next up previous contents
Next: Data Processing Up: Continuous GPS measurements in Previous: Introduction

Instruments and Data Transfer

In this section the history of continuous GPS measurements in Iceland and technical aspects of the operating stations are discussed. Figures 1 and 2 show where the stations are located. Tables 1, 2 and 3 summarize the main characteristics of the stations. Further technical information and photos from many of the stations are available at the ISGPS website: http://hraun.vedur.is/ja/gps.html.

The instruments used at permanent GPS sites are quite different from the handheld instruments used for navigation by many people today. The instruments are of the same type as those used in geodetic network GPS measurements and utilize both carrier waves (L1 and L2) from the GPS satellites along with the codes modulated on to the carrier waves. Using these instruments along with long (4-24 hours) observation periods and advanced processing methods, relative position of geodetic stations can be achieved with subcentimeter accuracy. The GPS system and how subcentimeter positioning accuracy can be achieved is not described in this paper. Interested readers are referred to e.g. [Leick (1990)], [Dixon (1991)], [Sigmundsson (1992)], [Hugentobler et al. (2001)], [Hreinsdˇttir (1999)] and [Jˇnsson (1996)].

Continuous GPS measurements in Iceland started when a station was installed by Bundesamt für Kartographie und Geodäsie (BKG) in ReykjavÝk (REYK) in November 1995. The station is operated in cooperation with the National Land Survey of Iceland (LandmŠlingar ═slands, LMI). REYK is still in operation and is a part of the International GPS Service (IGS) tracking network and used by many international data processing centers in their calculations, e.g. to determine the orbits of the GPS satellites. REYK is used as the reference station in processing of data from the ISGPS network. REYK is on the top of a three story concrete building, constructed in the 1970's, at the University of Iceland. The choke ring antenna is mounted on a tribrach on the rim of the elevator shaft which runs through the building and the receiver is inside the building. There is no radome mounted on the antenna. Data are collected continually to a Windows based PC computer and are transferred via an internet connection to BKG's data center on an hourly basis.

In May 1997 the second station, HOFN, was installed at H÷fn, Hornafj÷r­ur, by BKG and LMI. HOFN is on the top of a one story concrete building, otherwise the setup and data aquisition are similar to the one at REYK. The station was equipped with a Trimble groundplane antenna with a radome until September 21, 2001, when a Trimble choke ring antenna without a radome was installed (Table 2). This caused a significant offset in the time series (Section 4.1). BKG installed the third station, REYZ, a few meters from REYK in September 1998. REYZ tracks not only signals from NAVSTAR GPS satellites, but also from GLONASS satellites. GLONASS is the Russian counterpart of the American NAVSTAR GPS system. Presently there are 7 GLONASS satellites in operation. REYZ is equipped with Ashtech instruments and the antenna has a conically shaped radome from Ashtech.

