Sending data 500km with 150uW? - Page 1
Forum section: electronics / rf-microwave

author: nctnico
author_rank: Super Contributor
timestamp: September 19, 2016, 10:31:01 am
content: Someone keeps bugging me about a system to track small birds. Using GSM or Wifi are out of the question due to range and power consumption. The posts from DaveBev about tracking balloons over extreme distances (https://www.eevblog.com/forum/chat/you-might-find-this-interesting/) sparked an idea in my head. Even though he uses a world wide grid of receiving stations every now and then it seems his data get received by stations 20000km away. The system he uses is called WSPR which sends data in a very narrow band (several Hz) and appearantly a software defined radio solution and very low baud rates are used to demodulate the signal using a PC and soundcard. DaveBev's balloons use a 10mW transmitter in a 10-ish MHz band. Based on some quick calculations regarding weight and power usage I think 'my' system has to make do with 150uW transmitting power and use a license free band (lowest frequency is probably best for range but the antenna also has to be small). Since my knowledge about radio is rather limited I'd like some input on how feasible it would be to receive a signal transmitted with 150uW 500km away (from 100m to 300m) using an antenna on my roof and a 24bit soundcard.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: September 19, 2016, 11:17:34 am
content: I've had a play with WSPR. It takes two minutes to send maybe 12 characters. These are your amateur callsign, grid locator (alphanumeric) and your output power.There are online RF path loss calculators which might be helpful. Eghttps://www.pasternack.com/t-calculator-fspl.aspxBasically you need enough transmitter power to make up for path loss, receiver gain and to overcome noise on the frequency.As for frequencies, 7 MHz is a good frequency for 500 km but the antenna requirements are significant (a full sized antenna is 20 metres long). If it's near line of sight VHF might be possible. But 500km is still a tough ask unless, like WSPR, bandwidth and data rates are low.

author: Ammar
author_rank: Regular Contributor
timestamp: September 19, 2016, 12:40:46 pm
content: Use the Friis transmission equation to determine the receive power, giving 150uW of transmit power:https://en.wikipedia.org/wiki/Friis_transmission_equationThen, check if there are any commercial receivers sensitive enough to receive this sort of power. My hunch is that there isn't, but I haven't done the calculation.

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 19, 2016, 12:48:48 pm
content: 150 uW is about -8dBm, path loss over 500KM will be something like -110/-140dBm depending on frequency, the transmitter antenna will never be above 0dBm but more likely in the order of -10dBm and your receiving antenna 0-6dBm, so do the math and see what the noise floor is at your location on the chosen frequency.And then one more thing: tracking? Are you going to strap a GPS receiver on a small bird?

author: LaserSteve
author_rank: Super Contributor
timestamp: September 19, 2016, 03:50:11 pm
content: The tracking transmitter is easy. The required receiver network, not so easy. You either need something like HF Packet for the Domestic US, or Wispr, or Argos or Iridium for world wide. Bird tracking transmitters using Argos have been made.Commercial burst transmitting VHF tags are down to 0.3 Gram, if you have the massive ground receiver network needed... But no GPS at that weight.You have very few choices for the receive site, which is why LEO satellite clusters start to look good... One other answer is to use a chase aircraft, but that is hideously expensive.http://www.lotek.com/argos.htmhttp://www.lotek.com/pinpoint-argos.htmhttps://www.northstarst.com/tracking-birds/3.5 grams is going to be hard to beat...Steve

author: nctnico
author_rank: Super Contributor
timestamp: September 19, 2016, 05:10:54 pm
content: To me the WSPR system looks promising because it solves most of the problems at the receiver side using a PC for signal processing in order to retrieve the signal (message) from the noise. According to some information I read about WSPR it can deal with signal to noise ratios worse than -20dB (which means the amplitude of the noise is >10 times higher than that of the signal of interest).

author: rt
author_rank: Regular Contributor
timestamp: September 19, 2016, 05:50:34 pm
content: Depending on the spatial accuracy required, an optical geolocator may give enough resolution at very low weight.https://en.wikipedia.org/wiki/Light_level_geolocatorThe other key thing to know about the proposed study is can you get the tracker back (i.e. can the tag usefully store data) or does each data point have to be sent over the radio link?rt

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 19, 2016, 05:54:33 pm
content: Quote from: LaserSteve on September 19, 2016, 03:50:11 pmThat's still 20% of the weight of a small bird, and it uses satellites to retrieve the data only at the end of a set period. So line of sight communication and a frequency where you can build a reasonable effective antenna. If you really want to use HF (VHF will not do the range) then you have the antenna problem, the GPS receiver, the transmitter and the battery.And you need a bigger battery since you can't cover the wings with solar cells. Good luck ending up with even a weight equal to the bird and I don't think it will take off...

author: nctnico
author_rank: Super Contributor
timestamp: September 19, 2016, 06:21:52 pm
content: I know this project will take some serious math, simulation and experimenting before even committing to building a prototype. My target weight is 4 grams but it only has to run for a couple of days. The goal is to track a flock of birds for a couple of days to learn more about their flight patterns. The other solutions available are mostly to track a few birds/animals for a couple of months and are rather bulky.

