What Is Used To Give An Absolute Location

The quest to pinpoint our exact position on Earth has driven innovation for centuries, resulting in a variety of sophisticated techniques that rely on the principles of mathematics, physics, and technology. An absolute location, in its simplest form, is a precise coordinate that uniquely identifies a place on the Earth's surface. Unlike relative location, which describes a position in relation to other landmarks or features, absolute location provides a definitive address.

The Foundation: Geographic Coordinate Systems

At the heart of determining absolute location lies the geographic coordinate system (GCS). This system employs a spherical grid defined by two angles: latitude and longitude.

• Latitude: Measured in degrees north or south of the Equator, latitude lines (also called parallels) run horizontally around the Earth. The Equator is 0° latitude, while the North and South Poles are 90° North and 90° South, respectively. Each degree of latitude is approximately 69 miles (111 kilometers).
• Longitude: Measured in degrees east or west of the Prime Meridian, longitude lines (also called meridians) run vertically from pole to pole. The Prime Meridian, which passes through Greenwich, England, is 0° longitude. Longitude ranges from 0° to 180° East and 0° to 180° West. Unlike latitude lines which are parallel, longitude lines converge at the poles. This convergence means that the distance represented by one degree of longitude varies depending on the latitude. It is widest at the Equator and narrows to zero at the poles.

The intersection of a specific latitude and longitude creates a unique point on the Earth's surface, defining its absolute location. These coordinates can be expressed in various formats:

• Degrees, Minutes, Seconds (DMS): For example, 40°26'46" N, 79°58'56" W
• Degrees, Decimal Minutes (DMM): For example, 40°26.767' N, 79°58.933' W
• Decimal Degrees (DD): For example, 40.446° N, -79.982°

While GCS provides the framework, accurately determining these coordinates requires specific tools and techniques.

Tools and Technologies for Determining Absolute Location

Several tools and technologies have been developed over time to pinpoint absolute location with increasing accuracy:

1. Celestial Navigation

One of the oldest methods of determining absolute location, celestial navigation relies on the observation of celestial bodies – primarily the sun, moon, and stars – to calculate latitude and longitude.

• Sextant: This instrument measures the angle between a celestial body (usually the sun at its highest point, or a specific star) and the horizon. By knowing the time of observation, the date, and the angle, navigators can calculate their latitude.
• Chronometer: Accurate timekeeping is crucial for determining longitude using celestial navigation. A chronometer is a highly precise clock that maintains accurate time even at sea, allowing navigators to compare local time with the time at a known reference point (usually Greenwich). The difference in time is directly related to the difference in longitude. One hour difference equates to 15 degrees of longitude.
• Ephemeris: This table provides the predicted positions of celestial bodies for a given date and time. Navigators use an ephemeris to compare their observations with the expected positions of celestial bodies, enabling them to calculate their location.

Celestial navigation was the primary method of navigation for centuries, playing a crucial role in exploration and trade. However, it is weather-dependent, requires skilled practitioners, and provides relatively low accuracy compared to modern methods.

2. Terrestrial Radio Navigation Systems

Developed in the 20th century, terrestrial radio navigation systems utilize fixed ground-based radio transmitters to provide navigational information to ships and aircraft.

• LORAN (Long Range Navigation): LORAN uses the time difference between radio signals received from multiple transmitting stations to determine the user's position. By measuring the time difference of arrival (TDOA) of signals from at least three stations, a receiver can calculate its location as the intersection of hyperbolic lines of position. LORAN was widely used for maritime and aviation navigation, particularly during World War II and the Cold War.
• Omega: A similar system to LORAN, Omega used very low frequency (VLF) radio signals transmitted from eight stations around the world. It provided global coverage, but its accuracy was lower than LORAN.
• DECCA Navigator: This system used phase comparison of radio signals to determine position. It was primarily used in European waters for maritime navigation.

While these systems provided valuable navigational assistance, they were eventually superseded by satellite-based navigation systems due to their limited coverage, susceptibility to interference, and declining infrastructure support.

