Basic GPS tracker

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vance2loud
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Re: Basic GPS tracker

Postby vance2loud » Wed Jun 08, 2016 4:41 pm

Here is a RF900u connected as a tracker, sorry about the picture quality (I suck at macro :oops: )
DSC_0238a.jpg

DSC_0239a.jpg


and the complete tracker side, with a b6-4 for scale
DSC_0240a.jpg


I have used a different GPS chip this time, It is a GMS-G9 which is slightly bigger (19.5mm square)
It costs a tiny bit more ($25-30) but uses the American GPS satellites and the Russian Glonass satellites.
Using both types should allow for a better/faster fix.
This particular model also has larger pads for soldering (and less of them, though we still only use 4),
It also has a larger Antenna which should also increase it's performance.
TRA #15165 AMRS #50
I do it because i can, I can because I want to, I want to because you said I couldn't. - Unknown
I have not failed. I've just found 10,000 ways that won't work.
- Thomas Alva Edison

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Re: Basic GPS tracker

Postby SpaceManMat » Wed Jun 08, 2016 4:49 pm

Gee that's small.
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
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Re: Basic GPS tracker

Postby OverTheTop » Wed Jun 08, 2016 6:15 pm

Nice Vance. Looks good.

It costs a tiny bit more ($25-30) but uses the American GPS satellites and the Russian Glonass satellites.

Because the Navstar and Glonass systems use different frequencies, the pseudorange distance (used for determining the position of the satellite) uncertainty is greatly improved. The ionosphere has a bit of a random effect on the signals which translates to a meter or so of error on the ground. Because the two systems use different frequency bands the correction for this comes out with some fairly simple algebraic calculations and is eliminated from the position solution easily. That only leaves the tropospheric variations to cause the remaining gross inaccuracies IIRC. That means you can get some serious accuracy with far less reliance on slower statistical solutions.
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vance2loud
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Re: Basic GPS tracker

Postby vance2loud » Wed Jun 08, 2016 6:23 pm

according to the manufacturer these units have been known to transmit as far as 10km. No published data about what the settings were however.
I think they should be easily able to handle 15,000 feet flights landing up to 2km away.
Better antennas could increase that a bit.
If you need more range you can then upgrade to it's larger brother the RF900+ which has been tested to over 50km range with the right antennas at similar data speeds to what we need.
The larger RF900+ can output 1W (10x all the other radios I have used). Chris Barnes (sorry if I got that incorrect) made a tracker on here a while ago using these modules.
Once I finish posting the wiring required for the ground side with a RF900u I will move on to the RF900+ (actually RF900a - I have the older model but the wiring is the same)

I 3/4 understand what you wrote OverTheTop, and I am slowly changing most of my designs over for the increased range. I still use gps only chips but mainly only when the price determines.

I may need someone to do some side-by-side testing of the radios and GNSS chips, should only need a 54mm or larger airframe flying to 20,000 feet to give them a good shakedown
TRA #15165 AMRS #50
I do it because i can, I can because I want to, I want to because you said I couldn't. - Unknown
I have not failed. I've just found 10,000 ways that won't work.
- Thomas Alva Edison

vance2loud
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Re: Basic GPS tracker

Postby vance2loud » Wed Jun 08, 2016 7:04 pm

connecting the RF900u
To program the RF900u and to use it for your groundstation you will need to connect it to a FTDI cable (available here http://store.rfdesign.com.au/ftdi-cable-3-3v/ or from multiple places - this radio can use 3.3v or 5v not all devices can)
DSC_0241a.jpg

The connections are covered in the datasheet from RFDesign.
Basically however you need to connect the rx-tx, tx-rx, vcc-vcc and gnd-gnd just like you did with the gps units
If that fails to work try swapping tx and rx wire around as some units are marked incorrectly.
If you want to connect a bluetooth module to the unit it follows the same type of connection.

For all of these devices please read the datasheets that are available, my wiring worked for what I had but not all devices are the same.

