Understanding APRS: The Automated Packet Reporting System

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Introduction to APRS – Automated Packet Reporting System

The Automated Packet Reporting Service is a digital real-time communication system used to share tactical information and messages. The most common activity in APRS is using GPS information to plot the location of moving objects, but APRS can also be used for short messages, weather station telemetry, announcements, and displaying objects on a map. The power of APRS lies in its ability to geocode objects using mapping technology and display that information in a meaningful way. Thus, it is an excellent technology for tracking assets: such as checkpoints or runners in a marathon, or storm observers activated for SKYWARN.

APRS (Auto Packet Reporting System) has become familiar to many friends; it is a way of exchanging information through data communication in the amateur radio hobby, allowing the exchange of station geographic coordinates, weather information, telemetry data, short messages, and more.

The APRS system publishes data using wireless packet communication. In APRS, stations use connectionless packets to publish data in a one-to-many manner, similar to the relationship between a radio station and its listeners. As APRS has evolved, the devices for APRS are generally divided into APRS nodes, wireless digital repeaters (Digipeater), APRS gateways (IGate), and APRS servers.

In simple terms, APRS data is shared via the internet through APRS servers. How is it shared? It is through gateways that send APRS packets to APRS servers on the internet. How do APRS packets come about? They are collected by stations equipped with a Terminal Node Controller (TNC) from the APRS radio frequency network (APRS-RF).

First, I will outline the system, including a brief history, how it works, and the starter equipment. The goal is to help you understand APRS, so I won’t get too technical.

History of APRS

I have personally been involved in APRS since 2000, and much of what I share is based on my early experiences in Xiamen. Bob Bruninga, WB4APR, is considered the father and creator of APRS. His early work developed into a mapping system as early as the 1990s. Soon, GPS technology became available to the consumer market, and automated systems were developed. By the mid-90s, a robust APRS framework had developed. I refer to it as a framework because the next decade for APRS matured. However, to make APRS viable, certain things needed to be done, and by the early 2000s, dedicated APRS VHF frequencies were established. A full-time internet gateway was developed, and the digital and path protocols were formalized. The hallmark of APRS being ready for its golden era wasthe release of products with APRS functionality by radio manufacturers Kenwood and Yaesu. Those wild west days of APRS may have passed, but the Automated Packet Reporting System has become a well-established, functional, and very useful mode for amateur radio enthusiasts – especially for those interested in emergency communications.

How APRS Works

Thus, APRS works by sending connectionless packets containing call signs, paths, locations, and other information. APRS is based on packet radio technology, so transmissions use the AX.25 format with a baud rate of 1200 baud. Therefore, you need a device called a TNC or Terminal Node Controller to capture digital data and convert it into audio tones that an FM transceiver can transmit. In today’s world, this sounds incredibly outdated, but the genius of the APRS system lies in its robustness.

When I say connectionless, I mean sending APRS packets without expecting them to be received by another station. Back in the old days of packet radio, you could connect a TNC to another station, just as a computer and modem connect to another computer over a phone line. Therefore, for connectionless APRS packets, any number of receiving stations may receive that message and retransmit or digitize it. There may be conflicts, and these retransmitted packets may collide in the air, so filtering and packet discard methods built into digital repeater firmware can eliminate duplicate packets. The way to control the distance of APRS packets is through path information.

APRS Path Protocol

If you have ever looked at an APRS packet, you may have seen WIDE, WIDE1-1, etc. These are path protocols. The purpose of digital repeaters is to listen for packets and retransmit them. Since digital repeaters cover a wide area, they will automatically retransmit packets using the WIDE indicator. Therefore, when a digital repeater receives a packet marked WIDE, it will receive the packet, replace it with the WIDE call sign, and then retransmit it. Since the general WIDE term is no longer in the packet, another digital repeater will not retransmit it. The packet has now expired. Of course, multiple digital repeaters can receive packets and retransmit them, but the call sign replacement function of the protocol prevents the ping-pong effect (In mobile communication systems, if the signal strength of two base stations fluctuates sharply in a certain area, the mobile phone will switch back and forth between the two base stations, resulting in the so-called “ping-pong effect.”)

Paths like WIDE1-1 or WIDE2-2 work in the same way, except that 2-2 acts as a counter to extend the packet to multiple digital repeaters. WIDE1-1 will jump out 1 hop in all directions, and WIDE2-2 will jump out 2 hops in all directions. You never want to extend your packet more than 3 hops because each hop introduces more opportunities for collisions. Moreover, the goal of APRS is not to see how many maps you can light up, but to see if the packets from your travels can be received by the igate.

APRS IGates

IGates listen for signals in the air and inject packets into the APRS internet data stream. IGates can also obtain packets from the data stream and retransmit them wirelessly. This allows messages to be sent and received by any station that hears the internet data stream. Thanks to IGates, you can view local APRS traffic from almost any location.

So how do we view APRS information? The simplest way to get started is to use a website called APRS.fi. APRS.fi converts APRS packets onto Google Maps, making it very easy to view and query the APRS data stream. Certain features (like messaging) are not available, but you can track workstations and view their history, which is very useful.

