Basic Overview
For the longest time imaging using a UAV meant making sacrifices based on what sensor you had available. Often this meant mounting a camera in an unorthodox position and asking it to do things it wasn't made to do. This all changed with the GEMs sensor by Sentek System. This sensor is one of the first of it's kind, a sensor made specifically to be used with a UAV. GEMs stands for Geo-localization and Mosaicing System and it happens too do both quite effectively as you can see here. The GEMs manages to pack a dual 1.3 megapixel sensor into a very small form and includes GPS too geocode each image taken, an almost absolute must when it comes to effectively image an area. The Ground Sampling Distance or GSD is 5.1cm @ 400ft and 2.5cm at @ 200ft. These figures are important because with aerial imaging, location accuracy is key. Something that sets the GEMs apart from most sensors is the way images are stored. Most cameras use SD cards but the GEMs saves images to an external usb flash drive. Using a flash drive is nice because the cost associated with these is lower, they can write faster than most other storage formats and hold more data. Because the GEMs is lighter than most sensors (170 grams) and smaller it makes it possible to mount in even the smallest of spots but there are a couple of things to look out for and consider when placing the sensor especially on quadcopters. The biggest thing to remember is that the GEMs is meant to be placed downwards to the ground and flat to minimize distortion. Some other factors to consider is the amount of vibration the sensor experiences. If the sensor vibrates too much the accelerometers in the sensor can be thrown off, reducing the quality of your finished mosaic. Something else to watch out for is EMI or electromagnetic interference, luckily Sentek came out with a solution and includes shields on the sensor to prevent that from happening. When it comes to actually flying your mission and recording data there are some factors to consider. Each sensor is different and you have to find the perfect mix of height, speed of flight combined with your sensor specs to get the level of detail your project requires. When using mission planner software there are some things you need to input regarding sensor information. Fortunately, Sentek provides information pertinent to the GEMs for Mission Planning. (Figure A)
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| Figure A - Values to input in to Mission Planner for best results. |
There also happens to be a GEMs software package available for purchase or comes alongside with the GEMs sensor. The software takes your taken images from the usb drive along with GPS coordinates and according to Sentek Systems creates an orthomosaic of multiple forms including RGB, NDVI, NIR. In this case the GEMs software creates a georeferenced image which is great but is really only usable for measurements when GCPs (Field Activity 4) are used. Georeferenced Images
Using the Software
Using the software is pretty easy. Locate the folder where your images are located and insert them in to the GEMs software. The software automatically names the folder in this format - " Fight Data (Week=X TOW=H-M-S). Where X, H, M, and S are numbers specifying the instant that data collection began for the associated flight. X is the GPS week number of the starting instant. H, M, and S represent the hours, minutes, and seconds, respectively, into the GPS week of the starting instant." Using converters this can be turned in to an exact time frame. Once locating the folder click "run" and click "NDVI Initialization" Once this is finished running it can be imported into various programs such as ArcMap which is what we used to look at and work with the data. Once processed, bringing .tif images in to Arc allows you to match up the data you collected with a basemap and essentially covers the old imagery allowing you to see an up to date image of the area you imaged. This format is needed because .jpeg images do not include geolocations, essential for GPS accuracy. One thing to note with the data brought in using the GEMs is that the data does not have metadata associated with is so the user, in this case (me) has to go in and edit the metadata to preserve data integrity and provide information so others can see things such as when the data was collected and how it was collected.
Types of Images
The GEMs is unique in the sense that it doesn't limit you to just RGB and modded NIR. There are several formats included with the GEMs, RGB, NIR and NDVI.
RGB - Figure 1 - Represents true color, what the surface looks like with your naked eye.
Monofine Image - Figure 2 - The fine NDVI mono shows the reflectance levels as a high to low. The GEMS software does not assign a color scheme to this NDVI. Healthy high reflectance areas are white and low health is gray to black.
NDVI FC 1 - Figure 3 - Mainly used for monitoring vegetation, the red and orange representing vegetation and water content of that vegetation.
NDVI FC 2 - Figure 4 - Much like FC 1 except more visually appealing, where green represents vegetation much like the color of the vegetation. Red is min. NDVI while Green is max.
Normal NDVI Mono - Figure 5 - Black represents min. NDVI while White is max. this colorway is mostly used for finding small plant growth found in soil throughout a large area.
Flight Path - Figure 6 - This map shows the mission and path the Matrix Quadcopter took carrying the GEMs in order to image this particular area at the Eau Claire Soccer Complex.
