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The Fixed-Mount Rock Pin:
Site Selection, Design and Installation

by K. W. Hudnut
(last update 7/26/96)

In some cases, there are good reasons to install a fixed, forced-centering GPS antenna mount in rock. For this purpose, the rock pin design by Mike Bevis (at SOEST, HIGP) has been modified from the original. The installation of the pin itself is done in much the same way as for a Bevis pin. This page describes our modifications to the Bevis Pin design, how we install such pins, and things to consider during the selection of a good piece of rock to install such a pin into.

The continuous GPS station with the lowest random-walk type monument noise within the Southern California Integrated GPS Network (SCIGN) is of this type (Y. Bock et al., 1995 - Fall AGU talk; & m.s. in prep.). It is located at Lake Mathews, and is called station MATH. Although it is a somewhat different (higher mounting point) pin design than the subsequently designed one shown here, the selection of bedrock was made taking into account all of the factors discussed below. Also, it was installed as described below.

This is the least expensive way to install a highly stable monument for a GPS network. The largest challenge in doing this properly is in selecting a good piece of bedrock, so a discussion of this step is included here. Even if you plan to use a different type of pin, post, or mast antenna mount in bedrock, some of this section may be useful to you. If you are considering using this pin, you should also be aware of the tests indicating problems of using low (<0.5 meter) antenna mounting heights, and these are discussed here as well.

Index:
Site Geology: Selecting a good piece of bedrock
Modifications to the Bevis Pin
The Fixed-Mount Rock Pin Design: Technical Drawing and Notes
Installing the Fixed-Mount Rock Pin in Bedrock
Attaching the GPS Antenna to a Fixed-Mount Rock Pin
Acknowledgements

Site Geology: Selecting a good piece of bedrock

The type of pin described here should really only be used in situations where the bedrock is of extraordinarily good quality. If you find a site with really nice rock, and you aren't trying to discern differential rates between your stations at the millimeter per year level (roughly speaking), this pin is for you. When deciding whether the rock at your selected site is acceptable, keep in mind the following points.

  • The best rock is unfractured, unweathered, highly indurated rock that is free of clay minerals, (or minerals that chemically alter into clays, e.g., feldspars), such as a very clean quartzite. Clean, unfractured carbonates are good too, but tend to have more strongly developed bedding planes (i.e., fractures - keep reading).
  • Fractures fill with soil, including clay minerals, as well as water. Hence, these fractures will move with varying temperature (thermo-elastic strains as well as freeze-thaw), water saturation (expansion and contraction of clays), etc. If the rock is fractured, consider another monumentation strategy.
  • Weathering means there will certainly be expansion and contraction of clay minerals, etc. In some cases, we have installed driven-rod monuments successfully into late Tertiary conglomerates, etc. That would be a preferable approach. If the weathering is not too severe, or if you can find or create a patch of outcrop that has been excavated through to less weathered rock, you'll be better off.
  • Same goes for induration. A driven rod monument would attach you to a larger volume and would be preferable if it is logistically feasible and can be afforded. We have sites where we've set pins into rocks that don't ping under a sledge hammer blow, but if we had it to do over again with more resources, we'd seriously consider a drilled-braced monument at these sites (TRAK, CHTP, maybe even ROCK; all of these are in massive-bedded, late Tertiary sandstones).

  • NOTE: A quick and easy, though not necessarily reliable, test of the weathering and induration qualities of the rock in an outcropping is to stand on the outcrop with both feet, and give it a mighty whack with a heavy sledge hammer (6 or 8 lb.). If the hammer 'pings' and rebounds elastically, and if you feel no vibration through the soles of your boots, it's a good one. This is a gratifyingly low-tech geological field procedure well known to anyone who has done geological mapping in New England (for example), to help one decide whether a rock is glacial 'float' or actually bedrock.

    Beyond this, you should examine a rock specimen and make sure you understand the mineralogy - if there are feldspars in it, even really nice looking bedrock will chemically and mechanically weather more rapidly over the duration of your experiment, and the clays resulting from feldspar weathering will cause expansion and contraction.

    If what you really are looking at is a large boulder that is basically embedded in surrounding soil, this pin will work, but don't call it a 'bedrock monument.' These types of monuments are well known to wander around the countryside. You would be much better off in this situation to install a driven-rod-braced monument in the soil nearby rather than place a pin into the boulder. This is because the boulder is extremely well coupled to the surficial soil, so it will move around with the soil as the soil creeps downhill, expands, contracts, and so forth - this will happen even if it is a very large boulder.

