By Duncan Steel 25/12/2019


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Using knowledge of when the Sentinel-1A radar satellite was due to pass over New Zealand, a team at Christ’s College laid out specially-constructed radar retroreflectors across Hagley Park in Christchurch, in that way painting the Southern Cross and the Pointers in a huge array clearly visible in the derived radar image of the city. 

A special – but brief – post for Christmas Day (with more details to follow in a later post).

The image in the header above shows the Southern Cross (Crux Australis) and the Pointers (Alpha and Beta Centauri) painted large across Hagley Park in Christchurch, by Dr Andrew Taylor and a team of students from Christ’s College. Here is a wider view of the centre of Christchurch:

Now, this is actually a radar image collected by the European Sentinel-1A satellite as it passed over New Zealand. So, from where did the Southern Cross and Pointers appear?

The answer is that Taylor and his team fabricated metallic retroreflectors, and then positioned and oriented them such that a strong radar signal would be bounced back to the satellite. Clever, eh?

In the above image, the weaker radar reflections appear dark, scaling up through dark to bright green, then yellow, and then red. This scaling was deliberate, so as to match the colours of the stars on the NZ flag (though with the fifth star added, as in the Australian flag). Obviously the buildings in the city centre were strong scatterers of the radar waves, though the artificial retroreflectors out-powered them.

And what about the large red area at lower right in the above image? The strong scattering back to the satellite there is from buildings and other structures along Ferry Road and – here is the important bit – aligned with the angle of that road. That is, the buildings in central Christchurch are arranged on an east-west and north-south grid, but along the angle of Ferry Road the radar echoes are stronger because it happens that the angle closely matches that which the Sentinel-1A takes over the Earth below. The buildings then act more like mirrors than simple scatterers, producing a stronger signal and therefore the red patches above.

The retroreflectors positioned across Hagley Park, laying out the Southern Cross and Pointers depiction, were calculated in accord with the locations of the stars in the sky, as in the map below:

This, I hope all would agree, was a wonderful exercise in live science for the Christ’s College students, producing a magnificent visual result. Dr Taylor also had the various images collected and processed by him and the team printed up as Christmas cards, with all proceeds in support of the Christchurch City Mission.

As noted earlier in this blog post, I will say more about what was involved in this stellar, spacey effort in a later article. But right now, Christmas dinner awaits with my lovely hosts Brian and Robyn Anderson, and their friend Margot.