3. Satellite Navigation Systems (GNSS)

Global Navigation Satellite Systems (GNSS) represent the most accurate and widely used method for determining absolute location today. GNSS utilizes a constellation of satellites orbiting the Earth to provide precise positioning, navigation, and timing information.

• GPS (Global Positioning System): Developed by the United States Department of Defense, GPS is the most well-known and widely used GNSS. It consists of a constellation of at least 24 satellites orbiting the Earth at an altitude of approximately 20,200 kilometers.
• GLONASS (Global Navigation Satellite System): Developed by Russia, GLONASS is another GNSS that provides global coverage. It operates with a constellation of satellites in a similar manner to GPS.
• Galileo: Developed by the European Union, Galileo is a GNSS designed for civilian use. It aims to provide higher accuracy and reliability than GPS and GLONASS.
• BeiDou Navigation Satellite System (BDS): Developed by China, BDS provides regional coverage and is expanding to provide global coverage.

How GNSS Works:

GNSS receivers determine their position by measuring the time it takes for signals to travel from multiple satellites. This process, known as trilateration, involves the following steps:

1. Satellite Signal Transmission: Each satellite transmits a unique signal that includes its precise location and the time the signal was sent.
2. Receiver Signal Acquisition: The GNSS receiver on the ground receives signals from multiple satellites simultaneously.
3. Distance Calculation: The receiver calculates the distance to each satellite by multiplying the time it took for the signal to travel by the speed of light.
4. Position Determination: By knowing the distance to at least four satellites, the receiver can calculate its three-dimensional position (latitude, longitude, and altitude) using trilateration. The fourth satellite is needed to correct for receiver clock errors.

Factors Affecting GNSS Accuracy:

Several factors can affect the accuracy of GNSS positioning:

• Atmospheric Effects: The ionosphere and troposphere can delay or distort satellite signals, reducing accuracy.
• Multipath Errors: Signals can bounce off surfaces before reaching the receiver, causing errors in distance measurements.
• Satellite Geometry: The arrangement of satellites in the sky can affect accuracy. Better accuracy is achieved when satellites are widely spaced.
• Receiver Clock Errors: Even though receivers have accurate clocks, there can still be small timing errors that need to be corrected.

Augmentation Systems:

To improve GNSS accuracy, various augmentation systems have been developed:

• SBAS (Satellite-Based Augmentation System): SBAS uses geostationary satellites to transmit correction signals to GNSS receivers, improving accuracy and integrity. Examples include WAAS (Wide Area Augmentation System) in North America, EGNOS (European Geostationary Navigation Overlay Service) in Europe, and MSAS (Multi-functional Satellite Augmentation System) in Japan.
• GBAS (Ground-Based Augmentation System): GBAS uses ground-based reference stations to transmit correction signals to GNSS receivers, providing very high accuracy for applications such as aircraft landing systems.
• DGPS (Differential GPS): DGPS uses a network of ground-based reference stations to calculate corrections for GPS signals. These corrections are then transmitted to GPS receivers, improving accuracy.

GNSS has revolutionized navigation and positioning, enabling a wide range of applications, including:

• Navigation: Providing real-time navigation for vehicles, ships, aircraft, and pedestrians.
• Mapping and Surveying: Creating accurate maps and surveying land for construction and other purposes.
• Agriculture: Precision agriculture techniques that optimize crop yields by using GPS-guided equipment.
• Disaster Relief: Locating victims and coordinating relief efforts in disaster areas.
• Location-Based Services: Enabling location-based services on smartphones and other devices, such as mapping apps, ride-sharing apps, and geotagging.

4. Inertial Navigation Systems (INS)

Unlike the methods described above, Inertial Navigation Systems (INS) do not rely on external signals. Instead, they use internal sensors to track an object's motion and calculate its position and orientation.

• Accelerometers: Measure linear acceleration along three orthogonal axes.
• Gyroscopes: Measure angular velocity around three orthogonal axes.

By integrating the measurements from accelerometers and gyroscopes over time, INS can determine changes in position and orientation.

