LBA repairs

Well, as reported, the jänis had struck twice and that meant lots of repair work for us. Actually in total there were 14 x LBA aerials to fix.

Status on 27-Aig-2014

The storm damaged ones were the most work. There required fitting new connectors (the old ones had been ripped clean off during the impacts). The jänis-attacked LBAs were more numerous, but less time consuming.

Repaired LBA line (Photo: D. McKay-Bukowski)

The fix was adding a new length of nylon guy line. A double-fisherman's bend is used to join the new section of line to the old and a bowline is used to make the loop to hook over the peg.

To discourage the jänis from simply biting through these again, we smeared the lines with a combination of spray lubricant and grease. This is waterproof and should give a bad smell and texture, thus deterring further jänis attacks.

However, in the long-term, we'll still need to work out a better way of protecting the LBA aerial lines.

Air-conditioning fixes

Recently we've had some problems with the KAIRA RF-container air-conditioning again. The problem this time has been that the refrigerant has been too cold and has lost pressure. This has taken a little while to resolve, but we now have fixed the problem. This has been done by heating the airconditioner!

Yes, that's right, we have put in a heating coil which is variably heated according to temperature. Thus, in the bitterly cold winter nights, this coil warms up sufficiently to keep the partial pressure of the refrigerant at sufficiently high pressure to allow the compressor to continue working. It is a tough fix to make, but we hope to have improved reliability again.

Servicing the air-con unit at KAIRA. (Photo: D. McKay-Bukowski)

Toivo explains the changes and the new heating system. (Photo: D. McKay-Bukowski)

KAIRA frozen

Yes, it is the Arctic, but for the most part, KAIRA copes with the cold. Having a lot of warm signal processing electronics and computers keeps the inside of the RF container pretty toasty. However, in the early hours of this morning, we came unstuck.

Due to a computer failure, we scheduled a 5 minute UPS power cycle to reset the station. In the end the outage lasted a total of 7 minutes. Following the re-establishment of power, we did not recover the cooling system. Our suspicion (although we are still working on testing and checking this is that during the outage, the load on the air-conditioning dropped, thus allowing the low outside temperatures (approx. -35 deg C) to adversely drop the pressure of the refrigerant. This caused an air-conditioning failure.

Outside temperatures. The UPS was shutdown at approx. 0400 on 20-Jan-2014.

The irony is that, when the power was re-established, the internal computing and electronics overheated. Made even worse by the near-zero humidity, giving little heat transfer capacity to cycling air. The final error message before the system self-shutdown to protect itself indicated an internal temperature of 35 deg C... some 70 deg C greater than the outside ambient!

Within an hour of this shutdown, the temperature will have plummeted again.

We have checked the site today, but there is not a lot we can do without major work. Our best chance is actually to wait for some forecast warm weather ("warm" means > -25 deg C), which is expected later this week.

In the meantime we are off air awaiting the better conditions.

Such are the hazards of Arctic operation.

The repair in the data

Over the last few days, we've been reporting on the damage and repair work to KAIRA LBA aerial L17. This story started with the data and so that's how we'll end it. The image for today is the data trace from the day of the repair. Yes, the data acquisition was running throughout this time, so it is possible to see (in the radio spectrum) the repair taking place.

If you look carefully, you can see distortions in the data in data after the point when the aerial was re-erected. This was due me being right next to the aerial during that time as I attached the nylon line and O-ring and then did up the knots and attach the cable ties. In total the whole process took about 5 minutes... excluding the time before re-righting the aerial, as I prepared the various items.

What this does show, though, is that people walking around the aerials do make a difference! Thus wandering on to the field during observations is not without consequence.

However, more significantly for us is that the discovery and repair were made on the 1st August... over a month since the aerial fell. Of course one aerial in 48 is not going to be a dramatic effect in the beamformed data, which is what we normally look at, but still it is a concern that we didn't notice this earlier.

While there are some procedures that exist for the main LOFAR network to test data integrity, they are not really suitable for the operations at KAIRA. However, we are now devising some new software to allow us to easily and automatically monitor the individual RCU data and thus hopefully detect such problems if they occur in the future.

LBA repair work

Yesterday we showed the fallen LBA aerial #L17... the cause the mysterious step function in the data.The break itself had occurred in the elastic at the base of the northwest line. Subsequent examination suggests that this was premature deterioration (although an alternate hypothesis blames lemporidae).

Of course, upon discovering the problem, the aerial was quickly repaired. Without the exact replacement part, we had to improvise. And the photographs today show what we did.

The O-ring connects the line to the peg. Some degree of elasticity is required, as tension and flexibility are required... especially as ground movement can occur during the freeze/thaw cycle. The knot is a clove hitch and three half hitches. The line then goes back to the top. (Photo: D. McKay-Bukowski)

At the top, a knot connects the nylon to the dipole wire. Three cable ties and a fold back hold the remainder of the nylon line to provide two securing points and keep it neat. (Photo: D. McKay-Bukowski)

The repaired LBA aerial (Photo: D. McKay-Bukowski)