Global ionosphere maps (GIMs) provided by the global navigation satellite systems (GNSS) data are essential in ionospheric research as the source of the global vertical total electron content (VTEC). However, conventional GIMs experience lower accuracy and reliability from uneven distribution of GNSS tracking stations, especially in ocean areas with few tracking stations. The orbits of ocean altimetry satellite cover vast ocean areas and can directly provide VTEC at nadir with two different wavelengths of radio waves. Radio occultation observations and the beacons of Doppler orbitography and radio positioning integrated by satellite (DORIS) are evenly distributed globally. Satellite altimetry, radio occultation and DORIS can compensate GNSS data in ocean areas, allowing a more accurate and reliable GIMs to be formed with the integration of these observations. This study builds GIMs with temporal intervals of 2 h by the integration of GNSS, satellite altimetry, radio occultation and DORIS data. We investigate the integration method for multi-source data and used the data in May 2013 to validate the effectiveness of integration. Result shows that VTEC changes by ź11.0 to ź7.0 TECU after the integration of satellite altimetry, radio occultation and DORIS data. The maximum root mean square decreases by 5.5 TECU, and the accuracy of GIMs in ocean areas improves significantly.

Global ionosphere maps based on GNSS, satellite altimetry, radio occultation and DORIS

Ionosphere total electron content (TEC) from global ionospheric maps (GIM) is widely applied in both ionospheric delay correction and research on space weather monitoring. Global ionospheric modeling based on multisource data is an effective method to improve conventional GIM accuracy and reliability. In this study, a global ionospheric model is constructed from multi-GNSS (here, GPS/GLONASS/BDS), satellite altimetry and Formosat-3/COSMIC (F3/C) observations using a spherical harmonic (SH) function. The results show that compared to the conventional GIM derived from GPS/GLONASS data, the combined GIM performance from multisource data improves significantly; the RMS versus external data decreases from [2, 5] to [2, 3] TECU, and the BIAS decreases from [− 3, 1] to [− 1, 1] TECU. Specifically, BDS observations improve the IPP distributions, especially over the region of Australia; compared with GPS-based ionospheric TEC. Our calculated GIM with BDS data has better performance than that without BDS data. By combining JASON 2 and GPS/GLONASS data, the residual distribution is more concentrated, and the RMS is improved effectively in mid-high latitudes of the southern hemisphere and in the equatorial region. F3/C TEC also exhibits relatively minor improvements on GIM; the standard deviation reduces from 2.89 to 1.92 TECU, and the BIAS regarding extra F3/C data decreases from − 2.02 to − 1.71 TECU.

Global ionospheric modeling based on multi-GNSS, satellite altimetry, and Formosat-3/COSMIC data

. The Daedalus mission has been proposed to the European Space
Agency (ESA) in response to the call for ideas for the Earth Observation
program's 10th Earth Explorer. It was selected in 2018 as one of
three candidates for a phase-0 feasibility study. The goal of the mission is
to quantify the key electrodynamic processes that determine the structure
and composition of the upper atmosphere, the gateway between the Earth's
atmosphere and space. An innovative preliminary mission design allows
Daedalus to access electrodynamics processes down to altitudes of 150 km and
below. Daedalus will perform in situ measurements of plasma density and
temperature, ion drift, neutral density and wind, ion and neutral
composition, electric and magnetic fields, and precipitating particles. These
measurements will unambiguously quantify the amount of energy deposited in
the upper atmosphere during active and quiet geomagnetic times via Joule
heating and energetic particle precipitation, estimates of which currently
vary by orders of magnitude between models and observation methods. An
innovation of the Daedalus preliminary mission concept is that it includes
the release of subsatellites at low altitudes: combined with the main
spacecraft, these subsatellites will provide multipoint measurements
throughout the lower thermosphere–ionosphere (LTI) region, down to altitudes
below 120 km, in the heart of the most under-explored region in the Earth's
atmosphere. This paper describes Daedalus as originally proposed to the ESA.

