The the P.F.A.T.™ control system provides several innovative ways to control the messages present in every data stream. The PFAT features are available in the Pro, Basic, editions of SNIP. Lite copies of SNIP share the same code base, but many of these innovative features are disabled in the freely available Lite edition.
Using the PFAT Add Dialog Control
The PFAT Add control is used to add the selected RTCM3 message types found in any Base Station data stream to the current stream. The add action combines all the messages present in both streams (post any filtering that may be active) to form a composite. Various logic rules in SNIP check for, and warn on, creating conflicting content. The result is then forwarded on to any NTRIP Clients or to other users in SNIP (such as a PUSH_Out stream). All the messages are still parsed and may be used in various navigation and monitoring services. Whenever PFAT Add is active, the right-click menu for the stream shows a check mark. Selecting the menu item displays the Add dialog box. The message stream must be Parsed in order to Filter the messages.
The individual controls of the Add dialog is described in further detail here (link to be provided at release). The basic user interaction is to select one or more data streams from other streams present on the machine to which is then added to the subject stream. Multiple data streams can be added when required. Like all PFAT settings, the values selected are preserved and used each time the stream (or SNIP) is started.
Caution: The PFAT Add function is very powerful and it is quite easy to create composite message streams which make no logical sense or which are not internally consistent across all the combined messages.
The logic in SNIP will detect some but not all possible conflicts that can result. Among the things that SNIP will detect are:
- If more than one stream is sending the same message types (such as two streams sending 1004 observations).
- If more then one stream is sending conflicting messages with the same content (such as 1002 and 1004 at the same time).
If such conflicts are found among the proposed set of streams, a warning note is provided in the dialog. If the user attempts to save such a conflict, the warning is repeated asking the user to confirm his/her intent. SNIP does allow the creating of conflicting streams; this has some value to the GNSS research community that uses it in unique ways.
Note: When using Add and editing the streams, the subject stream will be re-parsed and message analysis process performed as the settings are changed. This allows SNIP to consider the composite data stream and its contents in real time. However, every time the message analysis is performed it can disrupt the message traffic seen by any NTRIP Clients who are actively connected to that stream, and it also resets the accumulated statistical data for that stream.
Typical Uses Cases for Adding
Common uses of the PFAT Add operation are described further in the examples below.
Adding Broadcast Orbital Data
A common use is adding orbital ephemeris messages to a stream.
As described in the article on filtering, PFAT filtering allows you to take a broadcast ephemeris stream with several GNSS types [GPS + GLO + GAL + BDS + QZS] and reduce that to only the types of interest. In the example below the common RTCM3EPH stream has been used to create a stream with only GPS+GLO messages (MTs 1019 and 1020). This stream has then be added to a the steam “AnExample” which is a common Base Station with the normal observation messages (MTs 1004 and 1012).
The resulting message stream for AnExample will now contain MTs 1004, 1005, 1012, 1019 and 1020.
Adding Precise Clock and Orbital Data
Another common use is adding precise orbital ephemeris message and / or precise clock data products to a stream. This is common with those developing various long baseline applications and hybrid PPP style navigation filters. Some deployments add this data to an observational data stream, others add it to ephemeris data stream.
When doing this, you (and your end user community) need to be sure if the precise corrections refer to the center of mass (COM) or the antenna phase center (APC) of the Space Vehicles (SVs). The final navigation filter needs this information to operate correctly.
Many of these data products can be obtained from IGS, here and from here. Registration is free but required for most of this data. God bless IGS and the folks at the MGEX effort that provide these data products. Truly an effort with many positive ramifications for the entire world.
Adding Private message Content
The PFAT Add function can also be used to add private message types to a stream. As an example SCSC (the developers of SNIP), have used this method to add RTCM 2.x messages encoded in a unique way into an RTCM3.x stream for comparison and validation testing. In another project, raw vehicle IMU data was inserted into a stream by this means. As long as both message streams contain well-formed RTCM3 message content, the Add function can be used.
- PFAT Filter; used for Filtering message types from a data stream
- PFAT Translate; used for Translating the messages in a data stream