 
Table 1: Permanently recording GPS stations in Iceland in operation as of May 2002. The first column describes the short names of the sites and the second column the full names. Position of the stations (columns 3 and 4) are ellipsoidal coordinates in decimal degrees (latitude and longitude). Station height (column 5) is the ellipsoidal height of the geodetic benchmark in meters. Antenna height (column 6) is the vertical height, as of March 1, 2002, from the benchmark to the lowest point of the antenna - sometimes referred to as the bottom of antenna. The operator (column 7) is the institute responsible for the daily operation of the stations. Start date (column 8) refers to the date when the station started collecting data on a regular basis.
        Height Antenna    
Station Full name Lat. Lon. [m] height [m] Operator Start date
AKUR Akureyri 65.69 -18.12 134 0.055 LMI 31 Jul 2001
HLID HlÝ­ardalsskˇli 63.92 -21.39 111 0.914a IMO 21 May 1999
HOFN H÷fn 64.27 -15.20 83 0.051b BKG/LMI 27 May 1997
HVER Hverager­i 64.02 -21.18 150 0.984 IMO 25 Mar 1999
HVOL Lßguhvolar 63.53 -18.85 265 1.044 IMO 19 Oct 1999
ISAK* ═sakot 64.12 -19.75 319 1.005 IMO 10 Jan 2002
KIDJ Ki­jaberg 64.00 -20.77 123 1.005 IMO 25 Jan 2001
OLKE Ílkelduhßls 64.06 -21.22 551 0.974 IMO 25 May 1999
REYK ReykjavÝk 64.14 -21.96 93 0.068 BKG/LMI 02 Nov 1995
REYZ* ReykjavÝk 64.14 -21.96 93 0.060 BKG/LMI 11 Sep 1998
RHOF Raufarh÷fn 66.46 -15.95 77 1.014 IMO/LGCA 20 Jul 2001
SELF* Selfoss 63.93 -21.03 82 1.011 IMO 06 Feb 2002
SKRO Skrokkalda 64.56 -18.38 982 1.076 IMO/LGCA 21 Sep 2000
SOHO Sˇlheimahei­i 63.55 -19.25 857 1.012c IMO 24 Sep 1999
THEY Ůorvaldseyri 63.56 -19.64 195 1.028d IMO 15 May 2000
VMEY Vestmannaeyjar 63.43 -20.29 135 1.069 IMO 27 Jul 2000
VOGS Vogsˇsar 63.85 -21.70 73 0.972 IMO 18 Mar 1999
*: Station not used in this study. a: Was 0.909 m until Mar. 15, 2000. b: Was 0.055 m until Sep. 21, 2001.
c: Was 1.011 m until Nov. 09, 1999. d: Was 1.027 m before Jan. 26, 2001.
 

Intensive seismicity in the Hengill area, associated with uplift at a rate of 2 cm/yr, started in 1994 [R÷gnvaldsson et al. (1998a)], [Sigmundsson et al. (1997)], [ Feigl et al. (2000)]. In 1998 the activity caused public concern and the initiation of the ISGPS network. The ISGPS network is a cooperation project between the Icelandic Meteorological Office (IMO), Nordic Volcanological Institute (NORDVULK), Science Institute, University of Iceland (SIUI), and University of Savoie (LGCA), France. Funding to purchase four GPS instruments to use for continuous measurements was obtained from the Icelandic Government and the ReykjavÝk Energy corporation. The main goal when designing the technical aspects of the ISGPS system was to maximize monument stability and operational security and minimize the installation and operational costs. It was originally planned to colocate the ISGPS stations with stations in the SIL seismic network [Stefßnsson et al. (1993)], [B÷­varsson et al. (1996)] to lower the operational costs. However, it was considered more important to be close to active deformation areas and to have solid bedrock for the ISGPS monument.

