01 RTD
RTD: English full name Real Time Differential, Chinese means real-time dynamic code phase differential technology. The working method is that the base station sends the pseudorange (or coordinate) correction value (differential value) to the user receiver, and the user receiver calculates the precise position based on the difference value and its own observation value.
When using RTK to measure before, the red RTD will appear first when CORS is connected, and then the RTK will be fixed. In fact, RTD and RTK are the same in some principles, so I will talk about them here.
First of all, from the same level, RTD (Real Time Differential) and RTK (Real Time Kinematic) are both differential GPS, or DGPS, and both are real-time, dynamic, and relative positioning.
Then, in different places, RTD calculates the pseudorange. According to the known coordinates of the base station and the coordinates of each satellite, the true distance from each satellite to the base station at every moment is calculated. Then compare with the measured pseudorange to get the pseudorange correction number and transmit it to the user receiver, which belongs to the code (C/A code, P code) differential technology.
What RTK calculates is the carrier phase observation value of the two stations, that is , the carrier phase collected by the reference station is sent to the user receiver , and the coordinate is calculated by the difference. Belongs to carrier (L1, L2, L5) phase difference technology.
In real-time dynamic measurement, differential technology was first introduced in code phase measurement, so real-time dynamic code phase differential measurement is called conventional differential GPS measurement technology. That is, general DGPS/differential positioning.
Because RTD uses pseudo-range measurement, the accuracy is only sub-meter low, usually in non-high-precision positioning fields such as car navigation.
02 SBAS
SBAS: The full name in English is Satellite-Based Augmentation System, which means wide-area differential augmentation system in Chinese. If you don’t know what this means, then I believe everyone knows the satellite-based augmentation system. The wide-area differential augmentation system is also the satellite-based augmentation system.
SBAS is a kind of GNSS enhancement. It is a satellite-based enhancement system. Relatively speaking, there are ground-based enhancement system GBAS and aircraft-based enhancement system ABAS.
The SBAS system is an enhanced system born to improve the positioning accuracy of the original satellite navigation system. It is mainly composed of four parts: ground reference base station, master control station, upload station and geosynchronous satellites. Let's take a look at the working principle:
The navigation satellites are monitored by a large number of widely distributed differential stations (locations are known), and the original positioning data (pseudorange, carrier phase observation value, etc.) are obtained and sent to the central processing facility (master control station), which is obtained by calculation Various positioning correction information of each satellite is sent to GEO satellites through the uplink injection station , and finally the correction information is broadcast to the majority of users, so as to achieve the purpose of improving positioning accuracy.
At present, all countries in the world have established complete SBAS systems, including WAAS in the United States, EGNOS in Europe, MSAS in Japan and GAGAN in India.
However, SBAS is mainly used in aviation and navigation. We rarely use satellite-based augmentation systems in our usual surveys, and we mainly use ground-based augmentation systems.
03 WAAS
As the editor said earlier, WAAS is a wide area augmentation system belonging to SBAS. The full name of WAAS is Wide Area Augmentation System, which is translated as Wide Area Augmentation System. A system developed is mainly used in aviation, such as aircraft GPS navigation.
The emergence of WAAS is to solve the problem that GPS cannot provide aircraft with very accurate altitude information and heading guidance, the distance from the runway and the elevation information of all points along the approach. Prior to this, the US air traffic control system mainly used ILS (Instrument Landing System) and MLS (Microwave Landing System). The former was difficult and troublesome to implement, and GPS receivers began to appear in the development of the latter.
WAAS is composed of three main parts: ground part, space part and user part. The part of the ground reference station receives and monitors GPS signals, makes corrections, and then sends them to the satellites in the space segment for rebroadcasting to the user segment.
WAAS Ground Reference Station
The emergence of WAAS has improved the positioning accuracy of aircraft navigation, while also reducing the cost of airport navigation facilities and the cost of a single receiver installed on the aircraft.
04 PPP
The full name of PPP is Precise Point Positioning, which is translated as precision single point positioning. It is also called absolute positioning . Precise single point positioning refers to the use of carrier phase observations and high-precision satellite ephemerides and satellite clock errors provided by organizations such as IGS. To perform high-precision single point positioning method.
PPP technology appeared later than RTK technology. PPP is another technological revolution after GNSS positioning technology relayed RTK/network RTK technology. Its appearance has changed the situation that only differential positioning mode can be used to achieve high-precision positioning. Global high-precision GNSS positioning provides an effective new method.
At present, the plane position accuracy of the points obtained by PPP based on one day's observations can reach 1~3cm, and the elevation accuracy can reach 2~4cm.
05 PPK
PPK stands for Post Processed Kinematic, which is a positioning technology corresponding to RTK . This is a dynamic relative positioning technology that uses carrier phase observations for post-processing, while RTK is real-time.
Because it is post-processing, the user does not need to be equipped with a data communication link (which can save traffic costs), and the observation is more convenient and free, and it is suitable for application fields that do not need to obtain positioning results in real time.
06 Wide Area Difference
Wide-area differential GPS, also known as WADGPS, is a type of differential GPS . Differential GPS has a single reference station differential GPS and a local area differential GPS of multiple reference stations according to the same model and principle classification method.
Why is there a wide-area differential GPS? As the name implies, when differential GPS needs to cover a large area (such as my country's mainland and adjacent sea areas), local area differential GPS with a single base station and multiple base stations will encounter some difficulties, and a large number of reference stations need to be established to group Internet, time-consuming and costly. Secondly, in some places, it is impossible to establish a base station and a signal transmitting station, which will result in a large blank area.
So at this time, wide-area differential GPS technology came out. In a relatively large area, a small number of reference stations are distributed more evenly to form a sparse differential GPS network. Each reference station independently conducts observations and transmits the observations to the data. The processing center is processed by the data processing center in order to separate various errors.
Then broadcast the satellite ephemeris correction number, satellite clock offset correction number, and atmospheric delay model to users. This differential system is collectively called a wide-area differential system.
World-wide DGPS is a special case of wide-area differential GPS. The WAAS technology mentioned by the editor above is the wide-area differential GPS technology, which further improves and supplements the wide-area differential GPS technology.
07 Summary
Regarding the relationship and difference between the above-mentioned technologies, here is a summary of RTK, RTD, PPK, SBAS, and WAAS as differential GPS technologies. PPP belongs to single-point positioning (absolute positioning is not differential).
The difference between RTK and RTD, one is carrier phase difference, the other is code difference, and RTK has higher positioning accuracy.
The difference between RTK and PPK is that one is to provide data information in real time, and the other is post-processing.
WAAS is a specific example of the SBAS system and is included in the SBAS system.
Article source: RTK House