About the Guppy Edge
We are constantly testing the latest low-power location technologies in our devices, striving for the perfect balance between device size, price, and performance.
We currently manufacture the Guppy LoRaWAN, our smallest LoRaWAN® geolocation tag for affordable indoor and outdoor asset monitoring. Powered by 2 x AAA batteries with up to 5 years of battery life, the Guppy LoRaWAN does not feature any location technology other than LoRaWAN geolocation from the gateways. With the Guppy Edge, we are testing Semtech’s LR1110 chipset with GNSS and WiFi scanning in the Guppy form factor, as well as exploring how well a chip antenna performs on smaller boards.
If you look at the image below, you can see that the PCB size is smaller and we have had to shrink the RF antenna, the WiFi antenna, and use a small GNSS chip antenna. Reducing the PCB size and the various antenna sizes has a direct impact on the efficiencies and performance of the antenna, so the purpose of this prototype is to measure how well (or not) the device performs.
Left: Guppy Edge PCB. Right: Yabby Edge PCB.
Inside the Guppy Edge
The Guppy Edge utilizes the same multi-technology chipset by Semtech as the Yabby Edge, the LoRa Edge™ Asset Management Platform (LR1110). The two GNSS systems that the LR1110 supports are GPS (USA), and BeiDou (China). These satellites are orbiting at roughly 20,000km, and up to 36,000km, above sea level, respectively. This means that signals are extremely faint by the time they reach devices on land. Therefore, all receiving circuitry must be well designed to reliably detect them.
Another issue with detecting GNSS signals is that they are right-hand circularly polarized (RHCP), as opposed to a linearly polarized signal. Polarization of RF signals works the same as it does with light. Think of polarized sunglasses; they work by blocking horizontally polarized light that gets reflected off the ground (water/road surfaces, etc), with the use of a vertically polarized filter in the glasses. This is the same with RF signals. If the antenna is not in the same polarization plane as the received signal, only a small part of the signal is received. This would be a big issue with global positioning systems, where the device may be at different angles depending on where it is installed. Engineers, therefore, designed the systems to use circularly polarized antennas where the polarization angle effectively rotates through space and can be picked up by antennas at different angles. In the case of GNSS systems, they use RHCP, which means the polarization is rotating counter-clockwise when looking into the source.
Antennas for polarized signals are more complicated and tend to be larger in size, compared to the standard linear polarized ones used by most other RF systems. The standard antenna used for detecting GNSS signals (and on all of our other devices), is the ceramic patch antenna. These come in a range of sizes, but the smallest we’ve seen is 10x10x2mm. This was too tall to fit into the Guppy enclosure, so a ceramic chip antenna was used instead, which is linearly polarized.
The trade-off is that there is an immediate 50% reduction in receiving performance due to the mismatch in polarization, on top of any further efficiency losses.
Luckily, the rest of the GNSS signal path (which includes a low noise amplifier (LNA) and RF matching and filtering) takes up a very small area, so it was easily possible to fit these components onto the board. In the Guppy Edge, we are also using smaller LoRaWAN and WiFi antennas, but these have already been verified on other designs. Other than the antennas (and overall dimensions), the design is exactly the same as the Yabby Edge, which we already know works well.
Testing in Progress
Two prototypes have been made by hand and testing is still in progress, pending firmware optimizations. While we were expecting a reduced performance, we can immediately tell that the device is less effective in scanning satellites compared to the Yabby Edge. In our testing, we have seen up to 18 satellites in a scan (full view of the sky), but the most satellites the Guppy has returned in a scan is 10, which affects location accuracy.
The Guppy Edge is also less tolerant in partial sky views (trees, buildings), only occasionally finding enough satellites to get a position, compared to the much more reliable Yabby, and testing in more covered areas (including office window), has yet to return a position. That being said, firmware work may be able to increase the reliability, as well as updates to the internal LR1110 software.