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Listening for the ISS with Raspberry Pi 3 and SoDeRa


Using the Raspberry Pi 3, SoDeRa board and SDR to receive signals from the ISS

With the Raspberry Pi 3 comes yet another boost in performance and once again this will in particular be welcomed by those with computationally intensive applications, such as more demanding media applications, computer vision and software-defined radio (SDR).

With the arrival of the Pi 2 back in February of last year I decided to take it for a test drive with GNU Radio, using the gr-air-modes SDR application together with a humble USB DVB-T tuner to receive and decode aircraft Mode-S transmissions. Which, I must say, it did admirably.

This time round I thought I thought I would aim a little higher — quite literally — and see if I could pick up transmissions from the International Space Station (ISS). Being privileged to not only get early access to the Pi 3, but also to brand new high performance SDR hardware, the SoDeRa board, pairing the two of these seemed like the obvious thing to do for the task at hand.

In this first post we'll take a look at look at the Raspberry Pi 3, SoDeRa and overall hardware setup, before considering what is involved in receiving signals from the ISS. In a follow-up post we'll then take a look at the software configuration, before finally putting the system to the test.

More bits!


The standout feature of the Pi 3's Broadcom BCM2837 SoC is that it's built around the ARM Cortex-A53 architecture, which means 64-bit ARMv8-A processor cores. In this case four of them clocked at 1.2GHz. Which, even if your application won't stand to benefit from the move from a 32 to 64-bit architecture, suggests a 33% performance boost based on the clock speed increase alone. Together with other architectural improvements this should lead to an overall 50-60% performance increase in 32-bit mode.


The SoC is not the only place where improvements have been made and the board also boasts new peripherals, namely 802.11b/g/n WLAN and Bluetooth. While this may well herald a drop in sales of USB wireless dongles, it's great news for, well, just about everyone — since even if an application does not make active use of networking, there is still the matter of installing and updating system software. While support for the Low Energy variant alongside classic Bluetooth, will undoubtedly find use in many IoT, home media and creative applications.


Spot the tiny ceramic antenna!

With a stated performance of 1Gpixel/s, 1.5Gtexel/s or 24GFlops, the Dual Core VideoCore IV GPU would appear to be the same as found in the Pi 2 SoC. Similarly, the latest addition to the family also has 1GB RAM, Micro SD for storage, 4x USB 2.0 ports and 100M Ethernet.

Programmable for any wireless standard


The SoDeRa board was only formally announced just last week at Mobile World Congress. Developed by Lime Microsystems and featuring their latest Field programmable RF (FPRF) transceiver, the LMS7002M, the SoDeRa is capable of supporting just about any wireless standard that operates between 100kHz and 3.8GHz. As if this wasn't enough, it's also dual channel — MIMO — with two each fully independent transmit and receive channels.


The board also includes an Altera Cyclone IV FPGA, enabling high throughput processing to be carried out in hardware, between the transceiver and FX3 USB 3.0 controller.

Hardware build


Happily, the Raspberry Pi 3 has the same form factor and mounting hole positioning as its predecessor and could be easily attached to the back of the official touchscreen, providing the display for our SDR application.

Since we're interested in listening to audio, a tiny amplifier PCB assembly was added to the project bill of materials, along with two small loudspeakers, a power switch and connectors. Once the BOM had been finalised my colleague, Stuart, put together a laser cut acrylic case design based on one for the Pi and Touch Screen that was covered in a previous blog post of his.


The SoDeRa board is mounted below the Raspberry Pi 3 and touch screen assembly, with a USB cable connecting the two. While the Pi 3 only supports USB 2.0, this should be more than sufficient for the bandwidth required. U.FL-SMA cables provide rear panel access to the SoDeRa RF ports.

VHF signals from space

titleThe ISS has been operating an amateur radio station since the year 2000, with various bands being used, ranging from 144MHz (2M) up to 2,400MHz (13cm). Of these VHF being the most active.

Seemingly the ISS equipment mostly operates in automatic mode, whereby it acts as a repeater for voice or AX.25 data packets. When not in automatic mode and astronauts put out a CQ (contact) call, this is on 145.800MHz, listening for replies 600kHz lower on 145.200MHz.

The low orbit of the ISS means that it will only be in range 5 or 6 times a day and for up to 10 minutes at the most each time. On top of this, the fact that it's travelling at in excess of 28,000 Km/h meaning that doppler shift will come into effect. However, one of the great things about using an SDR receiver is that we can visualise a slice of spectrum, covering above and below 145.800MHz, looking out for signs of promising peaks and quickly tuning to these.

A simple 3 element Yagi was decided upon for the antenna, a Cushcraft A148-3S. A directional aerial with 7.8dBi gain, this can be pointed at the appropriate part of the sky on that given day.

Stay tuned!

Now that we have the hardware set up, it should be just a matter of configuring the software and putting it all to the test — and perhaps a little patience...!  

  — Andrew Back


Open source (hardware and software!) advocate, Treasurer and Director of the Free and Open Source Silicon Foundation, organiser of Wuthering Bytes technology festival and founder of the Open Source Hardware User Group.
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