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Where is My Repeater Really Located?
By Eric Lemmon WB6FLY,
HTML'd and edited by Mike Morris WA6ILQ
The two most important physical pieces of information about any repeater, whether commercial, Civil Air Patrol, GMRS or Amateur Radio, are: Where on the planet is it located, and how high is the antenna? Judging from the typical responses to these basic questions, not every repeater owner or trustee understands exactly what is expected. Let's examine each of these parameters in detail.
Where? First of all, the "where" of a repeater should be expressed precisely in geodetic coordinates, not by some vague nickname. For example, one of the high points within a mile of my house is known as "Muffin Hill". Most of my neighbors know exactly what this hill is and where it is located. Unfortunately, none of the members of the local coordinating body have a clue where Muffin Hill is! In fact, there may be dozens of Muffin Hills throughout the State, or across the country, and which one are we talking about?
Note from WA6ILQ:
This has happened in other parts of the country... as an example, here in southern California there are two different locations named "Red Mountain" and two locations called "South Mountain" - and these aren't local names, they are on the topo maps. If you want some fun with maps sometime, look how many states have a city or town called Franklin, Fairview, Midway or Springfield.
The proper means of specifying the location of a transmitter is in geodetic degrees, minutes, and decimal seconds. This is often expressed when written or in print as DDD MM SS.s (where DDD is degrees, MM is minutes, and SS.s is seconds and a tenth of a second). Why the 1/10 second? The FCC requires that level of accuracy for commercial towers. One minute of arc at the equator equals one Nautical Mile, which is about 6,080 feet (about 1.85km). One second of arc is therefore about 101 feet, and 1/10 second is a little over 10 feet. The FAA goes one step further and requires obstructions to aviation such as towers to be located within 1/100 second, or about one foot.
How does one determine the exact coordinates of a tower to such accuracy without hiring a surveyor? The simple answer is: You can't. But, you can come close enough by properly using a good GPS receiver. I must emphasize the word "properly," because there are several pitfalls to avoid.
A good 12-channel GPS receiver with WAAS capability, such as the Garmin GPS76, is an ideal choice. When positioned so that it can receive WAAS differential signals, it is capable of providing a position within 5 or 6 meters. The key to achieving such accuracy is to leave it undisturbed for at least 15 minutes, at a point where all major obstructions are north of the GPS receiver. In the Southern Hemisphere, the major obstructions should be to the south of the receiver. It takes about 11 minutes for the GPS receiver to collect a complete almanac, so that it knows which satellites are reliable and which should be ignored. This is especially important if the GPS receiver has not been used for several days. A remote antenna on the GPS can be used to achieve a clear view of the sky - just remember that if you use one then your displayed location on the GPS is that of the antenna.
It is very important to set the GPS receiver to report its position on the NAD83 (North American Datum of 1983) or WGS84 (World Geodetic System of 1984) datum. The coordinates will be different when expressed on the older NAD27 (North American Datum of 1927) datum, which is still found on NGS topographical survey maps. The NAD83 datum is expected by all Federal agencies, as mandated by the Federal Geodetic Control Committee. The official notice in the Federal Register can be downloaded at http://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc89-14076.pdf. The GPS satellites themselves use the WGS84 datum, which is almost identical to the NAD83 datum used by surveyors, and we can ignore the miniscule error between them for our purposes here.
Now, let's consider the coordinate display. The GPS76 can display positions to the 1/10 second, in the DDD MM SS.s format, but not all receivers have that capability. An alternative is to select 1/1000 minute, in the DDD MM.mmm format. The latter format has a resolution of about six feet. Keep in mind that the display resolution does not necessarily equate to position accuracy.
Many repeater coordinating bodies are apparently unaware that geodetic coordinates, by themselves, are meaningless if the datum on which they were determined is not specified. Therefore, always include the phrase "NAD83 Datum" whenever you provide coordinate information.
Don't trust the coordinates shown on the FCC licenses posted by other users of the site! There are many instances where the first occupant of the site used coordinates estimated from a tour map, and each new operator reported the same coordinates on his license application. Remember, one geographic second is roughly 100 feet, so there should be obvious differences in the coordinates of multiple towers at a hilltop site. While the FCC may not dispatch the "Coordinate Police" if your antenna location is off by a significant amount, the Federal Aviation Agency (FAA) may find an erroneous position to be an air navigation hazard, and a $10,000 FAA fine hurts just as much as a $10,000 FCC one.
