Saturday, January 9, 2016

Spying on Antenna Farms

Let's have some fun to start off the new year. Have you ever tried looking at your or someone else's antennas on one of the mapping services? I have done so several times. It is surprising what you sometimes can, and cannot, see in those satellite images.

Since the viewing angle is usually quite steep towers, houses and other structures can appear strangely unfamiliar. Some, like Google, will give you a bird's eye view by massively processing the images. But for this exercise I'll stick with the "raw" images from Google Maps. Let's see what we can learn from those images. Are my antennas real? Are yours? We can find out.

Google Maps satellite image of VE3VN
The image at right is a cropped screen capture of my house and part of the lot from Google Maps, at maximum magnification, on January 5, 2016. However, that certainly is not the date of the satellite image. The watermark says 2014, which will be later than the true image date.

Although the resolution is barely adequate there is a great deal to see if you take some time to analyze the image. I'll walk through this so that you have more tools at your disposal for analyzing images of your own antenna farm, or others.

Antenna size

There is a correlation between true size of antennas and towers and what their owners claim. Most hams are honest reporters but there is a tendency to exaggerate as their size declines. The satellite images help us to verify the truth of those claims.

HF yagis are usually easy to see in these images. Only under ideal conditions will wire and VHF/UHF antennas be well enough seen to be evaluated.

At first I thought the adjacent image was of the boom on my Hy-Gain Explorer 14, with the elements washed out. A closer examination of the tower shadow and the image detail reveals that the image is older. That's the 30' Golden Nugget tower supporting my 4-band TH1vn (modified TH6 driven element), oriented broadside to Europe and the Pacific.

Some wires are visible. The sunlit portions of the ⅛" guy wires can be seen going north toward the house anchor and also southwest and southeast to their tree anchors. The inverted vee is not visible, although you can see the shadow of the house-bracketed 14 meter tall mast that supports it.

Tower height

This is a more difficult challenge. Restricting ourselves to linear measures on the image -- Google includes a scale on the image -- as our only data it is impossible to determine tower height. You must also know one or both of the satellite position and image date.

If you know the satellite position you can calculate the viewing angle. With that and the projected linear length of the tower the true height can be calculated with simple trigonometry.

If you know the date you can calculate the solar elevation from the time of day (shadow direction, like a sun dial). Measure the length of the tower's shadow (assuming level ground) and the height can, again, be calculated with simple geometry.

When Google constructs a bird's eye view they make use of the satellite position. You could try to figure out tower height from that construction. I haven't tried that. If you try it be aware that the images are distorted when processed in this way in order to provide a "sensible" result. Linear measures are often unreliable after image processing. Look closely and you'll see numerous instances of distortion in bird's eye view and Streetview.

Yet there are ways. These are somewhat in the manner of that old joke: how do you measure the height of a building with a barometer? For example, infer the tower type from the image or outside information. If the resolution is sufficient you only need to count off a few of the horizontal tower members on the tower's shadow and combine that with the dimension data from the spec sheet. Count them all or use the shadow length to calculate tower height.

Other height-measuring techniques using the satellite image are left as an exercise to the reader.

Dating the image

When was it taken? Can we find out? Often the answer is yes. I've left enough evidence of my antenna comings and goings on this blog that we can estimate the date. Although this is almost entirely useless for spying purposes it is interesting (to me at least) so let's proceed with this topic and see what a simple picture can tell us. At stations going through lots of changes you might even catch antenna work in progress, as we'll see.

First, we're facing north, so the sun is behind the satellite. Although there are many trees at the edges of my property, with branches often meeting in the middle (squirrels love it), everything of importance is sunlit. That is very helpful.

We can locate the satellite. Follow the shadow of the tower to its base. Compare the projected length of the tower to the shadow (~0.2), do a little trigonometry, and we see that the satellite is ~10° east of the zenith point. Image processing stretches these images so that the trapezoidal projection of a square patch of ground due to the oblique viewing angle can be stitched together with others to form a contiguous map. Some distortion can creep in, although it should be small in this image since the satellite is close to the zenith.

The TH1vn dipole was put up in early summer 2013 and taken down in early summer 2014. The image had to be from those 12 months -- the 2014 watermark is consistent with this. Notice that the dipole is sitting directly atop the tower. The extended mast for the 40 meter delta loop is not present. Since the delta loop went up in late October 2013 either the image was taken between July 2013 and early October 2013, or June 2014 as it was being disassembled. That the greenery is in full display confirms this.

The direction of the shadow tells us the image was taken shortly after 2 PM EDT. As for the time of year, well, the tower is 30' tall, the house is ~47' from the tower base and my latitude is 45.5° (grid FN25). Since the shadow is ~36' long the sun's elevation is ~40°. It's close enough to local noon that we can say the sun is slightly south of the celestial equator. Therefore it is most likely early fall of 2013.

Now I draw your attention to a small detail. Look at the upper roof. There is a faint arc running from the fireplace chimney on the left to the roof edge on the right. I quickly ascertained that it is a rope I had rigged as a safety line. The rope was arranged so that when clipped into my safety belt I could stand at the edge of the roof, and move right or left, without worrying about falling. This allowed me to safely work on the mast and inverted vee.

An blog article from September 2013 shows that I was putting up the first version of my multi-band inverted vee, including the mast. The bracketed pipe went up earlier in the summer and was first used to test a 20 meter delta loop. The shadow of the antenna mast is visible on the lower roof. There is no sign of the delta loop. This narrows the date range.

I left the safety rope on the roof for some time since I didn't want to redo it each day I was up there. Tuning the antenna and adjusting the mast supports dragged on for a while. It is therefore quite possible that it was a few weeks past the equinox, as initially estimated above. It is also possible that my measurement of the shadow or recollection of the distance between house and tower is inaccurate, or even that the aforementioned distortion is present in the image. So it could have been mid-September, right around the equinox.

Are you feeling yet like Sherlock Holmes?

Checking out the competition

Out of curiosity I checked out a couple of large stations located on rural acreages. I was to be disappointed. Instead of antenna farms all I saw were empty fields. Either there were no antennas or someone paid off Google to hide them from prying eyes.

Google Streetview image of VE3VN, dated August 2014
Of course it's the former reason. Acquiring satellite imagery costs money and fewer people want to explore open tracts of land than urban areas. Images of rural areas are infrequently updated to manage business costs. Looking closely for the dated watermarks confirmed this. The satellite images were quite old, predating tower construction.

Google Streetview did better, when their camera-equipped vehicles drove down those rural roads. Even there I ran into a peculiar problem. When I turned a corner the number of towers changed! I backed up and looked at the dates of image acquisition. They were different by two years. Less travelled dirt roads were covered later by Google.

The Streetview capture of my QTH is just above. It is dated August 2014, which is just after the DMX-52 was installed in July but does not yet have any antennas, which went up in September. Here you can clearly see wire antennas. If you are lucky and a recent Streetview image is available it can easily be superior to satellite images.

So go ahead and give it a try. Test your detective skills. See if the antennas some ham is bragging about are as advertised. They may be downplaying their size in an attempt to lull competitors. Now we have the tools to check on them from the comfort of our shacks. Satellite images are also useful for exploring land for a future antenna farm.

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