Iranian Permanent GPS Network: Strategy and Processing

Hamid Reza Nankali
Geodesy and Geodynamics Department
National Cartographic Center of Iran- Tehran
Azadi Sq.Meraj Ave. Po.Box:13185-1684
h-nankali@ncc.neda.net.ir

Yahya.DJamour
Geodesy and Geodynamics Department
National Cartographic Center of Iran- Tehran
Azadi Sq.Meraj Ave. Po.Box:13185-1684

Abstract:
A dense and wide permanent GPS station network has been established in Iran (Tabriz-Tehran-Mashhad) and other active part of the country by National Cartographic Center of Iran(NCC). Since first of the 2005 this network and is designed both for crustal deformation monitoring and to serve as a highly precise geodetic network in Iran and consist of 107 permanent stations in first phase. Average distance between dense parts is about 25 to 30 km.

Since we have collected about 1 year data , we estimated horizontal crustal displacement and velocity field with respect to the stable Eurasian plate. This new network will bring us more precise information on crustal information and geophysical phenomena such as ionosphere disturbances and water vapour too. Finally this network serve as active controlling system in Iran by GPS.

1) Introduction:
The present tectonics of Iran results from north-south convergence between the plates of Arabia to south west and Eurasia to the north east (Jackson & Mackenzie 1984).

It involves a continental collision (Falcon 1974) except along the Makran, its southeastern margin, where a remnant part of tetheys oceanic lithosphere subducts north works beneath south east Iran (Byrne etal.1992). Most of the deformation is accommodated in the major belts (Alborz,Zagros,Kopet-Dag) and along large strike slip lengths which surround blocks (Central Iran , Lut , South Caspian Sea) with moderate siesmicity (Jackson, Mackenzie 1984). The present-day deformation is associated with active siesmicity and destructive earthquakes that prodically shake Iran (Bam 2003). The understanding of the present day deformation will contribute to a more accurate and reliable assessment of the seismic hazard in Iran Fig(1).

Fig 1- Siesmicity of Iran

According to this situation, it is important to monitor the crustal deformation using geodetic data such as VLBI,SLR,In SAR, and GPS. Among them GPS has several advantates (continuous collection, cheaper, and more compact) and therefore it is easier to construct lots of observation stations. With the recent advance in GPS receivers technology and scientific software's (Bernese-Gamit/Globk-GIPSY OASIS) and using precise satellite orbit and clock, we can achieve the accuracy for station position their velocity for geodynamic applications.

So in 2005 National Cartographic Centre of Iran (NCC) started to built a GPS permanent observation network for crustal deformation monitoring and estimating geohozard in Iran.

The network consist of 107 GPS observation site which distributed in the active part at the country. To reach the goals we also benefited the studies and remark of GSI(Geological Survey of Iran) and IIEES (International Institute of Earthquake Engineering and Seismology).

2) Network Configuration
Iran is situated in a region of collision between two major tectonic plates:

Eurasian and the Arabian plates with the convergence rate 2.5cm±2mm (vernant etal.2004) The network consist of two part: base network and regional networks. The base network consist of 41 station which distributed in Zagros-Alborz-Lut-Kopet-Dag-Central Iran- Makran and east of Iran in order to monitor the total motion and geodynamics of plates boundary. Fig 2

Fig 2- Iranian permanent GPS Network

Regional networks are in tehran,tabrize and mashhad areas and the distance between the stations is about 25-30 km. Tehran as the capital with 12million population located in the southern mountain foots of central alborz in a highly active zone. In order to monitor tectonic deformation in this area 25 stations are established in different part according to geological and geodetical parameters. Fig 3

Fig 3- Tehran Network

In the western and eastern Azerbaijan and ardabil areas there are many active faults such as Tassouj and north Tabrize faults. Tabriz city is located at 100km distance from this fault and historical earthquakes with 6 to 7 Richter have occurred in this area. So in order to monitor this part 20 stations are established. Fig 4

Fig 4- Tabriz Network

The 3 cities of mashhad and neishaboor and sabzevar had also historically shaken by many earthquakes which destroyed the areas and killed many people due to neishaboor dasht-bayaz and kopet-dag faults. So 21 stations are established in order to monitor the active tectonic of this area. Fig 5

Fig 5- Mashhad Network

Fig 3 to Fig 5 Consist also base network stations.