 
Table 2: Receiver and antenna types that have been used at the continuous GPS stations in Iceland. Name codes are according to IGS naming conventions [IGS (2002)] where available.
  Receiver Antenna Valid period
Station Type Serial no. Type Serial no. From To
AKUR TRIMBLE 4700 221607 TRM29659.00 145519 31JUL2001 -
HLID TRIMBLE 4700 147819 TRM29659.00 148018 21MAY1999 15MAR2000
  TRIMBLE 4000SSI 28516 TRM29659.00 193254 21JUN2000 26JUL2000
  TRIMBLE 4000SSIa 26093 TRM29659.00 193254 17AUG2000 09NOV2001
  TRIMBLE 4000SSIa 26093 TRM33429.20+GP 168784 09NOV2001 21DEC2001
  TRIMBLE 4000SSIa 26093 TRM29659.00 193254 21DEC2001 -
HOFN TRIMBLE 4000SSI 09374 TRM22020.00+GP 008914 27MAY1997 21SEP2001
  TRIMBLE 4000SSI 09374 TRM29659.00 181800 21SEP2001 -
HVER TRIMBLE 4700 147815 TRM29659.00 148022 25MAR1999 -
HVOL TRIMBLE 4000SSI 26094 TRM29659.00 170423 19OCT1999 24JAN2002
  TRIMBLE 4700 219340 TRM29659.00 170423 24JAN2002 -
ISAK TRIMBLE 5700 268846 TRM29659.00b 262509 10JAN2002 -
KIDJ TRIMBLE 4700 221613 TRM29659.00 177334 25JAN2001 -
OLKE TRIMBLE 4700 147817 TRM29659.00 148016 25MAY1999 02NOV2000
  TRIMBLE 4700 194401 TRM29659.00 148016 02NOV2000 24NOV2000
  TRIMBLE 4700 147817 TRM29659.00 148016 24NOV2000 -
REYK ROGUE SNR-8000 T313 AOAD/M_T 434 02NOV1995 11JUL2000
  AOA SNR-8000 ACT T-396U AOAD/M_T 434 11JUL2000 -
REYZ ASHTECH Z18 ZX00111 ASH701073 CRG0102 11SEP1998 -
RHOF MARTEC MIRA-Z 633Z024 ASH701945C_M 1999040150 20JUL2001 29MAR2002
  ASHTECH UZ-12 220013831 ASH701945C_M 1999040150 29MAR2002 -
SELF TRIMBLE 5700 268934 TRM29659.00b 263955 06FEB2002 -
SKRO ASHTECH Z-XII3 LP03577 ASH701945C_M Unknown 21SEP2000 09NOV2000
  ASHTECH Z-XII3 LP03810 ASH701945C_M CR53903 09NOV2000 -
SOHO TRIMBLE 4000SSI 25992 TRM29659.00 170425 24SEP1999 09JAN2002
  TRIMBLE 4000SSI 26094 TRM29659.00 170425 24JAN2002 -
THEY TRIMBLE 4700 147819 TRM29659.00 170418 15MAY2000 -
VMEY TRIMBLE 4000SSI 28516 TRM29659.00 148018 27JUL2000 -
VOGS TRIMBLE 4700 147812 TRM29659.00 148019 18MAR1999 -
a: Receiver operated in semi-permanent mode.
b: The usage of TRM29659.00 with TRIMBLE 5700 requires an antenna power adapter.
 

Figure 3 shows a photo of a typical setup for the ISGPS stations. The actual physical point being measured at the stations is a classic geodetic copper benchmark cemented into solid bedrock or a concrete platform. The antenna is screwed on top on of an approximately 1 m high stainless steel quadripod, which is mounted directly over the benchmark (Figure 3). The quadripod structure is made by a local machine shop and is very stable on short and long timescales due to the endurance and thermal expansion properties of stainless steel. The thermal expansion factor for stainless steel is nominally a factor of 10 smaller than for concrete. The quadripod is fastened to the bedrock using two continuously threaded rods, 12 mm in diameter, for each leg (Figure 3). The rods are cemented in 11 to 22 cm deep holes, depending on bedrock type, using chemical anchor capsules (Spit Maxima M12). This method is for example used to secure large engine complexes. The top plate of the quadripod is levelled by adjusting the position of the legs on the threaded rods. The quadripod structure is only 1 m high thus inducing more multipathing into the measurements than a higher structure would do [Hugentobler et al. (2001)]. To minimize the effect of multipathing, choke ring antennas are deployed at all the ISGPS stations. To prevent snow and ice accumulation on the antennas they all have hemispherical plastic radomes from SCIGN (Southern California Integrated GPS Network) [SCIGN (2001)]. Although this precaution is taken, snow and especially rime have been observed to accumulate on the radome in certain weather conditions. The stations are attended to at least once per year to check if everything is working properly and to remeasure the antenna height.

 
Table 3: A summary of the power sources and data transfer for the continuous GPS stations in Iceland.
  Power Data  
Station Sourcea Voltageb transferc Tribrach
AKUR M 18 LC+ftp Y
HLID M 18 MM N
HOFN M u LC+ftp Y
HVER M 18 MM N
HVOL L 12 CMM Y
ISAK M 18 MM N
KIDJ M 18 MM N
OLKE L 12 CMM N
REYK M u LC+ftp Y
REYZ M u LC+ftp Y
RHOF M u MM N
SELF M 24 MM N
SKRO L u LC+X.25 N
SOHO L 12 CMM Y
THEY M 24 MM Y
VMEY M 18 MM Y
VOGS M 18 LC+X.25 N
a: M stands for municipal electricity and L stands for locally produced electricity.
b: Input DC voltage to receiver, in volts. "u" means unknown.
c: LC: Data collected to a local computer; ftp: Data transferred via ftp; MM: Data
transferred via a modem-modem connection; CMM: Data transferred via a cellular modem-modem
connection; X.25: Data transferred via a X.25 link.
 


