Small UAS Are Effective Remote Sensing Tools at Low Altitudes
Note: We are using this analysis for justification of airspace for small UAS. Please send feedback ASAP to make for the most robust analysis.
Remote sensing will be a major capability used in a variety of markets once commercial UAS operations are allowed. Using data from current commercial cameras and UAVs we can give a general profile of a UAS remote sensing mission (maximum altitude, coverage rate). For this analysis we use two sets of data:
- AUVSI Robot Directory:a listing of over 4000 robotic platforms including detailed physical specifications. Access available on a subscription basis. Your subscription directly supports the maintenance of this valuable data set and future analyses like this one.
- Go Pro HERO3+ Black Edition: Go Pro is a commonly used camera (~$400 retail) with readily available specifications on their website. We are not recommending this or any other specific camera. Feel free to use the below calculations for the camera of your choice. Notable specifications:
- Weight: 2.6 oz (4.8 oz with housing)
- Pixels: 4000 x 3000
- Frame rate: 15 fps at 4000 x 3000
- Pixel Size: 1.55um x 1.55 um
- Focal Lengths: 14, 21, and 28 mm
Disclaimer for data used in this analysis: Note that these are just example numbers for general reference, when developing/purchasing a system, you should do a more detailed analysis for your specific need.
What are you looking for?
All analysis that follows stems from what you are looking for. Let's give a couple of examples, you can tailor the analysis based off your mission need:
- Counting cattle: The body of a bull at maturity is around 40cm (16”) wide by 100 cm (40”) long (reference). Assuming at least a 2x2 set of pixels on each bull, a pixel resolution of 8” (20cm) is needed. For reference below, we will use 6" (15cm) x 6" pixel size.
- Corn health monitoring: For V3-V6 growth monitoring (reference), leaf size is not very large so a pixel size of 0.5” (1.3cm) or 1” (2.5cm) will be used.
- Power line inspection: An electrical power transmission line is about 4” in diameter (reference). A 0.5” pixel resolution would put 8 pixels on the line, likely enough to see if a single aluminum strand has broken off.
- For reference, we will also use 2” and 12” and 1, 10, 100 and 1000 mm.
Calculating Maximum Altitude
Slightly modifying a basic lens equation (read here for more details and related equations you may find useful) gives us a way to calculate maximum altitude:
Altitude / Focal Length = Pixel Resolution / Pixel Dimension
Altitude = (Pixel Resolution*Focal Length)/Pixel Dimension
Using the parameters above we can calculate maximum altitude to achieve specific pixel resolutions.
Field of View (FOV)
Calculating FOV is straightforward, a direct multiplication of the pixel resolution with the total pixels.
For the rest of the calculations, we need to bring in data from the Robot Directory (http://robotdirectory.auvsi.org). A previous analysis outlined some physical characteristics for small UAS. For this analysis, we use average data for a VTOL and hand-launched UAS. We used only platforms with:
- payload capacity >=5oz
- MGTOW <= 100 lbs
- Complete data on endurance, maximum speed, maximum altitude, and MGTOW
Leaving 92 platforms as a representative sample. Certainly there are hundreds more platforms that meet the needs of remote sensing missions, but this sample is adequate for our analysis.
The reasoning for these bins is forthcoming in a paper on a new taxonomy for UAS. Note that no hand launched UAS were greater than 40 pounds. The heaviest hand launched UAS is 23.6 lbs. It seems no one who flies a UAS is also into caber tossing.
Coverage rates (square miles per hour) and total coverage is calculated using resolution, number of pixels, endurance, and speed. Results are below. We add in an efficiency factor to account for power and time consumption included in takeoff, landing, turns, and differences between max speed and speed at maximum endurance.
Calculating Coverage for Given Endurances
The above numbers use averages for various weight classes. If we assume the maximum speed and efficiency factors presented above but vary endurance, we can plot endurance vs total coverage.
Note that the two graphs are identical except 2” resolution has 4 times the coverage. This makes sense since 2” resolution is one quarter of 0.5” resolution.
Commercial Applications of This Data
Let's look back at our earlier scenarios:
- Counting cattle: The needed 8” resolution allows a maximum altitude beyond the average max altitude so let’s use 2” resolution, that way we can count calves too. Note that one square mile equals 640 acres. Assuming a ranch size of less than 1280 acres or two square miles (average size varies greatly location to location), a rancher could map his/her entire farm on a single flight from any hand launched UAV or a VTOL UAV greater than 8.8 lbs.
- Corn health monitoring: This tasking requires 0.5” – 1” resolution. Assuming a similar farm size as above, <1280 acres, all but the smallest hand launched and VTOL UAS meet the needs of covering the entire farm in a single mission.
- Power line monitoring: US EIA has an interactive map showing major power transmission lines throughout the country. The needed resolution (0.5”) can easily be achieved by a variety of UAS. However, transmission line (and pipeline) inspection is unique due to the narrow and long strip of land that must be monitored. To better see small UAS utility, let’s look at total distance traveled. Assuming the aircraft will take off and land from a single spot, total distance traveled equals half of the endurance times max speed. Adding in the efficiency factor and we get the chart below. This mission type, where a platform has to traverse long distances, may prove to be better suited for larger UAS but the mission can be completed by small UAS.
Through this analysis we have shown two important points:
- Using standard commercial cameras UAS need to fly at lower altitudes
- At these low altitudes, UAS can effectively complete many commercial missions
From this and using the point that small UAS are much cheaper to operate, we can make a major conclusion:
- Many commercial operations do not need UAS to fly at high altitudes. High being altitudes that may affect current manned commercial operations.
A couple of items of note:
- Business models on using this new capability for remote sensing will be determined by market forces: We are not attempting to push a particular business model here, only to show the utility of small UAS at low altitudes.
- Efficiency factor: This factor will vary between systems and between missions, understand details of your system and mission.
Download the spreadsheet sheet containing all our calculations here.
Please provide feedback to make this analysis better.