M2-2 Image, above, captured by G8GKU at 15:25 July 31 2019, false colour. Orbit No 375. At 137.9 MHz OQPSK with 72 k symbol rate. Same image below in ” visible ” monochrome.
Along with the new Meteor M2-2 satellite images came the challenge of OQPSK in place of the QPSK utilized by Meteor M2.
The immediate situation was to have no working demodulation or decoding system for OQPSK.
Discussion with Les Hamilton revealed the option of using tools as found in Les’s pre-packaged archive tool sets. Please see http://leshamilton.co.uk/MeteorLRPTSuite.htm
A ” Thank you ” has to go to Les for assembling the various sets of tools.
As always, things are not so simple as may be thought. The tools chain as above is centred around using the LRPT Demodulation within SDR# as to be found in the archive.
Due to the quantity and strength of 137.x MHz data and ” pager ” signals in Oxford, it is not workable to use the well tried and tested RTL Dongle receiver. As G8GKU is located close to Oxford, a hybrid system was used to facilitate the reception and decoding of M2-2 images.
An M2 image article within this website has a description of the receiver chain used by G8GKU.
The three key components which allow G8GKU the reception of ( mostly ) unbroken images are as follow. Other methods exist and the use of the following products is merely descriptive as opposed to an endorsement to purchase.
First, after the crossed dipole RHCP aerial, is the use of a specific signal pre-amplifier, the Mini-Circuits PGA-103+ device. A Data sheet may be found on the Mini-Circuits web pages. There are other devices by other manufacturers but the PGA-103+ is excellent for the task. It has a very low noise figure and an impressive degree of strong signal handling, resulting in low, if at all, cross-modulation of the weak M2-2 signal by the local heavyweight data transmitters. The device has to be embedded within a circuit to be actually used, the device is not ” plug and play “, unless one has the evaluation module version. There is a useful article by G4DDK and WA5VJB, detailing the building of such an amplifier with the PGA-103+. See http://www.g4ddk.com/PGA103+2.pdf
To ensure maximum signal is obtained, the aerial is real-time tracked in azimuth and elevation with data obtained from Alex’s DDEtoSerial driver, to be found elsewhere in this web site.
Secondly is the use of twin screened 50 Ohm coax, between the pre-amplifier and the receiver position, silver plated coax screen is highly recommended.
Thirdly is the use of the SDR-Play Duo receiver. This receiver has a front end design which allows repetition of weak signals in the presence of strong close in band signals at an offset of only a few kHz. This is exactly the situation when receiving 137.x MHz images by G8GKU.
The complete signal chain for M2-2 uses the above for reception of the M2-2 signal and the received data is stored as a 16 bit I.Q. WAV file.
At this stage the tools within Les’s archive come into play. The SDR# application is fired up and the recorded dot.wav file selected as the signal source. The LRPT demodulator plug-in within SDR# is used to demodulate the OQPSK. Resulting in an ” s ” file, which is then decoded by the LRPT decoder within the tools set archive. There are options to be set in SDR#, the demodulator and the decoder, all of which are self descriptive.
By using this hybrid system the break up and loss of many images due to strong local close in-band interference has been very much reduced. Acknowledgement is made of all software names and such items.