    If the rock is less than ideal, it is a matter of considering whether you can or cannot afford something better, and so forth. If you have to think too hard, chances are you should try to get the funds for either installing a drilled-braced or a driven-rod-braced monument. You may want to go ahead and place a pin - the choice is yours. Even if you currently think your main goal is to figure out what is happening at relatively high velocity differences between stations in your network (>5 mm/yr), you don't want your data contaminated by noise from monument instability. It's best to install the monuments in your network really well at the outset of the experiment, if at all possible.

    In fractured, weathered, or poorly consolidated rocks, this pin should not be directly drilled into the rock, because it is better in such cases to attach to a larger volume of the rock, thereby minimizing the effects of local disturbances within the rock mass (due to thermal, mechanical, or chemical expansion and contraction, etc.).

    If this talk about geology is unfamiliar to you, you should have a colleague who is a geologist help you with selecting your rock sites. It would help you both if they would read this web page before going to the field, since they may not have considered some of the issues discussed here that are of importance to monument stability.

    One can attach to a large volume of rock in several ways. It is best to install a deeply anchored monument called a Drilled-Braced monument, designed and documented by Frank Wyatt and Duncan Agnew at Scripps. This type of monument is used, for example, at all of the sites in Brian Wernicke and Jim Davis's Basin and Range continuous GPS network, even though their sites all have excellent quality bedrock. There, the investigators are interested in differential velocities between stations that are on the order of millimeters per year.

    The best approach for your own experiment should be considered carefully, of course. For related information on GPS monumentation especially designed for continuous GPS stations, please see the UNAVCO Monumentation page. For information on the various factors to consider in deciding on the best type of monumentation to use in a given field situation, and with available resources, see the UNAVCO Monument Specifications page and the UNAVCO Monument Examples page by Jim Normandeau (at the UNAVCO Boulder facility).

    Notably, we have also used this pin design when installing an antenna on massive concrete structures such as the old building foundation, poured onto fractured crystalline rock that had been graded and excavated, at station Fire Camp 9 (CMP9).

    Modifications to The Bevis Pin

    The Bevis Pin. The Bevis rock pin is about 6" (15.2 cm) long, and is 1/2" (1.3 cm) in diameter. This makes the pin cheaper, easier to install, and so forth, than the one we describe here. It has a flat top surface with a drilled center hole, over which one sets up a tripod, tribrach, and GPS antenna. This is a great bedrock pin for campaign-style GPS work. On the UNAVCO Monumentation page, you can find diagrams showing a spike mount over a pre-existing Bevis rock pin (see Fig. A) , as well as the UNAVCO Boulder facility's solution to the design of a fixed-mount rock pin (see Fig. B - Antenna Mount Pin).

    The Fixed-Mount Rock Pin Design: Technical Drawing and Notes:

    Click on the image to zoom in on it.

    I designed this particular Fixed-Mount Rock Pin; other variations of design exist now and pin design is sure to continue to evolve. This design is simple and functional - it is simpler than any other fixed-mount rock pin designs I have seen. This should make it cheaper to machine, and easier to install, with no significant 'downside' to the design. Also, the mount itself is perfectly axisymmetric, and has as narrow a horizontal profile as possible in the volume beneath the antenna. These factors make this mount less likely to directly contribute to multipath or to asymmetric electromagnetic field interference, etc.

    The technical drawing of this pin, for machining purposes, was engineered and computer-drafted by Jeff Batten of the Caltech Seismological Laboratory. The cost of stock steel and machining will vary; Jeff estimates that it takes a couple of hours to machine each pin.

    For machining, pick up a postscript file of this technical drawing and print it out.

    The question of the height of this (and other) mounts to the ground surface has been investigated, and some conclude that low (<0.5 m) antenna mounts can contribute to decreased accuracy measurements (in the vertical component particularly), especially when the post-processing software is being used, as usual, to simultaneously estimate tropospheric delay parameters. A UNAVCO report entitled 1995 UNAVCO Antenna Height Tests is available from the UNAVCO WWW pages, and future test results can be found there. Their 1995 tests showed that low antenna mounts are more problematic than tripod height mounts, at least for microstrip antennas with standard flat ground planes. A further test, mentioned in that report, was to investigate the height effects on a Dorne-Margolin Choke Ring type antenna that is less susceptible to multipath errors. A preprint of Meertens et al., dated April 5, 1996, contains more information on this and related subjects.

