User's Manual

Theory of Operation/Technical Description
FCC ID:2ARKMNATLRA1
The device has only one user serviceable part, which is the battery. The device has a reed
switch as the only user control, which is used to trigger a position capture. All operation is self
contained. The device will only use one radio.
RF Circuit Function 915MHz
The main microcontroller assembles data packets for the Semtech SX1272 transceiver to
modulate the data using LoRa modulation. LoRa modulation is a derivative of Chirp Spread
Spectrum. The transceiver uses a pSemi 4259-63 RF switch to control if the custom trace
antenna is connected to the receive or transmit pins of the SX1272.
Antenna Setup 915MHz
The antenna is not changeable by the user, it is a copper trace in the PCB and contained within
the device enclosure. The antenna is connected to a filter network, then an RF switch (pSemi
4259-63), then either the receive or transmit filter network, then to the receive or transmit pins
on the SX1272.
Transceivers and Antennae
Chip
SX1272
Modulation
LoRa: Chirp Spread Spectrum
Antenna Gain
0dBi
TX Frequency
902.3-914.2MHz
915MHz Channels
There are 64 channels with 125kHz bandwidth each starting at 902.3MHz and increasing by
200kHz up to 914.9MHz. Additionally, there are 8 channels with 500kHz bandwidth each
starting at 903.0 and increasing by 1.6MHz up to 914.2MHz. The channel frequency specifies
the center frequency of the channel. Table shown on following pages.
The device only sends small packets of 255 bytes or less, infrequently. The device randomly
selects one of the 72 channels (0-71) when it needs to send data. Depending on the channel
selected it will use the bandwidth assigned to that channel (125kHz for channels 0-63 and
500kHz for channels 64-71). The device will send the entire packet on that 1 channel in under
400 milliseconds. The device will not attempt to transmit again until after 20 seconds have
passed (it will be a random time between 20 and 50 seconds) if another data packet has been
queued. The choice between 125kHz channel or 500 kHz channel is random, with 64 125kHz
channels and 8 500kHz channels the device is much more likely to transmit on a 125kHz
channel. The device will make sure not to reuse the last channel transmitted on when picking
the new random channel.