Satellite-based navigation is an essential part of all the technology-dependent applications, such as road transport, cell phones, the medical field, aviation or the shipping industry, etc. The performance of the navigation systems depends upon how quickly they can acquire and process the received signals for positioning solutions. However, in dense urban or indoor environments, signal acquisition can be a challenging task due to fading as a result of multipath and/or interference. This paper presents post-processing acquisition results on Global Positioning System (GPS) signals to study the relationship between data lengths used for signal acquisition and the achieved signal power using a Fast Fourier Transform (FFT)-based circular correlation method. Based on this study, the detection performance of the FFT-based method has also been analyzed by intentionally degrading the signal power levels. A new Adaptive Data length (ADL) method for acquisition has been proposed in this paper, which can be used for speeding up the acquisition process and uses adaptive data lengths rather than fixed data lengths. The ADL method works by estimating the threshold level based on the noise present in the signal and then comparing it with the signal power levels. Less difference between the threshold level and signal power level means less data length will be used while more difference means that more data length will be used for acquisition. The proposed algorithm can be used in commercially available receivers for adopting to an adaptive acquisition process for increased efficiency.

https://www.mdpi.com/2079-9292/10/14/1735/pdf

Adaptive Data Length Method for GPS Signal Acquisition in Weak to Strong Fading Conditions

. The emerging technique of GPS Radio Occultation has been used to detect the ionospheric irregularities prominent in the F-region known as equatorial plasma bubbles. The plasma bubbles are characterized by depreciated regions of electron density. For investigating the plasma bubbles, a nine-year (2008–2016) long time series of signal-to-noise ratio data are used from the vertical GPS radio occultation profiles. The variation in the signal-to-noise ratio of the GPS signals can be linked to vertical changes in the electron density profiles that mainly occur in line with the irregularities in the Earth's ionosphere. The analysis revealed that the F-region irregularities, associated with plasma bubbles occur mainly post sunset close to Earth's geomagnetic equator. Dependence on the solar cycle as well as distinctive seasonal variation is observed when analyzed for different years. In contrast to the other ionospheric remote sensing methods, GPS Radio Occultation technique uniquely personifies the activity of the plasma bubbles based on altitude resolution on a global scale.

/pdf/global-climatology-of-equatorial-plasma-bubbles-based-on-gps-1jhp38ynba.pdf

Global Climatology of Equatorial Plasma Bubbles based on GPS Radio Occultation from FormoSat-3/COSMIC

The ionosphere is a medium of free moving electrons where the presence of irregularities (electron density) may cause rapid fluctuations in the phase and/or amplitude of the received GPS signals also known as ionospheric scintillation which may result in reduced performance of the satellite-based navigation systems such as GPS, GLONASS, Galileo etc. This paper studies the occurrence of ionospheric scintillation and TEC variations over Pakistan using real-time GPS signals by installing Septentrio PolaRx5s GNSS receiver at Sukkur IBA University (27.73° N, 68.82° $E$ ). The study uses the data collected during the month of February and March 2019. The main objective of this paper is to study the scintillation effects on GPS receivers through loss of lock and carrier-to-noise ratio in relationship to TEC variations over Pakistan for the first time. The need for this study arises as Pakistan is located at mid-latitudes and, there are only very few scintillation studies exists for mid-latitude regions and hence this paper will help in understanding the scintillation morphology over Pakistan. The initial study on scintillation presented in this paper suggests that the navigational receivers at mid-latitudes (Pakistan) are more likely to be affected by amplitude scintillation due to high values as compared to the phase scintillation. This can be problematic for the high precision applications such as aviations industry because amplitude scintillation affects the signal-to-noise ratio (SNR) of the signal and therefore signal quality.

Investigation of Ionospheric Scintillations and Total Electron Content (TEC) over Mid-latitudinal Region (Pakistan)

The signal acquisition in GPS receivers is the first and very crucial process that may affect the overall performance of a navigation receiver. Acquisition program initiates a searching operation on received navigation signals to detect and identify the visible satellites. However, signal acquisition becomes a very challenging task in a degraded environment (i.e, dense urban) and the receiver may not be able to detect the satellites present in radio-vicinity, thus cannot estimate an accurate position solution. In such environments, satellite signals are attenuated and fluctuated due to fading introduced by Multipath and NLOS reception. To perform signal acquisition in such degraded environments, larger data accumulation can be effective in enhancing SNR, which tradeoff huge computational load, prolonged acquisition time and high cost of receiver. This paper highlights the effects of fading on satellite signal acquisition in GPS receiver through variable data lengths and SNR comparison, and then develops a statistical relationship between satellite visibility and SNR. Furthermore it also analyzes/investigates the tradeoff between computation load and signal data length.

/pdf/the-effects-of-using-variable-lengths-for-degraded-signal-mru5lqt8sl.pdf

The effects of using variable lengths for degraded signal acquisition in GPS receivers

The performance of navigational receivers using satellite-based navigation technology can be severely affected by the presence of time-varying electron-density fluctuations in the ionosphere which can cause amplitude and phase perturbations at the receiver resulting in loss of phase lock at the carrier tracking loop due to cycle slip and hence unavailability of the navigation services. This paper studies the effects of amplitude and phase fluctuations at high latitudes due to the irregular ionosphere and, their effects on the receiver performance by using real time raw data from the Global Positioning Systems (GPS) satellites. The paper also suggest the use of adaptive software-based receiver model or modified hardware receivers to mitigate the effects of amplitude and phase fluctuations due to irregular ionosphere.

Sunny Imam Ali

Papers

The Effects of Ionospheric Irregularities on the Navigational Receivers and Its Mitigation