C-band Sats – Part 5: 125E ChinaSat 6A, 128.5E LaoSat 1 (?), 132E Vinasat 1

This part of my satellite hunting adventure looks at ChinaSat 6A, what I suspect to be LaoSat 1 and Vinasat 1.

125°E ChinaSat 6A

ChinaSat 6A has a very similar role to ChinaSat 6B, carrying a large number of carriers mostly featuring provincial China TV stations with many free to air. It does provide service to Australia, although the beam pattern is noticeably weaker here. This satellite is particularly notable as one that suffers from a helium leak, resulting in pay-outs from insurance agencies due to reduced operational lifetime. Judging from the expectations, it might have another two years or so left of stationkeeping fuel.

It stays within the standard C-band, in fact, remaining entirely within the range of one-cable-solution type LNBs for good measure. The BLScan report is as follows:

TP N: 1
Frequency: 3721.115 Mhz
Symbol rate: 27499 KS
Polarization: Horizontal
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -42 dBm
Signal/Noise: 11.0 dB
Carrier width: 37.124 Mhz
BitRate: 38.014 Mbit/s
----------------------------------------
TP N: 2
Frequency: 3741.136 Mhz
Symbol rate: 27499 KS
Polarization: Vertical
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -40 dBm
Signal/Noise: 11.7 dB
Carrier width: 37.124 Mhz
BitRate: 38.014 Mbit/s
----------------------------------------
TP N: 3
Frequency: 3761.068 Mhz
Symbol rate: 28799 KS
Polarization: Horizontal
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -40 dBm
Signal/Noise: 10.9 dB
Carrier width: 38.879 Mhz
BitRate: 39.811 Mbit/s
----------------------------------------
TP N: 4
Frequency: 3781.121 Mhz
Symbol rate: 27499 KS
Polarization: Vertical
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -38 dBm
Signal/Noise: 13.0 dB
Carrier width: 37.124 Mhz
BitRate: 38.014 Mbit/s
----------------------------------------
TP N: 5
Frequency: 3800.991 Mhz
Symbol rate: 29499 KS
Polarization: Horizontal
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -39 dBm
Signal/Noise: 11.8 dB
Carrier width: 39.824 Mhz
BitRate: 40.779 Mbit/s
----------------------------------------
TP N: 6
Frequency: 3821.042 Mhz
Symbol rate: 29999 KS
Polarization: Vertical
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -36 dBm
Signal/Noise: 11.9 dB
Carrier width: 40.499 Mhz
BitRate: 41.470 Mbit/s
----------------------------------------
TP N: 7
Frequency: 3846.146 Mhz
Symbol rate: 17777 KS
Polarization: Horizontal
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -39 dBm
Signal/Noise: 14.2 dB
Carrier width: 23.999 Mhz
BitRate: 24.575 Mbit/s
----------------------------------------
TP N: 8
Frequency: 3858.158 Mhz
Symbol rate: 24999 KS
Polarization: Vertical
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -38 dBm
Signal/Noise: 10.1 dB
Carrier width: 33.749 Mhz
BitRate: 34.558 Mbit/s
----------------------------------------
TP N: 9
Frequency: 3867.119 Mhz
Symbol rate: 5424 KS
Polarization: Horizontal
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -43 dBm
Signal/Noise: 12.8 dB
Carrier width: 7.323 Mhz
BitRate: 7.499 Mbit/s
----------------------------------------
TP N: 10
Frequency: 3885.141 Mhz
Symbol rate: 5719 KS
Polarization: Horizontal
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -43 dBm
Signal/Noise: 11.7 dB
Carrier width: 7.721 Mhz
BitRate: 7.907 Mbit/s
----------------------------------------
TP N: 11
Frequency: 3889.134 Mhz
Symbol rate: 9989 KS
Polarization: Vertical
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -40 dBm
Signal/Noise: 13.3 dB
Carrier width: 13.485 Mhz
BitRate: 13.809 Mbit/s
----------------------------------------
TP N: 12
Frequency: 3894.159 Mhz
Symbol rate: 6879 KS
Polarization: Horizontal
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -41 dBm
Signal/Noise: 12.2 dB
Carrier width: 9.287 Mhz
BitRate: 9.510 Mbit/s
----------------------------------------
TP N: 13
Frequency: 3910.113 Mhz
Symbol rate: 8933 KS
Polarization: Horizontal
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -40 dBm
Signal/Noise: 11.7 dB
Carrier width: 12.060 Mhz
BitRate: 12.350 Mbit/s
----------------------------------------
TP N: 14
Frequency: 3913.150 Mhz
Symbol rate: 9899 KS
Polarization: Vertical
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -39 dBm
Signal/Noise: 13.2 dB
Carrier width: 13.364 Mhz
BitRate: 13.685 Mbit/s
----------------------------------------
TP N: 15
Frequency: 3923.114 Mhz
Symbol rate: 7249 KS
Polarization: Horizontal
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -38 dBm
Signal/Noise: 12.6 dB
Carrier width: 9.787 Mhz
BitRate: 10.022 Mbit/s
----------------------------------------
TP N: 16
Frequency: 3934.100 Mhz
Symbol rate: 6590 KS
Polarization: Horizontal
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -37 dBm
Signal/Noise: 13.5 dB
Carrier width: 8.896 Mhz
BitRate: 9.110 Mbit/s
----------------------------------------
