Drones and small satellites for next generation remote sensing
Introduction
Taking measurements from the Earth’s surface is by no means a new idea. The idea of remote sensing has been around for several decades. We should consider the imaging of the Earth in 3 resolutions; Spectral, Spatial and Temporal. The spectral resolution of an image was the growth area previously; the range of spectral bands combined with a ‘good enough’ pixel resolution meant that satellites could take new and interesting images of the earth. Very high resolution imagery (<0.3m) was previously acquired by aerial surveys, at considerable cost, in parts of the world that could justify this.
Today things are slightly different: Unmanned Aerial Vehicles (UAVs) are prevalent and able to achieve incredible detail and accuracy in the hands of skilled users. Small satellites sometimes referred to as ‘cube sats’ are being delivered into low earth orbits at vast numbers to address the third resolution, namely temporal. It is the growth of both UAVs and Small Satellites that we will discuss here.
UAVs
The Commercial UAV Show hosted the GeoConnect Show 2016 in London on 19–20 October. This came amid a flurry of news ranging from ‘drones for good’ (Figure 1) to the ‘here come the machines’ closing speech on Day 1 by Google’s Ed Parsons. The press spans from GIM International UAS Special Edition to US Dept. Homeland Security’s IoT fact sheet to name but a few.
And while oil refinery infrastructure surveillance by Cyberhawk was presented in 2013 or flight line mapping was touted in 2014 (Figure 2), two issues have emerged since that time.
Flight permission
Drone licensing and governance has proven to be difficult. In Kuwait five years ago, even working with the Air National Guard did not give permission to operate pipeline surveillance drones even though ground access was secured by the local distributor. UAV flight exemptions and new rules for line-of-sight flights below 150 m. /400 ft. are now detailed in the United Kingdom and the United States, for example. Drone pilot certification is offered in more and more locations, and pre-flight planning for aerial reconnaissance has been done since 2008 by British Antarctic Survey. That doesn’t solve the issue of the aerial extent of flight surveillance though.
Flight length
At the Commercial UAV Show, Boeing-owned Insitu pointed out how few fixed-winged drones there were. Recently near Brisbane, Australia, however, the Medical Express UAV Challenge required drones to cover in auto-pilot mode 23km each way to drop off a parcel, and most UAVs were fixed-wing with quadcopter enhancements. Quadcopters are currently all electrical, and depending on the size, flights are limited to under ½ hr. So fixed-wing with fuel motor are a must for any longer industrial use. Also, despite miniaturisation of on-board equipment, payloads are a key limitation — for example GPS and cameras fit in easily but LiDAR equipment is just coming on to the market. Advances in self-stabilizing gimbals, that let cameras and sensors point steadily in one direction, is just making these applications a reality today. But fixed-wing configurations allow for more precise gyroscopes and larger camera lenses and storage. And while line-of-sight real-time monitoring is available for smartphones, anything beyond takes you to industrial-grade piloting equipment toward the military realm. AGI Big 5 Data in October 2014 hosted a talk on UAVs where pilot cabins showed how drones were operated over-the-horizon (Figure 3).
Small Satellites
In 2014 the US government gave permission for satellite derived images of 25cm to be made commercially available. This has coincided with a vast increase in planned satellite launches; Euroconsult is predicting 1,450 launches of satellites > 50kg over the next decade compared with just 163 in total for the previous decade. September 2016 was a busy month for Earth Observation; Terrabella launched 4 more of its SkySats (Figure 4), bringing the total to 7. On the same launch vehicle (rocket) Peru Sat was launched; constructed in only 24 months it has already delivered its first image. Airbus announced that they plan to launch another 4 satellites to complement / replace / succeed their Pleiades satellites, and Planet (formally planet labs) launched 8 of its doves from the international space station.
These small satellites are cheaper to build, lighter and fly in a lower orbit than larger satellites (such as the worldview constellation) and this is causing a shift in temporal data. Small satellites are now attempting to provide high temporal resolution imagery. This data will mean that daily change detection will be possible. Spatial resolution of these satellites is in the range 1m-5m.
Planet hopes to have 100 doves in orbit by the end of the year. This, they claim, will lead to every part of the Earth’s surface being imaged every day. We are faced with a deluge of Earth Observation data. This data deluge is part of a perfect storm, namely that data storage costs are shrinking and machine processing is getting faster (GPU). These advances are driving and enabling companies like Orbital Insights to start analysing this type of data to derive analytics: construction change, counting cars in car parks and measuring oil storage in China.
UAVs vs. Small Satellites
A complementary pair
Small satellites and UAVs appear to be a natural pair. They can both complement each other’s data, and are stored in a similar fashion in a GIS. A study of a sand dune might be ideally suited to a UAV, or modelling/imaging a building. Deriving analytical data would be better suited to satellites who can guarantee temporal range and a large legacy of imagery in archive. In some climatic zones UAV’s can fly as to be unaffected by clouds, whereas optical satellites may well not get a clear picture for a long period of time.
Conclusion
The imaging of the Earth’s surface has changed and is changing. High resolution images, better temporal coverage, large decreases in data storage and advances in GPU computing are making a strong case for large and small scale projects to adopt the new imaging abilities. It is envisaged that the cost of data collection will decrease over time, so now seems to be an optimal moment to look at UAVs and small satellites for building up surface information systems.
Andrew Zolnai, zolnai.ca, and Andrew Cutts, ACGeospatial
