By Duncan Steel 20/11/2019 2


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Satellites pass over NZ all the time (literally). Here I focus on the 187 Planet Labs ‘Dove’ Earth-imaging satellites, and I show that one can determine in advance where they will be, enabling scientists on the ground to correlate their environmental and other data collection with opportunities to get imaging from space. That is, we can get ‘space-truth’ (rather than ground-truth) for the various types of scientific  investigation underway in the wonderful natural laboratory known as Aotearoa/New Zealand. 

When you work in a certain area of science and technology, generally you soon become rather blasé with regard to the reality of what your discipline entails. Familiarity breeds contempt, so the saying goes.

This was brought home to me over the past month or so, when I was giving two talks in Auckland to two quite different groups of people. All of them were sophisticated, well-educated folk… and yet they all seemed to be blown away, mentally, when I showed them brief movies I had composed containing the orbital movements of large numbers of satellites. To me, this was second-nature: of course there are hundreds of satellites passing over New Zealand, and only a few hundred kilometres above our heads, on a daily – nay, hourly – basis. To my audiences, though, this was news… and yet they were undoubtedly all smart, even brilliant, high-achievers in their own fields.

This sort of experience is humbling, for me, because it brings it home to me that although I may know a fair bit about space and astronomy, and a few other things, like my audiences I remain ignorant of the vast majority of human knowledge. Indeed that is a word I try to remember whenever I am going to give a public talk, with my guiding principle being the three I’s: as a speaker I must assume that my audience is Intelligent (as all humans are, to varying degrees), Interested (if they are not, my task is hopeless; and of course one must make one’s talk interesting for them), but also Ignorant — we are all ignorant of almost everything, and that is nothing to be ashamed about. Similarly, when I attend a public talk or research seminar to be given by someone else, I can gain most if the speaker makes the same assumptions: if the talk is about biochemistry, or ancient history, or 20th century novels, please start off by assuming my three I’s, most especially the third one.

A parallel to all this is my feeling when I enter a library. I almost drop to my knees as I look around me, because I know that I will only ever be able to read and appreciate (or profit from) a tiny fraction of all those books. Again, a humbling experience.

Before going further let me write this: being described (as I do myself) as being ignorant is not a pejorative label. It does not mean the same as stupid. Remember my three I’s, the first of which is intelligent, the opposite to stupid. Let’s just be suitably self-effacing, and admit that we know next to nothing about almost everything. We are ignorant of most things.


So, to satellite orbits and the like. The two talks I mentioned above were given at the 2019 Summit of the India New Zealand Business Council (held on 14th October at the Pullman Hotel) , and also an event for business people organised by the MartinJenkins company and staged (on November 6th) at the wonderful Civic Theatre on Queen Street in Auckland (and ably organised by the dynamo known as Kevin Jenkins, and his remarkable team). With regard to the INZBC, various kind mentions were made in the media, such as here and here and here and here. I had previously written a blog here about India’s prominent role in global space activity.

If anyone would like to take a look at them, the two movies that I prepared for my presentations (6 minutes 40 seconds and 8 minutes long, respectively) are available for download here and here. I would be particularly happy if those were used for educational purposes; a quick watch of those brief clips could tell young viewers (and old!) a great deal about what is happening in space right now, and what prospects there are for the future. And there are many!

Seriously, please do take a look at those short movies, which are in this Dropbox folder.


Okay, onto the main subject of this blog post at last. Orbiting the Earth, just a few hundred kilometres above your heads, there are now many hundreds of satellites. Just saying that leaves the average reader unimpressed, because space is big, is it not?

So let me try to impress you. In San Francisco was established just a handful of years ago (at the end of 2010) a company named Planet Labs. The core group forming the company came out of NASA-Ames Research Center in Silicon Valley, where I have spent a lot of time over the last 30-odd years working on various space projects. Ames is next door, as it were, to Google LLC. I love working at NASA-Ames: there are many really smart people there, and so it is a motivating, buzzing place.

(In passing I note that in the past year or so it has been arranged that a handful of New Zealand students every year become NASA Interns, generally at Ames rather that some other NASA research campus. That’s great, a wonderful and perhaps life-changing experience for the few successful candidates. Both my sons – who, like me, are joint Australia-UK citizens [apologies] – were interns at NASA-Ames with my long-term friend and work colleague, Christopher P. McKay. My sons are, let’s just say, not so dumb.)

Back to Planet Labs. They have had launched hundreds of imaging satellites into low-Earth orbit (LEO). These satellites (termed Flocks of Doves) are 3U cubesats; a standard module cubesat measures about 10 cm on a side, and so has a volume of one litre, and the Planet Doves comprise three such units (3U) in a row, making a satellite that on launch is about 30 cm (one-foot) long. Then, in orbit, their solar cells are deployed, and so their cross-sectional area becomes a bit bigger, intercepting the solar flux needed to power their cameras and electronics. Here is a schematic of a Dove 3U cubesat with solar cells extended; and, yes, that is artwork on the reverse side of the solar cells (why not?):

Using such satellites Planet Labs can map the whole Earth (cloud cover allowing) at a pixel size or ground sampling distance of three metres every day. Here is an image of Tolaga Bay, obtained using such a Dove cubesat:

For information about the Cooks Cove Walkway – well worth the hike – see this DOC webpage. (Note that previously I had the yellow caption incorrect, hence the comment from Maggy Wassilieff at the end of this blog post.) 

Sciblogs is not the place for adverts, but if you do want to know about how to get access to such photographs, please contact the company for which I work; we are the NZ agents for Planet Labs imagery.


How does Planet Labs cover the whole globe daily? The answer is that they have a lot of Doves in low-Earth orbit. Here is a picture I have drawn up with all the individual satellites (187 being functional) having their formal names shown:

All those names make the picture a bit confusing, so let me instead show a picture of where the satellites will be (with no labels) at 10:30 am NZDT on Friday 22nd November.

Did you see what I did there? The preceding graphic showed where the Dove satellites were at a certain time back in August. You might say: that’s easy, saying where they were after the fact. But in the graphic immediately above I have needed to derive where those satellites will be about 36 hours after I write these words.

Now, that’s a different thing. Imagine that you are a scientist monitoring lakes, or crop growth, or snow pack mass, or a pollution event, or shipping traffic, or… Might it not be useful to know when satellites are going to pass overhead, so that you can correlate your own data collection (of whatever type) with the satellite imagery that could be collected synchronously? It’s easy if you know how (and have some decades of experience).

Above I gave links to two movies that I had made for presentations over the past month. The graphic immediately above, though, is essentially a frame grab at a certain future time (precisely 10:30 am on Friday next) showing the locations (green dots) and orbital paths (green lines/curves) for the 187 Planet Labs cubesats in question. That movie – a bit simpler than the other two – shows just the motions of those satellites over a four-hour interval, between 09:00 and 13:00 NZDT on 22nd November (NZDT being 13 hours ahead of UTC, Universal Time, as shown on the above graphic and in the movie).

So, please enjoy the movie (56 MB). It is just 48 seconds long, and it shows those 187 cubesats (in bright yellow, now) as they pass over NZ (mostly) on Friday morning. Just imagine what uses we could put to the imagery they collect… and they do this every day.

 


2 Responses to “Earth’s artificial rings”

    • Thanks Maggy, you are entirely correct, and I appreciate you pointing this out. Mea maxima culpa. I should not have made that error, because I have visited both locations. Regarding the true ‘first steps by Lieutenant James Cook in Aotearoa’ site, see this webpage (and I guess we should forgive the photograph at its head, showing the silhouette of the statue of an Italian admiral, rather than Cook).