TP N: 17
Frequency: 3941.094 Mhz
Symbol rate: 29499 KS
Polarization: Vertical
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -33 dBm
Signal/Noise: 11.8 dB
Carrier width: 39.824 Mhz
BitRate: 40.779 Mbit/s
----------------------------------------
TP N: 18
Frequency: 3952.089 Mhz
Symbol rate: 13399 KS
Polarization: Horizontal
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 7/8
RollOff: 0.35
RF-Level: -35 dBm
Signal/Noise: 13.7 dB
Carrier width: 18.089 Mhz
BitRate: 21.611 Mbit/s
----------------------------------------
TP N: 19
Frequency: 3970.120 Mhz
Symbol rate: 11579 KS
Polarization: Horizontal
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -36 dBm
Signal/Noise: 11.8 dB
Carrier width: 15.632 Mhz
BitRate: 16.007 Mbit/s
----------------------------------------
TP N: 20
Frequency: 3972.074 Mhz
Symbol rate: 13399 KS
Polarization: Vertical
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 7/8
RollOff: 0.35
RF-Level: -34 dBm
Signal/Noise: 12.1 dB
Carrier width: 18.089 Mhz
BitRate: 21.610 Mbit/s
----------------------------------------
TP N: 21
Frequency: 3985.113 Mhz
Symbol rate: 3166 KS
Polarization: Vertical
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -37 dBm
Signal/Noise: 10.4 dB
Carrier width: 4.275 Mhz
BitRate: 4.378 Mbit/s
----------------------------------------
TP N: 22
Frequency: 3990.155 Mhz
Symbol rate: 9069 KS
Polarization: Horizontal
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -37 dBm
Signal/Noise: 12.0 dB
Carrier width: 12.244 Mhz
BitRate: 12.538 Mbit/s
----------------------------------------
TP N: 23
Frequency: 3993.133 Mhz
Symbol rate: 9579 KS
Polarization: Vertical
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -34 dBm
Signal/Noise: 11.9 dB
Carrier width: 12.932 Mhz
BitRate: 13.243 Mbit/s
----------------------------------------
TP N: 24
Frequency: 4000.058 Mhz
Symbol rate: 4419 KS
Polarization: Horizontal
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -38 dBm
Signal/Noise: 12.3 dB
Carrier width: 5.966 Mhz
BitRate: 6.110 Mbit/s
----------------------------------------
TP N: 25
Frequency: 4007.084 Mhz
Symbol rate: 4419 KS
Polarization: Horizontal
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -37 dBm
Signal/Noise: 11.7 dB
Carrier width: 5.966 Mhz
BitRate: 6.110 Mbit/s
----------------------------------------
TP N: 26
Frequency: 4014.121 Mhz
Symbol rate: 4140 KS
Polarization: Horizontal
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -37 dBm
Signal/Noise: 11.9 dB
Carrier width: 5.589 Mhz
BitRate: 5.723 Mbit/s
----------------------------------------
TP N: 27
Frequency: 4041.078 Mhz
Symbol rate: 30599 KS
Polarization: Horizontal
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -28 dBm
Signal/Noise: 12.2 dB
Carrier width: 41.309 Mhz
BitRate: 42.300 Mbit/s
----------------------------------------
TP N: 28
Frequency: 4069.073 Mhz
Symbol rate: 9899 KS
Polarization: Horizontal
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -38 dBm
Signal/Noise: 11.9 dB
Carrier width: 13.364 Mhz
BitRate: 13.685 Mbit/s
----------------------------------------
TP N: 29
Frequency: 4101.088 Mhz
Symbol rate: 27499 KS
Polarization: Vertical
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -38 dBm
Signal/Noise: 11.8 dB
Carrier width: 37.124 Mhz
BitRate: 38.014 Mbit/s
----------------------------------------
TP N: 30
Frequency: 4121.083 Mhz
Symbol rate: 27499 KS
Polarization: Horizontal
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -39 dBm
Signal/Noise: 11.7 dB
Carrier width: 37.124 Mhz
BitRate: 38.014 Mbit/s
----------------------------------------
TP N: 31
Frequency: 4132.125 Mhz
Symbol rate: 14799 KS
Polarization: Vertical
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -41 dBm
Signal/Noise: 8.9 dB
Carrier width: 19.979 Mhz
BitRate: 20.459 Mbit/s
----------------------------------------
TP N: 32
Frequency: 4146.149 Mhz
Symbol rate: 8329 KS
Polarization: Vertical
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -42 dBm
Signal/Noise: 9.9 dB
Carrier width: 11.245 Mhz
BitRate: 11.515 Mbit/s
----------------------------------------
TP N: 33
Frequency: 4149.132 Mhz
Symbol rate: 9579 KS
Polarization: Horizontal
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -42 dBm
Signal/Noise: 9.9 dB
Carrier width: 12.932 Mhz
BitRate: 13.243 Mbit/s
----------------------------------------
TP N: 34
Frequency: 4161.093 Mhz
Symbol rate: 9579 KS
Polarization: Horizontal
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -43 dBm
Signal/Noise: 7.8 dB
Carrier width: 12.932 Mhz
BitRate: 13.243 Mbit/s
----------------------------------------
TP N: 35
Frequency: 4173.058 Mhz
Symbol rate: 9899 KS
Polarization: Horizontal
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -43 dBm
Signal/Noise: 11.8 dB
Carrier width: 13.364 Mhz
BitRate: 13.685 Mbit/s
----------------------------------------
Total Scan Time = 688.413s