How INS Works:

1. Initialization: The INS is initialized with a known position and orientation.
2. Sensor Measurements: Accelerometers and gyroscopes continuously measure acceleration and angular velocity.
3. Data Processing: The INS processor integrates the sensor measurements to calculate changes in velocity, position, and orientation.
4. Position Update: The INS updates its estimated position and orientation based on the calculated changes.

Advantages of INS:

• Autonomous Operation: INS does not rely on external signals, making it immune to jamming and interference.
• High Accuracy Over Short Periods: INS can provide very high accuracy over short periods of time.
• High Update Rate: INS provides position and orientation updates at a high rate, making it suitable for dynamic applications.

Disadvantages of INS:

• Drift: INS accuracy degrades over time due to the accumulation of errors in the sensor measurements. This drift can be significant, especially for low-cost INS.
• Cost: High-performance INS can be expensive.
• Complexity: INS are complex systems that require careful calibration and maintenance.

INS are often used in conjunction with GNSS to provide more robust and accurate navigation. GNSS can be used to correct the drift in INS, while INS can provide navigation during periods when GNSS signals are unavailable.

Applications of INS:

• Aerospace: Navigation of aircraft, spacecraft, and missiles.
• Maritime: Navigation of ships and submarines.
• Land Navigation: Navigation of vehicles and autonomous robots.
• Surveying: High-accuracy surveying and mapping.

The Future of Absolute Location Determination

The field of absolute location determination is constantly evolving, driven by the demand for higher accuracy, reliability, and availability. Several trends are shaping the future of this field:

• Multi-Sensor Fusion: Combining data from multiple sensors, such as GNSS, INS, and visual sensors, to provide more robust and accurate positioning.
• Advanced Signal Processing: Developing advanced signal processing techniques to mitigate the effects of interference and multipath errors on GNSS signals.
• Improved Satellite Technology: Developing new satellite technologies that provide stronger signals and more accurate positioning.
• Quantum Positioning Systems (QPS): Exploring the potential of quantum technology to develop ultra-precise positioning systems that are immune to jamming and spoofing.
• Ubiquitous Positioning: Developing positioning systems that can provide accurate location information in any environment, including indoors and underground.

The ability to accurately determine absolute location is fundamental to many aspects of modern life, from navigation and mapping to transportation and communication. As technology continues to advance, we can expect even more sophisticated and accurate methods for pinpointing our position on Earth.

FAQ about Absolute Location

• What is the difference between absolute location and relative location?
  • Absolute location is a precise coordinate that uniquely identifies a place, while relative location describes a position in relation to other landmarks or features.
• What are the units used to measure latitude and longitude?
  • Latitude and longitude are measured in degrees, minutes, and seconds (DMS) or decimal degrees (DD).
• How does GPS determine absolute location?
  • GPS receivers determine their position by measuring the time it takes for signals to travel from multiple satellites and using trilateration.
• What are some factors that can affect GPS accuracy?
  • Atmospheric effects, multipath errors, satellite geometry, and receiver clock errors can affect GPS accuracy.
• What is an Inertial Navigation System (INS)?
  • An INS is a self-contained navigation system that uses accelerometers and gyroscopes to track an object's motion and calculate its position and orientation.
• What are the advantages and disadvantages of using an INS?
  • Advantages: Autonomous operation, high accuracy over short periods, and high update rate.
  • Disadvantages: Drift, cost, and complexity.

Conclusion

The journey to define absolute location has been a long and fascinating one, marked by ingenuity and the relentless pursuit of precision. From the ancient art of celestial navigation to the modern marvel of satellite-based systems, each method has built upon the knowledge and technology of its time. Today, GNSS stands as the dominant force, providing unparalleled accuracy and enabling a vast array of applications. However, the quest for even greater precision and reliability continues, with ongoing research and development pushing the boundaries of what's possible. As we look to the future, the ability to pinpoint our location with absolute certainty will undoubtedly play an increasingly vital role in shaping our world.