  
Figure 3: Author finishing the installation at VMEY. If the photo prints out well the brass geodetic benchmark is visible under the center of the quadripod. The antenna is covered with a SCIGN radome (grey plastic) and is mounted on a Leica tribrach. The receiver is mounted in a plastic box screwed to the side of the quadripod. The black cable visible on the ground is the power and data cable coming from a nearby house. The pins seen at the legs of the quadripod are drilled 11 to 22 cm into the bedrock. Concrete visible at the base of the legs is merely for cosmetics. The legs have uneven height from the ground because the top plate of the quadripod is levelled. (Photo: Jˇsef Hˇlmjßrn).
\begin{figure}
\centering
\mbox{\epsfig{figure=figures/008med.eps,width=10.8cm} } %
\end{figure}


  
Figure 4: Schematic overview of the installation and data flow at VMEY (see also Figure 3). The left part of the diagram notes instruments in the field. The left lower part notes instruments at the quadripod and the upper left part notes instruments in a nearby house.
\begin{figure}
\centering
\mbox{\epsfig{figure=figures/vmeynew.eps,width=14cm} }
\end{figure}

The data at all ISGPS sites are collected to the internal memory of the receivers in 24 hour long files, starting at midnight GMT. The receivers are set to log signals from the GPS satellites at 15 second intervals. The data files are downloaded automatically on a daily basis via a modem-modem connection during night hours (Figure 4). The communication rate is fixed at 9600 baud for all stations. The data files are 0.7 Mb to 1.6 Mb in size, depending on the internal receiver data format and elevation mask. Data from the Trimble receivers (Table 2) are downloaded using UNAVCO's (University NAVSTAR Consortium) download software, LAPDOGS [UNAVCO (2001b)]. Data from the Ashtech receivers (Table 2) are downloaded using Ashtech's remote33 software [Ashtech (2001)]. Both remote33 and LAPDOGS are based on Perl scripts which call communication routines that are specific for each receiver type. Data from REYK, HOFN and AKUR are acquired automatically on a daily basis via the ftp site of the National Land Survey of Iceland (LMI), ftp.lmi.is. The only electric equipment in the field are the receiver, antenna, modem and backup power (Figure 4). By avoiding to have a PC computer operating at each site the operational security is maximized and the number of objects that can break down in the field is minimized.

The first ISGPS station was installed at Vogsˇsar (VOGS) on March 18, 1999 (Table 1). VOGS is in a Holocene pahoehoe lava field. The station is colocated with a SIL station (vos) [B÷­varsson et al. (1996)], [Stefßnsson et al. (1993)]. The station is 12 m from the seismometer vault. The receiver is in the vault and uses power from the same source as the seismological instruments. The power consumption for the Trimble 4700 is approximately 5 W. Data and power are transmitted over an approximately 1200 m long ground cable. This cable length is too long for the RS-232 communication standard to work, so RS-232/422 converters are deployed at both ends of the cable. The power for the instruments is transmitted at 70 V DC over the cable and is converted to 15 V in the vault. This is a standard in the SIL system. However, the receiver at VOGS (a Trimble 4700) does not turn itself on after a power failure unless the input power is over 18 V. Thus a DC/DC converter is used to run the receiver on 24 V. The daily data files are downloaded to a Linux computer, that also operates the seismic instruments, using LAPDOGS. The data are subsequently sent to the data center in ReykjavÝk via a X.25 link [B÷­varsson et al. (1996)]. The LAPDOGS software did not support communications with Trimble 4700 receivers until in late 2000. Until then the data were continually logged to a Windows computer using the Universal Reference Station (URS) software from Trimble. In the beginning of measurements the data were transferred to a laptop computer every one or two weeks. This work was tedious and time consuming. Later the Windows computer was connected to the seismic computer (then operating on the Solaris system) and data transferred automatically to ReykjavÝk via the X.25 link. This method for data acquisition was unfortunate since the Windows computer tended to break down frequently and it was impossible to access the Windows computer from ReykjavÝk. Present setup is performing quite well except the X.25 link tends to break down. The X.25 communication software is not as robust in the Linux environment as in the Solaris or Interactive Unix environments (S. S. Jakobsdˇttir, personal communication 2002).

