Technical specifications
The transmitter model we developed to incorporate a geofence alert system was a CTT-1050a-PM Series GPS-GSM (2 Gen), Advanced Bird Telemetry System from Cellular Tracking Technologies™, LLC. These tags weigh ~50 g and are 80 mm × 62 mm × 10 mm (W × L × H). The tags are encased in a sturdy, weatherproof and nitrogen-purged polycarbonate + ABS thermoplastic housing. The antennae are internal to the hermetically sealed housing to improve ergonomics and aerodynamics. The tags can be configured as a backpack mount for large birds such as eagles, as a leg-mount device for long-legged species like cranes and herons, or as a patagial mount for condors (Fig. 1). The units are powered by a solar-recharging Lithium Ion Polymer (LiPo) battery pack and have an estimated operational lifespan of >5 years with a recommended operational temperature range of −40 to 85 °C.
GPS data are collected from sunrise to sunset, with start/stop times calculated from an astronomical formula based on the unit’s GPS coordinates. Under unobstructed skies the fix rate is near 100 %. Location accuracy is rated at <2.5 m horizontal, <25 m vertical, <0.001 m/s velocity and heading. If cellular coverage is unavailable, the unit can store >100,000 fixes until it reacquires a GSM link. The tags deplete their batteries if subjected to long periods of inclement weather with no recharging. However, if battery voltage falls to ~3.6 V, the tags conserve energy by ceasing daily GSM transmissions while continuing to collect GPS data. If power is <3.5 V, the tags enter a hibernate/recovery mode where all operations cease until they can recharge in sunlight.
Tag location data and diagnostic information are accessed from a secure CTT webpage. Acquired GPS location data are transmitted via a quad band GSM/GPRS Class 10 850/900/1800/1900 MHz world band radio. Maximum output power is Class 4 (2 W) @ 850/900 MHz, Class 1 (1 W) @ 1800/1900 MHz. The GSM cellular system allows the device to frequently update large batches of data at considerably lower costs than satellite-transmitting devices using Argos or Iridium systems. The two-way communication of the GSM cellular system also enables the tag duty cycle and firmware to be reprogrammed post-deployment while the tags are attached to free-ranging birds. The GPS fix rate interval is set at 15-min to provide a conservatively optimized balance between temporal resolution, power consumption, and data acquisition costs. Collected location data are transmitted once per day at noon PST. If a telemetered bird enters a geofence zone, the unit will transmit the most recent batch of data and the GPS fix rate will increase to 30 s. When the bird exits the geofence zone, the GSM system will again transmit the most recent batch of data and the fix rate will return to 15-min (Fig. 2). The two-way communication system enables the addition or modification of geofences as new management challenges present. Geofences can include up to three polygons consisting of up to eight vertex points each, as well as three circular geofences. A video animation demonstrating the geofence system can be found at this link: http://youtu.be/2oWodZpmbHo.
Performance field-testing
We field tested the efficacy of the prototype units geofence tags by driving them across geofences in a car and by flying the tags in a helicopter along the flight path of a California condor (Gymnogyps californianus) that we previous tracked via GPS telemetry. This condor was from a population reintroduced to northern Baja California, Mexico, by San Diego Zoo Global and its partner organizations [15]. As the condor flew north along ridgeline of the Sierra Mountains of Baja and crossed over the USA/Mexico international border in April 2007, its interpolated flight path intersected the Energía Sierra Juárez Wind-energy Project (ESJWP) being jointly developed by IEnova and Intergen [16, 17] (Fig. 3). Although no direct condor injuries or fatalities from wind turbine collisions have been confirmed to date, they are considered a species at high risk from wind energy impacts [18]. The ESJWP may pose a collision injury risk to the recovering condor population in Baja. Therefore, we programed into the GSM/GPS tag server two concentric circular geofences around the ESJWP site spaced 20 km (area = 1256 km2) and 40 km (area = 5023 km2) apart.
The spatial dimensions of the two geofence zones were based on the mean flight speeds (15 km/h) recorded from the telemetered condor population in Baja and were calculated to give wind farm operators sufficient warning to slow or shut down turbines if a condor crosses the geofences and flies towards a wind farm. If a condor crosses the outer 40 km geofence boundary it will trigger an automated SMS message sent to a list of select recipients. This alert relays the bird’s ID and geographic position relative to the wind farm site so that managers and wind farm operators can closely monitor its flight profile. If the condor continues and crosses the inner 20 km geofence zone a second SMS alert will be triggered indicating that the bird has flown into close proximity to the wind farm, prompting appropriate collision countermeasures. The 47 turbines installed at the ESJWP are Vestas™ model V112-3.3 with 112 m diameter rotors (3.3 MW) installed on 84 m towers. The average capacity of wind turbines installed in the US in 2012 was 1.95 MW [19], so those installed at the ESJWP lie at the larger end of the turbine scale. It will take 30–60 s to bring an ESJWP turbine to a full stop once the command has been implemented (IEnova, pers. comm. 2015).
We matched the helicopter speed, height and trajectory to the flight path characteristics of the tracked condor that crossed the ESJWP site so that we could gauge how quickly the geofence system would respond to the ingress of a simulated telemetered bird. We flew at the maximum speed (47 km/h) that we recorded for the condor and at its mean flight height (1240 m above mean sea level, 400 m above ground). We tracked our flight path at sub-meter accuracy using a Juniper Systems™ Archer Field PC GPS connected to a Hemisphere™ GPS XF101 DGPS receiver running ArcPad™ (v10.2.1, ESRI Inc.) with the effigis™ OnPOZ GNSS Driver. We drove the tags forty times across the geofences in a car at the mean flight speed recorded from the condor (14 km/h).