Watching from space if the ferry is on time
The challenge of observing change with satellites
Nowadays, the world is sensed with more satellite sensors than ever before. Especially, with the Dove satellites of Planet. With such a constellation of 190+ satellites, one easily starts to wonder about new applications. The possibilities might seem to be endless to you. But nature and society behave in different ways. Processes can be continuous, periodic or chaotic and this distinction makes the difference between success and failure. Therefore I give some handholds into the process of formulating new opportunities with earth observation systems. Because not every earth observation system fits the idea you have in mind. This text will highlight some aspects of change happening on the earth’s surface and how this can be sensed with different satellite configurations.
There is all kind of change occurring on the earth’s surface. To keep this piece short, I will only highlight analysis of qualitative change. That is, the displacement of an object or the deformation of the earth surface. Another type of change, which I will not cover, is surface property change or quantitative change. This is for example the greenness of vegetation, which changes over the season, or the coverage of the terrain with snow in winter.
On the nature of the natural
Natural change can be different dependent on which phenomena you look at. Some processes have a faster pace then others: glaciers creep down from mountain sides which can take years, while clouds can sweep overhead within minutes. So if you are interested in observing a certain phenomena you should keep in mind what its temporal behavior may be. However, not all change is continuous, it can be possible that a phenomena changes from one state to the other overnight and comes to rest again. Hence, phenomena can be grouped into three different behaviors; continuous, chaotic or episodic.
To clarify what is mend with duration and occurrence, examples of short lived natural events with different behaviors are given below. One continuous but changing force is the tidal loading on bays and estuaries. The mass of the moon and the sun pull the water towards themselves while they circle around the earth and by doing so, cause continuously displacement of water around the world. This effect is especially clear in a bay in Australia, where two small natural corridors exist. Between every cycle of ebb and flow (thus episodic), vast amount of water is pushed through the small alley. The whitewaters that are created are known as the horizontal waterfalls, and their tidal sequence can be seen by the Planet satellites:
Because, of the small funnel, this phenomena occurs twice a day, but does not take a lot of time, thus it is short-lived. Another phenomena coming from the same force can also been seen from space, it is much more rare and also short lived, it is a tidal bore:
A tidal bore is an estuary with a funnel-like entrance. Normally the water from the river flows into the ocean. But when the tide comes in, the force balance between in- and out-going stream flips. This flipping of forces results in a standing wave, that propagates up-river. The occurrence of this event happens twice a day, but its magnitude depends on the alignment of the moon and the sun (in respect to the earth). With high-tide the tidal bore is big and fast enough to surf upon!
The examples given above can be predicted to some extent. This is less the case for processes that are chaotic. For example the case below:
This is a dust cloud coming from the Piz Cengalo, in a short period of time four million cubic meter of rock came loose. The resulting rock resulted in a mud flow that caused several casualties and considerable damage in the down valley village of Bondo in the Swiss Alps. Authorities monitored this site but were unable to predict how much and when this part of the mountain would come loose. It is simply dependent on too many factors and hence a complex system, having a therefore a chaotic behavior.
From these examples, it should be clear that certain events do not need to take long, in order to be observable. However, it is necessary to know if the system is predictable. If so, a second piece of information is of importance: the timing of the sensing.
Matching a signal with a system
The second point to understand is the orbit earth observation satellites are flying in. Most of these satellites fly in a type of orbit that is sun-synchronous. This means that, every time the satellite passes over, its local time on the ground is similar to the last time it was sensed. A benefit of this property is a stable casting shadow, as the sun orientation is from the same angle. This helps with photo interpretation.
The local time of sensing depends on latitude and the inclination of the satellite orbit. To compare different sensors the equatorial crossing time is taken as parameter. The second axis is the revisit time, another comparable property for an observation system. This property is the time the satellite sees the same place again, also known as cadence. For Landsat, this repetition is twice a month. Any other system which wants to have a better revisit rate needs to deploy multiple satellites (Sentinel-2A/B, RapidEye, ..) or reduce the resolution of the instrument (Terra, Aqua, Sentinel-3, …). Another strategy to increase the observation capabilities is to equip the satellite with steer and stare capabilities. However, a system can only look at one place at the time. These system specifications are organized in the figure below.
Now these parameters can be used to match the satellite system with the phenomena you want to observe. For example with the tidal bore, the magnitude and the timing of the bore needs to overlap with the crossing time of the Dove satellites. But it are not only natural phenomena that have regularity, also in the human world there is episodic behavior, which can be sensed from space.
An example: the Arctic Circle ferry
When you have the opportunity to go to Norway, try to visit Helgeland. Route 17 follows the coastline full of mountain emerging out of the ocean, and beaches have sunsets that lasts for hours. Within this route, there are in total six rides by a ferry. One of them is crossing the Arctic Circle. Fortunately, when this ferry crosses the arctic circle, at the same time the Dove cubesats are overpassing as well:
At first sight it is clear the ferry is not at the same spot at every acquisition. However, the Doves have no propulsion, so are affected by differential drag. This results in some variation in acquisition time of some minutes, as can be seen in the annotation. However, still the ferry does seem to be spread considerably. But before we can say something reasonable, we might first have an idea how fast the ferry goes.
By coincidence, at one of the overpasses of a Dove, the ferry was just in the overlap of two images. Each dove as it flies-by takes every half a second a picture. These snapshots have a small overlap, and this can be seen in the figure on the left. When we look at the displacement of the ferry, which is about 3 pixels. Then the ferry is moving 18 meters per second (3*3/.5). Which is again equal to 65 kilometers per hour, or 40 miles per hour. So taking this into account, the clockwork of the ferry system seems to be very accurate!
Get your own timesheet!
While most satellite observation systems are designed to monitor continuous signals occurring on the Earth’s surface, other fast moving signals will become observable. With a higher cadence system, such as Planet, it might be possible to sense other systems which are episodic in nature and can be of importance for discovering, monitoring or inventories.
Another interesting aspect is the difference in time between the RapidEye constellation and the Doves. Both have a similar resolution, but the RapidEye constellation crossing type is approximately 1.5 hours later than Flock 3P. So if there is activity at a site this movement can easily be detected through change analysis, or visual inspection. Think for example of containers or airplanes. This is also the strength of the SkySat constellation, which has an overpass at several times a day. A selection of these overpasses are shown below. So now you are able to cross mate your time sheet with that of the satellites!
Further reading
Kääb & Leprince, 2014. Motion detection using near-simultaneous satellite acquisitions. Remote Sensing of Environment