author: Howardlong
author_rank: Super Contributor
timestamp: September 19, 2016, 06:52:09 pm
content: To give you some idea, I helped design a VHF telemetry system that's in use on several satellites in LEO. At the horizon they're of the order of 3,000km distant and have a downlink power of about 30mW and can be decoded with a simple dipole or 1/4 wave. The receiver channel bandwitdth is about 3kHz to help deal with doppler, but the occupied bandwidth is about 1.5kHz and the raw BPSK bit rate is 1200baud. After a concatenated FEC scheme the actual data rate is about 500bps.You can trade off bandwidth/bit rate against power, but keep in mind that the narrower bandwidth will mean it's harder and harder to correlate a signal, i.e., you need a lot of processing power to fish the signal out. Your reference oscillator in the transmitter will likely be not too accurate in relation to carrier and channel bandwidth.The choice of modulation and coding schemes is key here, but also in relation to the efficiency of your transmitter. You might find that operating FSK is more efficient to transmit than BPSK based on pure power, but BPSK can be demodulated at much lower SNRs.The FEC I use includes both a convolution and a block encoder with interleaving so that a transmision affected by single bit, burst and fading can all be recovered from.Also the choice of frequency band is key: there's an awful lot of noise at lower frequencies, and making directional antennas to reject noise from the warm earth is much harder. You may find a sweet spot higher up in UHF for example. Edit: and smaller antennas will be more efficient too.

author: Ice-Tea
author_rank: Super Contributor
timestamp: September 19, 2016, 08:32:41 pm
content: Where does the 150uW come from? Have you considered saving power elsewhere? Transit less (often) but with more power?

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: September 19, 2016, 08:42:51 pm
content: Quote from: nctnico on September 19, 2016, 05:10:54 pmOne consideration with WSPR is it requires extremely good frequency stability. This may be a challenge if the transmitter is exposed to temperature variations.Also WSPR uses 2 minute blocks (you're typically sharing a narrow range of frequencies with multiple other WSPR beacons). This requires a timing circuit that is accurate over time so that all transmitting stations come on and off in accurately spaced 2 min blocks. Although if you're on your own frequency and not sharing, maybe this is less critical?You give a 150 uW output power level? Is this based on drawing up a power budget? If so note that the duty cycle for WSPR can be set so that it is not transmitting for most of the time (eg a 20% duty cycle - ie transmitting for only 1 x 2 min block out of 10 minutes. That may change your calculations.Hans Summers G0UPL has done a lot of work with 'QRSS' (basically slow speed Morse code). His site may be worth a look.http://www.hanssummers.com/

author: nctnico
author_rank: Super Contributor
timestamp: September 19, 2016, 08:46:47 pm
content: Quote from: Ice-Tea on September 19, 2016, 08:32:41 pmI made a conservative power budget based on the capacity of a light weight battery, other electronics, continuous transmission and an estimate that half the power which goes into the transmitter ends up in the antenna. Maybe I can use more power for transmission and have long pauses between the messages but the project hasn't gone that far yet. I just needed to have a feel for the feasability in order to produce a project cost estimate.

author: Nuno_pt
author_rank: Frequent Contributor
timestamp: September 19, 2016, 08:55:21 pm
content: You can also use 1 minute cycle with PI4, CW, MGM, this is use in the beacons.

author: Ice-Tea
author_rank: Super Contributor
timestamp: September 19, 2016, 08:58:24 pm
content: You could log the position every minute or so but send the "readers digest" every hour or even every few hours. Might even allow you to use some off the shelf stuff like LORA or something...

author: Marco
author_rank: Super Contributor
timestamp: September 19, 2016, 09:36:25 pm
content: So what are the options for actually achieving what is asked in the first post? The propagation modes kind of limit the options AFAICS.- LF/MF with an electrically very short ferrite antenna ... how much "gain" could you expect to have with a couple gram ferrite antenna?-SATCOM bounce- ?

author: LaserSteve
author_rank: Super Contributor
timestamp: September 19, 2016, 10:16:55 pm
content: Does the species have any useful habits, like bedding down at sunset and staying down till morning?How many kilometers is the migration? Do they go to a known breeding ground that is physically small like an island?Do you have budget for custom silicon?Sounds like your flock is not even a radar target, let alone a cargo carrier. I keep thinking about transmitting a mode C ID in the blind, but at 150 uW that seems impossible, although the SSR receive antenna certainly has enough directivity and gain. But the bandwidth for the ID burst is very wide, and that reduces your Effective Radiated Power to nothing. Capturing a whole flock and mounting an antenna to act like chaff on them is probably impossible too.Here is 1 gram..http://www.lotek.com/pinpoint-gps.htmHere is the story...http://motherboard.vice.com/read/tracking-tiny-birds-using-tiny-gps-backpacksSteve

author: LaserSteve
author_rank: Super Contributor
timestamp: September 20, 2016, 01:59:08 pm
content: Here is 1 gram..http://www.lotek.com/pinpoint-gps.htmHere is the story...http://motherboard.vice.com/read/tracking-tiny-birds-using-tiny-gps-backpacksSteve

author: TheBay
author_rank: Unknown
timestamp: September 20, 2016, 11:27:19 pm
content: Quote from: Ice-Tea on September 19, 2016, 08:32:41 pmQuote from: vk3yedotcom on September 19, 2016, 08:42:51 pmWSPR will also need an accurate clock, another consideration with the design.Though both the time and frequency stability can be something the GPS can assist with though I can't see how it can be made light enough.Sent from my Harrier Tab from EE using Tapatalk

author: Lord of nothing
author_rank: Super Contributor
timestamp: October 17, 2016, 11:54:45 pm
content: @ blueskullThe problem is you cant receive any good Signal <30 - 50 Mhz because there are a lot of PLC Device who send there. Its hard to receive anything else arround Citys.Depend of the Country there are maybe some Animal Tracking Freq. Here in Austria its on the 142Mhz freq.