Listening To Satellites: A Journey With My RTL-SDR V3 | by Mohsen Tahmasebi | Apr, 2023

NOTE: That is the log of my private journey with satellite tv for pc RF. In the event you’re on the lookout for a tutorial, this would possibly sound boring to you because it has many pointless particulars, and I’m not an expert. Take a look on the finish of the publish and the referred hyperlinks.

For a very long time (to be particular, from once I was 13), I used to be fascinated with radio alerts and beginner radio hobbies, However you understand, it’s troublesome to apply such hobbies in Iran, the place most of the wanted elements and components aren’t obtainable, or very costly.

So I fully forgot my fascination with beginner radio, however it all modified once I acquired my palms on a brand new and sudden gem, The “RTL-SDR Weblog V3” dongle. I listened to all types of radio frequencies for days, however a brand new undertaking caught my eye: Listening to satellites (or, extra particularly, receiving their sign).

As you most likely already know, there are a bunch of (properly, greater than a bunch, because of SpaceX) satellites orbiting above us. A lot of them at the moment are lifeless and area particles, however lots of them are working (even partially. Among the satellites have damaged batteries and don’t work when orbiting the darkish facet of the Earth, however working when uncovered to daylight because of their photo voltaic panels). Most of those working satellites (if not all of them) talk with their floor stations utilizing radio waves (principally transmitting information again to Earth).

The enjoyable a part of the story is, you’ll be able to obtain the sign of lots of them; nonetheless, it’s most likely ineffective to you as a result of the info is encrypted. Fortunately, a few of these satellites are beneficiant sufficient to ship their information (or a few of it) with no or weak encryption.

What information? It ranges from telemetry information about their batteries and inner programs to pictures from the Earth and even relayed information from different folks (Some CubeSats and different satellites carry an beginner radio payload that receives the info transmitted to them on a selected frequency, and relays it again to the Earth on one other frequency. This fashion, Satellite tv for pc radio hobbyists can broadcast CW or audio and talk with one another because the satellite tv for pc passes above them).

These “information” are transmitted in varied transmission programs. Older energetic American NOAA satellites transmit in APT and HRPT (and a few extra for various varieties of information), Russian Meteor-M 2 satellites transmit in LRPT and HRPT (most of them at the moment are lifeless or malfunctioning, so no extra LRPT from Meteor till Meteor-M 2–3), and so on.

On this publish, we’ll look into climate satellites like NOAA and METEOR and obtain their reside pictures from the Earth, some small CubeSats and their relayed and telemetry information, and at last, the legendary QO-100, a satellite tv for pc within the GEO orbit that relays information to a large space.

First, I attempted the moveable dipole antenna set that comes with the RTL-SDR bundle however had no success. So I went to a freeway with a really open sight of the sky, opened the moveable dipole antenna at 120 levels, pointed it to the south, linked the dongle to my laptop computer, and opened SDR++. I tuned the SDR to 137.100 MHz and stared on the display screen. Primarily based on the n2yo calculation for my location, the satellite tv for pc have to be seen from my location proper now, and some seconds later, for the primary time, I heard the superb sound of the APT analog alerts. I rapidly began recording the alerts in SDR++ and waited for the satellite tv for pc go to finish. (Enjoyable sidenote: I used to be holding the antenna with my hand, and folks have been watching me like I used to be defusing a bomb; it was hilarious)

Now I had a WAV file containing the alerts. To transform the analog alerts to a picture, There have been two essential choices for me. First, it was the well-known WXTOIMG, and the second choice was noaa-apt. The primary one was extra superior and had extra choices and outputs, however I used to be operating Linux and didn’t want superior choices for now, so I used noaa-apt…

Trying from area at ourselves in real-time whereas a satellite tv for pc passes above us?

It. was. stunning.

Since I used to be VERY restricted on {hardware} and instruments, I rapidly crafted a V Dipole antenna based mostly on Adam 9A4QV design to get NOAA 15,18,19 satellite tv for pc photos. The design may be very easy and poor-man-friendly.

