Category Archives: GPS/GNSS

And while I’m at it

Last week I had the pleasure to spend in far-south Chicagoland, where it started out 48 °F and raining and ended up 18 °F with black ice everywhere. Not immediately, but once the ice formed, it stayed in place the rest of my visit. Made for some dicey walking.

However, my buddies at the RR, like the post office, work rain or shine, ice or sleet, snow or sand, and were busy installing my hoped-for array of antennas on a homebrew rack up on the roof.

Little hard to see in the photos, but it’s basically a triangular array of 3 tripod mounts held down with a bunch of concrete blocks, then two-sided unistrut forming two vertically spaced equal equilateral triangles, upon which it’s easy to set vertical pipes to attach antennas.

Right now the lab I’m helping to build (in the building below) needs far better cellular connections for the industrial modems that we’re testing, so the antennas on the roof will solve a lot of our mysterious connectivity problems.

The lab also needs a solid GPS antenna on the roof – I have a rack-mounted 8-way GPS splitter inside and I’ll feed one antenna to multiple devices in the lab that need GPS.

The runs from the antennas on the roof to the rack are all 1/2″ superflex, and we’ve got grounded lightning protectors at the rack and shield grounds just before the coaxes penetrate the building. We’re also way lower than many better lightning targets all around us, so we’re probably as safe as we can be.

Next step is to head back there and start assembling some LMR240 jumper cables to run from the rack connections to the final resting places for the signals received.

It’s fun working on the railroad.

Update on wardriving/walking/cycling setup

The stackup that I did inside the enclosure was fairly crude. The GPS receiver and its attached antenna are at the “top” of the stack, and stuck to the inside uppper surface of the enclosure using double-sticky foam tape. Immediately below that, the dual-band Wi-Fi board is component-side down. I’d removed both the SMA-RP female and the USB male board connectors to reduce height and length. The bottom of the stack is the USB hub, again with USB female sockets removed and component side down as there’s two electrolytic caps that stick up 8 mm or so.

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I’ve used double-sticky foam tape for things like this over the years, and as long as it doesn’t absorb moisture, it’s quite RF-transparent and the antenna has no issues.

The Wi-Fi board has a ground-plane on top and bottom, but I wanted to reduce to a minimum any local fields from the components, so the board goes in component-side down to isolate it (maybe a bit) from the GPS antenna.

The GPS receiver itself is completely encased in a shield, and is slightly larger than the antenna, so there may be some added attenuation of spurious emissions from the Wi-Fi getting into the GPS.

The USB hub, at the very “bottom”, also has a top/bottom ground plane.

The Wi-Fi sniffing performance is as good as my permanent mobile setup, and the GPS gets excellent PDOP (<2.0) when there’s a decent field of view of the sky.

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The apps used to grab the GPS data is 4river’s NMEA Monitor, a fine little program.

Highly Portable Wardriving, Warcycling, and Warwalking Setup Update

Before I left on a trip this past week, I was able to shoehorn all the components (GPS receiver w/integrated antenna, dual-band Wi-Fi module with external antenna input, 4-port USB hub, TTL to serial to USB adapters) into a single plastic case. It’s not yet weatherproof, but at least it’s splash- and rain-resistant.

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The original cable on the USB hub was only a meter, so I grabbed a 2 m cable from the box, whacked off the end, and replaced the shorter cable.

I used double-sticky foam squares to create an electronics sandwich, with the GPS antenna at the “top” of the stack, the Wi-Fi below, and the USB hub at the bottom. The cable passes through a silicone-sealed hole in the case, and I removed the SMA-RP female from the Wi-Fi dongle and replaced it with a short RG178 cable and bulkhead SMA-RP connector that pokes through the top of the case.

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As one can see, the box is a cheap one from Radio Shack or similar, it’s some kind of ABS. The box’s lid, which would usually be on the top, is now the bottom of the assembly. I silicone-glued 3 NdFeB rectangular magnets to the inside of the lid, and put 4 rubber-bumper feet to reduce any potential surface marring. Next, I took the thing out for a drive on a local freeway to see if it’d blow off. It did. I adjusted things a bit by getting rid of the rubber bumper feet, and replacing them with electrical tape on the outside of the lid for more of an anti-skid, compliant surface than the bare plastic alone. The next drive, and subsequent ones this week, proved that the widget is now fairly stable even at “high” highway speeds. I thought about painting it white to reduce heat absorption, but that would make it have higher visibility and I’d prefer to stay low profile.

In taking it for a drive or two around the neighborhood, it matches my mobile setup almost exactly in reception, and takes all of 30 seconds to deploy when getting a rental car.

I’m wondering if I can somehow add a temperature sensor inside and read it via the USB. But that’s not so important.

Trimming the fat from my wardriving setup

As you may remember, several weeks ago I wrote about my soon-to-be wardriving setup that centered around my new TW700 Winbook. I had a cheap USB hub, a TP-Link dual-band Wi-Fi stick, and an EM506 GPS receiver.

I finally took some time on Saturday to get rid of the fat, i.e., all the individual plastic housings, the USB connectors themselves (physically large and limited my layout options), and am about to repackage the whole thing into a single weathertight enclosure that can either be attached via magnets to the top of the rental cars or carried in a backpack/fannypack as I walk around an area.

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Here’s the setup without all the excess plastic. The GlobalSat EM-406A (SiRF Star III) modular GPS is at the left, with its built-in antenna the white/beige square; the TP-Link dual-band Wi-Fi dongle has been reduced to just the board, and as well I removed the USB connector to reduce the length; the USB hub has had the two dual-port USB assemblies removed to reduce height; and all the former USB connections have been reduced to bits of ribbon cable. The TP-Link dual-band antenna is quite good and so I’ll be keeping that. The 3′ USB hub cable still needs to be replaced with a 6′ version, and I think I’ll remove the RP-SMA connector from the side of the TP-Link and directly connect the coax to the board.

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The most difficult thing so far has been removing the excess USB paraphernalia and cleaning up the board. To that end, yesterday I bought a new tip for the soldering station, and more 63/37 solder. I use a solder sucker to remove the existing solder, but have found it’s quite difficult with the RoHS solder used nowadays. The 63/37 solder seems to dilute the RoHS solder and make it easier to remove with the solder sucker.

Today I’ll finish this up and put it in an enclosure.