NTRIP/CORS EXPLAINED

           NTRIP/CORS EXPLAINED HOW IT WORKS

NTRIP and CORS are different things.  However, the terms are often used synonymously in our industry.

NTRIP (Network Transport of RTCM via Internet Protocol) and CORS (Continuously Operating Reference Station) are forms of RTK differential correction that are done through the use of a cellular modem and base station network.  This means that instead of using the traditional base station and radio to send correction data to a rover, data is sent using the internet to a cellular modem with a data plan. 

In order to use this type of correction you must have a cellular modem, a receiver capable of RTK correction and a cellular data plan.  You are also required to register with your local NTRIP provider.  This will include creating a username and password as well as obtaining a port and IP address for the cell modem to access the network. 

One of the benefits to using NTRIP in areas where network is available is you are capable of achieving sub-inch RTK accuracies without having to purchase and manage a base station yourself.  RTK accuracy allows for a level of repeatability that cannot be achieved through most other types of correction.

There are four formats supported by NTRIP/CORS:  CMR, CMR+, RTCM 2.x and RTCM 3.x. CMR, CMR+ and RTCM 2.x are only capable of using GPS, whereas RTCM 3.x is capable of using GPS and GLONASS.

                 Basic Concept on how the systems work

The Hypertext Transfer Protocol (HTTP) is designed as an application-level protocol for

Distributed collaborative hypermedia information systems, HTTP is primarily used for bulk traffic, where each object has a clearly defined beginning and end. HTTP is widely used for IP streaming applications, which include the RTCM application. Ntrip, which uses HTTP, is implemented in three programs: Ntripsource, NtripServer and NtripCaster.

NTRIP SOURCE

The Ntripsource provide continuous GNSS data (e.g. RTCM-3.2 corrections) as streaming data. A single source represents GNSS data referring to a specific location. Source description parameters as compiled in the source-table specify the format in use (e.g. RTCM 2.0, RTCM 3, and Raw), the recognized navigation system (e.g. GPS, GPS+GLONASS), location coordinates and other information.

NTRIP SERVER

The NtripServer is used to transfer GNSS data of an Ntripsource to the NtripCaster. Before transmitting GNSS data to the NtripCaster using the IP connection, the NtripServer sends an assignment of the mount point. Server passwords and mount points must be defined by the administrator of the NtripCaster and handed over to the administrators of the participating NtripServer. An NtripServer in its simplest setup is a computer program running on a PC that sends correction data of an Ntripsource (e.g. as received via the serial communication port from a GNSS receiver) to the NtripCaster.

The Ntrip protocol may be used for the transport of RTCM data of a virtual reference station following the so-called VRS concept. Based on data from a number of reference stations, RTCM corrections are derived for a virtual point at the user’s approximate position. Data for this virtual reference station represent a single Ntripsource that can be transmitted by an NtripServer

NTRIP CASTER

The NtripCaster is basically an HTTP server supporting a subset of HTTP request/response

messages and adjusted to low-bandwidth streaming data (from 50 up to 500 Bytes/sec per stream). The NtripCaster accepts request-messages on a single port from either the NtripServer or the NtripClient. Depending on these messages, the NtripCaster decides whether there is streaming data to receive or to send.