What is the range of values of ionospheric delay and its uncertainty in GNSS positioning?
Dear Shanker
It depends on the data you use for computing the ionospheric delay. For example, if you apply the GNSS pseudo-ranges, then the computed ionospheric delay (which derives from geometry free combination) will have the error of the order of code measurements, and if you use phase observables (in continuous arcs) then the ionospheric correction will have lower noise level. GIM models also provide the ionospheric corrections. These models are very useful in altimetry processes, specially in shallow waters like coastal and inland waters and make better results than radiometer measurements. The most important property of the GIM values is the negative sign of the correction which is not true in some of the altimetry radiometer measurements.
as far as I know this goes down to a fraction of a nanosecond using all kind of correction rules
It is a great question, and it is impossible to give an exact and short answer. The magnitude of the ionospheric effect on GNSS-derived coordinates depends on many factors, including the frequencies of the receiver, solar activity, type of satellite positioning, implemented corrections in data collection and processing, etc.
However, if dual-frequency receivers are used, with ephemeris and basic ionospheric models applied, the ionospheric effect can be filtered very efficiently and is not a significant problem for the majority of applications. Only applications that require millimetric uncertainties, such as geodynamic studies, still require more advanced ionospheric modelling.
If you are interested in reading more about this topic, there are many online sources available, such as:
https://www.e-education.psu.edu/geog862/node/1715
https://gssc.esa.int/navipedia/index.php/Ionospheric_Delay
https://mdpi-res.com/d_attachment/remotesensing/remotesensing-11-00171/article_deploy/remotesensing-11-00171.pdf?version=1547721587
Hopefully, this answers your question.
The list of resources are pretty useful. Thank you very much Matej Varga .
In order to have an accurate position solution the ionospheric effect need to be mitigated; different approaches can be used depending on the receiver characteristics. In particular, ionospheric error can be reduced using multi-frequency measurements, exploiting corrections provided by an augmentation system, relying on a model such as Klobuchar, NeQUick, NTCM. In the first approach, the dispersive nature of the ionosphere is exploited: the ionospheric effect is mitigated using a linear combination of measurements from two frequencies. Although, this approach allows to remove up to 99% of the ionospheric error it requires the use of a multi-frequency receiver that is in general more expensive than a single frequency device; in addition, the combination of the measurements lead to an increased measurement noise. Single frequency devices could limit the ionospheric error using an augmentation system like European Geostationary Navigation Overlay Service (EGNOS) or Wide Area Augmentation System (WAAS) ; these systems broadcast ionospheric correction for specific coverage areas. With this approach, a receiver needs access the correction through an internet connection or processing the Satellite Based Augmentation System (SBAS) signals adding complexity to the device and to the processing chain. Single frequency stand-alone devices need to mitigate the ionospheric error using a model. Recently, an empirical model Neustrelitz Total Electron Content Model for Galileo (NTCM G) has been proposed as an alternative to Klobuchar and NeQuick-G (currently adopted by GPS and Galileo, respectively). The three models exploits different approaches and have different characteristics: Klobuchar model is quite simple to implement and it is able to correct up to 50% of the ionospheric error; some advantages have been demonstrated when using the NeQuick-G model, which is able to correct up to 70% of the ionospheric error but it has an higher computation load.
So the accuracy with which you can estimate the iono delay is function of the strategy adopted and the model used. Finally, in case of disturbances the discrepancy between the estimation and the current iono delay could be very large.
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