GPS, APRS, and serial (or how I came to realise that redundancy and data are both good things)

Discussions about anything electronics, from altimeters to cameras.

Moderator: Moderators

drew
Southern fried goodness
Southern fried goodness
Posts: 906
Joined: Sat Aug 01, 2009 10:20 pm
Location: Perth

GPS, APRS, and serial (or how I came to realise that redundancy and data are both good things)

Postby drew » Tue Aug 21, 2018 6:21 pm

So, long story short I've had issues of late with my TeleGPS, various versions of firmware, different computers for AltOS telemetry downlink, old TeleDongles, and the like.

Instead of breaking down the minutiae of that situation for all y'all I figured I'd instead share with you the value adds that I've come to develop for tracking.

Specifically, any AltusMetrum GPS capable device (excluding the TeleMetrum 1.x branch of boards) can support APRS packets ALONGSIDE the AltOS telemetry downlink. I like this for the following reasons.

1) I am already kitted up to handle APRS packets as my Yaesu VX-8GR has this inbuilt
2) having a "redundant" system of position reporting is a value add. IF my telemetry downlink isn't functional I can still find my rocket.
3) If both APRS and Telemetry aren't working then I've got a sure fire TeleGPS issue.
4) If I don't have telemetry functional I can still confirm I've got GPS lock before flight. This is important because even IF the telemetry link isn't working the TeleGPS will log the flight onboard for download post flight.

So I started by configuring my TeleGPS to output APRS packets as it's disabled by default. Below is the relevant config windows from AltOS.

Image
Initially I tested with the APRS format being "compressed" as can be seen above. This does NOT work with the VX-8GR. Like the Kenwood TH-D72 the Yaesu VX-8GR only supports Uncompressed format. The Yaesu FT1DR supports Compressed, so my assumption is that the newer FT2DR also supports that format.

Once I got the APRS format sorted output worked exactly as it does with my BigRedBee GPS trackers from a Yaesu downlink perspective. SWEET.

But one of the issues I've had in the past with my BigRedBee trackers is the logging file being corrupted on board post flight. So ideally I'd like to have my APRS TNC in my Yaesu output the Waypoint NEMA strings from my radio over serial. I've always owned the CT-143 PC Connection Cable for my radio, I bought them both at the same time. So I finally pulled it out of the box!

First off I needed to config my Yaesu to output NEMA strings over serial. Below is the configuration settings that are available.

Image
In my specific case, I configured the following.

STATUS: ON
SPEED: 19200bps (this isn't necessarly required, and I'm sure 9600 and most likely 4800 would both work, but I figure the faster the better)
INPUT: OFF
OUTPUT: WAY.P
WAYPOINT: NEMA9

IIRC there were a couple other options I set, but I don't have the radio in front of me to share them.

So, now that I had my Yaesu configured to output NEMA9 strings over serial I connected up the CT-143 cable to my radio and the other end to my USB to Serial adapter. I then opened up Putty and configured it for the COM port my serial adapter took (in my case COM9) and set my baud rate to 19200. Clicked open, and my NEMA strings started to appear in my console output once every 5 seconds!

Image
Now there's more to do but right now I can take the GPS output via serial, slightly modify it, and drop it directly into Google Maps for a Hybrid overlay of where my rocket is!

Specifically, the serial output is rougly in the following format.
32xx.xx,S,116xx.xx,E

I modify that to the following format in Google Maps
32 xx.xx S 116 xx.xx E

And press search and my GPS tracker location is displayed on my phone in Google Maps! Plus when I'm ready to depart for my recovery I just unplug my Yaesu from the serial port and use the inbuilt APRS TNC to provide me both bearing and distance values of the last packet as it has GPS and calculates where it and the rocket both are. Plus all the serial output modifications I've made should work "out of the box" with my BigRedBee hardware. BONUS!

