3.4.8.7. Differential GNSS (DGNSS) surveying

The DGNSS technique requires two identical GNSS units, and is used to improve the accuracy of a standard GNSS. It works by placing a GPS receiver at a known location, called a reference station, and another GNSS unit, known as rover station which is kept at unknown points to determine the coordinates.

Differential GNSS (DGNSS) improves the accuracy of standard GNSS equipment by utilizing two similar devices. One device, positioned at a known location (the reference station), calculates the positional differences. The second device, referred to as the rover station, is placed at an unknown location to determine its coordinates, using corrections from the reference station.

Imagine a teacher showing a student how to hit a target by adjusting their aim based on the previous attempts. The teacher has a fixed point known to be accurate (like the reference station) and helps the student adjust their aim (like the rover station) to hit the target accurately.

The reference station may be a bench mark whose exact location is already known. Thus, the difference between the GNSS derived position and the true position is determined at the bench mark. The reference station actually calculates the difference between the measured and actual ranges for each of the satellites visible from that station.

The reference station uses a known benchmark to calculate how far off the signals received from the satellites are. It determines the errors by comparing its readings from the satellites to their actual positions. These error differences, termed 'differential corrections', are then relayed to the rover unit to enhance its positional accuracy.

Think of this like a weather station, which provides accurate temperature readings. If you have a weather app on your phone that uses this station's data, it can improve its own predictions by adjusting for any discrepancies between its readings and the accurate weather station. This way, your app gives you better forecasts based on reliable data.

At rover station, the correction in the observed value can be applied using the differential correction. The DGNSS technique is based on pseudo ranges and code phase.

Once the rover receives the differential corrections from the reference station, it adjusts its calculations accordingly. This allows it to provide a position that is more accurate than what it would achieve using standard GNSS signals alone, utilizing the improvements based on the reference station's more precise measurements.

This can be likened to a GPS system in a car that adjusts your route when traffic updates are received. The initial route may consider only one set of data, but once it gets information regarding a traffic jam from a stable source, it recalculates the best route for you, improving your estimated time of arrival.

Although the accuracy of code phase applications has improved a lot with the removal of Selective Availability (SA) in May 2000, yet reduction of the effect of correlated ephemeris and atmospheric errors by differential corrections requires achieving the accuracy better than 2 to 5 m.

DGNSS has significantly achieved better accuracy ranging from 2 to 5 meters by applying differential corrections to GNSS data. The removal of Selective Availability (SA) in 2000 also enhanced standard positional accuracy, but DGNSS further minimizes atmospheric and satellite orbit errors that can affect the signals during transmission.

Consider using a magnifying glass to see fine details. Just as a magnifying glass helps improve your view, DGNSS sharpens the positioning data, making it possible to see and measure locations much more precisely than before.

The DGNSS based on C/A code SPS signals can however offer meter or even submeter accuracy. Pseudo-range formulations can be developed from either the C/A-code or the more precise P-code.

Using the C/A code for standard positioning service (SPS), DGNSS can achieve accuracy at the meter or sub-meter level. This precision is enhanced through pseudo-range measurements, which consider signals' travel time. Additionally, the more precise P-code can also be used for calculations, further enhancing accuracy.

Think of a photo taken with a camera. When using the basic settings, you might get a decent photograph. However, by adjusting the camera's settings more intricately (like using the P-code instead of C/A), you can capture a clearer and sharper photo with more details, just like how DGNSS captures more accurate geographic data.

--