    The station MATH, mentioned before, has the antenna mounted at about half a meter, between the 'really low' (approx. 10 cm) and 'tripod' (approx. 100 to 150 cm) heights - that station has been operating with a standard geodetic microstrip antenna. Clearly, despite it being classified as a low antenna mount, and its use of a non-choke antenna, this station's time series are very clean.

    This present version of the mounting pin places the antenna ground plane very close to the surface of the rock outcrop - for a typical installation of this pin, the ground plane will be approximately 10-15 cm (4-6") off the ground.

    It is straighforward to modify this current design to have either less or more space between the rock surface and the antenna ground plane, at least if you only want the antenna to be up to half a meter above the rock. For example, the station MATH has the antenna higher above the ground (approx. 50 cm) because of the uneven surface of the bedrock outcrop there. Similarly, of course, a very low mount is not desirable in any location where there is typically some snow accumulation, even if it is typically transient, during the course of each year. For any of the above reasons, one may want to position the antenna higher, and there are many ways to accomplish this.

    If you are going to modify this design to place the antenna higher, you should also make the portion that is in the rock deeper. We have used longer drill bits (up to about 16" long) with the drills mentioned here (even the lower powered Ryobi), without having serious difficulty. With the antenna up at half a meter, consider going to a larger diameter pin, and extending the below-rock portion to a half meter as well (if feasible).

    It may be preferable for logistical and possibly for technical reasons (especially for microstrip antennas) to mount the antenna higher than this present pin design would place it. If you want to place the antenna higher than about half a meter, it would be best to change to a larger diameter pin, say a 3" diameter pipe, in which case drilling into the rock will be much more difficult with the tools described here - perhaps even impossible. Some hand-operated coring drills may be available with a 3" diameter bit. Eddi Wheeler, Mike Bevis, and Fred Taylor have designed and recently begun installing tethered masts so as to keep the tools and hardware minimal while also getting the antenna positioned higher. This installation type is called a Geodetic GPS Antenna Mast, and the mast is installed over an existing Bevis pin.

    Alternatively, one could take the approach we did for our station at Chilao Flats (CHIL), where we knew that snow would typically accumulate on the bedrock ground surface each winter. Here, we used 3/4" stainless rod and a drill to fashion a permanently installed metal tripod. We drilled into the rock in 4 places, one for the vertical center leg, and 3 angled holes for the 3 slanted legs. These 4 legs converge about 1.2 meters above ground, and the central leg extends above that junction, and has a rotatable adaptor atop it for affixing the antenna. The legs were welded together at their junction with generator-powered portable arc welding equipment.

    Installing the Fixed-Mount Rock Pin in Bedrock:

    Materials: In addition to the pin shown in this diagram, the installation of this pin also requires one steel hex nut that fits the 5/8-11NC-2A threads on the top of the pin, as well as one washer that has internal radius opening of 3/4" and external radius of 1 3/4" (or larger). You will also need both some epoxy glue and some bolt-anchoring cement (expansive grout). Bring these items (and spares/extra) with you, as well as some containers and stirrers, etc. for mixing either the epoxy or the grout (note: you'll only need a small quantity of either one, that is, you don't need anything more than milk cartons or coffee cans). Also, bring along a drinking straw to blow rock powder out of the hole.

    Tools: For drilling into the rock, we advise use of an AC electric-powered rotary hammer drill, such as the Macho or Milwaukee ones we have. If AC power is not available from a nearby outlet or a portable gas-powered generator, then a gas-powered rotary hammer drill will do the job (but may prove to be under-powered, especially at high elevations). The only gas-powered drill we've found is made by Ryobi. In addition to the drill, you will need a carpenter's level, and an adjustable crescent wrench (or standard open-ended wrench that fits the tightening nut). Also, bring a metal file and a metal file brush. It is also a good idea to have the tools needed for servicing the drill, generator, etc. on hand.

  • Step 1: Select the exact piece of rock carefully, taking into account the rock itself as well as all the usual siting considerations (sky visibility, radio frequency interference, where and how far the antenna cable will need to run, where the receiver enclosure, AC/DC power and phone or other communications are available, etc.).

