The Global Positioning System (GPS) has become a powerful tool for ionospheric studies. In addition, ionospheric corrections are necessary for the augmentation systems required for Global Navigation Satellite Systems (GNSS) use. Dual-frequency carrier-phase and code-delay GPS observations are combined to obtain ionospheric observables related to the slant total electron content (sTEC) along the satellite-receiver line-of-sight (LoS). This observable is affected by inter-frequency biases [IFB; often called differential code biases (DCB)] due to the transmitting and the receiving hardware. These biases must be estimated and eliminated from the data in order to calibrate the experimental sTEC obtained from GPS observations. Based on the analysis of single differences of the ionospheric observations obtained from pairs of co-located dual-frequency GPS receivers, this research addresses two major issues: (1) assessing the errors translated from the code-delay to the carrier-phase ionospheric observable by the so-called levelling process, applied to reduce carrier-phase ambiguities from the data; and (2) assessing the short-term stability of receiver IFB. The conclusions achieved are: (1) the levelled carrier-phase ionospheric observable is affected by a systematic error, produced by code-delay multi-path through the levelling procedure; and (2) receiver IFB may experience significant changes during 1 day. The magnitude of both effects depends on the receiver/antenna configuration. Levelling errors found in this research vary from 1.4 total electron content units (TECU) to 5.3 TECU. In addition, intra-day vaiations of code-delay receiver IFB ranging from 1.4 to 8.8 TECU were detected.

/pdf/calibration-errors-on-experimental-slant-total-electron-43a91y0h7z.pdf

Calibration errors on experimental slant total electron content (TEC) determined with GPS

The main source of error in the estimation of TEC (total electron content) from dual Global Positioning System (GPS) data is the effect of the differential satellite and receiver instrumental delay biases. These biases are normally estimated simultaneously with the TEC. However, the additional estimation of the instrumental biases may constitute an insurmountable burden in some practical applications like real-time estimation of TEC, or the estimation may be difficult or correlated to the ionospheric parameters, particularly in situations where the TEC behavior may be harder to model (equatorial or auroral zone, ionospheric storms, etc.). A priori values of the instrumental biases, estimated under good conditions or with global networks, could solve those problems if we could determine how stable those instrumental biases are in time and how often we need to check or reestimate their values. In this paper we will present our estimation of the GPS satellite and receiver instrumental biases from 19 months of data and the study of their variation during that time. We will also show some situations of changes in the instrumental biases and the possible influence of antispoofmg (AS). The main conclusion of this work is that the variation of the estimated differential GPS satellite biases during the 19 months is smaller than 1 ns (1 ns = 2.86 × 1016 e/m2) in most of the cases, with a mean RMS of 0.15 ns. For the GPS receivers used, that variation is greater than for the satellites, with the larger variations corresponding to physical changes in the receivers. The difference of the estimated differential instrumental biases between two consecutive days is in practically all cases smaller than 0.5 ns for the GPS satellites and smaller than 1 ns for the GPS receivers. Regarding the influence of AS, we have detected some significant changes in the instrumental biases of some satellites and some stations whether AS is activated or not. Our main conclusion is that due to the stability of the GPS instrumental biases, only an estimation or calibration of them (under optimal conditions) from time to time is required.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97RS01457

Estimation of total electron content using GPS data: How stable are the differential satellite and receiver instrumental biases?

[1] Dual-frequency Global Positioning System (GPS) receivers present a plausible and cost-effective way of computing Total Electron Content (TEC) For accurate estimates of TEC, frequency-dependent satellite and receiver instrumental biases should be removed from GPS measurements properly Although instrumental satellite bias values are widely available through the internet from various International GPS Service (IGS) analysis centers, receiver biases (also known as differential code biases or interfrequency biases) are provided only for a very few GPS stations and a select number of days This makes it very difficult to compute TEC for a single station In this study, an online, single station receiver bias estimation algorithm, IONOLAB-BIAS, is developed and implemented to obtain daily and monthly averages of receiver bias The algorithm is successfully applied to both quiet and disturbed days of the ionosphere for stations positioned in high-latitude, midlatitude, and equatorial regions The receiver bias estimates are compared with two of the basic methods in the literature that can be applied off-line, and also with the receiver bias values provided from the IGS centers for a select number of stations It is observed that IONOLAB-BIAS is in excellent accordance with the sparse estimates from the IGS centers for all ionospheric states and regions IONOLAB-BIAS has a high potential to be an alternative receiver bias computation algorithm with its ease of implementation and accurate estimates for any single station GPS-TEC

