ESOC Ionosphere Monitoring Facility (IONMON)
Introduction
The ESOC Ionosphere Monitoring Facility (IONMON) software (version 1) has been developed at ESOC in order to use dual-frequency GPS data, that are collected from the International GPS Service (IGS) global ground station network, to produce ionosphere products in batch estimates. Since satellite signals are affected by ionospheric effects in dependence on their frequency, global dual-frequency GPS data offers a unique opportunity to observe directly the ionosphere's electron content and to use that information for the routine establishment of ionospheric electron density models. The IONMON is part of ESOC's GPS Tracking Data and Analysis Facility (GPS TDAF).
The IONMON has been developed to:
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Contribute to the ionosphere analysis within the IGS. On 28 May 1998 an IGS Ionosphere Working Group has been established to which ESOC is contributing as an Analysis Center with IONMON ionosphere products.
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Support of ESA missions (e.g. ERS and ENVISAT) by providing high precision information about ionospheric Total Electron Content (TEC) allowing the calculation of corrections to tracking data collected from ESA spacecraft, independently of a spacecraft's altitude and of up/downlink frequencies. These ESA spacecraft tracking data might be gathered at certain ESA ground sites, as well as might be regional or worldwide collected S-band & altimetry data.
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Further tasks of ionosphere monitoring and analysis at the Multi-Mission Support Section of the Flight Dynamics Division of ESOC.
Mathematical models
The parameters that can be estimated with the IONMON are the coefficients of mathematical models to represent the ionosphere's vertical TEC and transmitter and receiver differential code biases. The IONMON offers several two dimensional (2D) single layer models as well as a 3D Chapman profile-based model the represent the Ionosphere's TEC. For 2D single layer modeling local polynomials, global Gauss-Type Exponential (GE) functions and global spherical harmonics can be invoked. The Chapman profile approach describes the Ionosphere's electron density three dimensionally. The TEC itself between two points i and j is expressed by the analytical integral:
where
- X
- is the solar zenith distance,
- N0
- is the maximum electron density of the Chapman profile referred to X=0 deg.,
- h0
- is the height of maximum electron density N0 above Earth surface at X=0 deg.,
- h
- is the height above Earth surface at which the electron density along the profile is wanted,
- Z
- is the satellite's zenith distance as viewed from ground station.
This integral is calculated using the following relation:
To account for the changes of the zenith distances X and Z along the signal path, the integral is evaluated semi-analytically.
The layer of maximum electron density N0 and its height h0 are the key parameters of the Chapman profile approach. Both are estimated as functions of geomagnetic latitude and local time.
Global variation of N0 and h0 with geomagnetic latitude and local time thus causes also a global variation of the Chapman profile itself.
Routine Processing of Ionosphere Products at ESOC
At the beginning of 1998 the routine processing of ionosphere maps in final and in rapid mode has been started at ESOC. The first day that was processed in final mode was the 28 December 1997, and processing in rapid mode started for 19 March 1998. 24 hours of so called TEC observables, derived from carrier phase leveled to code data, are fitted to 2D single layer TEC models, as well as to 3D Chapman profile models. The processing sequence is as follows for one day of GPS-derived TEC observation data:
- A nighttime TEC data fit is made to obtain reference values for the Differential Code Biases (DCBs). The nighttime TEC itself is absorbed in this fit with a low degree and order spherical harmonic. In the other fits (2 to 4) these DCBs are then introduced as constraints for that day.
- A single layer GE-function is fitted to the TEC data. The results of this estimate are intended for the ESOC-internal interpretation of results and comparison with the other fits.
- A Chapman profile model is fitted to the TEC data, where the layer of maximum electron density N0 and its height h0 are estimated as GE-function resp. as extended sin-function. h0 is restricted to achieve values within a predefined height range only, currently 400 to 450 km. From these plots an animation showing the change in TEC along the day can be created.
- A Chapman profile model is fitted to the TEC data, where now h0 is fitted as a global constant for the 24 hours. This run allows us to observe the evolution of the height of maximum electron density.
Outlook
An External User Interface is under development to make updated TEC maps available to the external world in form of data files and evaluation software.
IONMON version 2 will provide additional services:
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Calculation of sequential estimates allowing for on-line updating of ionosphere TEC maps, real-time applications and navigation purposes.
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Usage of more sophisticated and physically funded mathematical models, e.g. improved ionosphere profiles.
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Processing of other than GPS data, namely GLONASS, PRARE, DORIS and SST data.
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