There are almost as many carriers as there was on ChinaSat 6B, although as expected, the SNR was quite borderline on some of them due to the reduced beam power and the limited size of my dish. Interestingly, it seems all carriers use the DVB-S mode rather than the more advanced S2, possibly to ensure cheaper equipment can tune in.

The service summary seems to show a few encrypted services in high definition – as if to limit HD reception to those who have paid (possibly) or to prevent people from pirating their material in high quality. One of the carriers seems to be carrying data service. Other than that, the other notable thing is that a number of services are encoding in AVS+ which is China’s own video encoding format.

Interesting Signals

The spectrum in horizontal and vertical polarity, respectively. On the whole, it seems the DVB-S carriers the card found are practically the only things using the satellite.

Beacons

At the high end of the band in the horizontal, a modulated beacon was found. The frequency is approximately 4193.142902Mhz. It appears to be the regular AM-type with sub-carriers. Because of the strength, it seems hard to pick, but I’d say the subcarriers are at 28khz and 48khz. Intermodulation effects seem to cause the other spurious results – the difference is 20khz (resulting in the inner weaker line M1) and the sum is 76khz (resulting in the weaker outer line M4).

Zooming in, we can see the LNB drift itself. The inner sub-carrier seems unmodulated, the outer is likely some FSK/PSK or similar. Some other intermodulation can be seen further out too.

Changing over to the vertical polarity, a strong beacon is seen at around 4200.136268Mhz, AM type with a single thin subcarrier at about 48khz.

It was a very drifty-day for the LNB so I’m not able to comment on absolute frequency, but a weaker beacon is spotted about 460khz below, seemingly too weak (?) to be sure that it is from this slot.

In the low end of the band, I spotted a sign of a beacon in the vertical around 3701.611394Mhz. This had unusually weak subcarriers as well, which appear to be about 48khz and 72khz.

128.5°E LaoSat 1 (?)

This next satellite was a bit of a chance discovery when maneuvering the dish away from ChinaSat 6A and looking for Vinasat 1. Instead, I found the presence of some weak signals which appeared to be a satellite, so looking at the charts, it seemed LaoSat 1 was the most likely candidate but I can’t be sure. LaoSat 1’s footprint does not cover Australia and as expected, no services could be locked. LaoSat doesn’t actually carry many services that we know of, at least, in the C-band.

Looking at the spectrum, there are a few fairly strong-looking carriers in the horizontal. Flipping back and forth, it seems that I might have got the skew quite out-of-whack as the strong peaks in the horizontal come through on the vertical in an attenuated fashion, implying that there is cross-polar interference. These peaks reside in the low end of the IF, or the high end of the C-band RF, which is not where the known services are. In fact, it seems that LaoSat 1 only broadcasts in the extended C-band (sorry to those with OCS LNBs) probably to avoid adjacent satellite interference.