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2007RS003785

Estimation of single station interfrequency receiver bias using GPS-TEC

Opportunities to use the Global Positioning System (GPS) for ionospheric total electron content (TEC) research are reviewed. The era of TEC measurements using very high frequency geosynchronous beacons is essentially over, and the new GPS TECs need to be treated in special ways if they are to augment the existing database of total electron content. Data taken at Boulder, Colorado, show that with appropriate smoothing, GPS TECs can be used to extend the existing database. The time delay data from the International GPS Geodynamics Service over central Europe are used to map the total electron content and reveal regional structures. Global electron-density profiles can now be measured using the GPS/MET (meteorology) system. The measurement of precipitable water vapor in the troposphere to an accuracy of 10% requires that 99.9% of the ionospheric delay be removed.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97RS00451

Studying the ionosphere with the Global Positioning System

[1] A novel regularization technique which can combine signals from all Global Positioning System (GPS) satellites for a given instant and a given receiver is developed to estimate the vertical total electron content (VTEC) values for the 24-hour period without missing any important features in the temporal domain. The algorithm is based on the minimization of a cost function which also includes a high pass penalty filter. Optional weighting function and sliding window median filter are added to enrich the processing and smoothing of the data. The developed regularized estimation algorithm is applied to GPS data for various locations for the solar maximum week of 23–28 April 2001. The parameter set that is required by the estimation algorithm is chosen optimally using appropriate error functions. This robust and optimum parameter set can be used for all latitudes and for both quiet and disturbed days. It is observed that the estimated TEC values are in general accordance with the TEC estimates from other global ionospheric maps, especially for quiet days and midlatitudes. Owing to its 30 s time resolution, the regularized VTEC estimates from the developed algorithm are very successful in representation and tracking of sudden temporal variations of the ionosphere, especially for high latitudes and during ionospheric disturbances.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2002JA009605

Regularized estimation of vertical total electron content from Global Positioning System data

When travelling through the ionosphere the signals of space-based radio navigation systems such as the Global Positioning System (GPS) are subject to modifica- tions in amplitude, phase and polarization. In particular, phase changes due to refraction lead to propagation er- rors of up to 50 m for single-frequency GPS users. If both the L1 and the L2 frequencies transmitted by the GPS satellites are measured, first-order range error contribu- tions of the ionosphere can be determined and removed by di⁄erence methods. The ionospheric contribution is pro- portional to the total electron content (TEC) along the ray path between satellite and receiver. Using about ten Euro- pean GPS receiving stations of the International GPS Service for Geodynamics (IGS), the TEC over Europe is estimated within the geographic ranges!20i4j440iE and 32.5i4/470iN in longitude and latitude, respec- tively. The derived TEC maps over Europe contribute to the study of horizontal coupling and transport proces- ses during significant ionospheric events. Due to their comprehensive information about the high-latitude ionosphere, EISCAT observations may help to study the influence of ionospheric phenomena upon propagation errors in GPS navigation systems. Since there are still some accuracy limiting problems to be solved in TEC determination using GPS, data comparison of TEC with vertical electron density profiles derived from EISCAT observations is valuable to enhance the accuracy of propagation-error estimations. This is evident both for absolute TEC calibration as well as for the conversion of ray-path-related observations to vertical TEC. The com- bination of EISCAT data and GPS-derived TEC data enables a better understanding of large-scale ionospheric processes.

/pdf/relationships-between-gps-signal-propagation-errors-and-33qhpenwek.pdf

Relationships between GPS-signal propagation errors and EISCAT observations

Radio beacon measurements have effectively been used in exploring the temporal and spatial structure of the ionosphere since nearly 3 decades.

About the Use of GPS Measurements for Ionospheric Studies

Real Time Monitoring of the Ionosphere

Relationships between GPS propagation errors and Eiscat observation.

Due to the recent developments towards an European Navigation Satellite System and the actual effords in aviation, a rising interest in the real-time monitoring of the ionosphere is noticed. The crucial task for the estimation of the total electron content (TEC) in real-time is in the separation of the ionospheric delay from the total error budget. Parameters affecting the total error budget have been identified to be the number of epochs before the first ambiguity fixing, elevation cut-off angle, the utilization of either predicted or real-time satellite/receiver instrumental biases, and the handling of multipath effects.

D. Klähn

Papers

Relationships between GPS-signal propagation errors and EISCAT observations

About the Use of GPS Measurements for Ionospheric Studies

Real Time Monitoring of the Ionosphere

Relationships between GPS propagation errors and Eiscat observation.

On the Implementation of a GPS Based Method for the Real-Time Measurement of the Total Electron Content of the Ionosphere