The use of ionospheric tomography and elevation masks to reduce the overall error in single-frequency GPS timing applications

Abstract

Signals from Global Positioning System (GPS) satellites at the horizon or at low elevations are often excluded from a GPS solution because they experience considerable ionospheric delays and multipath effects. Their exclusion can degrade the overall satellite geometry for the calculations, resulting in greater errors; an effect known as the Dilution of Precision (DOP). In contrast, signals from high elevation satellites experience less ionospheric delays and multipath effects. The aim is to find a balance in the choice of elevation mask, to reduce the propagation delays and multipath whilst maintaining good satellite geometry, and to use tomography to correct for the ionosphere and thus improve single-frequency GPS timing accuracy. GPS data, collected from a global network of dual-frequency GPS receivers, have been used to produce four GPS timing solutions, each with a different ionospheric compensation technique. One solution uses a 4D tomographic algorithm, Multi Instrument Data Analysis System (MIDAS), to compensate for the ionospheric delay. Maps of ionospheric electron density are produced and used to correct the singlefrequency pseudorange observations. This method is compared to a dual-frequency solution and two other single-frequency solutions: one does not include any ionospheric compensation and the other uses the broadcast Klobuchar model. Data from the solar maximum year 2002 and October 2003 have been investigated to display results when the ionospheric delays are large and variable. The study focuses on Europe and results are produced for the chosen test site, VILL (Villafranca, Spain). The effects of excluding all of the GPS satellites below various elevation masks, ranging from 5° to 40°, on timing solutions for fixed (static) and mobile (moving) situations are presented. The greatest timing accuracies when using the fixed GPS receiver technique are obtained by using a 40° mask, rather than a 5° mask. The mobile GPS timing solutions are most accurate when satellites at lower elevations continue to be included: using a mask between 10° and 20°. MIDAS offers the most accurate and least variable single-frequency timing solution and accuracies to within 10 ns are achieved for fixed GPS receiver situations. Future improvements are anticipated by combining both GPS and Galileo data towards computing a timing solution.

Ionospheric corrections for GPS time transfer

“…Interestingly, the Klobuchar time transfer is sometimes less stable than the time transfer with no correction. This concurs with the conclusions drawn by Rose et al [2011], which showed that the Klobuchar model can sometimes produce a worse result than if no ionospheric model were used at all.…”

“…This concurs with the conclusions drawn by Rose et al . [], which showed that the Klobuchar model can sometimes produce a worse result than if no ionospheric model were used at all.…”

“…AV is advantageous in comparison to CV over a long baseline because it makes use of all available satellites. This enables data from high‐elevation satellites to be included which provides more accurate results in comparison to using only low‐elevation satellites [ Rose et al ., ]. AV also allows time transfers to be made anywhere in the world using a GNSS system without requiring the satellites to be in common view.…”

Automated identification and tracking of polar-cap plasma patches at solar minimum

“…The patches are identified from the output of an algorithm, the Multi-Instrument Data Assimilation System (MIDAS). MIDAS is a tomographic imaging algorithm, with a ten-year history at the time of writing (Mitchell and Spencer, 2003), that has been applied to many ionospheric studies, for example Alfonsi et al (2011), Muella et al (2011, Rose et al (2009Rose et al ( , 2011 and Yin and Mitchell (2011). It solves an under-determined four-dimensional inverse problem in order to compute a gridbased image of the electron concentration of the ionosphere in a user-selected region.…”

Investigation into a GPS Time Pulse Radiator for Testing Time-Stamp Accuracy of a Radio Telescope

“…whilst maintaining good satellite geometry. A study done by [53] recommended that GPS timing receivers deployed in relatively mobile timing applications as presented in this thesis, are most accurate when satellites at lower elevations continue to be included with an elevation mask in the range 10 • to 20 • . It should be mentioned that it was a good thing that the GPS receiver maintained the Survey-In mode during the calibration because the GTR was designed to be portable instrument capable of dynamically determining its stationary position.…”