RTL-SDR Tracking of BRB (Big Red Bee) / HAM APRS GPS with Android

[JordanT]

Did I get enough TLAs in the thread title?

A rocket friend posted this from Amazon: RTL-SDR. I know, it's out of stock now - but there are lots of Realtek based tuners on the market. $20 for a Software Defined Radio that has a good deal of support in the OSS community seemed like a bargain. And (with prime) I could get it in 2 days. Instant gratification FTW.

I got it yesterday and after a bit of tinkering found that it will sync up with Android phones if you have an OTG (On The Go) cable. The $10 SDRTouch app can act as a tuner for the RTL-SDR dongle, and has a free companion app that provides the driver to allow that to happen. You can listen to your BRB chirp out its location over your headset. Now, APRSDroid (also not free, $5) can take an audio input and decode the AFSK stream received from a Big Red Bee (BRB) tracker, then plot the position on Google Maps (road or sat) or downloaded/offline maps.

Here's the rub - APRSDroid only takes audio input from the microphone and SDRTouch only outputs to the standard audio channel (speaker or headphones) and I'm not aware of any digital cable app that lets you pipe the output of the latter to the input of the former.

Luckily for me, I had parts on my workbench which were slated to make a conversion cable to connect my Baofeng HT to my phone - which just means I had a TRRS (headphone+mic) plug and a handful of resistors. By soldering a 500 Ohm resistor across the leads for the tip and sleeve, and leaving both rings open circuit, I made a talk-back connector that pipes the headphone signal back to the mic input on my phone. Huzzah!

tl;dr version: for $40 you can turn your Android smart phone into a GPS tracker for your rocket's BRB that requires no internet connection or cell service in the field.

Proof of concept video is here.

As a bonus, the SDRTouch software has a great signal strength graphic, so I suspect it would be the ideal companion for using a beacon (foxhunting) tracker like the Big Red Bee BeeLine or one you build yourself.


(sorry if this has been covered before - I searched on SDR and didn't see anything in the forums)

 

[KBlack]

SDR receivers like the RTL-SDR are not as sensitive as conventional receivers. You can add amplifiers to the front end, but it adds complexity. They're great tools when you have enough signal strength.

 

[Jeffmhopkins]

The greatest use I have for mine (SDR dongle) is the graphical display waterfall. The small beeps of my flight computer can be seen at the noise floor when just about nothing is actually heard over my transceiver. I'm running an LNA and preamp (70cm) with a 6 element yagi with good success.

Also, it's really good for scanning the spectrum before flight, never happened to me, but I've heard the stories of people launching on the same frequencies and I'm always for not letting that happen to me.

Good antenna will solve most problems of course

 

[ksaves2]

Jiggle the cable and the program can lockup. If triapsing around in a field, one wants to minimize cable connections as much as possible. Be careful with Baofengs for APRS. Their receiver is not that great. Fine for voice, not so good for APRS.
You get what you pay for. So don't be surprised if it drops packets. No matter what you use for APRS tracking make 100% sure you turn off the radios "battery saver" if it is equipped with one when using it to receive packets.

If one really wants to do APRS with a Droid on the cheap, do a Mobilinkd TNC: https://www.mobilinkd.com/.
Problem I found with APRS Droid is one has to transmit an APRS packet in order to get your base/tracking station to move on the map. Kinda stupid if you ask me.

One is better off getting a small 8" tablet and using one of the standard APRS tracking programs like APRSISCE/32 or some of the others out there:

https://www.rocketryforum.com/showthread.php?132634-900mhz-gps-trackers&p=1558916#post1558916

Can connect a used B/T GPS for base station position (There's few 'doze tablets out there with builtin GPS) and track the APRS rocket.

You'll need a box to put it all in. I have a flat black painted box to hold a tablet, a Mobilinkd, Kenwood THF6A and an outboard battery for extended runtime for the tablet. Can hold this with one hand and don't have to worry about jostling a cable. The box is to shade the screen because you won't be able to read diddly-squat in the sunlight on any screen. Alternatively, the box can hold an Android device, EggFinder LCD and extra batteries to track the EggFinder
with APRS Rocket Locator Android app. Use the EggFinder LCD (with Bluetooth) with a 'Doze tablet and with some manipulation can get APRSISCE/32 to display the positions in real time on a portable, cacheable photomap.