Instead, the known services are 3435V and 3638H which would place them at 1715Mhz and 1512Mhz. Looking closer, it seems the one in the H may well be visible to the right of the spike (likely a beacon) but the V one may well be hidden under the LTE noise. In the H towards the low-end of the extended C-band, there seems evidence of some wide carriers, but none were lockable.

Beacons

Not being able to positively identify the satellite by service didn’t stop me from at least sweeping around the band to look for beacons.

In the horizontal towards the top of the band, this appeared to be a CW beacon at 4200.727387Mhz. But knowing that LaoSat 1 doesn’t normally broadcast here … this might belong to another satellite.

Around the middle, toggling the polarity between vertical and horizontal, it seemed there are two beacon candidates spaced 660khz apart at 3752.663752Mhz (vertical) and 3752.003752Mhz (horizontal/circular?). But there seem too weak to be what I’m after.

Now, this seems to be what I’m looking for. Strong, in the vertical polarity, at 3700.1399Mhz (an expected place to find a beacon). But this beacon is particularly special variant of the AM type – it seems that this one changes subcarriers over time? For one, the 66khz subcarrier seems to be constant throughout. The inner subcarriers around 20khz and possibly 32khz seem to be discontinuous. When intermodulated with the constant subcarrier, it creates this pattern.

A telemetry cycle seems to last 25 seconds or so, split into about 6, 6, 12 and 1 second chunks. The first chunk seems to have unmodulated 32khz, the second has modulated 20khz, the third has unmodulated 20khz and the fourth has no inner subcarrier at all, while the outer 66khz subcarrier carries on throughout.

132°E Vinasat 1

Moving onto the Vietnamese Vinasat 1 satellite, I initially missed this satellite because it only broadcasts known services in the vertical polarity despite being capable of both. I normally align using the spectrum analyzer with LNB powered for the horizontal polarity, so without the carriers in view, I passed right by the slot! Once I realized my mistake, I managed to find it. I guess this is an equally valid lesson for anyone with a simple “beeper” style finder – you need to be in the polarity where there are services!

As this satellite only services in the extended C-band, I only scanned in that area. There seems to be some evidence of something in the horizontal, but I don’t know if that’s cross-talk from another satellite or spill-over from a beam that doesn’t cover me.

TP N: 1
Frequency: 3498.526 Mhz
Symbol rate: 19000 KS
Polarization: Vertical
Spectrum: Normal
Standard/Modulation: DVB-S2/QPSK
FEC: 3/4
RollOff: 0.35
Pilot: on
Coding mode: CCM
Long frame
Transport stream
Single input stream
RF-Level: -43 dBm
Signal/Noise: 5.6 dB
Carrier width: 25.650 Mhz
BitRate: 28.272 Mbit/s
----------------------------------------
TP N: 2
Frequency: 3548.279 Mhz
Symbol rate: 1999 KS
Polarization: Vertical
Spectrum: Normal
Standard/Modulation: DVB-S2/8PSK
FEC: 5/6
RollOff: 0.20
Pilot: off
Coding mode: CCM
Long frame
Transport stream
Single input stream
RF-Level: -45 dBm
Signal/Noise: 13.1 dB
Carrier width: 2.399 Mhz
BitRate: 4.956 Mbit/s
----------------------------------------
TP N: 3
Frequency: 3561.700 Mhz
Symbol rate: 1510 KS
Polarization: Vertical
Spectrum: Normal
Standard/Modulation: DVB-S/QPSK
FEC: 3/4
RollOff: 0.35
RF-Level: -48 dBm
Signal/Noise: 6.7 dB
Carrier width: 2.039 Mhz
BitRate: 2.089 Mbit/s
----------------------------------------
TP N: 4
Frequency: 3591.058 Mhz
Symbol rate: 19199 KS
Polarization: Vertical
Spectrum: Normal
Standard/Modulation: DVB-S2/8PSK
FEC: 2/3
RollOff: 0.20
Pilot: on
Coding mode: CCM
Long frame
Transport stream
Single input stream
RF-Level: -43 dBm
Signal/Noise: 6.8 dB
Carrier width: 23.039 Mhz
BitRate: 38.033 Mbit/s
----------------------------------------
Total Scan Time = 1567.428s

The BLscan only found four carriers, but that’s to be expected as the satellite is only known to be carrying three active carriers and other “occasional use” carriers.

