Yesterday, December 23, MELCO carried out some in-orbit tests of the Es’hail 2 Amateur radio transponders. Since Es’hail 2 is currently under commissioning, it was expected that at some point the Amateur transponders would be activated for testing, but no announcement of the tests was done in advance. At around 11:00 UTC, Rob Janssen PE1CHL, noticed that the narrowband transponder was active and a carrier signal was being transmitted through it.
Since then, I monitored most of the tests and sent updates on Twitter, together with other people (see also the posts in the AMSAT-DL forum). Without knowing the details of the test plans, we limited ourselves to watching and following the tests that were being made. If some schedule of the tests had been published in advanced, we could have thought, prepared and performed some interesting measurements on the downlink signals.
I understand that since these tests are carried out by MELCO, AMSAT-DL might not have the specific details, but still I think that AMSAT-DL is publishing very little information about Es’hail 2 events. It was only at 22:35 UTC that AMSAT-DL published a small note on Twitter about the tests. I think the greatest concern is that people start transmitting through the transponder, interfering with the tests. However, since news spread very fast these days through social media, I think that publishing more information rather than keeping things discreetly serves better to prevent people from using the transponder during the commissioning. In any case, I’ll repeat it here:
Es’hail 2 is currently under commissioning. The 2.4GHz uplink of the Amateur transponders should never be used until authorized by AMSAT-DL. The Amateur transponders will sometimes be enabled for in-orbit testing by the MELCO/Es’hailSat/AMSAT-DL engineers. Relax, sit back, and watch the tests on the 10GHz downlink.
I also think that publishing more information would be beneficial to educate the community of radio Amateurs. Some people have asked me about the concept of in-orbit tests. After a satellite is launched into orbit, the performance of all its systems is tested to ensure that it matches design specifications, simulations, and pre-launch tests done on ground. This is important to guarantee that any problems, malfunction or damage that occurred during the launch can be diagnosed and hopefully mitigated by activating backup systems or other reliability measures. In-orbit testing of large satellites can take several months, since there are many complex systems that need to be tested remotely.
In the case of the Amateur radio payload of Es’hail 2, MELCO is carrying out the tests, since the payload was built by MELCO according to the design specifications by AMSAT-DL. The kind of tests they are performing are related to the performance of the bent-pipe transponders. They sweep in frequency the transponders to make sure that the passband shape is as expected. They transmit carriers of different power levels to check for linearity of the transponder and AGC performance, and they try different gain/power level settings of the transponder power amplifier to make sure it performs correctly over all its working range.
This is a rough account of the tests that were made yesterday, using my tweets as a sort of activity log.
There are two Amateur transponders on Es’hail 2: the narrowband transponder and the wideband transponder. The narrowband transponder has an AGC and is designed for narrowband signals such as SSB, CW, etc. Nominally, it is 250kHz wide. The wideband transponder has no AGC and is designed for DVB-S/DVB-S2 and other wideband data signals. It has a nominal bandwidth of 8MHz.
The narrowband transponder downlink is at 10489.550 – 10489.800 MHz, using the vertical polarization. The wideband transponder downlink is at 10491 – 10499 MHz, using the horizontal polarization. Note that the design choice for this is to put the downlinks as close as possible to the edge of the 10.45 – 10.5 GHz Amateur satellite X-band (perhaps to make it easier to use unmodified Ku-band LNBs), and to use different polarizations so that IMD from the wideband transponder won’t disturb the narrowband transponder.
The firsts tests done yesterday involved the narrowband transponder. It was switched on and a carrier was transmitted on the central frequency of the transponder. Note the transponder noise, which shows the shape of the transponder passband. It doesn’t have steep skirts, so it is much wider than the nominal 250kHz. However, two beacons will be placed at the nominal edges of the transponder, marking the usable area of the transponder, so operating at the skirts of the transponder won’t be allowed.
Note that different power levels for the carrier are tested and when the carrier is stronger the AGC acts, so the transponder noise is reduced. Without AGC action, I had the transponder noise 7dB above my receiver noise floor. This is with a 95cm dish which is pointing to 26ºE (instead of 24ºE, where Es’hail 2 is currently stationed). PE1CHL indicated a transponder noise of 15dB with his 80cm dish. If this is representative of the final transponder settings, this means that a small dish (say 60cm) will be more than enough for the narrowband transponder. Keep in mind that once the transponder noise is above your receiver noise, nothing is gained by using a larger dish.
The narrowband transponder was tested several times, as can be seen in the tweets below.
The tweet below shows a very strong carrier being transmitted through the transponder. Note the effect of the AGC on the transponder noise.
In the afternoon, the wideband transponder was tested. I was away from home for some time, so I only caught back around 19:00 UTC. The tests involved sending a strong carrier through the transponder, trying increasing power levels. The strongest signal was almost 50dB SNR on my receiver (over a bandwidth of 76.3Hz, apparently). According to the documentation, the EIRP of the transponder near the centre of coverage is around 38dBW.
After this, they tested the transponder frequency response by stepping a carrier in frequency through the transponder passband. The weaker signal in the images below is just an IQ image.
Then they moved on to testing different gain or power settings on the power amplifier of the transponder. Note the differences in the noise floor of the transponder. Depending on the setting, it is visible on my receiver or not.
This kind of tests continued until 00:00 UTC approximately.
After this, the transponder has been off during the night and it seems that no tests have been carried out today, December 24.
2 comments