Index
Presurvey: choosing a site
The first (and most important) step in the realization of a GPS network is the localization of a good set of sites where GPS antennas and receivers will be installed. Data quality is strictly dependent from the stability of soil and minumentation, so a strong effort must be carried out to find rock outcrops and to achieve the best quality monumentation possible.
To decide where to place the GPS stations in the study area one must decide the ultimate purpose of the survey and the accuracy of the desired results. GPS surveys can be useful to:
- study soil deformations due to seismic deformation cycle (preseismic, coseismic and postseismic);
- study active faults and seismic risk in the region;
- study volcano deformations;
- contribute to the definition of the Italian GPS system.
Necessary presurvey tools:
- Palm GPS receiver (to calculate the coordinates of the site and estimate the tracking above the site);
- Topographic map (1:25000 to 1:100000);
- Compass (to measure obstructions);
- Hammer (to test the surface geology);
- Digital camera (for documentation purpose).
What to examinate in the presurvey phase
| Satellite configuration | To download quality data (to avoid signal blockage) GPS stations need to be placed properly. This means that no obstructions must be present above 15° elevation (360° azimuth): the site should therefore be chosen with no buildings, trees or other objects that could block the satellite signal in the near surroundings. |
| Geology | This is an extremely important parameter to deal with. Usually GPS stations should not be placed on clay or other kinds of soil that tend to expand or shrink depending on the water content. If no other possibility is available one should anchor the monument as deeply as possible to grant stability even in these conditions. Somehow it's always better to find some rock outcrops (limestone, granite, sandstone or tuff) to build the monumentation on. The rock should be as least fractured as possible and with no landslide zones or quarry in the surroundings. It is theresfore necessary to perform a geological analysis of the investigation areas. |
| Monumentation | As geodetic GPS network must describe very slow soil deformation, it's not usually convenient to put an antenna on the roof of a building, although this operation seems to simplify the instrument installation and maintenance. This should be avoided because of the termic deformation that causes building to expand and shrink daily or seasonally and might interfere with the geodetic datum. Another problem is represented by the damages and the permanent deformations that could affect a building if a strong earthquake happens. For similar reasons GPS antennas should not be mounted even on other constructions (bridges, etc.). The monumentation can be realized as a concrete column or a set of legs welded together and drilled deeply into the soil and must provide as much stability as possible. |
| Safety and access | To avoid damages or stealings to the GPS antenna it's better to locate the stations in protected zones (private areas, etc.). It will be then necessary to ask permission to build to the owner of the terrain and stipulate contracts to have full-time access to the site for maintenance, troubleshooting, or other tasks. If necessary the antenna can be surrounded with a wire-netting to protect it against vandalism. Another important thing is to install all the stations in easily reachable zones even with severe atmospheric condition (this applies mainly to high mountain sites in Wintertime). |
| Multipath | Reflecting surfaces in the vicinity of the antenna could lead to the multipath problem. Mutipath is the phenomena by which the GPS signal is reflected by some object or surface before being detected by the antenna. The signal can be reflected off a part of the satellite (for instance the solar panels) although this is usually ignored as there is nothing that can be done by the user to prevent this. Mutipath is more commonly considered to be the reflections due to surfaces surrounding the antenna and can cause range errors as high as 15 cm for the L1 carrier and of the order of 15-20 m for the pseudoranges (Rodgers, 1992). The surface most prone to multipath is water, whilst sandy soil is the least. For this reason it's necessary to choose sites free from possible multipath sources and to use appropriate instrumentation (e.g. choke-ring antennas) to reduce its effects. |
| Electromagnetic noise | The absence of nearby electrical installations (e.g. transmitters) is necessary to avoid signal disturbances. Such noises could irrietriviably damage the GPS signal since they work at similar frequencies. In the country it's often difficult to notice this kind of disturbing sources that could be placed several kilometers away. |
| Power supply and data transfer | More than one solution is available to provide power supply to the station and to perform the data tranfer procedures. It is therefore necessary to analyze what are the site logistic charachteristics. In general a GPS stations requres a power supply between 10W (without acqisition system) and 80W (with acqisition system). The possible alternatives to provide it are:
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| Duration of station | In order to obtain good solutions and modelization, as deformation rates in Italy are inferior to 1 cm/year, station lifetime should be 10 years or more. |
Setting up a GPS station
As an example the Zouf Plan GPS station installation will be described. Notice that there are often various different possibilities in the monumento building procedures, depending on the soil properties and on the avilable field equipment. It is therefore experience that will tell the most convenient way of proceeding in each situation. In this installation we used an Ashtech Choke-Ring antenna, therefore the most detailed descriptions (antenna/adaptor assembly) are referred to this instrument.