The second station in the ISGPS network was installed just outside Hverager­i (HVER) in March 1999 (Table 1). The station is sited just over 2 km southeast of the inferred center of uplift in the Hengill area [Feigl et al. (2000)]. Hverager­i is within the Grensdalur geothermal area. HVER is sited at the Hverager­i Golf Club hut and is equipped with a Trimble 4700 receiver. There were problems finding solid bedrock in the area since the bedrock is highly altered and fractured. A platform of reinforced concrete was built on the existing bedrock, of intermediate silica composition, after a backhoe had been used to scrape off soil and loose rocks. The platform is approximately 1.5x1.5 m wide and 0.5 m thick. 12 mm iron rods were driven 15-20 cm into the bedrock under the platform and the geodetic benchmark is fastened to one of the rods. An iron grid was constructed upon the rods driven to the bedrock. (H. Ëlafsson, personal communication 2002). The quadripod is secured to the platform. The receiver is inside an old nearby barn (a 30 m Rg214 antenna cable is used) in a plastic box, similar to the one shown in Figure 3, fastened to a wall. A power converter from Trimble supplies 18 V DC to the instrument. The receiver is connected to an external modem and data are downloaded using the LAPDOGS software. Before LAPDOGS supported communications with Trimble 4700 receivers, the data were continually logged to a Windows computer operating URS, as for VOGS. The data were then downloaded from the computer using a communication program called PolyPM (U. Hessels, personal communication 1999). The computer tended to break down every now and then and caused many gaps in the data collection. Present setup, with a modem connected directly to the receiver, performs well.

HLID (HlÝ­ardalsskˇli) was installed in May 1999 (Table 1). The quadripod is fastened into a Holocene lava field approximately 20 m from the HlÝ­ardalsskˇli building which houses the receiver. The site is not well chosen for the antenna sometimes gets covered with windblown snow that piles up on the leeside of the house. This is observed as spurious motion in the coordinate time series [┴rnadˇttir et al. (2000)] when the snow completely covers the antenna. The winter of 1999 to 2000 was accompanied by unusually much snowfall and the receiver was removed in March 2000. The antenna could not be found and was probably at 1 to 3 m depth. At that time the receiver had stopped seeing any signals from the satellites. The station will be moved to a better location in the future. HLID is presently operated in a semi-permanent mode, meaning that the receiver is used for network GPS measurements during most of the summer time. Initially, HLID was equipped with a Trimble 4700 receiver and the data transfer was similar to what is described for HVER before LAPDOGS supported communications with Trimble 4700 instruments. Data are now donloaded using LAPDOGS. The receiver was removed to be installed at THEY in March 2000. After the June 2000 SISZ earthquakes a Trimble 4000 SSI receiver, initially intended for network GPS measurements by NORDVULK, was installed. The antenna originally used was still at the site. In October 2001 the antenna was removed and sent abroad for calibration. A layer of corrosion was observed between the aluminium antenna and the stainless steel quadripod. Probably this does not affect the antenna height by more than 1 mm and can easily be prevented by having a thin plastic sheet between the antenna and the quadripod.

Ílkelduhßls (OLKE) was the fourth ISGPS station to be installed, in May 1999 (Table 1). The station is within a high temperature geothermal area, at the SW part of the Hrˇmundartindur system (Figure 2), 4 km north of the uplift center inferred by [Feigl et al. (2000)]. The quadripod is in a lava outcrop from Tjarnahn˙kur crater, which erupted in early Holocene [SŠmundsson (1967)]. The receiver is in a plastic box on the side of the quadripod, similar as in Figure 3. The site is a few kilometers from inhabited areas so electricity is produced at the site using a wind generator and a solar panel. The electricity buffer consists of four 115 Ah batteries, sufficient to support operation for over two weeks if electricity production fails. The wind generator type initially used was faulty at high windspeeds and many maintenance trips were required. In September 2000 a new type was installed and no maintenance due to power problems has been required since then. During the summer of 1999 the data were downloaded to a laptop PC every 5 days or so, since the Trimble 4700 receivers can only store about 5 days worth of data. In the autumn of 1999 a spread-spectrum radio link was established to a building on Hßhryggur (approximately 7 km north of OLKE) near Nesjavellir Power Plant. The building housed a Windows computer continually logging data from OLKE with the URS software. The daily data files were downloaded automatically during night hours using the PolyPM program. In September 2000 a cellular modem with a directional antenna was installed at OLKE and the data files collected directly from the receiver internal memory using LAPDOGS. The communication rate for cellular modems is presently fixed at 9600 baud. A program calls OLKE once per day to log the input voltage to the receiver and adds it to a plot on the internet. This enables us to monitor the power status of stations equipped with local electricity generators.

