LoRa APRS: Bringing APRS into the 21st Century
John Langner WB2OSZ and Jon Adams N7UV
LoRa APRS represents a significant advancement in the world of amateur radio communication. This innovative technology combines the long-range capabilities of LoRa (Long Range) with the well-established Automatic Packet Reporting System (APRS), offering enhanced performance and exciting new possibilities for ham radio operators.
Key Features and Benefits
Most amateur radio operators worldwide have access to the 70 cm band. In the US, the majority of hams are fortunate to have operating privileges in the 420-450 MHz spectrum. Worldwide, it's safe to assume that most other ITU regions and countries allow amateur radio use in the 430-440 MHz spectrum. As such, hams in the EU and elsewhere selected 433.775 MHz as a "good place to be", and here's a note about why that works well for US hams.
LoRa APRS is fully compatible and interoperable at the application layer with conventional 2 m FM AFSK AX.25 APRS. A quick look at aprs.fi, aprs.to, or other APRS websites will show both LoRa and legacy 2 m APRS.
Here's a snip from a greater San Diego California - on the right, there's WA6YVX-10, a conventional 2 m 1200-baud APRS digipeater atop Mt Otay, and on the left, a 70 cm LoRa digipeater KJ6BXN-10 in Coronado. The "L" in the KJ6BXN-10 green star is the secret to telling the difference! We also see another 70 cm LoRa APRS station, K6QM-1 that also functions as a digi.
Messaging between LoRa and conventional APRS is already enabled through services like aprs.fi and others, some hams are working on dual-radio devices, with conventional 2 m and 70 cm LoRa connected together, avoiding the Internet altogether! The aprx software, or direwolf, that run on a Raspberry Pi can also stitch together 70 cm and 2 m APRS systems, again without the need for the Internet.
LoRa APRS brings with it a bunch of advantages over conventional APRS:
1) Nearly every LoRa radio available can an operate at a wide range of over-the-air data rates, from 11 bps to 62.5 kbps. Low bit rates increase link budget, high bit rates improve channel capacity and throughput. All from one device. No radio hacking, fiddling with audio levels or RF circuits! Easy to change all parameters via a webpage or, for some devices, through their touchscreen. Some hams prefer higher link budget, others prefer greater channel capacity and throughput. There is no one setting, nor should there be, that will please all or extract the best performance for a given region or group of users. Importantly, this is NOT two-meter 1200 baud AFSK APRS, where there was little other choice 45 years ago.
2) LoRa can operate at SNRs moderately to significantly below the noise floor. This increases link budget and allows hams to get improved performance, especially in fast-fade mobile environments. Using marginal antennas and poor equipment placement wastes LoRa's high performance.
3) US hams can use LoRa technology in our 70 and 33 cm frequency bands. 70 cm is the overall best because of the fairly low noise environment at 433.775 MHz and the more than 6 dB better RF propagation performance of 70 cm. 33 cm is more challenging for US hams, as the 902-918 MHz band is allocated to FCC Part 15 (Radio Frequency Devices), Part 18 (Industrial, Scientific, and Medical Equipment), Part 90 (Private Land Mobile Services), and Part 97 (Amateur Radio Service). Hams are generally not primary. The noise floor is significantly higher on the 33 cm band due to the plurality of uses; finding a quiet channel really depends on your local RF environment, and is not something that should be established nationwide.
4) Radio modules/units with a display can be very inexpensive, starting at around US$20 - $25. While not a software-defined radio, there are multiple hams out there who've written firmware to operate APRS on these LoRa radios. It's easy to flash these images right to the hardware!
5) There are tens of millions of LoRa chips sold every year, so it's a market that is much larger than the ham market, making equipment availability high and constantly expanding! In the commercial world, there are over 400 million LoRa-enabled devices (about halfway down that page), incidentally providing hams with many competitive product options, and for those hams with the interest and capability, to hack commercial devices and make them work for the amateur radio world.
6) The technical characteristics of LoRa have been documented publicly, providing a large and growing corpus of technical work, through research papers and publications by hundreds of individuals and dozens of institutions, making it easy for nearly any ham with an interest to use and experiment with. As a result of this independent work and the prolific documentation from the semiconductor system manufacturer, LoRa meets the US FCC definition of a specified digital code (https://www.ecfr.gov/current/title-47/part-97#p-97.309(a)(4)). LoRa has many neat features that have spurred work to deploy this simple, mature, readily available, affordable technology in many very different fields.