Power supply and data transfer
The antenna needs to be powered, consequently it's necessary to run a power cable from the energy source to the monument. For this purpose we dug a 10-15 cm deep trench and set down a conduit where we inserted the power cable. If the temperature of the place where the site is located is often falling under 0°C it might be a good idea to provide the duct with a drainage pit. This will let water drain down into the underlaying soil without harming the conduit (water could infiltrate into the pipe and freeze, causing not easily recoverable damages). It is possible to use stiff or flexible conduits, somehow it's always better to insert the power cable before laying it into the ground. This is helpful to make the insertion process easier, as one can avoid problems coming from the pipe junctions or bendings.
Building the monument
The monument is she concrete column that serves as a support for the GPS antenna. In the buiding operations it's very important to keep perfectly vertical the steel rod on which the antenna adaptor will be screwed. One should also make sure that the rod cannot rotate on its axis. If these two steps aren't performed properly the antenna would lose its orientation and this would lead to lower quality data.
In the monument building phase it's necessary:
- Remove superficial ground until you find a rock outcrop, then clean the area from earth and dust.
- With a drill bore a 18-20 cm deep hole in the middle of the cleaned rocky area. This will be used to hold the central steel rod (monument pipe), so it should have a 4-5 cm diameter.
- Drill 4 others 18-20 cm deep holes with 2 cm diameter all around the rod holding hole. These will serve to anchor the reinforcement.
- Clean the area from dust.
- Try to insert the steel rod an the reinforcement into the drilled holes. Then set down the paper cylinder form that will be used to cast the concrete column so that the main rod stands at its center. Screw the adaptor onto the rod and check if there is enough space between this and the top of the column (almost 10-12 cm). Then remove the cylinder.
- Place the steel rod into the central hole, level it untill it stands perfectly vertical and then fix it in that position.
- Place the reinforcement around the main rod.
- Drill a hole in the main rod and screw the ground cable to it.
- Verify that the rod is still vertical.
- Prepare some cement.
- Pour it to into the holes containing the monument pipe and the reinforcement to fill the free space and block them in their positions.
- Using iron wire tie the cable conduct to the reinforcement.
- Wait some time (until the cement is partially dry and semi-solid), then place the paper cylinder form. Using a set of tie-rods or just pushing it into the semi-solid base cement make sure that it stands firm and vertical. Now, if everything has been done properly, the main rod should be on the Cylinder axis. In this phase it's very important to make sure not to let any free space between the Cylinder form and the basement. Otherwise when performing the column concrete casting some cement could come out and be lost.
- Wait for the complete hardening of the cement on the monument base.
- Prepare some new cement and cast the column.
- Make sure that the cable conduct gets out of the column in a proper position to connect with the hole on the plastic dome base. Obviously using flexible ducts gives you more tolerance. If necessary put some spacers.
Assembling the antenna
While waiting for the column cement to solidify one can proceed with the antenna assembly. Hence the parts to deal with are:
1. Plastic dome base
- Fix the cable conduct to the plastic dome base.
- Place the dome base so that the its central hole holds the monument pipe.
2. Adaptor bottom plate
- Using a compass determine the magnetic North direction, then locate the geographic North calculating the magnetic deviation of the area.
- On the bottom plate there is a North mark: screw the plate on the monument rod and point the mark to the North.
- Unscrew the plate, put some Loctite glue onto the screw thread and then re-screw it on the pipe making sure to correctly orient the North mark.
3. Adaptor top plate
- Assemble the adaptor top plate on the inferior one keeping in mind that the groove must be on the bottom side and North oriented.
- Insert and screw the clamping bolt with its two washers (with the hollow sides one against the other).
- Remove from the bottom side of the bottom plate the two leveling lock screws. Leveling screws, which stand into the same holes, are now visible.
- Using the leveling screws level the top plate.
- Once the top plate is horizontal lock the leveling screws with the leveling lock screws previously removed.
4. Antenna
- Unscrew the top plate from the bottom plate.
- Place the antenna on the top plate so that the security set screw correspond to the groove. When done the antenna should be oriented so that the North (indicated with an arrow on the antenna bottom side) is lined up with the North indicated on the adaptor top plate. Screw the antenna to the adaptor top plate without clenching too much. Do not use glue on this screw! Verify that during these operations the clamping bolt that keeps the two adaptor plates tied is inserted into its hole.
- Screw the antenna bolt to join antenna and top plate together.
- Screw the antenna-top plate block on the adaptor bottom plate.
- Verify leveling and orientation.
- Connect the cable to the antenna.
5. Dome
- Screw the plastic dome base to the adaptor bottom plate.
- Place the dome on the antenna.
- Screw the 12 screws needed to fix the dome to its base.
Final operations
Wait the necessary time for the monument cement to harden. Then remove the paper cylinder form and check if everything is OK. There should be no cracks in the concrete monument and the monument pipe must be stable and well anchored to the ground.
