New Chinese linear satellite XW-3 and its issues

Update: I have a video version of this article with a demo of the sat as well Update: The passband center frequency may actually be higher than what is published in the XW-3 user manual. I am trying to confirm the actual center frequency with other sat operators. See this for more information:

On December 25th, while most of the space community was excitedly discussing the James Webb Space Telescope (JWST) launch of that morning, the amateur radio satellite community was also anticipating their own new satellite launching that evening.

On December 26th at 03:11 UTC, XW-3 (CAS-9), or HO-113, was launched on a CZ-4C launch vehicle from the Taiyuan Satellite Launch Center in China, piggybacked with the ZY-1(02E) earth observation satellite.

XW-3 is a 6U cubesat, approximately 10 kg, with a VHF/UHF linear transponder for CW and SSB. It also has the ability to take pictures as commanded by users on the ground, though the command hasn’t been released to the public yet (see the XW-3 user manual for more info, FYI also see this frequency chart PDF). The CW beacon, GMSK telemetry, and transponder automatically turned on within one minute of separation from the launch vehicle, and the satellite is now operational and has been used by many amateur radio satellite operators. It does however, have some issues.

I’ve seen a lot of reports that the CW beacon and/or telemetry has a very strong signal, but also a lot of reports that the transponder downlink is weak compared to RS-44 or the XW-2 series of satellites (XW-2{A,B,C,D,E,F}). Originally, there was speculation that XW-3 had high SWR, which could explain the weak downlink.

Also, see the SWR on the SatNOGS dashboard for XW-3 (it shows 6:1 SWR currently).

However, Mike Rupprecht, DK3WN discussed the issue with Alan Kung, BA1DU, of CAMSAT. Alan stated something that seemed to imply that the telemetry indicating high reflected power (causing high SWR) might be incorrect since CAMSAT uses the same software as other satellites, but they used a different SWR bridge on XW-3.

It’s possible that a 6:1 SWR would destroy the power amplifier if operated continuously, so the idea that this is just a bad calculation is very plausible.

In fact, that’s exactly what this is. In another tweet, Mike Rupprecht said the value is ridiculous and the antenna was actually tested to have an SWR of about 1.3. In the future he may update his telemetry decoding software to compensate accurately.

Eliminating high SWR as an explanation, there are three real differences between XW-3 and the XW-2 satellites which could explain why XW-3 appears weaker. While both satellites are specified to have the same output power of 100 mW, XW-2A has a VHF downlink whereas XW-3 has a UHF downlink, and there is more signal path loss on UHF than VHF. Additionally, XW-3 has a wider transponder than XW-2A (30 KHz vs 20 KHz), so that same amount of power is distributed over a larger passband. Finally, XW-3 is in a higher orbit than XW-2A (770 km vs 551 km) so there is also a longer path.

Another issue is that the receiver is not as sensitive as would be desired.

While it can be worked with 5 watts and an Arrow II portable antenna, it can be hard, though Paul Stoetzer, N8HM did it the morning after launch:

I was also on that pass and heard Paul weakly make contact with Doug Papay K8DP (formerly KD8CAO). I didn’t have luck hearing myself though, and I was occasionally using 30-40 watts with my Arrow II.

I have since successfully made contacts on the satellite, first on a 25 degree max elevation pass over the Atlantic where I used 70+ watts, and also on a 75 degree max elevation pass where I was able to use 25-50 watts and hear myself on the downlink.

In addition to the power/sensitivity issues, the downlink passband sometimes has a lot of noise, making it hard to hear signals. That issue has been reported to occur around TCA (max elevation) and a fluctuating AGC has been implied to be the culprit.

Though I don’t understand how that could be since in another report the AGC seems to fluctuate in the opposite way.

I share all this in hope that it helps others have success on this new satellite. If you have any information or tips that could help others, please leave a comment. Congratulations to the Chinese Amateur Satellite Group (CAMSAT) and thanks for all your hard work to launch amateur radio satellites into space!

Correction: Originally this said Paul N8HM made contact on the first pass over North America, in fact he tried and didn’t succeed then, but did succeed on the following morning.

Update: I clarified there is not actually a high SWR issue.

Update: Removed reference to a possible incorrect published transponder frequency issue as only one person reported it and I think it was a misconfigured computer software setting for inverting/non-inverting. Update: I and one other person have reproduced this so it may actually be an issue.

2 thoughts on “New Chinese linear satellite XW-3 and its issues

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