Ionospheric corrections to precise time transfer using GPS

The free electrons in the earth's ionosphere can retard the time of reception of GPS signals received at a ground station, compared to their time in free space, by many tens of nanoseconds, thus limiting the accuracy of time transfer by GPS. The amount of the ionospheric time delay is proportional to the total number of electrons encountered by the wave on its path from each GPS satellite to a receiver. This integrated number of electrons is called Total Electron Content, or TEC. Dual frequency GPS receivers designed by Allen Osborne Associates, Inc. (AOA) directly measure both the ionospheric differential group delay and the differential carrier phase advance for the two GPS frequencies and derive from this the TEC between the receiver and each GPS satellite in track. The group delay information is mainly used to provide an absolute calibration to the relative differential carrier phase, which is an extremely precise measure of relative TEC. The AOA Mini-Rogue ICS-4Z and the AOA TurboRogue ICS-4000Z receivers normally operate using the GPS P code, when available, and switch to cross-correlation signal processing when the GPS satellites are in the Anti-Spoofing (A-S) mode and the P code is encrypted. An AOA ICS-Z receiver has been operated continuously for over a year at Hanscom AFB, MA to determine the statistics of the variability of the TEC parameter using signals from up to four different directions simultaneously. The 4-channel ICS-4Z and the 8-channel ICS-4000Z, have proven capabilities to make precise, well calibrated, measurements of the ionosphere in several directions simultaneously. In addition to providing ionospheric corrections for precise time transfer via satellite, this dual frequency design allows full code and automatic codeless operation of both the differential group delay and differential carrier phase for numerous ionospheric experiments being conducted. Statistical results of the data collected from the ICS-4Z during the initial year of ionospheric time delay in the northeastern U.S., and initial results with the ICS-4000Z, will be presented

Two generations of progress in ionospheric research

351-355Many of the great advances in ionospheric research in the last 40 years have come about because of improvements in technology, especially through modern electronics and optical and computational advances. The advent of the space age, with satellite and rocket capability, has provided a major improvement in our ability to measure the upper atmosphere and the ionosphere. The construction of realistic theoretical models of the upper atmosphere and the ionosphere have made great strides in our understanding of the relative effects of the various driving forces on the ionosphere. These are discussed in some detail along with prospects for future breakthroughs in technology which will benefit ionospheric research. One particularly significant example to be described in detail is the improvement of knowledge of the now well known equatorial anomaly, briefly described in S. K. Mitra’s book [The Upper Atmosphere (The Asiatic Society, Calcutta, India), 1947, 1952]. During the course of the 1950s, and even to the present date, the anomaly in F2-region electron density has been an active area of study, with much of this research being done by Indian scientists. Despite the many experimental results from both ground-based and satellite-borne sounders, and total electron content measurements, clearly showing the variability of the equatorial anomaly, it was not until the 1980s when a model of the low latitude ionosphere derived from first principles was able to reasonably well reproduce the observations discussed in Mitra’s book. Other examples of areas still needing additional experimental and theoretical research are also described

A First-order, worldwide, ionospheric, time-delay algorithm /

"Ionospheric Physics Laboratory Project 4643.""25 September 1975."Mode of access: Internet