Questions about receiving signals of Inmarsat I-4 Satellites

GNU Radio
1

Hi,

I am trying to receive signal from I-4 satellites of Inmarsat.

I found that I-4 satellites have one global beam with 25 KHz bandwidth
and
several regional beams with 50 KHz bandwidth. However, I have scanned
through frequency band 1525-1559 MHz and haven’t been able to find those
beams.

Anyone has experience of receiving Inmarsat satellites? Which downlink
frequency should I tune to?

I have tuned to frequencies listed on this website, still without any
luck.
http://www.uhf-satcom.com/lband/

Setup:

  • USRP N210 + WBX
  • LNA with 30dB gain (ZHL-1217MLN)
  • Iridium antenna (I am not able to get specific antennas for
    Inmarsat…Iridium is at 1616 -1626.5 MHz and Inmarsat is at 1525-1559
    MHz.
    I have used this antenna to capture signals around 1531 MHz
    successfully. I
    guess this antenna would not kill signals from Inmarsat.)

Best regards,
Cheng C.

2

I intend to make some tests with the antenna of my Inmarsat phone. When
the phone receives something, I keep the antenna position and simply
will plug the antenna to my SDR to see what happens. But I have no idea
when I will find some time for this test…

Ralph.

From: discuss-gnuradio-bounces+ralph=removed_email_address@domain.invalid
[mailto:discuss-gnuradio-bounces+ralph=removed_email_address@domain.invalid] On Behalf Of
Cheng C.
Sent: Friday, March 28, 2014 9:41 AM
To: GNURadio D.ion List
Subject: [Discuss-gnuradio] Questions about receiving signals of
Inmarsat I-4 Satellites

Hi,

I am trying to receive signal from I-4 satellites of Inmarsat.

I found that I-4 satellites have one global beam with 25 KHz bandwidth
and several regional beams with 50 KHz bandwidth. However, I have
scanned through frequency band 1525-1559 MHz and haven’t been able to
find those beams.

Anyone has experience of receiving Inmarsat satellites? Which downlink
frequency should I tune to?

I have tuned to frequencies listed on this website, still without any
luck. http://www.uhf-satcom.com/lband/

Setup:

  • USRP N210 + WBX
  • LNA with 30dB gain (ZHL-1217MLN)
  • Iridium antenna (I am not able to get specific antennas for
    Inmarsat…Iridium is at 1616 -1626.5 MHz and Inmarsat is at 1525-1559
    MHz. I have used this antenna to capture signals around 1531 MHz
    successfully. I guess this antenna would not kill signals from
    Inmarsat.)

Best regards,

Cheng C.

3

1531 MHz signals from what? If you are using a patch antenna then it
probably won’t have the bandwidth to cover 100 MHz.

Lou
KD4HSO

Cheng C. wrote

  • Iridium antenna (I am not able to get specific antennas for
    Inmarsat…Iridium is at 1616 -1626.5 MHz and Inmarsat is at 1525-1559
    MHz.
    I have used this antenna to capture signals around 1531 MHz successfully.
    I
    guess this antenna would not kill signals from Inmarsat.)

Best regards,
Cheng C.

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4

This is what I found for inmarsat :

http://i.imgur.com/KygwHdD.jpg

Some 200 kHz channels and some 50 kHz channels.

There is also some 30 kHz channels with a small ‘X’ periodically in
the spectrograme. Those are GMR-1 and probably leak through from
Thuraya (either reflections off buildings, or my antenna polarization
sucks). Or maybe inmarsat has some GMR-1 carriers, but I doubt it.

Cheers,

Sylvain
5

Did you use gr-phosphor for that plot?

Of course :slight_smile:

-F option of osmocom_fft enables it directly.

Cheers,

Sylvain
6

Did you use gr-phosphor for that plot?

7

Hi Sylvain,

May I know what’s the setup you used for receiving the signal? The
spectrum
looks very clear.

Best regards,
Cheng C.

8

Hi,

May I know what’s the setup you used for receiving the signal? The spectrum
looks very clear.

  • The antenna was a homemade helical antenna :
    http://i.imgur.com/c6JaySJ.png
  • It’s followed by a modified LNA23 (to be centered at 1550 MHz). This
    is a 30 dB LNA with a < 1dB NF with a built-in filter.
  • Capture was made with a bladeRF

Cheers,

Sylvain