How does Druid’s Argos-GPS device work?
Classic Argos PTTs provide global doppler locations, of which the accuracy varies between several hundred meters to several kilometers. GPS locations enjoy much better accuracy.
Different from Doppler method, GPS collects location data independently, meaning that the GPS data schedule is not related to Argos transmission.
To better under the device functions, we need to bear in mind some features of the Argos system:
Argos transmission capacity is limited. When a device enjoys sufficient power supply (big battery or good solar charging), it is able to collect hundred pieces of GPS data per day, which is overwhelming for Argos transmission.
Argos transmission relies on satellite pass, and satellite pass timing varies with latitude. The higher the latitude, the longer the satellite pass periods, thus the more chance for data reception. This means the devices for migrating species should not use a fixed Argos transmission schedule.
Argos satellite reception can be notably affected by weather (e.g., thick cloud could lead to more transmission failures). This means the Argos transmission can fail even during satellite pass period.
Argos satellite will not tell the device whether a piece of data is successfully received or not.
Given the above ground, Debut series Argos-GPS devices are designed with a scheme described below for researchers to maximum the valid data reception without jeopardizing the device energy balance.
The device is able to use its latest GPS locations to predict Argos satellite pass. When the timing arrives, the device will make continuous transmissions to the satellites.
The device has an advanced setting called “GPS data queue” which is to help you get more evenly distributed GPS data.
For example, you set the device to collect GPS every hour, while at your latitude, the Argos satellite pass happens during 1:10 pm ~ 2:50 pm and 9 pm ~ 11 pm. And you set the GPS data queue to 8.
In this situation, when the first satellite pass happens at 1:10pm, the latest 8 pieces of GPS data, which should be 6 am, 7 am, 8 am, 9 am, 10 am, 11 am, 12 am and 1 pm, will enter the transmission queue. The device will attempt to transmit the GPS data, one at a time and repeatedly, to the Argos satellites.
Such data queue setting makes sure that the data collected during the blank period (when there’s no satellite pass at all) can also have chances to be received.
With appropriate settings and good solar charging conditions, we have seen a lot of our Argos devices uploading over 40 valid GPS points per day at 38° latitude.