3) Observation Station:
The function of observational station is to receive signal from GPS satellite, to accumulate them and to communicate with data center. For this purposes each station is provided with a GPS antenna, GPS receiver, communication device and backup battery. The GPS antenna is fixed on a station pillar. The receiver and other equipment are stored in the rack near Pillar. A reinforce concrete pillar is 1.5m highs standing on the reinforced concrete base (1.20m * 1.20m)and 2m cube.The base of the pillar is reinforce concrete too. A rack near the pillar keeps the GPS receive, the modem and other instrument. A fan attach to the top of the rack is controlled to keep air condition suitable for instruments and also heather for temperature control. Cables and electric power supply and telephone line are buried underground in order to preserve diminish vibration effect by wind. Since it is expected that uneven subsidence of a pillar might occur a tilt meter installed in order to check any inclination from the initial state. 93 of the station also equipped with meteorological sensor for GPS meteorology. Fig 6

Fig 6- GPS receiver and equipment

GPS receivers installed in the rack are dual frequency from Ashtech uz12(ICGRS)and CGRS with 128 MB internal memory. All receivers are scheduled to receive dual band carrier phase data and code data every 30 seconds in daily mode(24h). Fig 7

Fig 7- GPS Station

Antenna is set on the top of the pillar coverd with a radom. 85% of remote stations are equipped with modems and commercial telephone lines and 15% was setup with GSM modem. A transmission of 9600 bps is available for the connection to the data centre.

Data Centre:
The Master Data Centre for controlling the network and data analysis is settled in Tehran at National Cartographic Centre of Iran. Also we have 4 data centre in Tabriz-Mashhad-Hamedan-Ahvaz that control the operation of their observation sites, data communications, data management and send them to the Terhan for final processing. Data stored by the receivers are downloaded once per day from data centres and master data centre according to define procedure in midnight. The size of data is about 1.6mb that needs 10minutes with digital line. Down loaded data is archived into the database in RINEX format using TEQC program from unavco and compress for final archive in the FTP site of NCC on the 2 server with 300 Gb capacity.

Data processing:
After archiving data, the data processing unit start network solution process of GPS data include the network and IGS stations.

The data processing unit consist of 6 engineering workstations with linux operating system. The data are processed by precise analysis software Gamit-Globk ver 10.20 which was develop by MIT and SIO, in two steps. In the first step Gamit analyzes these data from user specified. Clusters of stations to create loosely constrained (buty ambiguity resolved) estimates of station coordinates. In the second step Globk combine these Gamit output files to create solution file in the SINEX format, containing loosely-constrained estimates for entire network. Globk is basically akalman filter and uses the covariance matrices obtained by Gamit.

The process is performed twice: once with rapid orbit from IGS outputs and the final orbit(Precise ephemrids)after two weeks. The former is used to obtain rapid solution and the later for final solution of whole network.

Outputs and Results:
Several kinds of results obtained by routine analysis such as, time series group of coordinates sites, time series of relative position of two site (baseline time series) displacement vector and also velocity filed. The summery of results and reports can be seen on NCC web site. (http://www.ncc.org/English/geodynamic. Fig8, Fig9 shows the time series of Tehran station and also the present velocity field which produce by GMT. The new next product strain rate map which will appear soon.

Fig 8-GPS Velocity Filed estimated with respect to Eurasia

Fig 9 – Time Series of Tehran station

Conclusion:
IPGN (Iranian Permanent GPS Network) has been showing a enormous information about crustal deformation (plate motion-activity of active faults), and to serve as an active controlling system for GPS surveying. The usage of this network is not limited on crustal deformation. Actually in Iran GPS metrology project is also start and this is another application of this network. The next phase of this project is real time monitoring that start this year. This system will be expanded to more than 700 permanent observation site in the next years.

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