[aprssig] LoRa GPS tracking

[Scott Howard]

> Is there a document somewhere that walks through the spread spectrum
> rules and documents if LoRa counts as SS under Part 97 and if so (or if
> not) how it meets the rules?  I really have no clue about this and would
> like to read about it; I do remember somebody getting an STA but they
> were using a 50W amp and the STA might have been for higher power?

Here's a talk about the 50W amp with LoRa you mentioned (Dan Fay,
KG5VBY, at DCC). The link below will bring you right to the part of
hte talk where he discusses the Part 97 regulations and his STA
(0721-EX-ST-2018).
https://youtu.be/AAo2r3mGsBM?t=1024

Outside of Part 97, this is a summary of the Part 15 rules (for
completeness): LoRa is typically used as unlicensed ISM (Part 15
instead of Part 97) which allows for SS emission. LoRa typically is in
bands around 433 MHz or 900 MHz. In the US, the 900 MHz band (more
precisely, 902-928 MHz) is an unlicensed band that allows for SS
emission at higher powers that the unlicensed 433 MHz band, and the
900 MHz band also allows for optional frequency hopping SS to achieve
even higher powers.

The FCC rules (15.247):
https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A/part-15/subpart-C/subject-group-ECFR2f2e5828339709e/section-15.247

TI has a nice application note (see page 8, "Transmit Power Limits for
the 902–928 MHz Band")
https://www.ti.com/lit/an/swra048/swra048.pdf

Quick summary (for mostly isotropic antennas with gain < 6dBi)
- If the channel bandwidth is between 25-250 kHz: Power limit
(delivered to antenna) is +30 dBm. Total dwell time at a specific
frequency cannot exceed 0.4s in a 20s period (i.e., you have to hop to
50 different frequencies within 20s if continuously transmitting)
- If the channel bandwidth is between 250-500 kHz: Power limit is +24
dBm. Total dwell time at a specific frequency cannot exceed 0.4 s in a
10s period (i.e.,you have to hop to 25 different frequencies within
10s if continuously transmitting)

So you can avoid worrying about frequency hopping by designing the
system to transmit short messages at low frequency, which fits nicely
with APRS.


_____
Scott Howard, PhD
Associate Professor
Department of Electrical Engineering
College of Engineering Bioengineering Program
University of Notre Dame
http://ee.nd.edu

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