One of the biggest challenges in adding GPS capability to battery-powered devices such as vehicle trackers, asset trackers or internet of things (IoT) sensors is the shortened battery life. Increasing the size of the battery for longer runtime is, in almost all cases, not an option as this would increase the size, the weight and the cost of the wearable.
To reduce the power consumption of the GPS receiver, most designs keep the receiver off until the receiver’s position is required. As the receiver wakes up from what is referred to as a “cold start,” it acquires and locks onto the satellite signal and extracts the navigation message which includes the satellite orbit and atomic clock bias information (the ephemeris data). With a minimum of 4 satellite signals, it is able to determine position and time. However, since the navigation message data rate is a low 50bits/sec, the receiver must be powered on for several seconds to receive the broadcast data, normally 28 seconds or longer. It is only then that the receiver can calculate its position.
One way to help extend the battery life is to reduce the amount of time the receiver is on before a position is calculated. A snapshot receiver does exactly that. In a snapshot receiver implementation, the receiver is not on nearly long enough to receive and decode the ephemeris data. Instead, up to 28-day ephemeris data from a server in the cloud is pre-loaded into the receiver, allowing it to wake up and report its position within a couple of seconds. The reduction in the time the receiver is on can extend the battery life by an order of magnitude, making it ideally suited for applications where battery life has been a barrier to adoption of GPS technology.
This snapshot implementation is based on a patent granted in 2015 to Baseband Technologies Inc (BTI) of Calgary, Alberta. Maxim Integrated has collaborated with BTI and now offers a full GPS solution using the MAX2769C GNSS L1 receiver, as well as the MAX32632 Arm® Cortex®-M4 microcontroller running the BTI firmware.
It is helpful to think of the process of receiving the satellite signal to obtaining the position coordinates as a three-step process: Signal Capture, Measurement Generation and Position Estimation. In addition to the low power consumption, the Maxim Integrated/BTI solution provides the flexibility to perform both the Measurement Generation and the Position Estimation steps either on or off the board by offloading to a cloud server. This can further reduce both the power consumption and the cost of the system.
In addition to the power saving features of the snapshot algorithm, the use of the MAX32632 with its flexible power modes, an intelligent peripheral management unit (PMU), dynamic clock gating and firmware-controlled power gating further reduces the power consumption of the GPS system.
To illustrate the power savings that the snapshot receiver delivers, let us compare the energy usage of the Maxim Integrated/BTI GPS solution to a commercially available “low power” receiver module. Both receivers are turned on once an hour to receive the satellite signal, calculate the position and transmit wirelessly their position. The energy required for the position fix and the ephemeris download by the Maxim Integrated/BTI system is less than 20uWh, while the competitor consumes 170uWh, or 9 times more energy.
Another feature of the snapshot receiver is its ability to specify the acquisition window, or how long to capture the satellite signal. A longer capture window does improve the position accuracy, but also increases the number of digital samples to be processed and thus the power consumption. The firmware allows acquisition windows of 4ms, 6ms, 8ms, 10ms, 16ms, 22ms and 30ms. Using for example, a 16ms acquisition window, and updating the position every minute, our solution consumes 0.88mW. With a once an hour update rate, the power consumption will vary from 0.04mW to 0.06mW as the acquisition window is increased from 4ms to 30ms.
Using the data above, one can easily calculate how long a 100mAh coin cell would last. Depending on the capture window, a 100mAh coin cell can last up to 13 months.
In summary, Maxim Integrated/BTI’s snapshot GPS receiver offers much lower power consumption than traditional tracking GPS receivers, making it ideally suited for wearables and IoT applications where battery life is at a premium.
A more detailed version of this blog post can also be found at Maxim Integrated’s mgineer blog, which you can visit regularly to stay up-to-date on the latest analog and mixed-signal technologies, get tips to answer your design questions and gain insights into emerging design engineering trends.
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