The service summary is as above, and unfortunately, the first carrier just refused to give me any useful data because the SNR was too low. It seems that they did plan the frequencies just so that the LTE interference here didn’t wipe them out completely.

In some cases, even though the services were not lockable, I suspected a service may indeed be present and the IQ plot and SNR of 8dB is highly suggestive that this may be a data service (ACM/VCM) with idles.

Interesting Signals

As expected, all of the action is bunched towards the low frequencies (RF) in the extended C-band (hence to the right, being in the high frequencies (IF)). While I thought there was nothing in the horizontal, the spectrum analyzer begs to differ with a lot of spikes suggesting narrow data/satellite modems are using that polarity.

Beacons

Going beacon-hunting on VinaSat 1 was a bit of a mess. For one, at the low RF frequencies, we have the LTE interference to deal with.

This CW line was seen at 3406.651562Mhz in the vertical – maybe it’s a beacon? Or maybe it’s a spurious signal from the analyzer, or an uplinked pilot tone?

In the horizontal around 3700Mhz, there seemed to be a cluster of possible candidates … but are they? They seem way too weak … ah … that’s right …

… it seems they are in the vertical polarity. Two of them are strong, spaced 3Mhz apart. One weak one is spaced 1.5Mhz down, with a line spaced 2Mhz down. All of these figures seem quite deliberate, but I’m not sure about whether the latter two belong to this satellite at all.

The strong pair have virtually identical characteristics except frequency (3699.6377Mhz and 3696.634812Mhz). AM type with wide subcarriers, one at 49khz and the other at 74khz with about 8khz shift.

The other weak modulated one (3701.132979Mhz, vertical) has a wide subcarrier around 64khz, but I’m not willing to conclude it’s actually from VinaSat 1.

Conclusion

That’s another three satellites down – with one of them being a part mystery which is very likely LaoSat 1. It definitely has an interesting beacon compared to the others. Also, VinaSat 1 taught me a lesson to check which polarities are in use by the target satellite before going hunting for it … or risk passing right over it because you didn’t recognize the wide carriers you expected to see.

About lui_gough

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3 Responses to C-band Sats – Part 5: 125E ChinaSat 6A, 128.5E LaoSat 1 (?), 132E Vinasat 1

  1. darius says:

    Hi,

    do you mean helium-3 power fusion generator under development ?


    This satellite is particularly notable as one that suffers from a helium leak, resulting in pay-outs from insurance agencies due to reduced operational lifetime. Judging from the expectations, it might have another two years or so left of stationkeeping fuel.

    • lui_gough says:

      Um … no. Normally satellites are a rather conservative industry because it costs a lot of money to send a satellite into space, space itself is harsh (as is the ride to space) and you can’t fix anything that breaks when it’s out there. So fusion generators are not likely to be out there for these reasons alone, never mind the issues of nuclear-related things in space being governed by treaty.

      Normally, when broadcast satellites are concerned, their operating lifetime is often limited by the stationkeeping fuel. Such satellites “stay” in one spot in the sky (geostationary orbit) so as to enable ground users to used cheaper fixed dishes. However, the act of keeping a satellite in that same spot (or orbital slot) requires almost constant adjustment to compensate for drifting caused by gravitational influences, slight drag variations, maybe an occasional micrometeroite impact, etc. As a result, the satellite is sent up with a fixed reserve of stationkeeping fuel which is used to correct the drift under ground-station control (hence the need for telemetry and ranging signals). A certain amount of fuel is also kept to boost the satellite into a graveyard orbit at end of life (presuming no other major failures occur in-between). When the fuel gets low, an option is to let the satellite drift slightly in an “inclined” orbit. When the incline gets large enough, the signal at receiver dishes will fall enough to make the service unavailable for parts of the day. This allows a few extra years of revenue service, mainly for services with tracking dishes which can compensate.

      The stationkeeping fuel is often hydrazine but can also be liquid helium, etc. Some satellites also use other gases like xenon with an ionic thruster to increase the efficiency of the propulsion system by producing more momentum with the same amount of gas. Unfortunately, these have seemed to fail at unexpectedly high rates (Boeing XIPS failures – http://sat-nd.com/failures/index.html?http://sat-nd.com/failures/xips.html).

      Another use for liquid helium is cooling instruments on scientific satellites, but somehow, I doubt that is the actual use in the case of ChinaSat 6A.

      – Gough

  2. darius says:

    thank you
    I just read
    helium vs. xenon fed ion thruster discussion
    from
    https://www.quora.com/Can-we-use-helium-for-ion-thruster

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