Every smartphone uses an orbital technology whose infrastructure cost USD 12 billion to build, develop and launch. The first system was paid for by the US taxpayer, and now GPS is available for free across the world (Russia, China, India and Europe have built or are building their own GPS systems). Used first by the US military to keep track of every military asset from tanks and planes to soldiers on hundreds of bases in dozens of countries across the Earth, this technology became known as the Global Positioning System (GPS).
How Does GPS Work?
This constellation allows of GPS satellites allows for trilateral calculations that pinpoint your location anywhere on earth, due in part to predictable orbits and ground stations that constantly check and verify correct positions of each satellite. The satellites constantly broadcast a radio signal that the GPS receiver in your phone or navigation system captures. The signal travels at the speed of light and contains data including the exact time the signal was sent. Since the signal was sent at the speed of light, and each GPS satellite is outfitted with an atomic clock, the receiver can easily calculate the distance between itself and the GPS satellite.
Distance = Velocity of 299792458 meters per second x (Time Received – Time Sent)
The calculated distance represents a radius on the circumference of a sphere (the satellite signal’s geometry) starting at the location point of the satellite in orbit from exactly where the signal was sent. This calculation is repeated by three other satellites in distinct locations, and the intersection of each sphere yields the exact position on Earth in longitude and latitude.
Tracking Box Turtles with Innovative Open Hardware
In the open access journal Hardware X, a paper entitled “An open-source hardware GPS data logger for wildlife radio-telemetry studies: A case study using Eastern box turtles” was published recently by animal spatial ecologists Matthew Cross and Patrick Cain.
In the paper, the researchers discussed how useful GPS telemetry technology is to animal spatial ecology studies, but GPS loggers used to track different animals are expensive. GPS loggers can cost researchers from USD 2000 - 8000 dollars for each device, which limits the number of animals that can be outfitted and tracked in a sample size.
Cross and Cain set out to develop a “low-cost, customizable, open-source hardware GPS logger for use in animal movement studies” for use in field tests in northwestern Ohio with Eastern box turtles (terrapene Carolina). Devising a low-cost GPS logger has obvious benefits for researchers currently engaged in projects and would enable more studies to occur with less concern about budgetary limitations.
In wildlife biology, radio-telemetry allows researchers to collect information on species that would be very difficult to obtain using first-person observational techniques. A downside to using radio-telemetry to track focal species is the disruption to the normal behavior of the focal animal when measurements are required at a high frequency. GPS technology greatly advanced the ability of researchers to bypass these disruptions and enabled them to collect huge swaths of animal location data at differing spatial scales no matter how treacherous the condition of a focal animal’s environment became at any time.
The data gathered by remote observation of a focal animal outfitted with a GPS logger gave researchers the ability to infer more answers to questions about migration, home range size, resource selection, corridor mapping, human-wildlife conflicts and movement ecology.
The costly nature of GPS loggers creates statistical problems and constricts the quality of conclusions taken from a focal species because it only allows for restrictively small sample sizes.
Creating Low-Cost DIY GPS Loggers
In the paper, Cross and Cain classified low-cost GPS loggers into two categories: do-it-yourself (DIY) and modified commercial GPS loggers. DIY GPS Loggers are built from scratch and are advantageous in that they provide a higher degree of customization and cost-less than modified commercial ones. Modifications to commercial GPS loggers are typically enacted to increase the battery-life.
They first created a DIY prototype, dubbed “the original series” (TOS), and then created another DIY prototype with expanded memory they called “the next generation” (TNG).
Finding the Right Hardware
Cross and Cain chose lightweight components with a vertical profile and small footprint for the Eastern box turtles they would be tracking. Components of the TOS included a microcontroller, GPS module, transistor, on/off switch and a lithium ion battery. The TNG logger has the same microcontroller and lithium ion battery as the TOS, but in place of the other components is a custom-designed PCB with solder pads for connecting to the GPS module with surface mounted transistors, a JST battery connector and a memory IC.
They chose the Arduino Pro Mini as the breakout board for the Atmega28P-AU, which is widely known for its high degree of customizability, low power usage and faster learning curve due to its open source nature which comes with a prolific online community to reference.
At USD 15.95, the researchers used the GP-20U7 (56 Channel) receiver, which has a 2.5-meter circular error probable (CEP) accuracy, similar to commercial handheld units. Unlike the commercial units, which require users to connect their batteries to a power source for recharging or pay the company who sells them for replacement batteries, Cross and Cain purchased a 400mAh lightweight and small lithium ion battery for USD 4.95.
Instead of adding a USB or micro SD port, the researchers used an FTDI adapter for uploading and downloading code, which takes up less space and consumes less energy when writing to micro SD cards.
Testing the TOS and TNG loggers on Eastern box turtles
The researchers then deployed the TOS and TNG loggers on a set of 20 and a set of four box turtles. The loggers collected points during a variety of weather conditions and through different ecological habitats and collected more variation in the turtles’ movements than commercial handheld GPS loggers.
Though the TOS and TNG loggers consistently recorded points every three hours, two of the TOS devices recorded a few identical locations. The accuracy of the GPS loggers varied depending on the turtles’ habitat type, cloud cover and canopy cover.
The native memory of the Arduino Pro Mini allowed the TOS loggers to store 83 points of latitude, longitude, minute, hour, day and month. The TNG loggers were able to store about 16,000 points.
Bottom Line
Spending less than USD 50 for each DIY GPS logger versus USD 2000 – 8000 means that for every 10 GPS loggers built, tested and deployed, animal spatial ecology researchers save USD 19,500 – 79,500.