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What is dGPS | Differential Global Positioning System

GPS signals are affected by various sources of errors such as atmospheric delays, satellite clock inaccuracies and multipath reflections. The dGPS uses additional components and infrastructure compared to traditional GPS to achieve its high level of accuracy. dGPS addresses this by measuring the difference between the known positions of reference stations and the GPS-calculated positions, allowing it to correct for the errors in real-time.

As mentioned the full form of DGPS is Differential Global Positioning System. The dGPS is typically found in airports in order to bring a more precise aircraft position fix during the approach landing phases. As for the aircraft, it must be equipped with a dGPS receiver so that signals from both satellite and dGPS reference stations are observed to correct position errors.

DGPS Working Principle

The dGPS is an enhanced version of the Global Positioning System (GPS) used for more accurate and precise positioning and navigation. While GPS provides reasonably accurate location information, dGPS improves this accuracy by taking into account and correcting for various sources of errors and inaccuracies in the GPS signals. Let us understand how dGPS works with its various components.
➨Reference Station: It is set up at known and precisely surveyed location on Earth's surface. This reference station receives signals from GPS satellites just like a regular GPS receiver.
➨Error Calculation: The reference station calculates the difference between its known location and the location calculated by the GPS receiver using the received satellite signals.
➨Correction Data: The reference station then broadcasts this error correction data to nearby dGPS receivers, typically using radio waves or a cellular communication network.
➨GPS Receiver Correction: dGPS receivers, which can be mobile devices (aircraft, truck etc.) or other equipment, receive the correction data from the reference station. They use this data to adjust their own GPS position calculations in real-time.

DGPS,Differential Global Positioning System

Let us understand differential GPS i.e. dGPS working principle.
On the ground DGPS Tx and Rx system operates with following specifications:
•  Rx uses UHF band which is used to calculate DGPS position fix.
•  Tx uses LF/MF band(283.5 to 325 KHz), it is used to transmit correction signal to the Aircraft.
•  DGPS is stationary, therefore its actual LAT,LON,ALT is well known. This is used to transform it to XDO, YDO and ZDO.
•  Similar to GPS, here also four satellites are needed to obtain a 3D position fix for the DGPS. This is identified as XDO, YDO and ZDO.
•  The DGPS then takes the difference that exists between the actual known position and the calculated position obtained using GPS SATs.
δ = Calculated position - Actual Position
•  Ideally, δ should be roughly zero; however, most of the time this is not the case, and therefore the δ information is transmitted to the airborne GPS-Rx in the form of a correction signal.

In the Aircraft GPS receive system has following features.
•  It uses UHF band to calculate Aircraft position fix and LF/MF band to observe correction signal from DGPS.
•  Range of operation is about 370 Km.
•  Again, the A/C obtains its 3D position fix using the 4-SATs.
•  Then, the GPS receiver detects correction signal that contains the δ information and performs the following correction.
Actaul position = Calculated position - δ

By applying the correction data from the reference station, dGPS receivers can significantly improve the accuracy of their position estimates. While regular GPS can have accuracy within a few meters, dGPS can achieve centimeter-level accuracy or even better, making it valuable in applications where precise positioning is crucial, such as surveying and mapping, precision agriculture, marine navigation, autonomous vehicles, aircrafts etc.

Advantages of DGPS

Following are the benefits or advantages of differential GPS.
➨The dGPS improves accuracies achieved by standard GPS. GPS provides accuracies within a few meters where as dGPS can provide centimeter level accuracy.
➨dGPS enhances safety in aviation and maritime navigation which reduces risk of accidents and ensures more precise positioning during takeoff, landing and other phases of operation.
➨It allows more efficient route planning and execution. This leads to fuel savings and reduces travel times and increases productivity in agriculture, transportation and construction industries.
➨It helps to improve surveying and mapping. This makes it easier to create highly detailed and precise maps and perform topographic surveys and to assess land boundaries.
➨It can mitigate effects of signal interference such as multipath and radio frequency (RF) interference. This results into more reliable and accurate positioning.
➨It is used for environmental monitoring, tracking movement of wildlife and so on.

Disadvantages of differential GPS

Following are the limitations or disadvantages of DGPS (Differential Global Positioning System).
➨Most of the errors are either completely eliminated or made negligible after using DGPS; however, atmospheric (i.e. Ionosphere & Troposphere) and Rx-based (i.e. Rx-Noise & Multipath) errors would still exist. DGPS error sources are satellite clocks, orbital error, ionosphere, troposphere, rx noise and multipath.
➨Implementation and maintainance of dGPS is expensive which requires installation and operation of reference stations and supporting infrastructure.
➨To ensure greater coverage area more DGPS base stations or reference stations are required to be installed.
➨The position accuracy degrades as the separation between DGPS and aircraft GPS-Rx increases.
➨Transmit/receive of correction data introduces some amount of latency into positioning process. This is a major concern in real time applications such as autonomous vehicles.
➨Like traditional GPS, dGPS signals are vulnerable to jamming and spoofing. This compromises accuracy and reliability of the system.
➨dGPS is less suitable for indoor or underground applications where satellite signals are weak or unavailable.
➨Accuracy of dGPS relies on reference station data. If reference station experiences any issues or malfunctions, it can affect the entire dGPS network.

Difference between dGPS and GPS

Following table mentions difference between dGPS (differential GPS) and standard or traditional GPS.

Parameters Standard GPS Differential GPS
Accuracy within few meters centimeter level
Correction data does not use correction data from reference stations Relies on correction data from reference stations
Cost Affordable and widely available More expensive
Coverage Global coverage available worldwide depends on availability of reference stations
Latency Provides real time information without significant latency Introduces some latency due to transmission/reception of correction data.
Applications Suitable for general navigation and everyday location based services Used in precision critical fields such as surveying, agriculture, aviation and maritime navigation etc.

Conclusion : Overall, differential GPS (dGPS) significantly improves location accuracy compared to GPS, making it an essential technology in various industries and applications where precise positioning is required.

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