This article covers some key considerations when creating Single Baseline NEAR™ streams with the SNIP NTRIP Caster. A NEAR stream is a type of of RTK Network data stream where each end user (the rovers or NTRIP Clients) connects to a single mountPt (often with the word NEAR in its name) and then the SNIP NTRIP Caster automatically connects that user to be best (often the closest) Base Station meeting it’s individual needs. As the rover moves, this process is repeated, seamlessly connecting the user to the best Base Station at all times. This results in optimal single baseline data streams with less measurement noise than various virtual methods. And the end user need not become familiar with the names and locations of different Base Stations in the Caster Table. SNIP supports a wide range of feature and settings to create various NEAR streams for different user application.
Key Considerations for Creating and using NEAR Streams
In order to use a NEAR stream a small amount of planning is required, both on the NTRIP Caster Side and on the rover devices that will connect to it. The primary requirements and considerations are listed below.
On the Rover side:
- Each rover/NTRIP Client must report its estimate of position with the common NMEA-183 $GPGGA sentence. This information is used (each time it is sent) to determine what Base Station data stream, from that NEAR mountPts pool of base stations, to send to that rover device.
- Any rover/NTRIP Client that reports a position outside the service area of the NEAR pool is disconnected because there is no suitable data to send to it.
- Any rover/NTRIP Client that does not report its position within a reasonable period of time (~20 seconds or so) is disconnected. Often such users state they are “getting no data” because of this.
- The rover/NTRIP Client must report its position at least one time. It is common to repeat this every 5~30 seconds, but not required.
- The rover/NTRIP Client can report its position with any mix of NTRIP Rev1 and Rev2 protocol formats it desires.
- The first five NMEA-183 $GPGGA sentences seen for any rover/NTRIP Client connection are displayed on the console log (enable verbose logging to view) . Tip: If you do not see such messages, the rover/NTRIP Client is not sending the required data.
- If one or more of your rover/NTRIP Client sends ill-formed NMEA-183 $GPGGA sentences, enable the checkbox found on the Caster / Clients tab to instruct SNIP to be more relaxed in its decoding.
- The most recent NMEA-183 $GPGA data for any client can be seen in the Display Connected Users window along with the Base Station to which it is assigned.
- The movement reported from recent NMEA data for any client can be viewed on the MAP display, and this data can be sent out on the AVL server (see the AVL tab) for use by various 3rd party tools.
On the SNIP Caster side:
- Set up the NEAR pool with these general instructions:
- In the knowledge base the NEAR features have their own section:
- Each NEAR stream maintains a “pool” of Base Stations which meet its requirements for specific message content, location, types and numbers of satellites, and quality. When a user device provides its own location, one of the members of the pool is selected.
- Keep in mind the coverage area of the near pool is determined by the stations used at that time. Therefore the specific coverage region may change as stations come and go from the pool.
- If you observe positional “jumps” when a given rover connects from one Base Station to another, this almost certainly indicates that your Base Stations are not aligned to a common frame of reference. Use a post-processing tool like OPUS to precisely determine each Base position and confirm this is in fact what that Base is sending using SNIP‘s RTCM message viewer or various SNIP tool tips. If your network is made up of some Base Stations operated by another party (where you cannot set the station location values or the datum used), use a PFAT transformation to adjust the position to align with your other stations (see also this article).
- NEAR Pools all default to contain RTCM3.x message content, but you can set the content to be other data formats (CMR, uBlox, etc.), this is a Pro model feature only.
- The value set for “Max Baseline Distance” determines the farthest distance between a rover and a Base Station that will be allowed. The default is set to 60km but this value may need to be increased when supporting rover devices which are at the edge of your network coverage area. For further details see this article.
- If you desire to have rover devices connect to a given closest Base Station and then “stay” on that Base Station even when moving to another station that may be slightly closer, simply increase the Hysteresis Distance value (which defaults to 2km). The moving rover will not be switched over to the new nearest Base Station until the hysteresis value has been exceeded. This has value in some Precision Ag applications. Regardless of this value, should the Base Station go offline or have problems detected with it, all users connected to it are then reassigned to the best Base Station among the remaining ones for their individual needs.
- The values set in Stream Content settings (see this article) apply to every Base Station in the pool. For example if you check that “Galileo” (GAL) must be preset in the data stream, then a Base Station containing only GPS+GLO content would be excluded from that pool.
- You can also “draw a circle” around a given Lat-Long location, then set a coverage radius, and easily include all Base Stations within that region that match the specific Stream Content requirements you have set. This is useful where you have Base Stations in your SNIP Caster that cover widely separated operating areas such as both EU and Japan.
- If you have a specific set of Base Stations you wish to be in a NEAR pool, you can create a list of these when editing the pool settings. This has value where there is a need to exclude certain Base Stations for various reasons. See this article for further details.
- If you are running both a NEAR pool with legacy message content (to support older GNSS rovers) and another with MSM content, it is typical to add each type to its own list. You can then also hide or show the individual Base Stations in the public Caster table as you prefer. See these articles for further details on using PFAT to create either MSM or legacy message to use on such pools.
- The Base Stations used in the NEAR pools are expected to be stable and not subject to frequent short term drop-outs or corruption. SNIP measures and tracks each Base Station message stream for ~180 seconds before allowing it to be used in a pool. This quality checking causes a short delay when the Caster is first started to ensure suitable data.
- The SNIP Pro model allows 5 NEAR pools. And another near pool added for every 10th additional slot license found on the machine. The Basic model allows one NEAR pool with RTCM3.x content.
- A report summary of all the current NEAR pools and the stations that are used in each can be obtained with the menu item Reports → All Streams → NEAR Streams Summary. This report is also available directly from a button on the document viewer tab.
As always, setup support is only an email away when needed.