I entertained SDR for rocket tracking for awhile and discarded it as not being "portable" enough for my purposes. If one just wants to use it stationary at a base station then O.K. but I've dragged TNC's out in the field and trying to trouble shoot a setup parameter in the field is a real pain.

If I simply want to APRS track, I use a Kenwood D72A with a single (secure) cable connection to a Garmin 60Cs or 60CsX. Just a real time map in hand that works with or without a yagi. Yes there is a cost but it is 100% reliable with no muss or fuss.
The tablet with the F6A, Mobilinkd TNC and Nexus 7 2013 is pretty reliable too but entails carrying three or four devices (four if using the battery for the tablet). It is cheaper than a D72A if one already has a 'Droid device and a decent H/T. I found that not so hard to carry in one hand (in the open flat black painted box) since tablet computers are getting pretty small and light.

Tracking with a laptop I consider only workable if using it to record most of the flight until it gets out of range. Trying to use a full sized laptop pedestrian portable is a real PITA.

As far as scanning the band is concerned before flight? Not necessary. Simply turn on your receiver before you turn on your tracker. If you see data or hear the carrier, someones using it. You don't have to have an SDR station for that.
Yeah, SDR will show you a slice of the spectrum but is it really needed? No, it's just cool and that's about it. Kurt

 

[JordanT]

I've had bad luck with non-standard items over bluetooth with Android. Some connect, some don't, some don't stay connected.

I figure I always have my mobile phone with me, so it no extra gear. I connected the SDR to my home-built Yagi and - without any fiddling, and knowing only generally which direction she would be coming from - picked up my BRB in the back seat of my wife's car from a mile away. EDIT: actually, she showed up as an icon on the map a mile away; If I'd been listening I almost certainly could have heard the carrier tone from further, but I was busy talking with the neighbor while waiting for the school bus when the APRSDroid popped the location onto the screen.

I have everything on APRS Droid turned off but the audio modem, and I haven't transmitted a packet for my phone, and it seemed to track and bring up the map just fine.

While I appreciate the elegance of the Kenwood/Garmin combination, that's a $900 solution for which I don't actually have any other use. YMMV, of course. FWIW, tracking also works well with my Windows tablet, and all those utilities are free, but the phone is more portable.

 

[Jeffmhopkins]

I'm using a non APRS, 4800 baud GFSK serial link over the small APC220s.

Coming from a SATCOM background I prefer to look at the noise floor for more than just frequency reuse interference, by viewing the available frequency range on the SDR I can choose the best center frequency in that environment, hopefully around my 435MHz tuned yagi / preamp (mini-circuits).

Not necessary, but insures the best signal I'll get out there.

I'm constantly surprised on how the 70cm spectrum is full of crap, often intermittent.

 

[ksaves2]

I've had bad luck with non-standard items over bluetooth with Android. Some connect, some don't, some don't stay connected.

I figure I always have my mobile phone with me, so it no extra gear. I connected the SDR to my home-built Yagi and - without any fiddling, and knowing only generally which direction she would be coming from - picked up my BRB in the back seat of my wife's car from a mile away. EDIT: actually, she showed up as an icon on the map a mile away; If I'd been listening I almost certainly could have heard the carrier tone from further, but I was busy talking with the neighbor while waiting for the school bus when the APRSDroid popped the location onto the screen.

I have everything on APRS Droid turned off but the audio modem, and I haven't transmitted a packet for my phone, and it seemed to track and bring up the map just fine.

While I appreciate the elegance of the Kenwood/Garmin combination, that's a $900 solution for which I don't actually have any other use. YMMV, of course. FWIW, tracking also works well with my Windows tablet, and all those utilities are free, but the phone is more portable.

Haven't had trouble with a Mobilinkd nor EggFinder LCD via Bluetooth as long as I don't try to pair with an external keyboard on a Nexus 7 2013.
Yeah, the D72A is pricey but before it was a D7A and D7A(g) which was just as pricey. Historically, APRS tracking was the only game in town.
Now the Garmin 60Cs series can be had used in the $150-ish range and if one picks up a used Yaesu VX8GR, it has to be a GR, the Garmin/D72A cable will work
to get the rocket waypoint into the Garmin. Plus, one doesn't have to buy the Garmin maps as Open Source Maps are free to download for the 60 Cs series and others. The POI database isn't complete but the maps are decent. There are ways to get the price down for APRS but if one doesn't want to go the Ham route, the EggFinders are very cost effective. Kurt