ANALYSIS OF SOFTWARE BASED GPS DATA FOR STUDYING SOME CHARACTERISTICS OF IONOSPHERIC IRREGULARITIES

Aritri Debnath (Ghosh)Electronics and Communication Department

Sikkim Manipal Institute of Technology,Sikkim Manipal University

Majitar, Rangpo, East-Sikkim, Indiaaritridebnath06@gmail.com

Tanushree Bose (Roy)Electronics and Communication Department

Sikkim Manipal Institute of Technology,Sikkim Manipal University

Majitar, Rangpo, East-Sikkim, Indiacontact_tanushree@rediffmail.com

Abstract— In this work, analysis of Software based GPS Receiver data for the possible identification of different Ionospheric propagation effects is done. An algorithm is developed for data ordering from the raw input files received from the GPS receiver, in a manner suitable for studying against a time baseline. Analysis of the Carrier to Noise Ratio (CNO), Total Electron Content (TEC) and Phase data from the software based GPS receiver to estimate the accuracy of position fixing and to identify Ionospheric Irregularities is also done.

Keywords-GPS, Software based dual frequency GPS Receiver, TEC, Ionospheric propagation, Ionospheric Scintillation.

I. INTRODUCTION

The Global Positioning System (GPS) is a space-based radio navigation system; operated by USS Department of Defense that provides reliable positioning, navigation, and timing (PNT) services to civilian users on a continuous worldwide basis [1,2]. GPS is made up of three parts: satellites orbiting the earth; control and monitoring stations on Earth; and the GPS receivers owned by users which provides three-dimensional location. User parameters transmitted by GPS are: (i) Latitude position from -90º to 90º (ii) Longitude position from -180º to 180º (iii) Altitude position in meters (iv) GPS system time. The Universal Time Count (UTC) advances the GPS system time by 5:30 hours. Presently all GPS satellites uses CDMA technique for broadcasting at only two frequencies: L1= 1575.42MHz and L2 = 1227.6MHz. It aims to improve the accuracy of the GPS system by involving new ground stations, new satellites and four additional navigation signals (civilian signals: L2C, L5 and L1C and military code: M-Code) by 2013.

System segmentation: GPS consists of three major segments. (i) Space segments: consists of 32 orbiting GPS satellites at 20,200km, in 6 Medium Earth Orbits, having approximately 55º inclination (tilt relative to Earth’s equator) and are separated by 60º right ascension of the ascending node (angle along the equator from a reference point to the orbit’s intersection) and with orbital radius of 26,600 km. (ii) Control segment: having five control stations at Hawaii, Colorado Springs, Colorado, Ascension

Island, Diego Garcia and Kwajalein, as shown in figure 1, with the master control station at Falcon Air Force Base, Colorado Spring, Colorado for maintaining satellites in proper orbits, adjusting satellite clocks. (iii) User segment: is the user’s GPS receiver, consisting of an antenna tuned to satellite transmitted frequencies, receiver-processors, and a highly-stable clock.

Figure1. Different GPS monitor control stations situated across the globe.

2. APPLICATIONS OF GPSGPS is extensively used in military services involving

navigation, fleet management, target tracking, missile and projectile guidance, search and rescue, reconnaissance and map creation, etc. Many civilian applications also uses GPS involving navigation, finding absolute location, relative movement, time transfer used in communication, measuring the motion of faults in earthquakes, GPS embedded mobile phones, etc.

3. ADVANTAGES OF SOFTWARE BASED GPS

RECEIVER OVER COMMERCIAL GPS RECEIVERS

Software GPS receiver having open architecture can be reconfigured as desired, which is not applicable in commercial GPS receivers. Any configuration of the software based GPS receiver can be built and implemented by arranging the existing modules through a simple point-and-click graphical user interface and also by using standard C++ coding. The architecture of the software based GPS receiver is suitable for many scientific applications requiring additional specialized processing modules or modification of the existing modules. The software based

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GPS receiver can be configured to provide very stable operation under scintillation condition and are suitable for Ionospheric monitoring, while the operations of the commercial receivers are unstable under these conditions. The locking range, sampling rate of the signal and the coordinate system can be changed and new models can be created according to the requirements in the software based GPS receiver. All these facilities are absent in the commercial GPS receivers.

