Why is the SPARTN ionospheric term negative?

[ferraragianluigi97-source]

Hi everyone,

I am working with SPARTN HPAC ionospheric corrections and I am trying to understand how the ionospheric term should be interpreted from the receiver/user side.

In my case, I have messages with no grid information, so I assume I_G = 0, and with SF054 = 0, meaning the ionospheric polynomial reduces to the constant term c_{00} (from SF057).

For some satellites, SF057 is negative, so the resulting slant ionospheric value is also negative.
My question is: should this quantity be interpreted as an absolute slant ionospheric delay (similar to STEC-derived delay), or as a signed correction/residual to be applied to the pseudorange/carrier measurements according to the SPARTN convention?

An example:

{ "identity": "SPARTN-1X-HPAC-GPS", "timeTagtype": 1, "gnssTimeTag": 505521300, "SF005": 143, "SF068": 12, "SF069": 0, "SF030": 0, "atmareablock": [ { "SF031": 143, "SF039": 0, "SF040T": 1, "SF040I": 1, "troposphereblock": { "SF041": 0, "SF042": 1, "SF043": 2.32, "SF044": 0, "SF045": 0.08000000000000002 }, "SF054": 0, "ionosphereblock": [ {"PRN": 1, "SF055": 1, "SF056": 0, "SF057": -27.919999999999995}, {"PRN": 2, "SF055": 1, "SF056": 0, "SF057": -32.71999999999999}, {"PRN": 3, "SF055": 1, "SF056": 0, "SF057": -23.559999999999995}, {"PRN": 4, "SF055": 1, "SF056": 0, "SF057": -8.079999999999998}, {"PRN": 8, "SF055": 1, "SF056": 0, "SF057": 12.36}, {"PRN": 17, "SF055": 1, "SF056": 0, "SF057": -20.359999999999992}, {"PRN": 28, "SF055": 1, "SF056": 0, "SF057": -17.439999999999998}, {"PRN": 32, "SF055": 1, "SF056": 0, "SF057": 0.44000000000001194} ] } ] }

More specifically:
• If c_{00} is negative, does that mean the physical slant ionospheric delay is negative?
• Or does it mean that SPARTN is providing a negative correction term relative to an internal/background model used by the service provider?
• When comparing SPARTN with a GIM-derived STEC delay, should I compare them directly in meters/TECU, or should SPARTN first be interpreted as a correction in the observable domain rather than as an absolute ionospheric delay?
I am asking because my SPARTN-derived values can be negative, while the GIM-derived ionospheric delay is naturally positive, and I want to make sure I am comparing quantities with the same physical meaning.

Thanks in advance

[semuadmin]

Hi @ferraragianluigi97-source ,

This is probably a question for the protocol authors (i.e. u-blox) or a more specialist technical forum.

The protocol defines the ionospheric delay coefficients as follows, and the pyspartn parser simply implements this format:

[ferraragianluigi97-source]

Thanks a lot anyway. I'll try contacting the provider.

[ferraragianluigi97-source]

Sorry, I have another question. Is SF057 already scaled with the resolution by the parser? For example, if SF057 = 20, do I have to scale it by 0.04, or does the parser do it automatically?

Thank you.

[semuadmin]

Sorry, I have another question. Is SF057 already scaled with the resolution by the parser? For example, if SF057 = 20, do I have to scale it by 0.04, or does the parser do it automatically?

pyspartn applies the documented scaling factor and range automatically as part of the SPARTN_DATA_FIELDS data dictionary, e.g. in the following, FL signifies 'float', 12 signifies 12 bits, 0.04 signifies scaling factor (resolution) and -81.88 signifies range minimum:

pyspartn/src/pyspartn/spartntypes_core.py

Line 151 in 6f2356f

"SF057": (FL, 12, 0.04, -81.88, "Small ionosphere coefficient C00"),

[ferraragianluigi97-source]

Thank you so much

[ferraragianluigi97-source]

Hi, I apologize again for the inconvenience. I can't find a parser for BPAC messages in the documentation. Can you help me?

[semuadmin]

Hi, I apologize again for the inconvenience. I can't find a parser for BPAC messages in the documentation. Can you help me?

pyspartn is cabable of parsing BPAC (Basic-precision atmosphere correction) messages - it covers all the message types defined in the SPARTN 2 protocol specification - but testing has been limited as none of the subscription SPARTN services available to me in the UK output the BPAC message type. They use the HPAC (High-precision atmosphere correction) message instead.

I'm going to close this discussion stream now as the original question has been answered. If you have any further queries, could I ask you to raise them in a new thread? This makes it easier for me to monitor responses. Thanks