Using APRS

So you want to get involved in APRS, I think the easiest way is to use a handheld radio, as shown below with the Btech APRS-K1 cable and smartphone. The APRS cable connects to the 2-pin connector on the radio and plugs into the audio port of the phone. On the phone, you will run applications like APRSDroid or APRS Pro Deluxe. The GPS in the phone will provide location information, and the app will simulate the Terminal Node Controller. This setup allows you to view and transmit to the local APRS channel and also view the APRS internet stream. Additionally, as you move the phone, the radio will signal your location. You can purchase the BTech APRS-K1 cable.

When I first started using APRS, I built a tracker using a mobile radio, GPS module, and TNC. Due to lack of experience, APRS at the time utilized a lot of cables and connectors. A large number of connectors led to some interfaces always disconnecting or failing to work. I wanted to carry APRS with me while biking, so I bought the Yaesu VX-8R handheld device. This small radio integrates GPS, TNC, and transceiver into one package, so there’s no need to worry about cable issues. Kenwood also created APRS integrated radios, and the widespread use of these devices has made APRS a useful protocol.

But the commonality of APRS is the need for a TNC or Terminal Node Controller. Whether you are using tracking devices like Tinytrack or Argent Data System, applications like APRSdroid, or radios like the Kenwood D-710. Local APRS stations typically rely on hardware TNCs like the Kantronics KPC-3+, or older TNCs like PK-12 or MFJ 1270. The new KPC3+ TNC has become very expensive, and this value has permeated the second-hand market. But there are still deals to be had, and you can buy items like PK-12, MFJ 1270, PK-232 at reasonable prices on the second-hand market. Usually, the key is to have a keen eye for quality. But once you have a TNC, you can use one of the standalone APRS applications, like the new PinpointAPRS on your cabin computer.

Currently, a more common gateway setup among enthusiasts primarily uses a regular radio connected to a TNC (Terminal Node Controller), ultimately connected to a computer or router to form a setup. Common products include Argent Data Systems’ Open Tracker (OT), Tracker2 (domestically known as OT2M), Tracker3, and domestic 51 TNC, among others.

Using this setup has the advantage of being economical, cost-effective, and very suitable for long-term fixed gateway stations. Its disadvantage is that the setup process is more complicated, with many cables. Sometimes, we need to take the radio to other places for leisure, activities, or temporary stay, so a quickly deployable APRS gateway is also essential for related amateur hobby activities, outdoor exploration activities, and emergency rescue command.

source:https://www.jpole-antenna.com/2018/09/17/introduction-to-aprs-the-automated-packet-reporting-system/

Arduino + TNC + GPS = APRS Station???

After getting my Arduino, I recently returned to amateur radio, and I thought I could use simple equipment (2-meter handheld or base station), TNC (the cheapest one can be purchased), an Arduino, and a GPS receiver (they have one or some modules, consider a Garmin) to have the Arduino parse the information from the GPS, capture the location, format the correct sending string, and then send it to the TNC for transmission.

Arduino is a convenient, flexible, and easy-to-use open-source electronic prototyping platform. It includes hardware (various models of Arduino boards) and software (Arduino IDE). Developed by a European team in the winter of 2005, its members include Massimo Banzi, David Cuartielles, Tom Igoe, Gianluca Martino, David Mellis, and Nicholas Zambetti.

One major problem I face is that I need multiple serial ports, but I think I can simply use relays to switch between the TNC and GPS’s RX and TX lines.

Any ideas? Has anyone else really done something like this (without implementing a packet modem on Arduino)?

That should be simple. Most microcontroller TNC/APRS projects also choose to implement TNC in the microcontroller.

This is a non-Arduino APRS tracker built on ATmega8: http://garydion.com/projects/whereavr/

This is a pseudo TNC in shield form for Arduino (actually an OpenTracker processor with special firmware): https://www.argentdata.com/catalog/product_info.php?product_id=136&osCsid=0ec7ccea78f7f35aede742c52263b8e7

If I use Arduino simply to create APRS packets and use an external TNC, I would use TNC-X or Opentracker and connect the Arduino with GPS using newsoftserial and tinyGPS. I might use a serial Arduino based on MAX232 instead of a “standard” USB Arduino for easier connection to TNC.

Another option is to use an MX614 modem IC to generate tones and eliminate the TNC. MX614’s alternatives include XR2211 and XR2206 for tone detection and generation, as well as other discontinued but still available 1200 baud modem ICs.

One major reason I want to go the TNC route is that I don’t want to program the TNC into Arduino (which would be cumbersome for implementing something like TNC-X), and it’s best if current amateur radio enthusiasts already have a TNC (rather than having to acquire a lot of new things).

In my thinking, hams just need to buy an Arduino, some basic components, and a cheap GPS receiver with continuous output.

I am also considering using my Android phone and Bluetooth to transmit serial data from the GPS in the phone to Arduino.

More about APRS:

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[Product Explosion] The world’s smallest APRS transceiver | Matchbox size, with GPS PicoAPRS

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[Car Club] APRS outdoor application practice – APRS amateur radio and professional GPS two-way data transmission setup application

[Ham DIY] Setting up APRS digital repeater (DIGI), internet gateway (IGATE), and weather station (WX)

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