You simply want some coaxial cable, two aluminum/copper rods, a choc block, and one thing to mount the antenna on. It will need to have a ways from metallic issues, an acceptable peak from the ground (we’ll attain the calculation half later), and an open sight of the sky. Additionally, having an LNA will SIGNIFICANTLY enhance your efficiency (In the event you use it proper).

In my first design, I used two copper rods, fastened the antenna to a water pipe with some cable ties, and ignored the LNA.

After some trial and error, I put in the raspberry-NOAA-v2 on my RPI, linked the SDR dongle, and let it run. The consequence was pretty…

The above pictures have been transmitted from NOAA 19 utilizing the APT analog transmission system on ~137 MHz. These NOAA satellites additionally transmit higher-resolution pictures utilizing the HRPT digital transmission system on a better frequency. The high-resolution pictures are more durable to obtain and require a directional antenna, so let’s neglect it for now.

As you’ll be able to see, the unique pictures are black and white, however you’ll be able to add a “false coloration” to them.

I had enjoyable with NOAA satellites for a couple of days after which determined to get the METEOR-M2 LRPT alerts that are additionally on ~137 MHz. In contrast to the NOAA’s APT, LRPT alerts are digital, so whereas NOAA’s APT transmits two channels with 4 km per pixel decision, METEOR-M2 LRPT transmits three channels with 1 km per pixel, which ends up in a considerably higher picture.

The recording half is analogous; nonetheless, the decoding is a little more advanced, so I let the raspberry-NOAA-v2 automate the entire course of. You may also use SatDump.

Regardless of getting the photographs efficiently, they have been removed from excellent or perhaps a good seize. u/PhaseRay prompt a radiation sample subject with my antenna whereas I suspected the wirings.

He warned me concerning the peak of the antenna. I didn’t have a clue that this was an essential issue, so my data was too brief about how an antenna works. I began studying about it, and there have been two choices for my setup. First, to decrease my antenna, and second so as to add a reflector to my setup, however I selected choice 3: constructing a brand new antenna.

One of the essential steps in constructing an antenna is testing the antenna. There are numerous methods and strategies to check an antenna, like testing with VNA and even merely with trial and error. Sadly, I used to be very restricted on this step. Getting my palms on a VNA in Iran was very exhausting and costly, and I even could not go along with trial and error as a result of there have been only a few passes per day, and I had not another reference sign in that frequency.

So I had just one choice left, simulation. There may be an previous and in style antenna modeling system named NEC. It’s not straightforward to be taught, however when you’ve realized it, you are saved.

I selected 4nec2 and began modeling my present V dipole antenna. It turned out that including a reflector to my antenna at 80 cm from the bottom would give me a fairly good antenna sample in some instructions. So, in principle, a few of the passes (based mostly on their path within the sky, from my viewpoint) will end in good captures.

So I fastened some wiring points, added a reflector to the V dipole antenna, and waited a couple of days to see if I may seize such a go, and it labored.

And as anticipated, this seize was simply an exception due to its path, and on the similar time, different passes with the identical elevation weren’t good.

Earlier than I transfer to the following half, let’s take a look at how wiring and optimization are essential:

With the assistance of the 4nec2 and the alicja.space design for his or her 435 MHz antenna, I calculated the numbers for the 137 MHz mannequin and adopted the earlier poor-man-friendly design to a turnstile to keep away from 3d printing. Observe that my model of the antenna may be very soiled and worse than the unique alicja.area design, simply to simplify the method and use the identical choc blocks and water pipe (sure, I am very lazy).

The brand new turnstile antenna benefited from a wider antenna sample, which helped me scale back the noise and nulls within the captures.

The outcomes have been stunning, even with out an LNA:

Later I added a Nooelec LaNA LNA, which I purchased for double the value and waited three months for it to reach; sure, having this stuff in Iran is difficult. (and I foolishly broke it not too long ago)

After three months, I made a decision it was time for Cubestats and telemetry information. So I modified my turnstile antenna for 435 MHz (the precise mannequin of alicja.area).

Earlier than seeing the outcomes, let’s speak about Cubesats and 435 MHz. On 137 MHz, we had 5 or fewer satellites transmitting climate pictures. Whereas it’s spectacular to have a look at satellite tv for pc pictures above you, area satellites have way more to have enjoyable with. In contrast to the 137 MHz, there are dozens of satellites that transmit information on ~435 MHz, principally Cubesats and principally telemetry information (no extra pictures, unhappy). So I didn’t have to attend till dawn to check my setup. One hour after ending the antenna, I acquired my first sign.

