There are several commercial beacons available for the areas without good GSM coverage. Good and relatively inexpensive one is tBeacon ( tBeacon.org ). The idea is to get coordinates from craft’s GPS and transmit it to any UHF radio, or search the plane using signal strength. Since standard UHF radio is used, the beacon must use voice modulation (actually PWM modulation, but for simplicity we call it analog).
The idea is great but has some drawbacks: time of transmission is long (battery consuming) (the beacon says “one”,”two”,”three” etc by voice just to transmit a few digits. Second, voice is hard to recognize in a noise when signal is weak.
So I decided to do it all digital (packet exchange). To compare with analog method, I made the beacon compatible to tBeacon firmware and performed a test with using both types of links.
The beacon core is RFM22B radio, it can provide +20dBm (100 mW) of power, has built in modem for packet handling with CRC check. The opposite side is a Finder which uses RFM23BP version; the radio is the same, but it has power amplifier up to 1W (+30 dBm). More power at the Finder side is convenient when calls from the Finder side are made (to be sure we always reach the Beacon). RFM23bp radio, Arduino Mini Pro, the buttons, Nokia 5110 screen are mounted in 3D printed case.
The Finder unit has possibility to set and save permanent settings in the beacon’s EEPROM, display last coordinate and display 3 bars showing last three values of signal strength (for signal strength search). The potentiometer adjusts scale of the bars.
Here are the tests:
As expected, digital method shows longer range (2.2 km) vs 1.2 km for analog. As per estimations, unobstructed line-of-sight should be around 4 km.
After the test, more compact version of the beacon was done (LiPo charger+buzzer+ArduinoProMini (1/3 was cut to make it smaller)+RFM22B. All parts connected with magnet wire, liquid tape and shrinkable tubing are used to seal. Here is the beacon, battery is 50 mAh. The size is 22×18 mm.
The beacon can be switched-off with the command from the Finder, in deep sleep mode current is only 2 uA ( power LED and LDO should be removed from ArduinoMini, they draw current a lot) . From the deep sleep it can be awaken with powering the Mavic (5V). After some tests, I set LiPo charge current to 100 mA, so battery can be fully charged during one flight time. With these parameters we should not care about battery at all.
Space in Mavic is very limited, so put everything inside and do not block ventilation slits is tricky, but possible. In the picture the buzzer actually does not block the slit, it is above, its horn directed toward the slit for better sound.
Next, I’m going to test LoRa modules (RFM98), expected range should be doubled.
(Done, not working see Is RFM98 LoRa a Fake?