[chbot] Planning for the next few weeks - SDR Experts?

Sun Mar 22 19:09:10 GMT 2020

If you are using an RTL-SDR + RPi then that is a reasonable approach.

Snapshot 1 sec of data then go from there.

Yup, like GPS you're probably shooting for a 2 step approach:
Acquisition: finding the signal frequency offset (similar to GPS doppler
determination) + beep repeat rate (similar to GPS code).
Tracking: keeping synced to these.

Uncle Google should help you find some good texts.

https://berdiansklib.blogspot.com/2011/07/the-hobbyist-guide-to-rtl-sdr-really.html
https://www.rtl-sdr.com/gnss-sdrlib-real-time-gps-positioning-rtl-sdr/

On Sun, Mar 22, 2020 at 6:34 PM Trevor Wignall <zl3adz at gmail.com> wrote:

> Yes, some form of iterative or parallel technique sounds like a workable
> approach.
> I was thinking of interfacing the SDR USB dongle to a Raspberry Pi and
> remoting into it from a laptop to do the display. Once a signal has been
> acquired and locked, I am guessing the processor load can be reduced, so
> the processor can move on to other transmitter frequencies and keep a lock
> on multiple transmitters. Useful to help find people that you learn are
> missing some time later...
> Trevor
>
>
>
> On Sun, Mar 22, 2020 at 6:01 PM Charles Manning <cdhmanning at gmail.com>
> wrote:
>
>> Hi Trevor
>>
>> If you don't know the carrier frequency accurately enough then it sounds
>> like one of those cases where you want to store the signal, then iterate
>> over it beating with various carrier frequencies until you find one that is
>> close to zero.
>>
>> Similar techniques are commonly used in GNSS/GPS processing and I've
>> played with techniques that can find satellites in 10milliseconds of
>> snapshotted data.
>>
>> On Sun, Mar 22, 2020 at 2:09 PM Mark Atherton <markaren1 at xtra.co.nz>
>> wrote:
>>
>>> I would start off with a PC and a dongle capable of a waterfall display
>>> centred on the nominal carrier, with scan width set to maybe 3kHz, then you
>>> may be able to find the carrier.
>>>
>>> Then you can improve centre frequency so you can drop scan width, and IF
>>> bandwidth.
>>>
>>> Repeat until carrier is within a couple of Hz, and IF is maybe 20Hz
>>> wide. You now have the possibility of a very sensitive receiver.
>>>
>>> If you can get an SDR handheld with an IF bandwidth of 20Hz, all you
>>> have to do is transfer the exact frequency of the carrier, and you have a
>>> very sensitive portable device.
>>>
>>> In terms of automation, initially don't. Get the hang of a process that
>>> works, and equipment that will do the job. Once you have ironed out the
>>> fiddling to make the system work, then you can transfer your knowledge into
>>> an algorithm to automate the process.
>>>
>>> Required laptop and SDR sounds easy to set up. Handheld SDR sounds
>>> expensive / difficult.
>>>
>>> Having said that, you could set up a network of laptops + SDR dongles at
>>> fixed locations and manage some kind of tri/quad/hex-angulation. Each
>>> dongle must have an accurate and stable timebase. Laptops must be connected
>>> to the same network.
>>>
>>> Even a 50Hz wide IF for a CW receiver might do the job, but you will
>>> need around 10Hz accuracy at 300Mhz which 33 x 10-9 which sounds like a
>>> Rubidium reference standard.
>>>
>>> The other option is to load a calibration number into the handheld each
>>> day to adjust it's reference which will hopefully stay put for 24 hours.
>>>
>>> Just wondering about a homebrew Zero-IF RX, you can easily crank down IF
>>> bandwidth on such a device...
>>>
>>> Hopefully some of that makes sense.
>>>
>>> -mark
>>>
>>>
>>>
>>> On 22/03/2020 1:22 PM, Trevor Wignall wrote:
>>>
>>> The frequency is around 300MHz and is known to within a couple of kHz -
>>> later models are a bit more stable, so perhaps within +/1 1kHz. Repetition
>>> is 1-2 seconds. So essentially a CW (morse) transmitter sending the word
>>> "e". Transmit power and aerial are not relevant to trying to improve the
>>> receiver - I'll take any improvement I can get! There is a test transmitter
>>> operating with a coverage of most of Christchurch - clearly heard at the
>>> Idris Rd clubrooms.
>>> I don't want to put too many specifics in an open thread - anyone
>>> seriously interested can contact me and I will give them more specifics
>>> (after I have cleared it with the Wandersearch Trustees).
>>> Trevor
>>>
>>>
>>> On Sun, Mar 22, 2020 at 12:59 PM Mark Atherton <markaren1 at xtra.co.nz>
>>> wrote:
>>>
>>>> Some parameters please
>>>> - max and min limit of carrier frequency, or Fnom ± Ftolerance
>>>> - repetition rate of the 100ms bursts ?
>>>> - transmit power
>>>> - transmit antenna
>>>>
>>>> Do you have a pendant in hand to take apart ?
>>>>
>>>> So unmodulated carrier, and no ident modulation ?
>>>>
>>>> - Mark
>>>>
>>>>
>>>>
>>>> On 22/03/2020 12:44 PM, Trevor Wignall wrote:
>>>>
>>>> Hi all
>>>> As a Wandersearch responder (i.e. someone who goes out with a
>>>> directional receiver and tries to locate the small transmitter
>>>> pendants worn by people who tend to go wandering e.g. dementia patients), I
>>>> am looking at using a Software Defined Radio (SDR) to receive these
>>>> signals. However as the signals are weak, I am looking for ways of
>>>> extracting the short transmitted pulses (around 100-200 milliseconds of
>>>> unmodulated carrier) from the background noise using narrow band filtering.
>>>> The problem is that the exact transmitter frequency is not known due to
>>>> tolerances, temperature changes etc.
>>>> If anyone familiar with SDRs would like a challenge to combat boredom,
>>>> any help would be appreciated.
>>>> Trevor
>>>>
>>>>
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