Segment of the first Meteor M-N2 image successfully received
The Russians recently (2014) launched the second in the line of Meteor M weather satellites. Meteor M-N2 transmits Low Rate Picture Transmission (LRPT) format pictures round the clock on 137.100Mhz. This format is of higher resolution that the older APT format.
There have been two excellent people helping with software and advice on how to receive images from this satellite. These are Raydel Abreu Espinet, CM2ESP, and Oleg Bekrenev.
Oleg has provided a program to process the captured signal and produce images like the one you see above. Raydel has provided a GNUradio program to capture the signal.
I don't intend to go into detail about what each program does or why as I don't have time but purely to describe how I received the images and offer pointers to Oleg's website where more information is available. The confusing aspect of receiving these images is that there is more than one way of doing it.
So, firstly my equipment:
- Antenna - homebrew QFH antenna
- Pre-amp - old Microwave Modules 144Mhz preamp
- Receiver - RTL-SDR dongle
- Computer running Ubuntu 14.04, xfce windows manager
The strength of the signal from the satellite is not strong and I did not receive a good signal until I added the pre-amp. The RTL dongle has the necessary bandwidth, it may be possible to use an original Funcube Dongle Pro but this only has 92khz of bandwith and this may be too narrow. I am sure a Funcube Dongle Pro+ will work very well. Not only does this have 192khz of bandwidth but the FCD's are both very much more sensitive than the RTL dongles. Other SDR receivers will also work of course.
So, first thing to do is optimise your receiving set-up and adjust the gain of your SDR device using a known signal source. I use London Heathrow ATIS as this transmits all the time. You need to find the sweet spot of your dongle between LNA gain and IF gain, to maximise gain without overloading the SDR. This is crucial, I copied an 88 degree overhead pass on Sunday and only received a short amount of picture. The next day the image you see here was at about 66 degrees elevation and I got very good results. I have a feeling that finding the sweet spot of the gain controls might be difficult.
Once you have optimised your settings use a program like GPredict or your favourite satellite tracking program to identify suitable Meteor M-N2 passes.
Use Raydel's GNUradio soft symbol decoder to capture the data stream from the satellite and decode to a format that Oleg's image processing program can understand. Raydel's instructions (and a link to the GNUradio program are here, note that this document covers several different method of receiving and decoding the signal).
The received constellation should look like this is you are receiving the signal correctly.
Once the soft symbol.s file has been received then it is time to open Oleg's LRPTOffLineDecoder program to produce the image. This is a Windows program but runs well under Wine in Linux.
The image above is a part of the window of Oleg's program. If you have captured the signal using the GNUradio program then the processing of the data follows these steps (I think it is different if you have captured the signal in a different way):
- Click the "72k" button (as the satellite seems to be sending data at a symbol rate of 72k but it has also sent at 80k recently).
- Navigate to the place on your hard drive where the .s output file from the GNUradio program is stored, select the file and click ok.
- You should see the two progress bars start to move and the constellation of the received signal show in the top left hand side of the window.
- This step can take some time.
- If the image can be decoded the infrared/visible channels received will be shown to the right hand side.
- Click "RGB" to generate a false colour image, you need to select the RGB drop down lists to correspond to the wavelengths that were received otherwise you will just get an all white image. You can also change the RGB settings to change the colouration of the image. At this point you can also save the image.
Oleg's website has other examples of images received plus links to the software etc.
The picture below is the full image I received on 16/9/14. The white bar occurs every 6.5 minutes and is a programming error of the satellite code that apparently cannot be resolved.
This is not a soup to nuts description of how to receive these images. You will need to read the instructions produced by Raydel in particular but it should enable you to capture these images quicker than the several weeks that is has taken me to learn how to do it.