Next up I plan on investigating YAAC but in the interim I'm quite pleased with the progress I've already made.
Andrew Hamilton
AMRS 28 L3
AMRS Records Committee Chairman
Max Alt AGL - 23,908ft - K300 - Balls 22
Max V - 2,488 ft/s, ~Mach 2.2 - M2250 - THUNDA 2015

drew
Southern fried goodness
Southern fried goodness
Posts: 906
Joined: Sat Aug 01, 2009 10:20 pm
Location: Perth

Re: GPS, APRS, and serial (or how I came to realise that redundancy and data are both good things)

Postby drew » Thu Sep 06, 2018 6:26 pm

So I've been meaning to provide an update to this thread. Here we go.

First off, my above TeleGPS Configuration screenshot isn't correct as the APRS Interval(s) value is 0. 0 effectively means it's off. Below is the correct "working" configuration for my VX-8GR. Notice APRS Interval is at 5 seconds and the APRS format is uncompressed.

Image
Next up, below is a screenshot of the YAAC Raw Packet Sniffer AND my TeleGPS App both running simultaneously and both producing GPS location data. And these are two different live location data feeds, one APRS, one Altus Metrum Telemetry. This is what I'm after so to speak. As mentioned, if Altus Telemetry has issues I can always fall back on the APRS location data. Furthermore having APRS working will allow me to use just my VX-8GR for rocket recovery instead of attempting to lug a Teledongle + laptop into a paddock. Sure, I could use AltosDroid but I have the most connectivity issues with that app plus if I'm being honest USB-C doesn't really tickle my fancy. The cable connection is a bit flaky on the phone when compared to MicroUSB that had a more positively retained attachment for OTG dongles.

Image
YAAC is nice and is very easy to setup and configure. That said I'd ideally like to have a satellite or hybrid overlay for my location window (not pictured) and I'm not sure if that's available. You can easily get OpenStreetMap data (less useful) into YAAC as well as topographical data (more useful) but ideally having a satellite image would make recovery more simple. I'll investigate that more soon.

I haven't been able to get altitude data to output over serial. I don't think I'll be able to do that with NMEA. That said I'm going to grab a Mobilinkd and have a play with using APRSDroid + a Baofeng + Mobilinkd to provide a backup APRS receiving capability for the field. That update will take some time though as I ordered a UV-3R from AliExpress and it'll take a month or so to get here given it's free shipping.
Andrew Hamilton
AMRS 28 L3
AMRS Records Committee Chairman
Max Alt AGL - 23,908ft - K300 - Balls 22
Max V - 2,488 ft/s, ~Mach 2.2 - M2250 - THUNDA 2015

drew
Southern fried goodness
Southern fried goodness
Posts: 906
Joined: Sat Aug 01, 2009 10:20 pm
Location: Perth

Re: GPS, APRS, and serial (or how I came to realise that redundancy and data are both good things)

Postby drew » Wed Sep 19, 2018 5:54 pm

So I've been meaning to chime in with another update, so here we go. First off, I think it wise to take a step back and look at what I'm actually trying to accomplish. Bear with me, we'll get there in the end.

It's worth mentioning that there are two things that we require to receive and decode APRS data from an APRS capable GPS tracker. First off, we need a radio to receive the Audio Frequency Shift Keying (AFSK) signals being transmitted from the APRS tracker. You might not have heard of AFSK before, but you've most likely heard it before, either on the flight line or an incredibly old dial up modem. Here's an example of a 1200 baud AFSK-modulated signal from Wikipedia.

https://en.wikipedia.org/wiki/File:AFSK_1200_baud.ogg

Next up, we need a Terminal Node Controller, or TNC. Basically the TNC is what translates the AFSK transmission from the tracker into a digital format that we can then read to get our APRS transmitted GPS position data. Inbuilt TNCs are what make high end handheld radios like the Yaesu VX-8GR, FT2DR, or the Kenwood TH-D72A able to decode APRS transmissions from our various APRS capable GPS trackers. Obviously those devices are quite expensive and normally run in excess of $500AUD at a minimum. So that's a pretty high bar from a financial perspective. The Mobilinkd I've been looking at recently is a purpose built APRS TNC that leverages serial over Bluetooth to transmit decoded AFSK signals from a tracker over any given frequency to APRSDroid on an Android device. So it sits in between a radio and a Android device and provides TNC functionality in a reasonable package.