4. IONOSPHERIC SCINTILLATION AND TEC-TOTAL ELECTRON CONTENT

Ionosphere is defined by the ionized shells encircling the earth within an altitude range of (60–1000)km. The ionosphere is divided into different layers: (i) D-region: extending from an altitude of (60-90)km and causes most of the absorption encountered by HF signals. (ii) E-region: extending from (90-130)km and is prominent during daytime which becomes unnoticeable after sunset due to decreased ionization. (iii) F-region: during daytime, F-region remains over some locations and at certain epochs of sunspot cycle divides into F1 region extending from (130-210)km and F2 region extending from (200-1000)km.

Scintillations are caused by naturally occurring Ionospheric plasma density irregularities, moving from west-to-east. When they intersect satellite links, scintillations are encountered resulting in loss of signal and errors in navigation. The plasma depletions may occur over a considerable height range and sometimes have quite large amplitudes, affecting the region lying at low-latitudes. These effects are more significant mainly during post-sunset hours of equinoctial months. Ionospheric scintillations produce fluctuations in the amplitude and phase of both L1 and L2 frequencies of GPS signal, which degrades the performance of GPS receiver [3-6].

Therefore GPS receiver, which is immune to the degradation of signal, caused by these scintillation-related effects has been developed.

Total Electron Content (TEC) is the total number of electrons present within an imaginary cylinder of unit cross-section, which is extending from the base of the ionosphere 60km to 1600km. 1 TEC unit (TECU)=1016 electrons/m2. Ionospheric TEC could be calculated by measuring group delays and carrier phase advances of received radio signals transmitted from satellites [7-9]. TEC is strongly affected by solar activity. It gradually starts increasing from sunrise, reaches a peak value at around (1200-1400)hours, Indian Standard Time (IST), then gradually starts decreasing in the post sunset hours, and reaches a minimum value from sometime after midnight and remains at the minimum value from midnight till pre-sunset hours, and then again gradually starts increasing with sunrise. TEC is never zero. At sunset TEC shows a sharp decrease, which causes a bite out region as shown in figure 2. Signals propagating through this bite out suffer fluctuations.

The accuracy of position fixing using GPS in the equatorial region is mainly affected by:

1. Group delay – which is proportional to total electron content (TEC).

2. Inospheric Scintillations

Figure 2.Variation of TEC with respect to the time.

TEC is related to the group delay in the following way.δ(Δt) = 40.3 NT (1/f2² – 1/f1²) C Where, NT = total electron content (TEC) electrons/m2

= δ(Δr ) 40.3 (1/f2² – 1/f²) C =velocity of light (3x108m/s), Δr =ionospheric correction, Δt=group delay, f1=1575.48 MHz and f2 =1227.6 MHz.

5. RESULT AND DISCUSSION A comparison between the software GPS receiver

and the commercial GPS receiver has been done, which indicates the merits of the dual frequency software based GPS receiver over the commercial GPS receivers.

The present project is based on dual frequency software based GPS data, recorded at Kolkata (Latitude = 22°34.69´, Longitude = 88°22.33´). In this project the effects of the diurnal variation of the range error on the receiver positioning accuracy has been studied.

Simultaneously the geo-stationary very high frequency (VHF) link from the satellite FLEET SAT COM (250 MHz, 73° E) at 42,000km is been recorded continuously at the ionosphere field station of the University of Calcutta (set up since 1954), located at Haringhata at 50 km north-east of Kolkata, in a relatively RF interference free environment. Geostationary VHF scintillation has been observed on this link. In order to investigate the effects of these scintillations on the GPS links at Kolkata, the amplitude scintillation index S4 and corresponding elevations of different GPS satellites are noted on the same days.

The Dual Frequency Software Based GPS Receiver has a resolution of 2.4millimeters at the frequency L1. The GPS system uses four satellites for calculating the position of a receiver, where three satellites provides the position information and the fourth satellite provides the timming information. Errors in position calcualtion arises due to presence of scintillations which might turn off some of the satellite signals used for position calculations and presence of high TEC which gives high group delay, due to which range error is high and value of latitude and longitude deviates from actual value.

For the above purpose an algorithm has been developed using JAVA in the UNIX platform to order the data from the raw input files received from the GPS receiver, into a manner suitable for studying against a time baseline. Further modification of the algorithm developed

was been done to make the process of ordering the data interactive through user-defined parameters.

For the analysis, datas obtained from the three days of a month for a 24 hours time, when scintillations were absent as well as datas from three days from another month when scintillations were present, by selecting a day from the starting of the month, a day from the middle of the month and one from the end of the month. The modified algorithm was used for ordering the GPS receiver datas on these selected days, and the output gives the GPS system time, satellite vehicle no., ionoshperic correction, receiver latitude, longitude and PDOP in Microsoft Excel 2007 sheet.

