To HAM or not to HAM

[waltr]

Just powered on the Eggfinder Mini and my RFExplorer to look at the transmission spectrum.

Single frequency at 913.714MHz and MiniTX sends a short packet every few seconds. NO FHSS.
Bandwidth is about 100kHz.
The miniTX uses an FCC approved module.

 

[Adrian A]

Is that legal in that band?

Yes, the Featherweight GPS system is fully certified and compliant, which was verified by certification testing. It uses spread spectrum with >500kHz bandwidth and therefore does not need to channel hop.

 

[FredA]

spread spectrum with >500kHz bandwidth and therefore does not need to channel hop.

Spreading = Hopping.
You typically do not change channels, you need to move the frequency.

 

[Grant_Edwards]

Spreading = Hopping.
You typically do not change channels, you need to move the frequency.

Frequency hopping is one type of spread spectrum. There are other types of spread spectrum that are not the same as frequency hopping. You can use direct sequence spread spectrum to increase the bandwidth and reduce the energy density without doing any channel/frequency hopping. FHSS changes the carrier frequency in some predictable pattern. DSSS does not change the carrier frequency, it just spreads out the data signal to widen the modulation.

https://en.wikipedia.org/wiki/Spread_spectrum

 

[plugger]

Frequency hopping is one type of spread spectrum. There are other types of spread spectrum that are not the same as frequency hopping. You can use direct sequence spread spectrum to increase the bandwidth and reduce the energy density without doing any channel/frequency hopping. FHSS changes the carrier frequency in some predictable pattern. DSSS does not change the carrier frequency, it just spreads out the data signal to widen the modulation.

https://en.wikipedia.org/wiki/Spread_spectrum

And given the carrier frequency doesn't change, you can RDF that signal.

 

[schworer]

Technically, yes you could RDF the signal, but to do that you would really need a receiver with the same bandwidth as the transmitter. Trying to use a conventional receiver would most likely be unsuccessful because the spread spectrum bandwidth of the transmitter is spreading out the transmitted power density over a much broader area, so the instantaneous signal detected by a conventional receiver bandwidth would be low. Also, a spread spectrum signal into an analog receiver generally sounds like static, so you would have to be able to audibly discern changes in static levels. An analogy is a thrust curve comparison for equal total impulse kick-ass Warp-9 motor (narrow band analog or digital transmission) vs. a very long burn motor (spread spectrum). Both have the same total power in NS, but the Warp-9 has a lot more thrust at any point on its curve.

To goose the reception range on a Featherweight tracker for a very high altitude Balls flight last year, the ground receiver was married up with a three element yagi. After the rocket landed on the playa a number of miles away the yagi definitely showed directionality in that if the receiver was not pointing within roughly 60 degrees of the transmitter the signal was not received. Of course the last received position under chute prior to landing was right where the rocket was found.

 

[plugger]

Technically, yes you could RDF the signal, but to do that you would really need a receiver with the same bandwidth as the transmitter. Trying to use a conventional receiver would most likely be unsuccessful because the spread spectrum bandwidth of the transmitter is spreading out the transmitted power density over a much broader area, so the instantaneous signal detected by a conventional receiver bandwidth would be low. Also, a spread spectrum signal into an analog receiver generally sounds like static, so you would have to be able to audibly discern changes in static levels. An analogy is a thrust curve comparison for equal total impulse kick-ass Warp-9 motor (narrow band analog or digital transmission) vs. a very long burn motor (spread spectrum). Both have the same total power in NS, but the Warp-9 has a lot more thrust at any point on its curve.

To goose the reception range on a Featherweight tracker for a very high altitude Balls flight last year, the ground receiver was married up with a three element yagi. After the rocket landed on the playa a number of miles away the yagi definitely showed directionality in that if the receiver was not pointing within roughly 60 degrees of the transmitter the signal was not received. Of course the last received position under chute prior to landing was right where the rocket was found.

Are you sure? I thought carrier and data were able to be separated? Or, at least they can be is what I should say. I routinely look at satellites with sdrs now and can often see carrier at a high SNR despite not having enough dish to get the actual data transmission out of the noise floor. Elektro-L3 in GEO is a good example, I can see it's HRIT carrier signal very strong (6dB, which isn't bad given it's 38,000km away) but I don't have enough dish to get sync to demodulate the data.
Full disclosure though, I've never attempted to RDF a LoRa signal, much less looked at it in the FFT or waterfall. But now I really want to.

 

[schworer]

In my reply I was assuming the use of an analog scanner or ham radio which have a limited bandwidth around any selected frequency. Any received spread spectrum signal received in that bandwidth will sound like static. That is pretty cool you can see the satellite transmission on your SDR display, are you sure its actually spread spectrum vs a broad bandwidth high data rate signal? I couldn't find much on HRIT transmission specs in a quick search.