Seismic unrest at Mřrdalsj÷kull and Eyjafjallaj÷kull accompanied with a small j÷kulhlaup in J÷kulsß ß Sˇlheimasandi in July 1999 [Sigur­sson et al. (2000)] led to funding from the Icelandic Research Council for purchase of three Trimble 4000 SSI instruments for continuous GPS measurements in the area. Initially the stations were planned to monitor Katla volcano. GPS network measurements indicated that an intrusion event had occurred beneath the southern flanks of Eyjafjallaj÷kull [Sturkell et al. (2002b)], so one station (THEY) was installed close to the inferred intrusion center. The stations were originally intended to be operated in a semi-continuous mode, with the receivers being used for GPS network measurements by NVI and SIUI during summer time. However, activity at Katla and Eyjafjallaj÷kull required the instruments for near real-time monitoring and the instruments have been fixed at the sites since they were installed. As will be discussed later, a grant from the the Icelandic Research Council was provided in 2001 to change the receivers to make the instruments available for GPS network measurements.

The station at Sˇlheimahei­i (SOHO) was installed in September 1999 (Table 1). It is only 5 km SSW of the subglacial Katla caldera rim and is thus well suited to monitor magma movements beneath Katla. The station is sited in a glacially eroded lava outcrop. The bedrock was hammered and polished to level the quadripod to within 2$^\circ $, so the top plate of the quadripod is not precisely levelled. The quadripod legs stand directly on the bedrock as opposed to at most stations where the legs actually stand on the threaded rods. To level the antenna a Leica tribrach is used, same type as can be seen in Figure 3. The hole in the top plate of the quadripod for the bolt to secure the antenna is 1 to 2 mm wider than the bolt. Thus the antenna cannot be replaced excactly at the same position if it is removed. The tribrach, along with an optical level, allows the antenna to be precisely (to within 0.5 mm) centered over the benchmark. Tribrachs were used in the installation of stations SOHO, HVOL, THEY and VMEY. The use of tribrachs in the installation process was discontinued, but left at the stations already installed with a tribrach, since the structure is more fragile and it is easy to accidentally tamper with the settings of the tribrach. SOHO is remotely located and no municipal electricity is available within kilometers. Thus electricity is produced at the site in the same manner as at OLKE, also sharing a similar history of problems. Data are collected in the same way as at OLKE. Sˇlheimahei­i is a very windy place with high precipitation, icing conditions and rapid changes between freeze and thaw causing a significant strain on the instruments. A new type of wind genearator was installed in December 2000 that is still working. The receiver used at SOHO was a Trimble 4000 SSI until it was swapped for a Trimble 4700 receiver in the summer of 2002 and used for network GPS measurements as originally planned. The same choke ring antenna is still used. The Trimble 4700 instrument consumes only half of the power that the 4000 receiver uses.

HVOL (Lßguhvolar) was installed in October 1999 (Table 1). It is 12 km SE of the Katla caldera rim on a palagonite hill. The site is colocated with a SIL station (hvo). Initially this station was intended to be colocated with another SIL station at SnŠbřli (snb), approximately 20 km east of the glacier. A quadripod was installed at SnŠbřli (SNAE) and the point has been measured in several GPS network campaigns. The antenna at HVOL is mounted on a tribrach and electricity is generated with a wind generator and a solar panel. Data are collected via a cellular modem, with a directional antenna, of the same type as at OLKE and SOHO. There are large sand plains deposited from the glacier in the surroundings and in high winds the instruments are battered with airborne sand. Plastic surfaces such as the antenna radome and the receiver box show signs of extensive wear. The Trimble 4000 SSI receiver was changed for a Trimble 4700 receiver in January 2002 (Table 2).