Another point worth mentioning is that The RF900u states a minimum voltage of 4.0v, I have tested them below this point but range may suffer.
Always use a freshly charged battery with these devices as some of them are only capable of functioning on the top 50% of a lipo battery's capacity.
Expected battery Life with the 600mAh batteries mentioned For reference;
3dr style - 4-6 hours
RF900u - only tested to 2 hours so far.
RF900a - requires 5v to operate and more than a computer USB likes to provide - recommendations coming soon
TRA #15165 AMRS #50
I do it because i can, I can because I want to, I want to because you said I couldn't. - Unknown
I have not failed. I've just found 10,000 ways that won't work.
- Thomas Alva Edison

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Re: Basic GPS tracker

Postby Lister » Wed Jun 08, 2016 9:43 pm

Hi Vance, I plan on testing it by driving to the mountain near here and putting the air unit and attached gps at a friends house which is near the top and has a clear view to my house. Then drive home and see if I still have signal. Its about 12km away so it should give a fair indication of what it can do. If Its too far I will drive back with the ground module hooked upto my phone and see how far It will go and will be able to check the accuracy.

If you want any of the others tested the same way just send them over and I'll do them all at the same time.

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Re: Basic GPS tracker

Postby SpaceManMat » Thu Jun 09, 2016 9:36 am

How well do these GPS units determining altitude Vance? One of the issues with the I've seen with the eggfinder's GPS was that it really could calculate an accurate altitude.
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
Largest Motor: CTI 1115J530 IM
Current Project: X Wing

vance2loud
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Re: Basic GPS tracker

Postby vance2loud » Thu Jun 09, 2016 10:19 am

GPS altitude tends to be hit and miss, generally they are within 10-20 feet but may drift as much as 50 feet on a bad day.
It depends a lot on how good a GPS fix you have and how many satellites you are receiving. (having Glonass capabilities should help here)
The other thing to remember is that the Altitude reported is 'above Mean sea level' so doesn't factor in the launch site altitude.
I learnt that the hard way when I thought I had lost signal when the rocket was still 200+ meters up only to discover it had landed and was where the coordinates said. :oops:
Generally they should be slightly more accurate than the eggfinder, as the eggfinder uses Sirfstar chips which have a reputation for being inaccurate in altitude.
I prefer the Ublox or Mediatek chipsets myself but still need to test the mediatek units a bit more.

If you look at the list of approved GPS units for Tripoli records you will notice that nearly all of them specify the Ublox 7 series chipsets.
I would go for the Ublox Neo-8m over the 7 series for the Glonass capability and the flash memory which saves any settings.
They cost a bit more than the units that I have been using for this write-up (around $40) and are slightly larger in size.

My main aim when I started this was to see how small I could make these while keeping the cost as low as possible.
I have got them down to fitting in a 20mm tube, 70mm long plus the antenna for under $70 all inclusive.
If you have more room to spare and are willing to pay a bit more then accuracy and range can improve.
Last edited by vance2loud on Thu Jun 09, 2016 10:47 am, edited 1 time in total.
TRA #15165 AMRS #50
I do it because i can, I can because I want to, I want to because you said I couldn't. - Unknown
I have not failed. I've just found 10,000 ways that won't work.
- Thomas Alva Edison

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OverTheTop
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Re: Basic GPS tracker

Postby OverTheTop » Thu Jun 09, 2016 10:35 am

Altitude determination is about a factor of three times worse than the horizontal position accuracy when using GPS units. This is due to errors in the pseudoranges calculated from the satellites. It is quite dependent on where the satellites are in the sky (where and how their error bands overlap) and which ones are visible at the time of determination. Not usually a problem for rockets, although the airframe could potentially affect this.

As stated before, the use of both Navstar and Glonass systems gets rid of one of the main contributors to position noise. The altitude accuracy will still be about a factor of three worse than position, but both will be significantly better than on a single-system GPS. We can't change that geometry!
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vance2loud
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Re: Basic GPS tracker

Postby vance2loud » Thu Jun 09, 2016 11:16 am

The other consideration is that some of these GPS units will lose lock/fix on the way up.
This can be due to incorrect settings in their firmware (look up cocom limits for more information), doppler shift, or just that they aren't designed to move this fast.
Due to this sometimes they won't reacquire a 3d fix until after apogee.
If this occurs then your apogee altitude will have to be incorrect.
TRA #15165 AMRS #50
I do it because i can, I can because I want to, I want to because you said I couldn't. - Unknown
I have not failed. I've just found 10,000 ways that won't work.
- Thomas Alva Edison

vance2loud
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Re: Basic GPS tracker

Postby vance2loud » Fri Jun 10, 2016 4:29 am

Cheap 5v power source
For those of you who wish to use the larger RFDesign radios, you will need a 5v power source.
Ensure that whatever GPS module you use can handle the increased voltage before hooking it up.

Some of you may have noticed these cheap phone powerbanks appearing at most supermarkets and service stations,
DSC_0254a.jpg


with a little disassembly
DSC_0258a.jpg

You might note that the battery states 5000mAh or similar, this is most likely false as good quality 3000mAh versions of these batteries cost more than these units do. 2000mAh is a more likely true capacity.

If you de-solder the original battery terminals from the board and solder a connector to suit your own 1s Lipo you will have a basic usb charger for your batteries and a way to charge your phone from your tracker batteries. Note do not hook this up to any Lipo below 500mAh or the charge and draw current may exceed safe limits.
DSC_0266a.jpg


From here we can then connect into the USB port for 5v power or solder wires directly to it. Double check polarity for your device before soldering.
DSC_0268a.jpg


These powerbanks are usually rated to 1A output but for the smaller units I would keep it below 800mA.
The output voltage is also not highly regulated and can vary between 4.8-5.5v.
That voltage and current rating is capable of running the RF900u and the RF900+, I would however recommend using a powerbank rated for 2A output for the larger radios.

If you are testing the voltage output from these devices expect a lower voltage (3.8V on the unit shown) with no load attached, they have current sensing to lower their battery draw if they don't sense a load attached.

Also on power connections, some of you may have noticed that I tend to use the red JST for most of these trackers, I had been using them for my altimeters and other devices as well but have decided to cease that practice.
The reasoning behind this is trying to keep track of what devices require what voltage, these trackers may be destroyed rapidly if you connect a 7.4v 2s battery to them (no I didn't learn the hard way)
I have now started using balance lead extensions for low current devices that require a higher voltage to avoid any lipo confusion.
Most Hobby Lipo packs will already have these connectons on them for attachment to a quality charger.
Here is a 2s connection beside a JST for comparison
DSC_0270a.jpg
Last edited by vance2loud on Fri Jun 10, 2016 4:55 am, edited 1 time in total.
TRA #15165 AMRS #50
I do it because i can, I can because I want to, I want to because you said I couldn't. - Unknown
I have not failed. I've just found 10,000 ways that won't work.
- Thomas Alva Edison

vance2loud
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Re: Basic GPS tracker

Postby vance2loud » Fri Jun 10, 2016 4:49 am

For reference,
A 600mAh battery, radio air unit and a bluetooth module (HC-06 shown) will fit inside the case when you remove the original battery.
DSC_0273a.JPG

Ensure to insulate all the electronics with masking tape or heatshrink if doing this. (wiring and insulation not shown for clarity)
Note I run the charge cable, battery cable and a power cable for the radios outside the case to allow disconnecting the battery when not in use and for the ability to use this as a charger for my tracker batteries.
This makes for a very compact bluetooth beacon to link the tracker to your phone.
TRA #15165 AMRS #50
I do it because i can, I can because I want to, I want to because you said I couldn't. - Unknown
I have not failed. I've just found 10,000 ways that won't work.
- Thomas Alva Edison

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Re: Basic GPS tracker

Postby Lister » Fri Jun 10, 2016 10:26 am

Very helpful thread. Nice work Vance.

The power bank I got from big W is 2200mah.. considering they look the same and cost $10 I highly doubt they would be a higher capacity.

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Re: Basic GPS tracker

Postby natty » Tue Jun 14, 2016 9:28 pm

G'day Viewers

Well I took delivery of my transmitters (900u) from RFdesign and they worked right out of the box.They are slightly different than whats on the website, it looks like theres been an update. My GPS chip turned up in the afternoon (https://www.hobbygo.com.au/receiver-tra ... results=76) and I've wired it up.

I went into a terminal program and all I'm getting is crazy stuff
screenshot.JPG
screenshot.JPG (43.78 KiB) Viewed 2159 times


Can anyone make sense of this?

Thanks
L3
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vance2loud
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Re: Basic GPS tracker

Postby vance2loud » Tue Jun 14, 2016 9:41 pm

Check the default baud rate of the GPS module and set the radio modules to the same.
It looks like what I get if one of the baud rates is mismatched.
It looks like the GPS should be on 9600 according to that website.
If you set the RF900u to 9600 as well on both modules, you can also then lower your airspeed to increase your range.
I tend to keep the airspeed at around 2-3x the baud rate.
Also make sure your terminal program is set to the correct baud rate.
ucenter (the program I use to read the GPS) has an autobaud setting for ease of use.
TRA #15165 AMRS #50
I do it because i can, I can because I want to, I want to because you said I couldn't. - Unknown
I have not failed. I've just found 10,000 ways that won't work.
- Thomas Alva Edison


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