Global Navigation Satellite System Wildlife Tracking Collar Positioning Accuracy

1. What they did

Global Positioning System (GPS) collars let researchers follow animals without being in the field, but the places animals live (under trees, below cliffs, in canyons) are the places where satellite signals struggle. This thesis asked how accurate wildlife tracking collars really are in those conditions. Chapter 1 explains how Global Navigation Satellite Systems (GNSS) work and where their errors come from, as a critique for wildlife researchers.

In Chapter 2, six Televilt Simplex collars were mounted on a pipe at fixed points in Connecticut whose true positions had first been surveyed to within 4 cm with a dual-frequency receiver. The sites ranged from open sky on a rooftop to 40%, 65% and 85% of the sky blocked by forest canopy, measured with hemispherical photos. The collars recorded a position twice an hour while their antennas were turned through eight angles, from pointing straight up to pointing down, for at least 24 hours each, and the pipe was turned north to south and east to west. In 2008 the test was repeated with the one collar that had performed well.

In Chapter 3, collars from three manufacturers (SirTrack, Advanced Telemetry Systems (ATS) and BlueSky) were tested on surveyed control markers in Torres del Paine National Park, Chile. A SirTrack collar was then carried, together with a survey-grade rover receiver, along “simulated animal movement corridors” in three habitats of increasing difficulty: open steppe, a scrubby canyon, and steep forested cliffs near a glacier.

2. What they found

Connecticut, 2007

• Only 6,743 of 15,863 scheduled positions were recorded, a 43% success rate. One collar recorded nothing and another under 10% of its schedule.
• Of 6,740 positions (after three extreme outliers linked to a failing battery were removed), 54% were within 15 m of the true point, 29.6% were 15 to 30 m off, 15% were 30 to 100 m off, and a few were 100 m to just over 2 km off. No position was ever closer than 2 m, even in the best conditions.
• Mean error rose with canopy: 10 m under open sky, 19 m under 40% cover and 70 m under 65% cover. Antenna angle mattered less (19 to 26 m on average).
• 72% of positions were 2-dimensional (2-D), with a mean error of 27 m; 3-dimensional (3-D) positions averaged 13 m.
• Removing all 2-D positions would have discarded 72% of the data. Removing positions with a Position Dilution of Precision (PDOP) above 5 discarded only 8% and took out 76 of the largest errors; positions above that threshold averaged about 51 m more error.

Connecticut, 2008

• The best collar recorded 92% of its positions.
• Error and missed positions both increased with canopy, with the antenna turned away from the sky, with 2-D positions and with higher PDOP.
• 58% of positions were 2-D (mean error 29 m), against 15 m for 3-D positions.
• All three models predicted mean error reasonably well, but individual positions varied too much to predict any one of them.

Patagonia

• On open control markers, the three collars differed significantly in mean error, but the differences were small for wildlife work. All three showed directional bias; the ATS collar's positions were the most tightly clustered.
• In the corridors, the SirTrack collar recorded 30 of 45 attempted positions: 21 of 30 on the steppe, 6 of 7 in the canyon and 3 of 8 in the cliff forest.
• Each corridor had one extreme error (395 m, 413 m and 528 m). With these, mean error was 38 m, 80 m and 186 m; without them, 20 m, 14 m and 14 m. The overall mean was 101 m with the outliers and 16 m without.

3. What they concluded

• The Televilt claim of 15 m for 90% of 3-D positions is true but misleading, because most positions in wildlife habitat are 2-D and less accurate.
• Dropping 2-D positions loses too much data and could make it look as though an animal does not use an area. The author suggests keeping them and showing each location as an error ellipse sized by its PDOP, rather than as a point.
• PDOP, which collars record anyway, predicted mean error almost as well as measuring canopy and antenna angle, making it a practical screening tool. Positions with a PDOP above 5 should be treated as highly suspect, though not automatically discarded.
• Accuracy depends more on what blocks the sky (trees, rocks, topography, or the animal itself, such as a bear digging with its head in a hole or a cat sleeping on its back) than on antenna angle.
• An animal's behaviour and choice of habitat therefore shape the movement data: species that are reclusive, nocturnal, or use dense cover and steep terrain are more likely to have missed or inaccurate positions, which can distort home range, movement and habitat-use estimates.
• Collars should be tested where they will be used, and before each deployment.
• GPS collars can be used for wildlife research in Torres del Paine, with care over the map scale and the species.
• GPS data are best used for gross movements and locations, followed by ground-truthing with camera traps and tracks and signs surveys to confirm whether a species is present.

4. How tracking was or was not used

Tracking was not used. The thesis describes tracking (citing Liebenberg 1990 and Stander 1997) as identifying an animal's tracks and signs and then following its trail, notes that radiotelemetry allows researchers who may be unskilled with tracks and signs to monitor animals, and recommends tracks and signs surveys as one way to ground-truth GPS results.

5. Tracker or observer certification

Not needed.

6. Other technologies, skills or innovations

• Televilt Simplex, SirTrack, ATS G2100 and BlueSky GPS collars.
• TOPCON Odyssey and Javad dual-frequency survey receivers, post-processed with TOPCON Pinnacle and tied to Continuously Operating Reference Stations (CORS) or International GNSS Service (IGS) stations.
• A stop-and-go base and rover method for real-time survey positions in the field.
• A custom 12-volt battery unit powering six collars at once.
• Hemispherical canopy photos analysed in Gap Light Analyzer (GLA).
• A repeatable method for testing collar accuracy against geodetic control.

7. Implications for conservation (from the paper)

• Wrong locations can lead to costly or harmful management decisions, for example concluding that a habitat is not needed by a species.
• Errors can be larger than the pixels of the satellite images positions are mapped onto, which can distort habitat and corridor maps, especially for species with small, specialised home ranges.
• Budgets should allow for collar failures, recapture and battery refurbishment.
• GPS may not be the best tool for cryptic species, or for species whose behaviour and habitat block satellite signals.