If there is more than one tower at a site, EACH ONE has a coordinate location - so the site owner needs to provide the coordinates for the tower you are going to be on... something like "If your antenna is on the North Tower, use these coordinates; if on the West Tower, use these coordinates, etc.", and the coordinates should be the position of the center of the tower. Remember, each 1/10 of a coordinate second is about ten feet, so no tower leg position is going to be the center of the tower, and there might be a difference of 2/10 second (or more) between two legs of the same large tower! The FCC doesn't care which leg of the tower a particular antenna is on, just so the coordinates are accurate enough to determine which tower of several at a site is defined. Obviously, things get real murky when one operator quotes his coordinates on the NAD27 datum, while another operator quotes on the NAD83 datum. In my area, those two points will be about 270 feet apart! Here's a PDF file (about 99 kB) from the USGS on the difference between NAD83 and NAD27 offsets.
Note from WA6ILQ:
Location errors - no matter what the source - can be significant - I know of one radio site that was actually about three miles from where the site coordinates had it... the first tenant (back in the 1950s) cited a latitude and longitude on his license application, supposedly from a surveyor. A 3-by-5 card saying "You are at (lat/long)" was posted on the wall inside the building door between the door frame and the adjacent circuit breaker panel. Everybody used those coordinates, and nobody ever verified them (it was a single-tower site) and in over 30 years nobody had any reason to doubt them. When a friend's radio group put a 220 MHz system in there in the early 80s they did the same thing and quoted the same coordinates. One day we were doing a site visit to add a channel to the 2m remote base and a group member brought along his son who worked at a Sharper Image type of expensive-toys store, and he had the store demonstrator of a new toy: a GPS receiver... and once it was set up and had a fix everybody was scratching their heads at the difference... A phone call was made to the site owner (who was a ham) and after explaining what a GPS was (imagine that conversation - "You've got a WHAT?"), we gave him the numbers off the GPS display. The site owner appreciated the tip and had the tower location professionally surveyed... over the next few months, about 45 commercial licenses (i.e. every tenant) plus a half-dozen ham licenses) were quietly corrected.
In other words, don't trust what someone says. These days GPS receivers are common enough that it's easy to do a sanity check on the information provided by the site manager. There's a saying in the intelligence community that is also used by the police detectives that applies here: "Trust, but verify".
Many hilltop radio sites are also good locations for Geodetic Survey Monuments, since a surveyor can swing angles over great distances from an elevated reference point. If you have a National Geodetic Survey marker at the site, you can make offset measurements from that marker to determine the exact location of your antenna. I will cover the use of NGS survey markers in a separate article.
Note from WA6ILQ:
King County, in Washington State had an interesting web page on Survey Markers, including photos.
How High? There are two height figures associated with antennas, and they are not the same. When the FAA and FCC ask for antenna height, they usually mean the top point of the antenna radome or the mast, whichever is taller. This height establishes the minimum clearances for air navigation. The other height figure is the height of the antenna's radiation center, or "RC". The RC height is used to perform propagation studies and to calculate the Height Above Average Terrain, or HAAT. Usually the RC is considered to be the physical middle of the antenna, but it's not always.
Before you erect an antenna, you need to measure and record two critical numbers: First, the distance between the bottom of the antenna and the radiation center (RC), and Second, the overall length between the bottom of the antenna and its tip. Both numbers are fairly easy to measure when the antenna is on the ground, but very difficult to do once it is in place on the tower. Once mounted, determine and record the distance between the bottom of the installed antenna and some reference point at ground level, such as the top of the concrete abutment supporting one of the tower legs. This is the antenna height Above Ground Level, or AGL of the lowest point of the antenna. Add the RC to this to get the height of the antenna's radiation center above the ground.
The next height figure we need to determine is the elevation Above Mean Sea Level (AMSL) of the tower, using the same tower leg abutment as for AGL, above (AMSL is the height of the ground or the concrete footing at the base of the tower). You should not use the elevation shown on your GPS receiver for this figure, because it simply is not accurate enough (altitude is the least accurate number supplied by a GPS). If your site has a National Geodetic Survey marker, as many hilltop sites do, you can find out the precise elevation AMSL in meters of that marker, and use simple techniques to translate the elevation of the marker to the elevation of your concrete abutment (or othe reference point). As noted earlier, I will discuss these techniques in a future article.
If your geodetic coordinates are accurate, you can determine the approximate AMSL elevation using one of several mapping programs such as TOPO! or TopoQuads. Many commercial propagation programs have the capability to determine the Height of Average Terrain, or HAT, given the exact GPS coordinates of the antenna tower. Then, the physical height of the antenna's radiation center above ground level is added to the HAT to give the Height Above Average Terrain, or HAAT. It is possible to manually determine the HAT by drawing radials on paper topographical survey maps, but that is beyond the scope of this article. Just be certain to use the RC and not the antenna's overall height (total length) when calculating and reporting HAAT.
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Article text copyright © 2005 by Eric Lemmon WB6FLY
Hand coded HTML and italicized text copyright © 2005 and the date of last update by Mike Morris WA6ILQ
The information presented in this web site, on these web pages and in these modifications and conversions is © Copyrighted 1995 - current by Kevin Custer W3KKC and multiple originating authors.