Measurements from Mobile GPS for Ionospheric Awareness
ABSTRACT: Propagation Research Associates, Inc., (PRA), teamed with the Jet Propulsion Laboratory (JPL), proposes to design and evaluate an algorithm to produce useful measurements of the state of the ionosphere from Global Positioning System (GPS) equipment used on mobile platforms. We will address how methods of TEC estimation that have been designed for multiple ground-based, stationary receivers recording data over long time periods work for single GPS receivers on moving platforms. These methods will be modified as necessary to allow for a receiver-based estimation of TEC and a solid understanding of the uncertainties. To this end, in Phase I, PRA will first do a survey of the computational and memory capabilities of DoD equipment to determine the required operating parameters of the algorithm. PRA will also gather field data in scenarios that reproduce the motion and signal dropout that might be encountered by equipment mounted on vehicles. Finally, PRA and JPL will design a robust, standalone algorithm that works internal to DoD equipment to produce both a measurement of TEC and its uncertainty estimate. PRA will study the results of applying this algorithm to the GPS field data collected under this effort. BENEFIT: The advent of the Global Positioning System (GPS) was a boon to ionospheric science. The two initial GPS signals, known as L1 (1575.42 MHz) and L2 (1227.60 MHz), are separated enough in frequency to allow direct measurement of the total electron content (TEC) of the ionosphere, a layer of ionized gas located in the Earth"s upper atmosphere. In the years following the launch of the GPS satellites, physics-based models have been developed that assimilate TEC from GPS measurements and calculate both the current state of the ionosphere and predictions of its future state. These ionospheric models are only as complete as the data fed into them. One source of standardized GPS data is the International GNSS Service, which collects and distributes data from GPS receiver locations around the globe. These stationary and ground-based receivers probe the ionosphere within a roughly 1200 km radius in ground range depending on the elevation angle cutoff. Ionospheric locations such as the middle of the African continent and out in the Pacific Ocean are not well sampled by this network. Space-based orbiting GPS receivers, such as the FORMOSAT-3/COSMIC constellation, cover these areas but are expensive to operate and subject to satellite availability. It would be a great advantage to increase the sampling of the ionosphere using GPS equipment that may already be in these areas but not currently tasked to produce ionospheric measurements. Dual-frequency GPS receivers are used on a wide variety of mobile platforms, both civilian and military. Aircraft and ships routinely travel through areas undersampled by ground-based receivers, and military personnel in ground vehicles are often located in areas where ionospheric effects on communication are of great interest. Collecting ionospheric measurements from these receivers would significantly expand the data ingested into ionospheric models and could improve their accuracy, provided the accuracy of the TEC measurements was sufficient and well understood.
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