/pdf/daedalus-a-low-flying-spacecraft-for-in-situ-exploration-of-3gyx91hnb8.pdf

Daedalus: a low-flying spacecraft for in situ exploration of the lower thermosphere–ionosphere

The ionosphere is a dispersive medium for space geodetic techniques operating in the microwave band. Thus, signals traveling through this medium are—to the first approximation—affected proportionally to the inverse of the square of their frequencies. This effect, on the other hand, can reveal information about the parameters of the ionosphere in terms of Total Electron Content (TEC) of the electron density. This part of the book provides an overview of ionospheric effects on microwave signals. First, the group and phase velocities are defined along with the refractive index in the ionosphere and the ionospheric delay. Then, we focus mainly on the mitigation and elimination of ionospheric delays in the analysis of space geodetic observations, specifically for Global Navigation Satellite Systems (GNSS) and Very Long Baseline Interferometry (VLBI) observations. In particular, we summarize existing models as well as strategies based on observations at two or more frequencies to eliminate first and higher order delays. Finally, we review various space geodetic techniques (including satellite altimetry and radio occultation data) for estimating values and maps of TEC.

https://www.springer.com/cda/content/document/cda_downloaddocument/9783642369315-c2.pdf?SGWID=0-0-45-1404781-p174970686

Ionospheric Effects on Microwave Signals

The Compass/BeiDou system is currently being built as a navigation constellation consisting of 16 navigation satellites. Construction of these satellites will significantly increase the number of visible satellites over the Chinese mainland and improve the geometry of satellite positioning. We obtained data by simulation and measurements to analyze the influence of BeiDou regarding the longest observation arc and the ionosphere piercing point distribution. A regional ionosphere delay model is built using data measured by BeiDou only, global positioning system (GPS) only, and the dual-satellite system. The results show that the model accuracy for BeiDou only is as accurate as the single GPS system in the middle and lower latitudes, while a deviation becomes noticeable at high latitudes and over marginal areas where observations are fewer due to lack of BeiDou satellites. With the current distribution of the satellites and tracking stations, it appears that the dual-satellite system could significantly improve the ionospheric model in China and the accuracy of differential code bias (DCB) determination. The experimental results also show that the BeiDou satellite DCB is quite stable, with a monthly maximum change of 1.8 ns.

/pdf/modeling-regional-ionospheric-delay-with-ground-based-beidou-35qb0pg2bx.pdf

Modeling regional ionospheric delay with ground-based BeiDou and GPS observations in China

For space geodetic techniques, operating in microwave band, ionosphere is a dispersive medium; thus signals traveling through this medium are in the first approximation affected proportional to inverse of the square of their frequencies. This effect allows gaining information about the parameters of the ionosphere in terms of Total Electron Content (TEC) or the electron density (Ne). TEC or electron density can then be expressed by means of spherical harmonic base functions to provide a Global Ionosphere Map (GIM). The classical input data for development of GIMs are obtained from dual-frequency observations carried out at Global Navigation Satellite Systems (GNSS) stations. However, GNSS stations are in-homogeneously distributed around the world, with large gaps particularly over the oceans; this fact reduces the precision of the GIM over these areas. On the other hand, dual-frequency satellite altimetry missions such as Jason-1 provide information about the ionosphere precisely above the oceans; and furthermore Low Earth Orbiting (LEO) satellites, such as Formosat-3/COSMIC (F/C) provide well-distributed information of ionosphere globally. This study investigates on global modeling of TEC through combining GNSS and satellite altimetry data with global TEC data derived from the occultation measurements of the F/C mission. The combined GIMs of vertical TEC (VTEC) show a maximum difference of 1.3–1.7 TEC units (TECU) with respect to the GNSS-only GIMs in the whole day. The root mean square error (RMS) maps of combined solution show a reduction of about 0.1 TECU in the whole day. This decrease of RMS can reach up to 0.5 TECU in areas where no or few GNSS observations are available, but high number of F/C measurement is carried out. This proves that the combined GIMs provide a more homogeneous global coverage and higher reliability than results of each single method. All comparisons and validations made within this study provide vital information regarding combination and integration of various observation techniques in the Global Geodetic Observing System of the International Association of Geodesy.

http://meetingorganizer.copernicus.org/EGU2009/EGU2009-6927.pdf

S. Todorova

Papers

Global ionosphere maps of VTEC from GNSS, satellite altimetry and FORMOSAT-3/COSMIC data