The small sign bursts within the above image are digital frames that must be decoded, however how?

Think about a community of beginner radio hobbyists linked to look at the completely different satellites and share the info. Now make all of it computerized (even higher!), It’s SatNOGS.

SatNOGS presents open-source software and hardware for floor stations to make their station computerized and even connect to their network. They’ve a very good database of satellites and their standing and transceivers.

They’ve a prepared ISO picture for RPI. You write it to an SD Card, join your dongle to RPI, join the RPI to the Web, insert the token you generated from their web site, and also you’re completed!

If you create a station within the SatNOGS community, you specify your antenna’s working bands and your station’s location. Then the community will robotically schedule the passes that your station will be capable to obtain and will get again the consequence out of your station. You’ll be able to take a look at some of the successful observations from the network.

I made a decision to make use of their system to automate issues, however because the Iranian authorities may be very strict in radio comms, connecting a floor station to a international community could have extreme penalties. They merely received’t perceive or care (which turned out to be proper once they arrested me the final October due to researching Web censorship); they are going to rely it as spying.

Anyway, thankfully, the SatNOGS community itself is open supply, so I launched a non-public occasion on my house server and linked my RPI to it. It labored completely!

The above seize is a few telemetry information from the GRIFEX satellite tv for pc, which incorporates info just like the satellite tv for pc’s reminiscence standing, CPU standing, Energy standing, and so on.

SatNOGS even have a dashboard for satellites which have a public decoder for his or her telemetry. You’ll be able to see the most recent standing of those satellites based mostly on observations from the SatNOGS community worldwide.

Till now, all the mentioned satellites have been orbiting the LEO orbit, which means their location relative to Earth continually modifications and strikes. So you need to calculate when they are going to go above you, their path, and their max elevation above the horizon. A better elevation means a greater go.

However as you already know, there are satellites which might be within the GEO orbit. Their location relative to Earth is fixed, and we see them in a set place. In 2018, QO-100 supplied the primary beginner radio transponders within the GEO orbit, protecting from Brazil to Thailand. QO-100 is VERY greater than LEO satellites like CubeSats (500 km VS 35000 km), and its downlinks are working within the X band, so clearly, no extra omnidirectional antenna; we want a dish antenna.

Happily, since QO-100 is within the GEO orbit, we don’t want to maneuver the antenna (in contrast to the upper bands for LEO satellites like NOAA HRPT, the place you need to level a directional antenna on the transferring satellite tv for pc). We want a daily satellite tv for pc dish and a customized LNB to obtain the downlink.

QO-100 receives narrowband alerts on 2400.050–2400.300 MHz and relays it again on 10489.550–10489.800 MHz (2401.500–2409.500 MHz and 10491.000–10499.000 MHz for wideband). Our LNB receives the downlink sign and downconverts it by 9750 MHz so we are able to get it on RTL-SDR.

For this satellite tv for pc, I wanted much less crafting and extra spending. I ordered a suitable LNB (additionally for double the value and with two months of ready) and put in it on my LNB. Achieved! Oh wait, I forgot one thing…

LNBs must be powered; this one wants 12–19V, so RTL-SDR V3’s 4.5V can’t energy it on. Even worse, to change the LNB’s polarization, you have to change the voltage. To inject energy into the cable with out destroying the sign or your dongle, you have to inject the facility in a method and block the DC present within the different manner (to the RTL-SDR). This energy injector is called BIAS-TEE, and I forgot to purchase one, and it was not obtainable in Iran, and I had not the boldness to construct one, however I needed to.

It’s a easy circuit. Whereas blocking the DC from going to the RTL-SDR, it should enable the RF to go.

So I learn this post and created one with poor high quality (RF circuits are very delicate, so you need to deal with them rigorously, otherwise you’ll get no sign).

I powered the circuit, pointed the dish, and it labored!

You’ll be able to hearken to QO-100 on-line here.

See Also

MAIM | Wildergarden