signal from GPS Satellites.
Differentially Corrected Positions
Differentially corrected positions in GPS refer to a method of improving the accuracy of GPS positions by using a reference station, also known as a base station, to calculate and correct errors in the GPS signals.
This method involves comparing the signals received by the base station with the signals received by the mobile receiver. The base station’s known location is used to calculate the
errors in the GPS signals, which are then transmitted to the mobile receiver. The mobile receiver can then use this information to correct its own position, resulting in a more accurate location.
This method is particularly useful in areas where the GPS signals are weak or obstructed, such as in urban areas or near tall buildings. It is also commonly used in surveying and mapping applications, where high precision is required.
Overall, the use of differential correction in GPS allows for a more accurate and reliable navigation experience in the global navigation satellite system
Positioning Techniques
1. Triangulation:
Triangulation is a technique used in GPS to determine the position of a device by measuring the distance between it and multiple satellites. The device uses the distances to calculate its position on a map using trigonometry.
2. Doppler shift:
The Doppler shift technique uses the change in frequency of a signal as it travels to and from a satellite to determine the device’s position. This technique is based on the fact that the frequency of a signal will change as it travels toward or away from the satellite.
3. Time of Arrival (TOA):
This technique measures the time it takes for a signal to travel from the satellite to the device, and uses this information to calculate the device’s position. The device uses the time of arrival to determine the distance from the satellite, which it then uses to calculate its position on a map.
4. Angle of Arrival (AOA):
AOA is a technique that uses the angle of a signal as it arrives at the device to determine its position. The device uses the angle of arrival to determine the distance from the satellite, which it then uses to calculate its position on a map.
5. Assisted GPS (A-GPS):
A-GPS is a technique that uses information from nearby cell towers and other sources to speed up the process of acquiring a GPS signal. This technique is especially useful in urban areas or other areas where the satellite signal is weak.
6. Real-Time Kinematic (RTK):
RTK is a technique that uses a real-time correction signal to improve the accuracy of GPS measurements. This technique is typically used for surveys or other high-precision applications where accuracy is critical.
Precise Point Positioning
Precise point positioning (PPP) is a technique used in the Global Navigation Satellite System (GNSS) to determine the precise location of a receiver on the Earth’s surface. This technique relies on the use of multiple satellite signals and advanced algorithms to correct for errors and provide highly accurate location data.
The basic principle behind PPP is the measurement of the pseudo-range, or the distance between the receiver and the satellite, using the time of arrival (TOA) of the signal. The pseudo-range is measured by the receiver’s GPS receiver clock, which is compared to the satellite’s atomic clock to determine the distance.
To improve the accuracy of the position, PPP uses a combination of data from multiple satellites and advanced algorithms to correct errors caused by ionospheric and tropospheric delays, satellite clock errors, and other factors. The receiver also uses precise ephemeris data, which is information about the precise location and motion of the satellites, to calculate the position.
In addition, PPP also utilizes a technique known as carrier-phase positioning, which uses the phase of the satellite signal to determine the position with even greater precision. This is done by measuring the phase of the signal at the receiver and comparing it to the phase at the satellite, which is known to be extremely accurate due to the use of atomic clocks.
Overall, the working principle of PPP in the GNSS is to use a combination of multiple satellite signals, advanced algorithms, and precise ephemeris data to determine the precise location of a receiver on the Earth’s surface with high accuracy.
Satellite Geometry
Satellite geometry is an important aspect of the working principles of GPS (Global Positioning System) in a global navigation satellite system. The GPS system relies on a network of satellites orbiting the Earth to provide location information to users on the ground. The satellites are arranged in a specific geometry that allows for accurate positioning and navigation.
The GPS system uses a minimum of 24 satellites in orbit around the Earth at an altitude of 20,200 km. The satellites are arranged in six orbital planes, each containing four satellites. The satellites are positioned at an inclination of 55 degrees, which means they are tilted 55 degrees from the Earth’s equatorial plane.
The satellites orbit the Earth every 12 hours, moving at a speed of approximately 14,000 km/h. As they orbit, they emit a continuous signal that includes their precise location and the current time. The signals are received by GPS receivers on the ground, which use the information to determine the user’s location.
The satellite geometry in GPS allows for multiple satellites to be in view at any given time, providing a clear signal and allowing for accurate positioning. The geometry also ensures that there are always at least four satellites in view for the GPS receiver to determine the user’s location.
Overall, the satellite geometry in GPS is an essential aspect of the system’s working principles, allowing for accurate navigation and location information to be provided to users on the ground.
Mask and Azimuth Angles
The mask angle is the angle between the horizon and the lowest point at which a GPS satellite can be tracked by a receiver. This angle is affected by factors such as the receiver’s location and the satellite’s elevation. The mask angle is important for determining the minimum elevation at which a satellite can be tracked and used for navigation.
The azimuth angle is the angle between the true north and the satellite as seen from the receiver. This angle is important for determining the position of the satellite relative to the receiver and is used in calculating the receiver’s position. The azimuth angle can be determined by measuring the angle between the satellite and the horizon, or by measuring the angle between the satellite and a known reference point, such as a nearby landmark.
Both the mask and azimuth angles play important roles in the working principles of GPS in the global navigation satellite system, as they are used to calculate the receiver’s position and track the satellites for navigation.
Differential Global Navigation Satellite System (DGNSS) is a technology that uses a network of ground-based reference stations to improve the accuracy of GPS signals. The reference stations receive the same GPS signals as a user’s receiver and then use the information to calculate the errors in the GPS signals. This information is then transmitted to the user’s receiver, which can use it to correct the errors in its own GPS signals. This allows for a more accurate location determination. DGNSS is commonly used in surveying, navigation, and other applications where high accuracy is required.
Satellite Based Augmentation System (SBAS)
The Satellite Based Augmentation System (SBAS) is a technology that enhances the accuracy, integrity, and availability of GPS signals for users on the ground. It works by receiving signals from a network of reference stations on the ground and a geostationary satellite and then uses this data to generate correction messages that are transmitted to users via the GPS satellites. These correction messages are designed to correct for various errors that can affect GPS signals, such as atmospheric effects, clock errors, and satellite orbit errors.
SBAS uses a variety of techniques to improve the accuracy of GPS signals, including differential correction, integrity monitoring, and precise orbit determination. Differential correction involves comparing the GPS signals received at a reference station with the known location of that station and then broadcasting the difference as a correction message. Integrity monitoring involves monitoring the GPS signals for any potential errors or anomalies and providing an alert to users if a problem is detected. Precise orbit determination involves using data from multiple sources to accurately determine the position of the GPS satellites, which helps to improve the accuracy of the signals.
Overall, SBAS plays a crucial role in ensuring the reliability and accuracy of GPS signals for a wide range of applications, including transportation, navigation, surveying, and mapping. It is a vital component of the global navigation satellite system and helps to ensure that users can navigate with confidence and precision.