Category Archives: Desert Roaming

Nice filter for 1090 MHz ADS-B site

Over a year ago I decided that I needed some good filters for ADS-B reception on mountaintops. Not that I had an immediate need, I didn’t have anything on a mountaintop, but I suppose I had a little extra cash and felt excited to be able to imagine a good ADS-B receive site atop an Arizona mountaintop (or hilltop) location. So, I drafted up what I thought would be a reasonable spec and went into Alibaba to find a filter manufacturer to build one. I ended up with 5 filters, all exactly to my spec, and for a bargain price (well, relatively speaking).

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Above is the finished ADS-B receiver assembly, complete with Raspberry Pi, RTL-SDR 1ppm TCXO SDR dongle, a eBay-purchased LNA, an eBay-purchased 12 vdc to 5 vdc DC-DC converter, and some coaxial cabling (also from eBay). The black square in the center of the image is the 1090 MHz filter, and it’s a quite good one.

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It’s a straightforward cavity filter, a little aluminum brick with fine performance.

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Solid out of band rejection, and I suspect around -100 dB ultimate rejection. The SA just doesn’t have the range to see it.

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While the signal of interest is only a MHz wide, I wanted a filter that was wider so that temperature and mechanical variation would never haunt me, and I wanted a low bandpass loss (the above shows less than 1 dB loss) across the band.

Behind the filter is a run-of-the-mill eBay wideband LNA with a 1 dB NF, and somewhere around +30 dBm IP3. The RPi is running the most current version of FlightAware’s PiAware, rev 3.0.4, and supports just about any off-the-shelf USB SDR dongle.

After setting it up, it looked like I needed to reduce the overall gain a bit, so I discovered how to go into dump1090 and change the gain from “automatic” (really not, I think it’s just max) to 42 dB. That gave me best range and most received a/c.

The antenna for the site is a FlightAware fiberglass stick, about 12′ above the ground, mounted on the side of the tower.

HeyWhatsThat_30aug16Coverage seems to be pretty close to the model generated by HeyWhatsThat.com (above). The blue line is the 40,000′ contour, while the orange line is the FL300 contour.

24 hours or so of actual flight logs produces the following plot, which is more or less pretty similar to the HeyWhatsThat plot.

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The primary notch in the pattern, in the SE, is the higher part of the ridge on which the radio site sits. It ends up blocking any coverage of flights in and out of Tucson, over 110 miles away, until said flights get to FL300 or so.

It will be interesting to see how the coverage shapes out over the next few weeks – I hope that it will get up near the top of all the local receive sites in performance.

ADS-B Update

Installed an ADS-B receiver up on White Tank Mountains over the weekend. Coverage seems pretty darned good, and I lose a/c only when they’ve gone about 1.5° below the theoretical horizon.

Below is a screenshot of the display. The a/c out beyond the 200 nm range ring are all at FL300 or greater, but at least according to heywhatsthat.com, they’re all significantly below the local horizon from White Tank.

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Also seems that I have about a 10 dB dynamic range on the receiver. Signals are never stronger than about -1.6 dBFS and I always lose the a/c when the signal drops below about -12 dBFS. Using HeyWhatsThat.com, it’s very apparent when it drops out.

Here’s AAL110 headed NE from LA. I lost it as it was about due east from Cedar City UT.

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The RSSI is below the minimum required for this particular station to decode successfully. Not that the a/c is over 220 nm distant, but it’s at FL410. So why did it suddenly fade out? HeyWhatsThat gives an indication.

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From this plot, it’s obvious that the a/c, even at 41k feet altitude, is well below the local horizon and the actual path is probably ducting or multiple knife edges. Either way, it seems to explain for me why I can’t hear farther than this even with the receiver atop a significant “hill” in the Phoenix area.

ADS-B Receiver for mountain-top comms site

Monitoring aircraft via ADS-B is a terrific hobby and super easy to do. I get to have a display here that shows sometimes hundreds of aircraft (both commercial and general aviation) out to about 200 miles from the house.

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The above screen capture was from my Raspberry Pi running PiAware and connected to a homebrew 1090 MHz antenna up on the roof of the house. Nothing special in the setup, but look at the range! A/C at altitude are hearable out over the Grand Canyon and into California. Occasionally I get a/c into or out of Mexico, and I can see traffic in the southwest corner of New Mexico.

I’m planning to put one of these receivers on a local hilltop, about 3000′ above my house and the Valley floor, and yesterday installed the antenna on a temporary mount on the tower at the site. I connected it to my most recent ADS-B receiver setup, seen below, and was awestruck at the coverage. Was seeing a/c over Los Angeles and Albuquerque!

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It’s a bit tricky to put a cheap SDR dongle anywhere near radio transmitters, and the hilltop that I was on is loaded with them. In fact, the building in which I have some current monitoring equipment is only 50′ away from a huge comms tower with dozens of two-way radio antennas, and a lot of potential interference. The ADS-B receiver antenna is right in the center of the picture, on the end of a piece of unistrut attached to the tower legs. In the background, nearly a dozen towers bristling with antennas.

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The secret to success is a very good filter in front of the SDR receiver. That black square near the middle of the picture is exactly that. It’s a custom-made cavity filter, only 50 MHz wide, centered at 1090 MHz. Extremely sharp rolloff and ultimate rejection about 100 dB. Really helps the RTL-SDR receiver.

