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KC2NJV > ALL 27.03.90 22:44l 187 Lines 8299 Bytes #999 (0) @ WW
BID : 596_KC2NJV
Subj: How APRS Works: A Simplistic Explanation of a Complex System
Path: IZ3LSV<IK6IHL<IK7NXU<HB9ON<DK0WUE<PD0LPM<PI8ZTM<PY2BIL<KC2NJV
Sent: 260327/2034Z 596@KC2NJV.#NASS.NY.USA.NOAM LinBPQ6.0.25
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How APRS Works: A Simplistic Explanation of a Complex System
by Wayne Spivak, KC2NJV
Prologue
I have simplified the concepts contained herein. While factually correct,
there are additional aspects that can modify and amplify the information or
the way APRS works. It is up to the reader to decide if they want more
detailed information.
What is APRS?
APRS stands for Automatic Packet Reporting System. It is a special way that
ham radio operators can send messages and share information using their
radios. APRS is a digital communication system based on AX.253, a
communication protocol. Like all computer languages and protocols, it has
grammar, structure, and rules.¹
How Does APRS Work?
APRS sends small bits of computer data through radio waves. Your radio works
with a computer or special device called a TNC (Terminal Node Controller)
that changes the computer information into radio signals and back again.¹
When you send an APRS message, it goes out on the radio frequency and other
APRS stations can receive it. These stations can be other ham radio
operators, or they can be special gateway stations that connect to the
internet.¹
APRS is delivered in real time, and if your device is not on, you will miss
the message.¹ How far a message will travel depends on normal radio signal
travel, your radio and antenna, and how you set up your APRS equipment.¹
The "path" the packet is sent on will decide if the packet stops locally (1
Hop), regionally (2 Hops), or possibly around the world (3 Hops). A hop is a
single retransmission of a data packet by a digital repeater, or
"digipeater". Also important is whether that packet is picked up by an iGate
(which sends radio signals to the internet) or a Digipeater (which sends
signals between radio and internet).¹
Just remember that many radios, Digipeaters, and iGates may hear the same
message and forward that message on, based on how the path is set up.¹
The Four Main Types of APRS Messages
APRS has four main ways to send information: Beacons, Bulletins, Text
Messages, and Bots. Each one works differently and is used for different
purposes.
1. Beacons
Beacons are like automatic announcements that your radio sends out on its
own. A beacon can be set to automatically send over the air at fixed times
without you having to do anything.¹ The most common type of beacon tells
other people where you are located.
For example, if you have a GPS connected to your APRS radio in your car, it
can automatically send out your location every few minutes. Other ham
operators can see on their computer or radio screens exactly where you are
driving! This is really helpful for emergency workers, people on road trips,
or during special events like parades.¹
Beacons can also share other information like the weather at your location,
how fast you are traveling, or which direction you are going. Some people
set up weather stations that automatically beacon the temperature, wind
speed, and rainfall in their area.¹
The same radio may beacon one time a minute or one time an hour. If you are
in range and your radio is on, you might get up to 60 beacons an hour.¹
2. Bulletins
Bulletins are messages that go out to everyone in an area, not just one
person. Bulletins are simply messages sent to call signs that start with BLN
followed by a number or letter. Any grouping starting with BLN with a
maximum of 9 characters is a valid Bulletin name.¹ Think of bulletins like
announcements on a school loudspeaker - everyone can hear them.
Ham radio clubs often use bulletins to tell people about meetings, emergency
situations, or important news. For example, if there is bad weather coming,
someone might send a bulletin that says "SEVERE STORM WARNING - TAKE SHELTER
NOW." Everyone with an APRS radio in that area would see this message.¹
Bulletins are sent whenever the author chooses them to go. A rule of thumb
is every 20 minutes, but some people send their bulletins every 4 hours.¹
What's different with bulletins is the ability to turn them off or filter
them to only a few types. There are standard bulletins, and the major ones
are BLNx, BLNxNET, and BLNA through BLNZ.¹
In fact, many radios and smartphone apps allow you to adjust/mute ringers,
vibration, reception and other aspects of APRS. What you can adjust is
unit/software dependent.
3. Text Messages
Text messages in APRS work a lot like text messages on your phone, but they
go through ham radio instead of cell towers. You type in who you want to
send the message to and what you want to say.¹
You can send a message to just one person by typing their ham radio call
sign (like W1ABC-xx) and then your message. The APRS system will try to
deliver your message to that person. When they get your message, their radio
can automatically send back a receipt to let you know they received it.¹
These messages are great for staying in touch with other ham operators when
you cannot talk on voice radio, or when you want to send exact information
like addresses or phone numbers that might be hard to understand over voice
radio.¹
Text messages are just sent. You cannot block text messages - you just have
to ignore them if you don't want them.¹
4. Bots
Today, we also have mailing lists in APRS, commonly called Nets. These are
run by a computer program called a Bot.¹ Most Bots will just take a message
and send it out to everyone on their list.
I created a special Bot that has "Store and Forward" abilities2. This means
if you miss the message, it can be sent again at a later time.¹ This
happens either when the Bot hears that your radio is on the APRS system or
at a scheduled time.
Bots are different because you can subscribe and unsubscribe. So, if you no
longer want the text messages, they stop.¹ You can also, on my program,
turn the Store and Forward feature off or on.
Why These Different Types Matter
Each type of APRS message serves a different purpose:
• Beacons work automatically to share your location and other information
• Bulletins send important announcements to everyone in an area
• Text Messages let you talk privately with one person
• Bots create mailing lists where people can join or leave as they want
This variety makes APRS very useful during emergencies. When hurricanes,
earthquakes, or other disasters happen, regular phones and internet might
not work. But ham radio often still works, and APRS can help emergency
workers know where people are and what help they need.
APRS is also fun for everyday use. Many ham operators enjoy tracking their
friends' travels, sharing weather information, or just trying out this neat
technology.
Conclusion
APRS makes ham radio more powerful by adding computer smarts to regular
radio signals. With its four main features - Beacons that automatically
share information, Bulletins that send announcements to everyone, Text
Messages for personal communication, and Bots that create smart mailing
lists - APRS helps ham radio operators stay connected and share important
information quickly and easily.
The best part about APRS is that much of it works automatically. Once you
set it up, your radio can share information and receive messages without you
having to do anything. It is like having a smart radio that knows how to
send text messages and share your location all by itself!
Whether used for emergency communication or just for fun, APRS shows how
creative ham radio operators can be in finding new ways to communicate with
each other around the world.
________________________________________
Footnotes:
1. Information from the provided APRS technical document
2. Github: https://github.com/wspivak/APRSBOT-SF
3. AX.25 Link Access Protocol for Amateur Packet Radio
https://www.ax25.net/AX25.2.2-Jul%2098-2.pdf
Additional Sources:
• Wikipedia - Automatic Packet Reporting System
• Trail Roster - APRS Automatic Packet Reporting System
• HamRadioPrep.com - Beginner's Guide to Using APRS for Ham Radio
• APRS.org - APRS: Automatic Packet Reporting System
• APRS.fi - APRS Bulletin Information
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