L-Band

Alphasat 25°E (Inmarsat-4A F4) FFT.

There is a lot of interesting traffic on Inmarsat including AERO, frequency hoppers etc.

What can be found on the L-Band?

There is quite a bit of interesting stuff that can be explored on the L-Band. For example, you can find GPS satellite downlinks, Inmarsat downlinks, low-earth orbiting weather satellites that are transmitting high resolution weather images (HRPT), Iridium downlinks and more. 

What hardware do I need to receive L-Band frequencies?

You basically need four things: 

  • a receiver that covers 1 -2 GHz, I like to use SDRs (AirSpy or RTL-SDR)
  • a low noise amplifier that has a low noise figure and a good amount of gain on the desired frequencies
  • a bandpass filter that only lets through the desired frequencies (1.5 GHz to 1.7 GHz)
  • a suitable antenna (patch antenna, satellite dish + feed etc.)

If you’ve got all of the above you should be able to pick up L-Band satellites easily. 

Which LNAs can be used on the L-Band?

There are quite a few good options. Personally, I found the LNA4ALL made by 9A4QV to work well. It is a good idea to use two of them in series, although I would recommend to put a bandpass filter in between them to avoid amplifying unwanted signals. Another good option are the SawBird LNAs produced by NooElec. They are producing the SawBird+ for Inmarsat and GOES/HRPT reception. They all have an integrated filter which makes them a great value for the money spent. 

My LNA4ALL in a metal enclosure.

Which antennas can be used to receive L-Band frequencies?

The best antenna for the L-Band is a satellite dish. It has a lot of gain into the direction it is pointing. Due to that you get the best SNR on the wanted signal. 

Of course you need an antenna to “feed” the dish. There are multiple options depending on the polarisation of the signal. For circular polarisation (used on most LEO and GEO satellites) you should use a helical antenna feed, for linear polarisation it is recommended to use a cantenna or other similar waveguide antennas. Instructions for a helical antenna feed can be found here (credit: UHF-satcom).

Note: For offset dishes it is recommended to use a 7 – 8 turn helical antenna feed with 20mm spacing, but I found that 3 – 4 tuns with 40mm spacing work just as well if it’s tuned properly. Instructions for a 7 – 8 turn helical antenna feed can be found here.

My L-Band dish.

My L-Band feed.

What about L-Band LEO weather satellites?

Once you have a properly working L-Band setup you could try receiving weather satellites on 1.7GHz. All of the 1.7 GHz weather satellites are using right-hand circular polarisation for their downlinks, for the best result you should use a helical feed. You have to make sure that your L-Band filter and LNA also cover 1.7 GHz, else you won’t have much luck decoding any useful data. 

Another important thing to remember is that you need to be able to track the satellites in their orbit because they are constantly moving. The best option is to put your satellite dish onto a rotator. Many people (including me) also attempted to handtrack these satellites – it works well once you have built up enough skill for tracking. 

usa-satcom sells an excellent HRPT decoding software called XHRPT (150$), but you can also try to demodulate and decode the signal in GNU Radio (free). 

NOAA-18 HRPT signal.

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