However, I wasn’t able to leave the receiver up on the hill yesterday, I was troubleshooting other issues and didn’t have time to set up the network connection to my receiver. Next time I will hopefully get it installed and on the air!

My first mobile computing setup

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This Epson notebook, an MFJ TNC and a Yaesu FT209 handheld 2 meter radio set on 145.01 MHz allowed me to have data communications while motoring around the southwest. Back then, hams had assembled a tremendous network of mountaintop digital packet repeaters (digipeaters) that provided amazingly good coverage of California, Utah, Arizona, New Mexico and into Texas.

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I don’t have a picture of it, but I’d built a floor-mounted stand for the Epson in my Rabbit diesel pickup, and a terminal program ran on the Epson talking to the TNC. I could send and receive short messages. The Epson had the tape-drive memory, which made it easy to write my log and store it on the tape.

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The MFJ was pretty cool at the time, and could even do HF packet, though that was especially painful. The Rabbit had a FT757 HF transceiver as well, the antenna (not shown in the above image) was on a ball mount on the left rear side, about midway between the wheel well and the taillight.

It was my first foray into the world of digital, and especially digital mobile communications. From that point on, I always had a computer of some type in my trucks.

A Looney Tunes Tribute on an FAA Plate

The statement, famously spoken by Tweety Bird, is immortalized on the GPS Rwy 16 approach for Portsmouth NH airport KPSM.

“I tawt I taw a puddy tat”

Spelled out over 4 waypoints on the approach!

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And if one misses the approach and needs to go in a hold, there’s IDEED, as Tweety always(?) follows the aforementioned with the exclamation

“I deed, I deed taw a puddy tat”

Nothing nearly as interesting here in Arizona, where we have lots of references to baseball, hockey, football, golf, desert animals and plants, and the weather.

And darkly and non-sequiturly, if the aviator approaches from the NE, they’ll have to pass through SATAN…

The Mojave Phone Booth

I stumbled on this link when engaged in some otherwise pointless search on-line for something entirely different.

Like my desert-dog buddies, Bart R., Pete C., Greg M., and others, we all had our adventures wandering around the Mojave Desert as kids and then young and not-so-young adults. Some more than others.

The string (yes, there’s more than one) of phone booths in the Mojave was one of those fascinating, improbable, and curious marks of humanity and technology in an otherwise fairly natural and wild scene. The first time I stumbled across one was in a weekend adventure with Bart.

We’d been up in the Mountain Pass area, I think trying to suss out the Kokoweef mine/cavern mysteries, and decided to venture south into the Landfair Valley. We’d seen on the topo map an old railroad grade heading up a canyon from Ivanpah, and a bunch of old mines in the area, so it looked worthy of a visit.

Working our way up the path through the New York Mountains, passed intermittent signs of humans, some very abandoned, others fairly fresh. There were the occasional cattle, and sometimes a corral or stock tank. Even more rarely, there was what looked to be a cluster of buildings like a ranchhouse, but not much in the way of people.

Arriving at the north end of the Landfair Valley on the Goffs-Landfair-Ivanpah Road (aka Landfair Rd), it was just miles and miles of miles and miles of empty, serene desert. It was winter, so there were signs of some recent and rapidly vanishing snow over on Drum Peak, the high point at the west end of the New Yorks, and even some on the Mid Hills, to the southwest of Drum.

Anyway, we ambled along down the road until we discovered our improbable phone booth, either at the intersection of Old Government Rd or the intersection with Cedar Canyon Rd.  I’m sure that perhaps somewhere I have my own photo of it, but there are many photos of one of the booths and the surrounding desert available on the inter-webs!

Stopped and picked up the handset, and lo and behold, a dial tone. Pretty cool. We saw a sign adjacent cautioning about digging and damaging a cable underground, so it appeared that the phone may get its connection from the underground cable.

Once we got back to LA and to work the next week, I was determined to figure out how to call that phone booth and why it was there. Set me on a mission to find someone somewhere who knew. First stop was dialing “0”.

Nice operator answers, I tell her that I’d like to call this phone booth in the middle of the desert, in the Landfair Valley. I think she hands me over to a super-operator, or or maybe there’s a couple of handoffs, but ultimately I get to speak to a lady who seems to know how to contact the phone there. Sadly, I have no documentation now on this exploration from 40 years ago, but it was pretty awesome, as far as I can remember. First, that a snotty-nosed 20 year old kid could call the OPERATOR, and there was someone there who could navigate me through the arcane world of party lines and microwave links that somehow got to that one spot in the desert, and spend an hour doing it; second, that she then handed me over to someone at AT&T Long Lines (an engineer, I am pretty sure) who was excited to find a young person who was actually interested in their arcane but extremely vital universe of long-distance telephony and data communications; and finally that doing this didn’t trigger all sorts of alerts and alarms that I was interested in things like this. In today’s atmosphere, it seems like an invitation from the MIB or worse.

The story I got was that the booth was there because of tariff/regulatory rules. There was a long-distance underground cable buried there, along the old Government Road, and the rules required a public access for phone service no less than every 20 miles. This happened to be an empty spot in the desert where there were a few people living, there was a major road through here, so they put the booth there.

At this time, there wasn’t a direct-dial number for the booth, it was something like having to talk to a special operator and then asking for “Landfair 1”, or something like that. Awesome!