Final Critiques
Anytime you purchase a first generation device you have to be willing to accept there may be flaws and often a high upfront cost, especially when there are not many other options available specifically for this application. The GEMs sensor is no different. Alternatives to the GEMs Sensor are cheaper, but are not necesarily created for UAVs yet people have found ways to use them, avoiding the high price tag of the GEMs. To purchase the GEMs you have to contact Sentek Systems directly and they will assist you with purchasing. As stated on their website the GEMs is 100% designed, tested and manufactured in the United States.
The price set aside the GEMs is a very capable sensor and does what Sentek claims it does. Although only 1.3 megapixel the images are clear and effective but for agricultural purposes this is good enough. I'm a little disappointed they did not include a better camera just knowing with how advanced phone cameras have gotten considering the size factor. Sentek markets the sensor for agriculture and as you can see in the processed images here, it will do an effective job. As competitors enter the market and availability increases the cost of sensors such as the GEMs will decline.
If you have the funds and do not want to fiddle with modifying other sensors to accomplish what the GEMs does it will serve you perfectly. The GEMs plugs directly in to the UAVs power and works fluidly. We did not have any issues with processing the data and it was very capable. I would recommend the GEMs Sensor as of right now and will revisit this review if I feel otherwise once processing data with the other sensors. It is a sensor made for UAVs and delivers what Sentek claims, the software it comes with pairs with the sensor and prepares the captured images in to .tifs in a timely matter.
Using the Software
Using the software is pretty easy. Locate the folder where your images are located and insert them in to the GEMs software. The software automatically names the folder in this format - " Fight Data (Week=X TOW=H-M-S). Where X, H, M, and S are numbers specifying the instant that data collection began for the associated flight. X is the GPS week number of the starting instant. H, M, and S represent the hours, minutes, and seconds, respectively, into the GPS week of the starting instant." Using converters this can be turned in to an exact time frame. Once locating the folder click "run" and click "NDVI Initialization" Once this is finished running it can be imported into various programs such as ArcMap which is what we used to look at and work with the data. Once processed, bringing .tif images in to Arc allows you to match up the data you collected with a basemap and essentially covers the old imagery allowing you to see an up to date image of the area you imaged. This format is needed because .jpeg images do not include geolocations, essential for GPS accuracy. One thing to note with the data brought in using the GEMs is that the data does not have metadata associated with is so the user, in this case (me) has to go in and edit the metadata to preserve data integrity and provide information so others can see things such as when the data was collected and how it was collected.
Types of Images
The GEMs is unique in the sense that it doesn't limit you to just RGB and modded NIR. There are several formats included with the GEMs, RGB, NIR and NDVI.
RGB - Figure 1 - Represents true color, what the surface looks like with your naked eye.
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| Figure 1 - RGB Image |
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| Figure 2 - Mono Fine Image |
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| Figure 3 - NDVI - FC 1 Image |
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| Figure 4 - NDVI - FC2 Image |
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| Figure 5 - NDVI Mono Image |
Flight Path - Figure 6 - This map shows the mission and path the Matrix Quadcopter took carrying the GEMs in order to image this particular area at the Eau Claire Soccer Complex.
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| Figure 6 - Flight Path |
Final Critiques
Anytime you purchase a first generation device you have to be willing to accept there may be flaws and often a high upfront cost, especially when there are not many other options available specifically for this application. The GEMs sensor is no different. Alternatives to the GEMs Sensor are cheaper, but are not necesarily created for UAVs yet people have found ways to use them, avoiding the high price tag of the GEMs. To purchase the GEMs you have to contact Sentek Systems directly and they will assist you with purchasing. As stated on their website the GEMs is 100% designed, tested and manufactured in the United States.
The price set aside the GEMs is a very capable sensor and does what Sentek claims it does. Although only 1.3 megapixel the images are clear and effective but for agricultural purposes this is good enough. I'm a little disappointed they did not include a better camera just knowing with how advanced phone cameras have gotten considering the size factor. Sentek markets the sensor for agriculture and as you can see in the processed images here, it will do an effective job. As competitors enter the market and availability increases the cost of sensors such as the GEMs will decline.
If you have the funds and do not want to fiddle with modifying other sensors to accomplish what the GEMs does it will serve you perfectly. The GEMs plugs directly in to the UAVs power and works fluidly. We did not have any issues with processing the data and it was very capable. I would recommend the GEMs Sensor as of right now and will revisit this review if I feel otherwise once processing data with the other sensors. It is a sensor made for UAVs and delivers what Sentek claims, the software it comes with pairs with the sensor and prepares the captured images in to .tifs in a timely matter.
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