THEY (Ůorvaldseyri) was installed in May 2000 (Table 1). The station was initially intended to be at Mi­m÷rk, west of Eyjafjallaj÷kull, where a SIL station (mid) is operating. Before the installation of Mi­m÷rk (MORK) was completed, results from GPS network measurements showed significant deformation in the southern flanks of Eyjafjallaj÷kull and it was decided to install the station as close to the source of the signal as possible. THEY is located approximately 5 km WSW of the intrusion center inferred by [Sturkell et al. (2002b)]. The quadripod is fastened in a pre-Holocene lava layer from Eyjafjallaj÷kull. The site is deep in a valley at the Koltunguvirkjun local power plant. The nearby mountains mask the sky up to 15$^\circ $ in all directions but south. The receiver and antenna are approximately 100 m from the turbine housing and power (at 24 V) and data (RS-232) are transmitted to and from the turbine housing via a cable. The antenna is mounted on a tribrach. Initially it was planned to have a Trimble 4000 SSI receiver at THEY, but since HLID was not working properly at the time of the installation of THEY, the Trimble 4700 receiver from HLID was used at THEY (Table 2). There was no telephone connection at Koltunguvirkjun before installation of the instruments and a telephone line was established in August 2000. The data were initially logged continually to a Windows computer running URS. The environment in Koltunguvirkjun was hostile for computers because it was damp and the regulators of the turbines were old. The power plant has been greatly renewed. In January 2001 the computer was removed (actually that was the third computer tried at the site) and a modem connected directly to the receiver. Data have been downloaded using LAPDOGS since then.

The State Disaster Relief Fund (Vi­lagasjˇ­ur) supported installation of a SIL seismic station and an ISGPS station to monitor seismicity and crustal movement at the Westman Islands. Westman Islands are a central volcanic area at the tip of the propagating Eastern volcanic zone (Figure 1). An eruption in 1973 occurred in Heimaey, the largest island, covering the town in Heimaey with ash and devastating a significant part of the inhabited areas. Presently around 4500 people live in Heimaey. VMEY started recording data on July 27, 2000 (Table 1). The installation is shown in Figures 3 and 4. The station is in a Holocene lava field in the middle west part of Heimaey. The antenna is mounted on a tribrach as at stations SOHO, HVOL and THEY. A modem and a Trimble power supply are located in a nearby house (Figure 4) and data are transferred on a daily basis using LAPDOGS. The station has been working very well and almost no data have been lost since the station was installed.

A French group from the Laboratoire de GÚodynamique des Chaines Alpines (LGCA), University of Savoie, led by Thierry Villemin, has been conducting GPS network measurements in North Iceland since 1995. They have contributed to the buildup of the ISGPS network and obtained funding from the French Polar Institute (IFRTP) to install a station at Skrokkalda (SKRO), in the interior of the Iceland (Figure 1). The station was installed in September 2000 (Table 1). It is set on top of a small mountain. The quadripod was fastened in what looked like solid bedrock, but spurious motion recorded at the station indicates that this is not the case. This will be discussed more in Section 4.1 along with the time series from SKRO. The antenna is secured directly to the top plate of the quadripod and has a hemispherical radome from SCIGN (part number 0010-1). The antenna is connected to the receiver via a 70 m long Rg-214 antenna cable and an amplifier. The receiver is in a hut, owned by the National Power Company, that also houses various communication hardware. The instruments are powered by a diesel engine. SKRO is colocated with a SIL seismic station (skr). The daily data file in the internal memory of the receiver is downloaded during night hours to the SIL computer, running a program called remote33 on a Linux platform. The file is subsequently transmitted to the data center in ReykjavÝk via a X.25 link. A telephone modem was connected to another serial port of the receiver in the summer of 2002 to have an alternative communication link if the computer breaks down.

