Some people think that DGPS only applies to GPS code measurements techniques. In contrast, the phase measurements are called as relative. Is it true?
DGPS refers to making an observation model by differentiating two distinct (non-differenced or differenced) observation equations while relative positioning refers to determining the target position relative to another object (i.e. positioning in a local user defined coordinate system).
Actually, some people like to call DGPS when corrections are sent to the rover so it can calculate its position accurately. On the other hand, they consider the term relative positioning when the base sends its raw measurements to the rover. The rover then will do the differencing and the position will be estimated relative to the base station position.
The other trend, which I usually prefer, is that sending corrections is Augmented-GPS while sending the measurements it self is DGPS, which is divided into Code-based DGPS and Carrier-based DGPS.
"Some people think that DGPS only applies to GPS code measurements techniques. In contrast, the phase measurements are called as relative. Is it true?"
I think there is a mix of terms that should be clearified, so better think of Concepts and Methods / Techniques.
There are two concepts to obtain positions (and other estimates) out of GPS/GNSS observations:
- Absolute positioning (error budget has to be known, estimated for any terms in the observation equation; zero-differencing)
- Relative positioning (error budget is canceled out or reduced to a minimum due to observation differences)
Differential GPS is a techniques that is used with code observations only, the accuracy is around 0.5 - 1.0m depending on the equipment used in the application. With this technique, its possible to achieve respectable estimates by using single-frequency receivers only. Nevertheless, differential GPS is a relative method as observations are improved (error budget is reduced) due to differences of observations.
You are also right that the term of relative positioning is ofthe used in combination with the phase measurements, for example during coordinate estimation in networks (dual frequency, code+carrier, single/double differencing). One prominent example for real time applications is RTK (real time kinematic), which is also a relative method, but uses the combination of code and phase and is applied to dual-frequency equipment/observations.
I think from my side, there is always the need to name the concept and the method / technique:
- concept: absolute positioning: (method: single point positioning (SPP) for code; precise point positioning (PPP) for code+phase observations)
- concept: relative positioning (method: Code - DGPS/DGNSS, Code+Carrier: Double differencing (DD), single differencing (SD), real time kinematic (RTK), etc.)
Hope this helps to clearify the issue.
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