Scintillations in FSC signal has occurred at 16:40 UTC – 17:40 UTC and at 19:00 UTC – 19:40 UTC. Data analysis is done on the received GPS signal from 16:00 UTC – 20:00 UTC. The satellite vehicle no. with Elevation angle ≥50º, are considered for further analysis to avoid any disturbance due to multipath effects. Since local time of Kolkata is 06:00 hours advanced to UTC, local time for each day is calculated by adding 06:00 hours to the GPS sytem time. TEC is calculated from ionospheric correction, using the formula given above. Whenever we measure any GPS parameters, there are both spatial variation (due to the movement of the satellite and geometry of the Earth) and temporal variation. The spatial variation has a significant effect on the temporal variation and vice-versa. Only the temporal variations are considered in this project, so moving average of TEC is calculated for both 10 minutes and 6 minutes in order to remove the spatial variations.

By setting Elevation angle ≥ 50º, TEC is calculated within a time period of 03:00 – 04:00 hours and a reference value for the receiver latitude and longitude is found out. The TEC value is considered to be minimum during this time period. Then the deviation in latitude and longitude (in meters) from the reference values of latitude and longitudes are evaluated. Finaly the TEC, deviation in latitude (deltalat), deviation in longitude (deltalon) and Carrier to Noise Ratio (CNO) has been plotted against the time baseline, for studying the variations of these parameters with respect to the local time (LT). The plotting was done in Microcalorigin61, which is suitable for plotting double Y- axis.

A particular case has occurred on 02/02/08, on which high scintillation has occurred from 20:44 IST – 20:50 IST.

Figure 3. Variation in the deviation in carrier to noise ratio (CNO) and TEC (10-16 eletrons/m2) with local time (LT) of 02/02/08, Station : KOLKATA.

The deviation in carrier to noise ratio (CNO L1) was evaluated by performing the moving average of 10 minutes of the carrier to noise ratio and is plotted with TEC with respect to local time (LT) as shown in figure 3.

6. CONCLUSIONA significant fluctuation in the carrier to noise ratio is

observed almost near the peak of the TEC, after which the ambient level of the carrier to noise ratio is found to be quite stable.

TEC moves from West to East and GPS satellites are moving from East to West. Hence the signals from the GPS satellite will have a very minimum encounter with the ionospheric scintillations. The intensity of the ionospheric scintillation called the scintillation index S4, is inversely proportional to the square of the frequency; (S4 α 1/f²). Thus the intensity of scintillation present at 1.5 GHz for the GPS satellite signal is very small as compared to that present at 250 MHz for the FSC signals. It is seen that signature of the scintillation present in FSC is also present in GPS, with much reduced intensity. And the software based dual frequency GPS receiver, present at Kolkata is capable of tracking such low levels of scintillations.

We get seven satellite vehicles, which have Elevation ≥ 50º, these are ; SV3, SV16, SV19, SV20, SV23, SV31, SV32. After calculating and plotting TEC with respect to the local time in Mirosoft Excel 2007, only four satellite vehicles SV3, SV16, SV20, SV23 were found to have significant scintillation, among which only SV3 and SV16 are shown in figure 4 and figure 5.

Figure 4. Variation in the TEC deviation of 10 minutes with respect to time for SV3. Station: KOLKATA

Figure 5. Variation in the TEC deviation of 10 minutes with respect to time for SV16. Station: KOLKATA.

By analyzing the different characteristics of these GPS parameters, the dual frequency software based GPS receiver was found to have satisfactory accuracy in position fixing. The calibration of the software based GPS receiver was also analyzed by observing that the software GPS receiver can track very small scintillation in the GPS signals. Satellite Path Tracks at Kolkata station is shown in figure 6.

Figure 6. Satellite Path Tracks at Kolkata station

As the software based dual frequency GPS receiver has an open architecture and its internal working are available to the users, the software can be reconfigured by C++ coding or by adding new block to further improve the receiver performance in position accuracy under scintillation condition.

ACKNOWLEDGMENTThe authors would like to acknowledge the significant

support and extensive guidance provided by Dr. Ashik Paul and Dr. Asish K. Dasgupta of Institute of Radio Physics and Electronics, Calcutta University, Kolkata.

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