  • Step 2: Drill into the rock, keeping the drill vertical so that the hole is vertical. This is so that when you attach the GPS antenna, it will be 'leveled-up.' To keep your drill hole vertical, use your carpenter's level while you are drilling. With an under-powered drill, this will all be more difficult. If you don't get the hole drilled quite vertically, then you will need to over-drill the hole by spinning the bit in the rock, and this makes for a poorer installation overall. The ideal is to get a vertical hole that is just the diameter of the bit, so that when the pin is inserted you obtain a friction fit between the pin and the rock. If you stray from vertical, you are better off over-drilling the hole and setting the pin vertically (so that the antenna is plumbed) than you would be if you accepted an off-vertical pin. The hole needs to be drilled to a depth such that the 1/16" machined groove (below the threads) is exposed above the bedrock surface one inch or so. For the dimensions given in the accompanying technical drawing, this means drilling a 6" deep hole. After drilling is done, clean out the hole. This is best done with air. Just bring a drinking straw with you and blow it out - but watch out for inhaling rock dust or getting it in your eyes.

  • Step 3: Try setting the pin into the dry, empty hole. If it will not slide in easily, and yet the pin is vertical, you are ready to continue. Mix up some epoxy, smear it liberally on the part of the pin that will be in the rock, and drip some of it into the hole. Then, put the pin in the hole as far as you can, spinning it as you do. Now, use a small sledge hammer to tap, or even forcefully pound, the pin into the rock. The narrow part of the pin above the threads is there so that you can pound pretty hard without damaging the threads.

  • Step 4: If, instead, the pin slides easily into the hole and can be wiggled around when in the hole, or if the pin had to be over-drilled in order to make it vertical, then you should use bolt-anchoring expansive grout (a.k.a. pour-stone) instead of epoxy. Mix this up, pour it into the hole, and then emplace the pin and hold it vertical with the carpenter's level as the grout cures. You may want to fashion a brace to hold it vertical as the grout sets up.

  • Step 5: Now that the pin is in place, and after the epoxy or grout has fully cured, you may need to clean up the top of the pin (remove any burs, etc.) and the threads. Use a metal file and metal file brush to flatten the top surface of the pin, and then clean off the threads.

    Finally, a step that is not recommended (but does work) - if the pin is not quite vertical after the glue or grout has fully cured, and before attaching the antenna, you may want to bend the pin slightly to make it truly vertical. If you have done the previous steps well, you should not need to do this. But, if you do, just fit a larger diameter pipe over the exposed part of the pin and lean on it as a lever. You may damage the threads even if you place a nut on them first. Do this gently and try not to over-bend the pin. A pin that is adjusted in this way will be less strong, and the force could damage the glue or grout bond with the rock.

    Attaching the GPS Antenna to a Fixed-Mount Rock Pin:

    Once the pin is in place and set, you attach the GPS antenna as follows. First, place the nut onto the threads and spin it all the way down to the bottom of the threads. Next, place the washer on top of the nut. Now, spin the antenna clockwise onto the threads until it is screwed on as far as it will go. Now back the antenna counterclockwise until it is pointed exactly to true north, keeping in mind the magnetic declination at your site. Now, tighten the nut up against the base of the antenna. Use the crescent wrench to torque it slightly, but don't over-tighten it. You can now use the carpenter's level to make a final and more accurate check on the leveling of the antenna.

    To measure the antenna height, use a metal ruler or caliper to measure from the base of the antenna pre-amp to the top edge of the machined 1/16" groove in the shank of the pin.

    Finally, you can stand back and admire your work. Good job!

    Acknowledgements

    I would like to thank Mike Bevis, Brian Wernicke, and Jeff Batten, for extensive discussions, all of which led collectively to this document and the technical drawing that accompanies it.

    I hope that this design and documentation will be widely used and that others will continue to improve on this. Please send me your comments or suggested links to related WWW sites that are not already included here. If you get a nice photo, have some experiences to write about, etc., please put it on the WWW and send me a note with the link. I'd like to help develop a WWW catalog of examples of these types of installations.

    This report is preliminary and has not been reviewed for conformity with U. S. Geological Survey editorial standards. Any use of trade, product, or firm names is for descriptive use only and does not imply endorsement by the U. S. Government.

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