This system offers several advantages over conventional APRS:
- Extended Range: LoRa is an FSK system that's novel in that the signaling is based on the rate of change of carrier frequency with respect to time, which is a departure from typical ham and commercial radio. This frequency-limited chirp spread spectrum modulation allows for reception of signals far below the noise floor due in part to matched filter reception, which can significantly increase the communication range compared to traditional APRS, and easily offset the added RF propagation loss in moving from 2 meters to 70 cm.
- Low Power Consumption: The most common LoRa APRS radio boards typically transmit at +20 to +22 dBm (100 - 140 mW). Newer, higher power LoRa products on the market put out as much as +30 to +34 dBm (1 - 2.5 W), especially valuable for digipeaters and mobiles. There are efficient bidirectional amplifiers that can boost the +20 to +22 dBm output by as much as 11 dB, and simultaneously improve receiver noise figure. While higher-powered radios consume more power during transmit, the much shorter transmit times result in similar overall power consumption, and the reduced transmit time improves channel availability, allowing more users to communicate.
- Improved Performance: The system can provide location updates, weather information, and text messaging over longer distances than conventional APRS, in no small part because LoRa includes extensive forward error correction, which is one of the important reasons why a LoRa packet is far more robust than an AX.25 AFSK FM packet, where one millisecond-long noise burst can corrupt an entire 1 second-long packet, forcing a retry.
- Cost-Effective Hardware from Multiple Vendors: LoRa trackers and iGates can be quickly built using readily available, low-cost consumer off-the-shelf (COTS) hardware.
While LoRa APRS offers these advantages, it's worth noting that traditional APRS benefits from a well-established infrastructure and network effect, having been in use for decades. As LoRa APRS continues to grow, it promises to significantly enhance the capabilities of APRS technology, especially in scenarios requiring either higher channel throughput, or greater link margin. Nonetheless, there's no substitute for quality antennas and digipeater placement to take best advantage of this incredible technology.
Applications in Amateur Radio
LoRa APRS has found particular favor among Summits on the Air (SOTA) operators. Its ability to function effectively in areas with limited cellular coverage makes it ideal for remote locations and mountainous terrain. The system's low power requirements and long-range capabilities are well-suited for portable operations, allowing SOTA activators to send position reports and receive spots without relying on patchy mobile data connections.
Technical Specifications
LoRa APRS firmware written by many hams tends to default to the lowest performance in order to improve the newcomer's chances of immediate success. Thus, these firmwares typically use the following parameters:
- Center Frequency (Fc): 433.775 MHz, 434.855 MHz, 439.125 MHz
- Bandwidth (BW): 125 kHz
- Spreading Factor (SF): 9 or 12
- Coding Rate (CR): 4:5 or 4:7 (also seen abbreviated as CR5 or CR7)
- Output Power: Depends on the device, default range from +20 to +30 dBm
The above settings of BW125 SF12 CR4:5 provide an over-the-air data rate of 293 bits per second, while the BW125 SF9 CR4:7 settings provide 1256 bps over-the-air.
The IARU International Amateur Radio Union (IARU) Region 1 VHF Handbook 8.50 recommends 433.775 and 433.900 MHz with a bandwidth of 125 kHz. Be sure to check whether the frequency and bandwidth are legal in your country before transmitting. There is no recommendation for the US; however, this article argues for a modernization of the US 70 cm band allowing a well defined range for digital modes.
Rather than using the AX.25 frame format, the LoRa frame payload contains:
· Three byte sequence to distinguish from other LoRa applications: 0x3c 0xff 0x01
· APRS packet in TNC-2 Monitoring format.
Building a LoRa APRS Network
Development so far, has concentrated on two main components:
- Trackers: These devices transmit geographical positions in real-time using LoRa modulation. They are often built using ESP32 LoRa modules, which include a 433 MHz transmitter/receiver, GPS receiver, WiFi module, microcontroller, USB port, OLED display , and often a battery charger.
- IGates: These systems receive frames sent by trackers and inject them into the APRS-IS network via internet access. Forwarding in the opposite direction is usually limited to “messages” addressed to stations recently heard over RF. IGates are typically installed in fixed, elevated positions to ensure wide coverage.