The June 2000 South Iceland seismic zone earthquake sequence (Section 4.6) called up on densification of the ISGPS network in the SISZ. IMO funded instruments for installation of one new station in 2001. The station was installed at Ki­jaberg (KIDJ) in January 2001 (Table 1). The quadripod is secured in breccia from the Hreppar formation using 22 cm deep holes for the threaded rods. The receiver is in a plastic box on the side of the quadripod and power and data are transmitted via a 50 m long cable. The station has been performing well since its installation and only 4 days of data are missing since the start of measurements as of May 2002.

Funding from the Icelandic Research Council supported installation of two permanently recording stations in the SISZ and installation of new receivers at SOHO and HVOL to make the preexisting instruments there available for GPS campaign measurements. The stations are located at Selfoss airport (SELF) and at Stˇrˇlfshvoll (STOR), Hvolsv÷llur. SELF started collecting data in February 2002 (Table 1). The station is in the Ůjˇrsßrhraun lava field. The communication link does not yet support automatic downloading of the data using LAPDOGS. Data are logged to the internal memory card of the receiver, a Trimble 5700, which can store around two months worth of data. The data are downloaded to a laptop every two months or so. To use a choke ring antenna with the Trimble 5700 requires an antenna power adapter (part number 43216-00). Installation of STOR has not yet been completed. A quadripod has been installed in palagonite surroundings and the site has been included in GPS network campaigns. A volumetric strain station [Stefßnsson et al. (1983)] is located within 1 km from the monument.

The French Polar Institute funded installation of a station in Raufarh÷fn (RHOF). The station was installed in July 2001 by LGCA and IMO (Table 1). The quadripod stands on a glacier polished lava outcrop at the northern edge of the town. The receiver is inside a nearby house. In March 2002 the receiver (a Martec Mira-Z) was swapped for an Ashtech $\mu$Z-12 receiver (Table 2). The receivers are near identical, since the inside of the Martec receiver is mostly provided by Ashtech. Data are downloaded on a daily basis via a modem connection using the remote33 software.

AKUR (Akureyri) was installed in July 2001 by LMI and the University of Akureyri (Table 1). The antenna is on an approximately 10 m high concrete chimney at the University of Akureyri. Data are continually logged to a Windows computer running the Trimble Reference Station software. The data are collected into 1 hour long files (at 5 s recording intervals) that are subsequently sent to LMI's data center in Akranes via ftp. At LMI the data are converted into 24 hour long files (at 15 s recording intervals). Both data sets are publicly available at ftp://ftp.lmi.is/GPS/AKUR (24h 15sec) and ftp://ftp.lmi.is/GPS/AKUR/1h5sec (1h 5sec for 90 days) (M. Rennen, personal communication 2002).

The National Power Company supported installation of a new permanent station at ═sakot (ISAK). The station was installed in January 2002 (Table 1). ISAK is located near the intake reservoir for B˙rfellsvirkjun power plant in Ůjˇrsßrdalur. ISAK is approximately 15 km NW of the summit of Hekla and should be able to detect major magma movements beneath the mountain. The quadripod was installed over an existing geodetic benchmark that has been included in network measurements since 1986 and used as a reference station in network campaigns around Hekla and Torfaj÷kull. The receiver is in a plastic box on one side of the quadripod. A modem and a power supply are in a hut 150 m from the quadripod. Although this cable length is on the verge of being too long for the RS-232 standard, there have been no problems with data transfer since the installation.

In the summer of 2002 a station was installed at ┴rholt (ARHO), Tj÷rnes peninsula, North Iceland, in cooperation with LGCA. Initial tests for installation of a permanent GPS station at GrÝmsfjall, Vatnaj÷kull, in cooperation with LGCA, have started. Installation and operation of a station at GrÝmsfjall is technically very challenging. The place is known for extreme icing conditions during all times of the year. A method to deice the antenna radome utilizing local geothermal heat resources is being devoloped at IMO (J. Hˇlmjßrn, personal communication 2002). The data will possibly be transmitted with the same spread-spectrum radio link as the SIL station at GrÝmsfjall (grf) uses.


next up previous contents
Next: Data Processing Up: Continuous GPS measurements in Previous: Introduction
Halldor Geirsson
2003-03-21