Re the carrier and data separation (demodulation) question, the only way that could be done is if you know the spreading sequence code and have a receiver designed for it. If you look at a direct sequence spread spectrum signal you won't be able to see any carrier, what you would see is a higher "noise" level over the spread spectrum transmission bandwidth. Here is a "back of the envelope" explination that is approximate enough. The reason the signal looks like noise is the carrier is modulated by a very high speed pseudo random code (PRC) of "chips", which mathametically resembles white noise, and that is what spreads the signal out...the faster the chip rate the wider the signal spread and the lower the power level at any discrete frequency. The digital intelligence in the signal, which is sent at a data rate far lower than PRC chip rate, is combined with the PRC in a mixer circuit and then sent to the transmitter. To extract the intelligence the received signal is processed in reverse using the same time synchronized PRC (think of encoding as addition and decoding as subtraction, if you know what the add and subtract number is at any given time you can recover the intelligence). Because the PRC rate is much faster than the data rate, "pieces" of the received SS signal for a bit of information/intelligence (0 or 1) can be missing or corrupt and the correlator circuit can still decode it, i.e. if 60% of the PRC chips for a given data bit of intelligence are present or correct the correlator would assume what the bit was even though 40% of the chips were lost. That ability to loose PRC chips and still extract the intelligence is what gives spread spectrum processing gain and allows some incredible ranges at low transmit power (there are also other error correction protocols built in too that are not part of SS). So multiple spread spectrum radios can share the same center frequency and bandwidth because they will be using different PRC sequences. The way the codes are constructed a SS receiver will see all other received SS signals as background noise while its decoding the intelligence using the PRC of the transmitter. The older Verizon and Sprint cell phones used this type of system. Hopefully this explination is not as clear as mud If you want to learn more just search on direct sequence spread spectrum.

 

[plugger]

In my reply I was assuming the use of an analog scanner or ham radio which have a limited bandwidth around any selected frequency. Any received spread spectrum signal received in that bandwidth will sound like static. That is pretty cool you can see the satellite transmission on your SDR display, are you sure its actually spread spectrum vs a broad bandwidth high data rate signal? I couldn't find much on HRIT transmission specs in a quick search.

Re the carrier and data separation (demodulation) question, the only way that could be done is if you know the spreading sequence code and have a receiver designed for it. If you look at a direct sequence spread spectrum signal you won't be able to see any carrier, what you would see is a higher "noise" level over the spread spectrum transmission bandwidth. Here is a "back of the envelope" explination that is approximate enough. The reason the signal looks like noise is the carrier is modulated by a very high speed pseudo random code (PRC) of "chips", which mathametically resembles white noise, and that is what spreads the signal out...the faster the chip rate the wider the signal spread and the lower the power level at any discrete frequency. The digital intelligence in the signal, which is sent at a data rate far lower than PRC chip rate, is combined with the PRC in a mixer circuit and then sent to the transmitter. To extract the intelligence the received signal is processed in reverse using the same time synchronized PRC (think of encoding as addition and decoding as subtraction, if you know what the add and subtract number is at any given time you can recover the intelligence). Because the PRC rate is much faster than the data rate, "pieces" of the received SS signal for a bit of information/intelligence (0 or 1) can be missing or corrupt and the correlator circuit can still decode it, i.e. if 60% of the PRC chips for a given data bit of intelligence are present or correct the correlator would assume what the bit was even though 40% of the chips were lost. That ability to loose PRC chips and still extract the intelligence is what gives spread spectrum processing gain and allows some incredible ranges at low transmit power (there are also other error correction protocols built in too that are not part of SS). So multiple spread spectrum radios can share the same center frequency and bandwidth because they will be using different PRC sequences. The way the codes are constructed a SS receiver will see all other received SS signals as background noise while its decoding the intelligence using the PRC of the transmitter. The older Verizon and Sprint cell phones used this type of system. Hopefully this explination is not as clear as mud If you want to learn more just search on direct sequence spread spectrum.

I'm pretty certain it's not spread spectrum, but it's a digital mode with a carrier. Obviously my smooth brained understanding around this is minimal at best. But I've been playing with these things for a year now so I'm at the point where I know that I don't know very much at all. Another (probably better) example is the Chinese FY-2x GEO birds. Below is a screenshot from SDR++ showing the carrier signal and data on the sidebands.

I need moar dish and a linear feed if I want to attempt sync on that bird. My helical feed just doesn't cut it.

And here's the waterfall screenshot from a TeleGPS from another thread I wrote here. It shows a combination of FSK and APRS transmissions. I am dropping it here because to my understanding the Altus Metrum FSK digital mode is the same across Altus hardware. I've RDF'ed a TeleMini v3.0 before and they're not able to have APRS enabled OOB as they're not GPS enabled.


I recovered a rocket via RDFing that FSK signal with my Yaesu VX-8GR. Gross overkill of a radio for RDFing, but as long as your squelch is turned up you can definitely hear the packets and use the signal strength meter combined with a yagi to get a bearing. I'm pretty sure the FSK transmission is quite wide and is probably a wider bandwidth than what my Yaesu is able to sample, but I was able to do it none the less. And when looking at the Sigidwiki there's nothing to a LoRa transmission that would make me think you couldn't RDF it.


Finally, if a HAM radio isn't something you have or can afford, SDR++ has a full client for Android. Combine that with a RTL-SDR v3 or similar (via a USB OTG cable) and a yagi and you'd effectively have a real time waterfall capable scanner with a bandwidth of 2.4MHz, which should be ample for this use case.