Describe the purpose and working principle of a Differential Global Positioning System (DGPS) in mine surveying.

A Differential Global Positioning System (DGPS) is used in mine surveying to improve the accuracy of GPS positioning by correcting errors caused by atmospheric interference, satellite clock errors, and other factors. The purpose is to provide more precise location data than standard GPS alone. Its working principle involves using a stationary base station at a known location to calculate corrections and transmit them to a rover receiver in the field. GPS, or Global Positioning System, uses signals from a constellation of satellites to determine a receiver's location on Earth. However, GPS signals are subject to various sources of error that can reduce the accuracy of the positioning. Atmospheric interference, such as variations in the ionosphere and troposphere, can delay or distort GPS signals, causing errors in the calculated position. Satellite clock errors, which are small inaccuracies in the timing of the GPS satellites, can also contribute to positioning errors. Multipath errors, caused by GPS signals reflecting off of surfaces before reaching the receiver, can further degrade accuracy. A DGPS system minimizes these errors through a differential correction process. A base station is a GPS receiver that is set up at a known, precisely surveyed location. Because the base station's location is known, it can calculate the difference between its known location and the location calculated by the GPS satellites. This difference represents the error in the GPS signals. The base station then transmits these error corrections to the rover receiver, which is the GPS receiver being used in the field to collect survey data. The rover receiver applies these corrections to its own GPS measurements, significantly improving the accuracy of its position. The corrections can be transmitted in real-time or can be applied later during post-processing. Real-time DGPS provides corrections as they are being measured, allowing for immediate access to accurate positioning data. Post-processing DGPS involves collecting the raw GPS data in the field and then applying the corrections later in the office. Differential correction removes common errors. The key to DGPS is that many of the errors affecting GPS signals are correlated over relatively short distances. By using a base station that is located close to the rover receiver, the DGPS system can effectively cancel out these common errors. Types of DGPS systems include local base stations and wide-area systems. Local base stations are set up by the user at the survey site. Wide-area DGPS systems, such as the Wide Area Augmentation System (WAAS) in the United States, use a network of ground stations to broadcast corrections over a wider area. DGPS is often used in mine surveying for establishing control points, mapping surface features, and monitoring ground deformation. It provides a cost-effective and efficient method for obtaining accurate spatial data in the field.