But given the relative computing power of PCs, laptops, and even phones and tablets we now have the option of using software based TNCs to do the decoding for us. APRSDroid supports three different methods of APRS transmission mediums.

1) APRS-IS (Internet based APRS) - this doesn't require a radio of any description, you share your APRS data over the Internet. But it requires an Internet connection and isn't really applicable from a rocketry perspective (unless you want to fly your phone).
2) Bluetooth - As mentioned earlier Android phones support serial over Bluetooth connections. This allows a Bluetooth enabled TNC like the Mobilinkd to work with APRSDroid. The TNC receives the AFSK signal from a VOX capable radio over a physical audio cable, decodes it, and transmits the data over Bluetooth which is "seen" as a serial connection from an Android perspective. This would work for us but requires a serial to Bluetooth capable APRS TNC. Good, but not ideal. Also, Also, Apple iOS does not support Serial over Bluetooth.
3) APRSDroid supports direct audio input/output. One of the features of APRSDroid is that it can operate as a software based TNC So basically we need to connect the handheld radio directly to an Android device running APRSDroid with the appropriate audio cable. Remember effectively all we need to do is output the AFSK signal from our handheld radio to the audio jack of the phone.

Big shout out to DaveW6DPS over on TRF for pointing this out. That thread can be found here.

https://www.rocketryforum.com/threads/very-simple-and-inexpensive-way-to-receive-aprs-data.147891/

And the original post from a 4x4 forum in 2012.

http://www.4x4ham.com/showthread.php?2078-APRS-with-just-an-Android-phone-and-an-HT-that-supports-VOX

Finally, here's the page DaveW6DPS wrote up for this use case for rocketry.

https://www.qsl.net/w6dps/APRSDroid.html

So given this I've decided to not buy a Mobilinkd TNC and instead buy a couple of their TNC cables instead. These cables are made to interface with various handheld radios on one side and 3.5mm TRRS connectors on the other side. From my understanding most Android devices that have a headphone/hands free connector port are TRRS connectors (4 pole connector). Probably the easiest way to confirm this is to look at the wired hands free headphone/microphone that came with your phone.

Here's the Baofeng UV-3R cable Mobilinkd sells which shows each side of the cable is a 4 pole connector.

Image
I've also ordered a TNC cable for my Yaesu VX-8GR so I can test on a "good radio" as well as the Baofeng just in case all the ranting about those radios being useless is truly correct. That said the UV-3R is sitting on an average rating of 3.9/5 over 131 reviews over on eham.net so I'm not too sure as to whether they're as bad as the vocal minority make them out to be.

Regardless, the cables and a few 4mm Ferrite Beads are making their way down here as I type this. I suspect I'll get them at the end of the month. Hopefully my UV-3R will show up soon as well. I'll update the thread once I've done some testing.

Thanks for reading.
Andrew Hamilton
AMRS 28 L3
AMRS Records Committee Chairman
Max Alt AGL - 23,908ft - K300 - Balls 22
Max V - 2,488 ft/s, ~Mach 2.2 - M2250 - THUNDA 2015

drew
Southern fried goodness
Southern fried goodness
Posts: 906
Joined: Sat Aug 01, 2009 10:20 pm
Location: Perth

Re: GPS, APRS, and serial (or how I came to realise that redundancy and data are both good things)

Postby drew » Thu Sep 20, 2018 2:50 pm

The UV-3R just arrived! Not bad considering I ordered it on Sept 3rd with free shipping.

Man this thing is small!
Andrew Hamilton
AMRS 28 L3
AMRS Records Committee Chairman
Max Alt AGL - 23,908ft - K300 - Balls 22
Max V - 2,488 ft/s, ~Mach 2.2 - M2250 - THUNDA 2015


Return to “Electronics”

Who is online

Users browsing this forum: No registered users and 2 guests