Home
Remote sensing of the upper atmosphere of earth has historically included giant radars costing $100M (e.g. Arecibo). We propose an inexpensive radar network costing less than $200 per node that citizen scientists and primary school science classes can build and deploy. The radar nodes work together as an infrastructure-less self-organizing network, transmitting pseudonoise waveforms hundreds of kilometers in the shortwave radio bands. The waveforms simultaneously measure atmospheric characteristics and contain data relayed to the cloud for offline processing for purposes including:
- improving ionospheric models vis measurements as an alternate or complement to GNSS TEC measurements
- 4-D imaging of the Earth’s atmosphere/ionosphere
- data relay from isolated sites (e.g. flood alarm, tracking animals)
- Solar storm impact detection and quantification
Hysell, Mill, Vierinen "A multistatic HF beacon network for ionospheric specification in the Peruvian sector" describes a 3-site system comprised of one dual-frequency transmitter using similar coding at 1/2 Watt, and two receiver sites, all synchronized via GPS. One of the key factors noted in the work was the immense oversampling of the received signal.
Initial experiments show that the CPU & PLL on the Raspberry Pi 3 may be capable of transmitting HF radar waveforms with sufficient spatial resolution to resolve interesting atmospheric disturbances. System architecture is highly flexible, and nearly any single-board computer may be suitable. $10 DDS chips like the AD9834 allow synthesizing frequencies from DC to VHF with arbitrary modulation.
An RF receiver is necessary; one option is using a $20 RTL USB stick with a $50 upconverter board. The radar system will be documented and described sufficient for a “hackaday” type article enabling citizen scientists and school teachers to build and deploy their own. Designing a custom PCB will allow substantial cost reduction, critical for adoption in economically disadvantaged areas.
The PiRadar system has two primary functions
- data transceiver-long range (> 100 km) broadband HF
- ionospheric radar
The problems these systems have in common for our experiments and deployments include
- too expensive radios ~$1000 ham radio transceivers
- too little instantaneous bandwidth: < 3.5 kHz typically.
These systems fail to use Shannon-Hartley theorem optimally. One can trade bandwidth and/or time for power<-->SNR. In essence we are proposing a more optimal use of scarce RF resources by using low-power pseudonoise broadband waveforms that other users will scarcely detect.
We don't think of these systems as competing but rather complementing. Other types of instruments e.g. optical are also experiencing the revolution of dense networks of small cheap sensors completings large single site sensors.
- SuperDARN: excellent global network running for over 20 years, costing just over $1 million per site. SuperDARN observes on timescale of minutes in the 8-20 MHz range. We propose time resolution on timescale of seconds in the 3-10 MHz range.
- Arecibo this is a UHF incoherent scatter radar studying the ionosphere from a single site and with mechanical steering. Naturally, a 330 meter dish antenna in the mountains is very expensive.
- FAST and GBT similar: cost/size/single site
- HackRF One $300, 20 MHz bandwidth. HackRF One block diagram
- BladeRF x40 $420, 40 MHz bandwidth. BladeRF block diagram
- Ettus B20x $750, 56 MHz bandwidth. Several of the Ettus radios are sophisticated implementations of an AD9364 ecosystem.
The above are all good to excellent choices for their purposes, which often is on-air cellular, Wifi, bluetooth or high-resolution radar. However:
- Inexpensive <$100 PCs currently have trouble handling the full bandwidth of these radios.
- we need < 1 MHz bandwidth
These radios take the approach of using a frequency synthesizer and IQ mixer for highly frequency-agile (several GHz) broad bandwidth (tens of MHz) transceivers.
By distinction, our application requires only several MHz of frequency agility and < 1 MHz bandwidth. It is possible that we can omit the IQ mixer and external DACs and simply modulate the frequency synthesizer itself, saving a great deal of cost and complexity.
To do so probably requires a microprocessor or FPGA to clock in the preprogrammed modulation sequence. The AD9834 eval board has a Blackfin MPU enabling control and modulation.
- the Red Pitaya is a new SDR with onboard CPU and FPGA and ~ 50 MHz bandwidth. It is currently priced at $330 for the bare board.
Normal COTS amateur radio gear struggles to pass more than about 2.5 kHz instantaneous bandwidth with reasonable amplitude flatness.
- most nations allow at least 6-10 kHz instantaneous bandwidth for HF amateur radio transmissions, and bandwidth ~ range resolution, so we want the maximum legal bandwidth -- and may employ experimental licenses such as MIT Haystack currently holds to use more than 10 kHz bandwidth.
- due to multiband coverage from 160-10m and beyond, ham radio oriented SDRs cost >$600 and require a desktop PC to operate.
- a partial SDR transceiver is the Pocket SDR for $500 early adopter price, but it is a test device not yet ready for general use according to the creator. We could see if the pricing will change for a future run.
Numerous other crowd-sourced radio science projects out there, including:
- Radio Jove 20 MHz receiver network listening to Jupiter's decametric radiation. Seems to use spectrum vs. time.
- Aurorasaurus actually for optical measurements, but has great citizen use and visualization.
DDS waveform generation (we might be interested in Figure 6 operation)