Orbit and Clock corrections feeds

The number of SNIP users, both SNIP Caster operators, and those connecting to SNIP nodes with NTRIP Clients, have been asking about orbital and clock corrections (the so called ‘State Space Representation’ (SSR) messages found in RTCM 3.3 messages and beyond).  SNIP handles every defined and adopted SSR message, as well as those still in the experiential range and those send in proprietary message assignments.   If you wish to send any non-standardized messages, simply disable parsing and send the message using the bent-pipe mode.

This article discusses how to set orbital and clock corrections up on your own copy of SNIP or how to get them from either NTRIP.use-snip.com or from the RTK2go.com open Caster. If you also need a source for a broadcast ephemeris stream, you can connect to the RTCM3EPH stream for that content.

Aside: It is typical to connect to a RTCM3EPH stream once just after rover power-up (to obtain the data quickly) and then disconnect.  Long term connections can put a usage cost on cellular links to be avoided. The same data is transmitted for long periods without any change until the orbital update (~2hr GPS, ~30min GLO, etc.).

Corrections to Broadcast Ephemerides and Clocks, background

The basic format for sending clock offset and drifts and describing the orbital data for each GPS space vehicle (SV) was developed over 35 years ago.  Times (and politics) have changed and we now provide much more accurate data than the original message definition allowed.  Many of the firms with claims along the lines of “download our orbital data and use them for 2 weeks with better accuracy” exploit this fact.  Other aspects of the problem (clock drifts and walks) can be bounded but cannot be predicted, hence a semi real time data link is wanted.

Thankfully we have IGS to save the day and provide the entire world with various data products in several ready-to-use formats.

To use their own words about this wonderful and vital group:   Mission

The International GNSS Service provides, on an openly available basis, the highest-quality GNSS data, products and services in support of the terrestrial reference frame, Earth observation and research; positioning, navigation and timing; and other applications that benefit science and society.  To learn more about the IGS Mission, and how it is benefitting society, please refer to the IGS Strategic Plan, and the IGS Progress Report.

These two articles provide a very good job of explaining the issues and mention which RTCM message are used for various needs.  Keep in mind that you need to be sure what reference frame is being used with the different orbital data feeds as well.


Connect your Caster Directly

The above two links also include how to register and obtain these data streams directly.  Please consider doing this as it also helps IGS to know who depends on this data, and that in turn helps when budget time comes around.

Date Products on Open SNIP Casters

You can also connect to our casters. (updated Dec 11th 2017)

On the NTRIP.use-snip.com node we run these products:

  • IGS01 with the 1060 message  (GPS Combined Orbit and Clock Correction)
  • IGS03 with the 1057, 1058, and 1064 messages (GPS Orbit, Clock and GLO Clock)

On the RTK2go.com node we run the same products.

Both sites also run RTCM3EPH which provides world-wide broadcast orbits from all the different GNSS systems.

With 1019, 1020, 1043, 1044, 1045, 1046 and 1042 messages

The message rates vary, but clocks are typically updated at 5 second intervals while orbits are typically every 60 seconds.

For RTCM3EPH everything is repeated every 5th second, with the intent that once you have a copy your rover will drop off  (you do not need to, it is just wasteful).  Note that this is every SV from all GNSS systems in the world, including the ones your rover cannot see at present.

The  IGS RTS product streams refer to the International Terrestrial Reference Frame 2014 (ITRF2014).

We will at times use both the Antenna Phase Center (APC) aligned streams, and the Center of Mass (COM) aligned.  Today APC is what the RTCM 3.x standard calls for, but much of the PPP type software out there can deal with either. The caster table entry ‘city name’ tells you which alignment format is used in a given stream.

In the past the stream CLK00 with the 1057 and 1058 messages, and a COM alignment, was also run.

To What Benefit

To state the obvious.  Obtaining better (precise) orbital and clock data means the rover device can better determine (both with or without the presence of local correction) its own positional estimate (the covariance matrix has a smaller residual error).  Many people want to express this only in terms of the Precise Point Positioning (PPP) mode while others recognize its value to traditional RTK well.   The point of view expressed often turns on how rural the GNSS operation is.  If it is covered by cellular, but not in range of a base station, the PPP mode has great attraction.  In the absence of a real time data link, clock updates are not possible.  But in the presence of such a link, a local Base providing RTCM observations is increasingly cost effective.  People often do both.

[For what it is worth, SCSC, the people behind SNIP, feel the real value is found in the urban market supporting the automotive safety community where the distribution of RTCM observations can be frequently interrupted from radio drop outs.   An optimal navigation filter for this mission uses corrections when present, then downgrades to an autonomous filter mode with clock and orbital corrections, and finally becoming fully autonomous with WAAS corrections when out of range.  RTK for everyone is not just a saying.]

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