DGNSS

A commonly used technique for improving GNSS performance is differential GNSS.

About DGNSS

Differential GPS involves two receivers:

  • One stationary(base station) and another
  • b.Roving(rover station) makes position measurements
Base station transmits corrections to the rover station for improved positioning.
Base Station: The GNSS receiver which is acting as the stationary reference . It has a known position and transmits messages for the rover receiver to use to calculate its position.
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Purpose

The purpose of differential correction to improve GNSS accuracy

IF a stationary GNSS receiver detects the same satellite signals as rover receiver, it can send correction data based on it's precisely surveyed location.

Process

In differential GNSS, the position of a fixed GNSS receiver is determined to a high degree of accuracy using conventional surveying techniques or taking average position of several weeks / months etc. as per accuracy requirements.
Then, the base station determines ranges to the GNSS satellites in view using:

  • The code-based positioning technique
  • The location of the satellites determined from the precisely known orbit ephemerides and satellite time.
The base station compares the surveyed position to the position calculated from the satellite ranges. Differences between the positions can be attributed to satellite ephemeris and clock errors, but mostly to errors associated with atmospheric delay. The base station sends these errors to other receivers (rovers), which incorporate the corrections into their position calculations. Differential GNSS. The base station computes the GNSS errors by differencing the ranges measured from the above methods. The base station sends these computed errors as corrections to the rovers, which will incorporate the corrections into their position calculations. A data link between the base and rover stations is required if corrections need to be applied in real-time, and at least four GNSS satellites in view at both the base station and the rovers. For corrections to be applied, the base and rover must be tracking a minimum of 4 common GNSS satellites (recommend atleast 6 common satellites for best results) The absolute accuracy of the rover’s computed position will depend on the absolute accuracy of the base station’s position.
Differential GNSS works very well with base-station-to-rover separations of up to tens of kilometres.

DGPS Technique

The idea behind all differential positioning is to correct bias errors at one location with measured bias errors at a known position. The stationary receiver is the key. It ties all the satellite measurements into a local reference. GPS receivers use timing signals from at least four satellites to establish a position. Each of those timing signals is going to have some error or delay. One receiver measure the timing errors and then provide correction information to the other receivers that are roving around. This way virtually all errors can be eliminated from the systems. The idea is simple. Put the reference receiver on a point that's been very accurately surveyed. This reference station receives the same GPS signals as the roving receiver but instead of working like a normal GPS receiver it solves the equations backwards. Instead of using timing signals to calculate its position, it uses its known position to calculate timing. It figures out what the travel time of the GPS signals should be, and compares it with what they actually are. The difference is an "error correction" factor. The receiver then transmits this error information to the roving receiver so it can use it to correct its measurements Since the reference receiver has no way of knowing which of the many available satellites a roving receiver might be using to calculate its position, the reference receiver quickly runs through all the visible satellites and computes each of their errors. Then it encodes this information into a standard format and transmits it to the roving receivers. It's as if the reference receiver is saying: "OK everybody, right now the signal from satellite #1 is ten nanoseconds delayed, satellite #2 is three nanoseconds delayed, satellite #3 is sixteen nanoseconds delayed..." and so on. The roving receivers get the complete list of errors and apply the corrections for the particular satellites they're using.