Considerations for USA
FCC Part 97 (Amateur Radio) restricts transmission bandwidth to 100 kHz for the 70 cm band so it is not legal to use the usual 125 kHz bandwidth here.
The US LoRa frequency range is 902 - 928 MHz. This is an unlicensed Industrial Scientific Medical (ISM) band with all sorts of random consumer, commercial, government, and military stuff. It is also available for Amateur Radio use.
Some US hams use LoRa APRS on this band, sometimes with discarded Meshtastic devices after the users become disillusioned with it. There is not yet consensus on a standard frequency. Since these are used under Amateur Rules, the ARRL band plan should be respected. The edges are reserved for repeaters and weak signal work. This leaves 909 to 927 MHz.
Around here, I'm seeing a continuous signal centered around 924 MHz with peaks every 75 kHz. This vicinity would not be a good choice. 909 to 920 is generally quiet with few sparse signals. How about sharing what you see in other parts of the country?
Future Prospects
As LoRa APRS continues to gain traction, it promises to drag APRS into the 21st century. The technology's superior performance, especially in challenging terrains and low-power scenarios, makes it an exciting prospect for the future of amateur radio tracking and communication. While LoRa APRS is primarily focused on position reporting and short messaging, its integration with the existing APRS infrastructure allows it to leverage decades of development in the APRS-IS system and visualization tools like aprs.fi.
Those who think that APRS is watching trackers on aprs.fi do not understand the philosophy behind APRS. In the words of its creator:
“APRS is not a vehicle tracking system. It is a two-way tactical real-time digital communications system between all assets in a network sharing information about everything going on in the local area. On ham radio, this means if something is happening now, or there is information that could be valuable to you, then it should show up on your APRS radio in your mobile.”
“APRS is a real-time tactical digital communications protocol for exchanging information between a large number of stations covering a large (local) area. As a multi-user data network, it is quite different from conventional packet radio.
APRS is different from regular packet in four ways. First by the integration of maps and other data displays to organize and display data, second, by using a one-to-many protocol to update everyone in real time, third, by using generic digipeating so that prior knowledge of the network is not required, AND FORTH, since 1997, a worldwide transparent internet backbone, linking everyone worldwide. APRS turns packet radio into a real-time tactical communications and display system for emergencies and public service applications (and global communications). Normal packet radio is useful in passing bulk message traffic (Email) from point-to-point, but it does not do well at real time events where information has a very short life time and needs to get to everyone quickly.
APRS is a LOCAL RF network. Although the Internet monitors APRS worldwide, this is not the primary objective. But like all of our other radios, how we use APRS in an emergency of special event is what drives the design of the APRS protocol. Although APRS is used 99% of the time over great distances, and benign conditions, the protocol is designed to be optimized for short distance real-time crisis operations on RF.
APRS provides universal connectivity to all stations in the net by avoiding the complexity and limitations of a connected network. It permits any number of stations to exchange data just like voice users would on a voice net. Any station that has information to contribute simply sends it, and all stations receive it and log it. Secondly, APRS recognizes that one of the greatest real-time needs at any special event or emergency is the tracking of key assets. Where is the Event Leader? Where are the emergency vehicles? What’s the Weather at various points in the County? “
– Bob Bruninga, WB4APR (SK)
“APRS is not a vehicle tracking system.” … “APRS is a LOCAL RF network.”
To realize this goal, bridges are needed to connect the two radio networks without requiring Internet connections.
As more radio amateurs adopt this technology and contribute to the growing network of IGates and bridges to traditional APRS, LoRa APRS is poised to become an increasingly valuable tool for the amateur radio community, particularly in applications requiring long-range, low-power communication in challenging environments.
References:
Is LoRa (Long Range) Technology Magic?
Finding 70 cm equipment for LoRa APRS
Chirps Aren't Just for Birds and Bats (no bees)!
LoRa - The Settings Under the Hood
How LoRa Modulation really works - long range communication using chirps
Exploring LoRa APRS: VHF vs UHF Performance
RPi-LoRa-KISS-TNC: a simple software for enjoying LoRa APRS
Optimizing LoRa APRS Tracker for Maximum Battery Life