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What they mean, how they are identified and which one to use
Mountpoint is a term used to describe the type of positioning service available from a Real Time GNSS network that is communicating to the user with Networked Transport of RTCM via Internet Protocol (NTRIP). While the concept of a Mountpoint is simple enough, there are various network capabilities involved and several types of protocols for streaming data. This multiplicity of parameters makes the subject a bit confusing without a clear understanding of the terminologies, abbreviations and acronyms involved.
NTRIP protocol is a variant of Hypertext Transfer Protocol (HTTP) the protocol used for transmitting data back and forth through websites and is familiar to everyone from website addresses where it always precedes the address with the familiar http:// symbol.
NTRIP is a protocol that is a subset of HTTP and is designed specifically for GNSS networks. It allows for a much greater throughput of data and, provides security features through user logins and password checking. NTRIP is the protocol used to transfer data throughout most GNSS networks from the base stations to the networks server software and the user in the field. The data streamed through all the mountpoints of the ARTGN is streamed using NTRIP. The user in the field seldom needs to know anything about NTRIP except that it is a setup option within the survey controller setup choices. When there is a choice of NTRIP or something else, the user needs to choose NTRIP, not something else.
The various types of services available are categorized according to four types of characteristics:
1. Positioning accuracy needed.
2. Rover equipment manufacturer.
3. Whether they produce a rover position from a Single Base Station or whether they produce a rover position from a "virtual reference station" (VRS).
4. Whether they use only GPS satellites or both GPS and GLONASS satellites.
1. Positioning accuracy needed:
The lower accuracy mountpoints are designated with the term DGPS (Differential GPS). All the other services are high accuracies. Differential GPS (DGPS) provides accuracies better than one meter. This type of differential positioning is often used for GIS data collection where higher accuracies are not needed. It was originally used primarily for positioning ships and aircraft. The mathematical process involves just the pseudorange data and is usually thought of as being the corrections applied to the pseudoranges to the base station that would position the point exactly at the known high accuracy values of the base station position. These same corrections to the base station pseudoranges are also applied to the pseudoranges at the rover unit to get a corrected position at the rover. This type of correction achieves accuracies better than one meter when the rover is within sixty miles of the base station. All the mountpoints that contain the term "DGPS" provide this kind of service. This service is completely independent of rover equipment manufacturer.
All the other services provide higher accuracies that are usually around three to seven hundredths of a foot. This type of positioning is called Real Time Kinematic and is produced by double differencing of the phase observations of the GNSS data.
So in summary, if you only need one meter accuracy and need to position objects up to sixty miles from the nearest base station, use a DGPS mountpoint. For higher positioning accuracies at distances much closer to a base station use one of the other non-DGPS mountpoints.
2. Rover Equipment Manufacturer.
All the ARTGN high-accuracy mountpoints are currently setup either for Trimble or Topcon rover equipment. They are differentiated because of the way the rover unit software deals with the vertical distance between the bottom of the base station antenna and the L1 phase center. One manufacturer works with the phase center and the other the base of the antenna (ARP). If any user would like to use equipment from another manufacturer, please contact us and we will work out a solution for your needs. Otherwise, the choice is self-explanatory.
3. Single Base or Virtual Base.
High accuracy positioning can be done either by connecting to a datastream from a single base station of the network or by positioning relative to a nearby "virtual" base station. The effect of using a single base station is the same as standard Real Time Kinematic (RTK) operations using a user supplied base station. Most users are familiar with this process which predates network positioning. Single base mountpoints are identified by the abbreviation "1Base". The virtual base station choice will create a virtual base station near to the user current position. This process will generate much shorter baselines and slightly higher accuracy. These mountpoints are identified by the abbreviation "VRS".
As a temporary expedient for Topcon user, we are recommending they only use the 1Base option until changes are made in Topcon survey controller software to properly deal with antenna vertical distances.
4. GPS only or GPS and GLONASS
Both the high-accuracy and low accuracy mountpoints have been set so that some provide data only from GPS satellites and some provide data from both GPS and the Russian GLONASS satellites. There is much debate about which type provides the best positioning. We provide both types of data and let the user decide. The data which only contains GPS data is identified by the abbreviation "GPSOnly" and the data with both type is identified by "GPSnGlonass".
If you have any questions about mountpoints please contact us.
When users connect to the ARTGN they currently presented with the following list of mountpoints: