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Old Jul 03, 2013, 02:06 PM
Stuart
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UK, Cardiff
Joined Dec 2008
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Lost Model Locator - with long range capability

This is my take on a lost model locator (LML).

Documents and plans will be found here;

https://www.dropbox.com/sh/ompujkefda543w6/NXgepgXrmW

It uses an RFM22B radio transceiver and in its basic form the LML acts as a simple UHF RF locator beacon. Gets power from the main RC supply and\or a backup Lipo battery. The Lipo charges from the main RC supply.

It will go into beacon (lost) mode on expiry of a timer, or when a RC servo pulse fails or goes into fail-safe or hold mode.

Hear the tones on a radio, a cheap hand-held UHF FM one will do, and go search for your model.

Add a GPS, and the LML will transmit its location as RFM22 data packets on a regular basis. There are a range of other options for receiving the location, some with very long range reception.

There are two versions of the LML, one small one using SMT components, and a larger on using easier to assemble wired components. Both the small and large LML can be used as a transmitter or receiver for the data packets. The large LML can be fitted with an Openlog logger which will log the data packets to a micro SD card for reading on a PC.

One of the pictures is of the large LML being used with an attached LCD display to receive data packets, showing you where the model is. The display has the location, altitude, speed, track, how far away and in what direction the model is. For a model in flight and at altitude, expect a range of tens of km using simple antennas.

There is the option of sending the location information as fast (120WPM) Morse, just hook the audio from the UHF radio up to a PC and decode with free PC software designed for Hams. Should have a range of about 3 times that of the data packets.

The distance and direction information can be also be transmitted as a few digits of slow FM Morse which you can listen for on the UHF radio. If you cant read Morse numbers yourself, record the audio and send it to someone who can, or give them a ring on the phone and let them listen.

The slow FM Morse will give you the information to locate the lost model, and will be heard at a distance that is more than 10 times greater than the RFM22 data packets will manage.

If you have the necessary receiving equipment the LML can transmit the GPS co-ordinates as FSK RTTY data at 200 baud. That should get you about 20 times further than the data packets and give a position read out on a PC display. Expected line of sight range should be more than 1000kM for this mode.

As a last resort the LML can be set-up to transmit the distance and direction as slow FSK Morse, that should have a range of around 45 times that of the data packets, say in excess of 2000kM LOS, satellite to ground for instance.

The LML uses a PICAXE 28X2 and PICAXE basic, so the software can easily be customised.

Initially I will have some PCBs to sell, and I am preparing a document pack describing what the LML it is, how it works, how to build, test it and use it, with notes on field range tests.

Parts cost, around 22, plus LCD, plus GPS.

I have a few PCBs available, for the large LML and the small LML.

The small LML PCB also includes the PCB for a Mediatek MTK3329 GPS, although you will need to separate the PCBs yourself.

Prices for the initial PCBs are 3 each , plus 3.50 for Airmail, with Signed for on delivery being 5.60 extra.

If you want to take the risk that the package does not arrive, and not get a replacement, you can drop the signed for on delivery charge.

UK delivery, including signed for on delivery is 1.60.

Send me a PM if interested.
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Last edited by srnet; Feb 16, 2014 at 10:45 AM. Reason: Added Micro version picture
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Old Jul 03, 2013, 02:07 PM
Stuart
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UK, Cardiff
Joined Dec 2008
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The tested range, true line of sight for the RFM22 data packets at 100mW, was 40km, on 1/4 wave antennas either end.

The frequency in use 434Mhz, means that there is a ready supply of gain antennas, bought or DIY, as well as effective low noise amplifiers (LNA).

A reasonable yagi would extend the distance for reception of data packets by a factor of 3 or 4. A good LNA adds about a further 12dB of signal gain to the RFM22 reciever which should improve reception even further.

Thus if you wanted to track a high altitude balloon, implying good line of sight, one of the LMLs, plus the above kit, ought to be trackable for 200km or more.
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Old Jul 03, 2013, 02:08 PM
Stuart
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Old Jul 03, 2013, 04:52 PM
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Quote:
Originally Posted by srnet View Post
should have a range of around 45 times that of the data packets, say in excess of 2000kM LOS
HEY! if you guarantee that range put me down for 10. even though im here on east coast my plane is always wandering off course and crashing in new mexico.

seriously, great project. it would be very interested to see a diy. maybe ill try an avr version if not too difficult.
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Old Jul 04, 2013, 12:19 AM
Stuart
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Quote:
Originally Posted by dave1993 View Post
HEY! if you guarantee that range put me down for 10. even though im here on east coast my plane is always wandering off course and crashing in new mexico.
Never any guarantees in life, sadly.

The point of testing for these ideal situations is to understand the difference between the range under ideal conditions and worst case conditions.

Put a model on the ground in a forest or urban area and the range of the data packets (for the GPS stuff) may be as low as 200M, whilst its 40km LOS. Thats a range difference between worst case and ideal of 200:1.

200M limit searching in an urban area is not great, but if you show that you can improve search range by a factor of 4 or more, by adding a gain antenna, low noise amp, or just simple morse code, then that makes a big difference.
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Old Jul 05, 2013, 04:35 PM
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Is there a way to hook up the LCD to a receiver to decode and display location quickly in the field?
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Old Jul 05, 2013, 11:46 PM
Stuart
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UK, Cardiff
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The third picture in the first post shows the larger LML acting as the receiver.

The LCD in that picture is displaying the last recorded position of the 'lost' model.

If you fit the open logger you will get a file of the models track stored on a micro SD card.
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Old Jul 06, 2013, 04:39 AM
FPV in Hawaii
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United States, HI, Kailua
Joined Feb 2012
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Im sure you remember me. I've been bugging you about this project for ages! I'm very happy to see you pursuing it.
It's a bit overkill for my needs, but I still want one. All I really want is a small FMKit type device that can transmit gps coordinates in morse code that I can copy down into an phone app or google, and know the location. To speed things up, you can omit the first digits of the coordinates since I'll always know which continent I'm in.
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Old Jul 06, 2013, 07:56 AM
Stuart
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Quote:
All I really want is a small FMKit type device that can transmit gps coordinates in morse code that I can copy down into an phone app or google, and know the location
.

It will act as a simple beacon of that type of course. You can cut the PCB down, to about 25x30mm, you dont have to have the pin header or the SMA antenna socket if you dont want them

Quote:
you can omit the first digits of the coordinates since I'll always know which continent I'm in
The GPS co-ordinates themselves are normally sent as fast morse, but its a simple matter of changing a flag bit to have them sent as slow morse.

In most cases you can indeed omit the degrees, as you normally know which degree of latitude or longitude you are in.

Confusion could of course occur at the equator, or at the Greenwhich meridian in the UK, when the W\E indicator is needed.

Myself I find the distance and direction more useful, up to 9.999km all you have to listen for is at most 7 numbers.
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Old Jul 06, 2013, 11:19 AM
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I would certainly be interested in a board or two when available.

Quote:
Originally Posted by dave1993 View Post
...maybe ill try an avr version if not too difficult.
Second the interest in an AVR version.
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Old Jul 06, 2013, 03:23 PM
FPV in Hawaii
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United States, HI, Kailua
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Quote:
Originally Posted by srnet View Post
.

Myself I find the distance and direction more useful, up to 9.999km all you have to listen for is at most 7 numbers.
The issue with this is that it isn't easy to quickly enter into your smartphone and get a location.
It would be nice to have the distance and direction while searching for the plane, but it would be safer to assume that the beacon will die at any moment and the exact coordinates need to be captured ASAP.
Also, how does it know where home is?
Either by extra equipment at home, or by memorizing the launch point. Both can add to the possible points of failure.

Will the beacon be able to be programmed for numerous gps bauds so we can use the gps already onboard for our OSD? Perhaps the ability to take 2 gps units in diversity. One is already there with the OSD, and the beacon has another facing downward. The beacon picks the data with the best HDOP, so even if the plane lands upside down we can get a good fix.
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Old Jul 07, 2013, 12:40 AM
Stuart
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Quote:
The issue with this is that it isn't easy to quickly enter into your smartphone and get a location. It would be nice to have the distance and direction while searching for the plane, but it would be safer to assume that the beacon will die at any moment and the exact coordinates need to be captured ASAP.
From the outset, one of the design ideas, was to able to find the plane with the absolute minimum of support equipment.

Apart from the radio, with distance and direction, all you then need to find a plane is a compass. A map makes things easier, and distance and direction will give you the location on the map, and you get that by sending the least amount of information.

If anyone wanted to send the slow Morse of the co-ordinates, then it will do that, but then you also need to carry a GPS. A smartphone is one way of doing it, I have one of those small 'MINI GPS' in my bag, which does the job for a fraction of the cost.

Quote:
Also, how does it know where home is?
It just does, that bit is not difficult

Quote:
Will the beacon be able to be programmed for numerous gps bauds so we can use the gps already onboard for our OSD?
Well I did realize that there is no standard baud rate for GPS, so its easy to configure for separate baud rates.

Quote:
Perhaps the ability to take 2 gps units in diversity.
No.
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Old Jul 07, 2013, 05:09 AM
FPV in Hawaii
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Thank you for humoring my brain storming ideas, I certainly don't mean to criticize your design, I'm very excited about it.
I wasn't thrilled about one of your answers. "How does it know where home is? " - "It just does".
Ok, perhaps it's an elementary question. However it serves my understanding to know whether the GPS must acquire a "lock" before departure in order to calculate the bearing. Since you state only a compass is required to locate the beacon, then this must be the case and it adds another preflight check to be certain that it's ready. If the GPS were to lose its lock during flight, would it still remember where home is when it comes back?

Also, the reported bearing only in reference to home since there is only one way communication, right? I mean if it would continually update the bearing/distance during the hunt, that would be very useful indeed. Without this capability, you really need to plot it on a map before you start the search, just like lat/longs.
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Old Jul 07, 2013, 05:17 AM
FPV in Hawaii
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I also want to mention that in my experience with navigation, a coordinate system is more precise than a radial and distance. It is more difficult to draw accurate radials and small errors magnify over distance. If you are dealing with a difficult search in a forest or tall grass it can make a big difference. I'm happy you are offering the slow morse coordinates as an option.
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Old Jul 07, 2013, 05:26 AM
Stuart
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If you turn up at the flying field and turn the LML on, like all GPS that are powered on after a period off, the GPS will take a time before it gets a fix. Usually between 30 and 60 seconds, no way around this.

You need to wait for this first fix as its the only way you know that the GPS is actually receiving signals.

The radio sends out a sequence of pips when the first (home) fix has been aquired, your then good to go.

As long as the LML does not loose power, the home fix and last fix are stored. So if the GPS looses contact, it carries on using the last good fix until a new fix is acquired.
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