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APRS


APRS [ 1 , 2 ] is a digital amateur radio protocol. Based on this protocol, a global communication system is built. Its main tasks are: transmitting information about the coordinates of objects in space, exchanging messages, transferring data from weather stations, and much more.

What is this article about? In general, APRS is a large, complex, and incomprehensible topic even for most radio amateurs. But on Habré radio amateurs are not very many. Therefore, I would like to show that the standard itself is very good and can be applied outside of amateur radio. There are many systems where the issue of exchanging messages about coordinates is invented again and again (transport), formats for transmitting information, for example, about weather from weather stations, are invented, and methods for sending text messages are being developed. However, if the creators of these systems knew about the APRS standard, they could not only save time (at least on the development of the protocol), but also apply a number of ready-made software and hardware solutions.

Most of my knowledge of the system was gained not from the practical use of existing programs and equipment, but from the development of my own programs and utilities for working with it [ 3 , 4 ]. I drew information from the standard [ 5 ], Xastir source codes [ 6 ], amateur radio forums [ 7 , 8 ] and from communication with radio amateurs (you can’t count them all).
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If you do:

And in almost any case, when you want to build a data transmission system over the radio channel and invent a protocol for this, radio amateurs have practical experience in the form of various digital types of communication and the APRS protocol.


What is APRS?


In essence, it is a standard that describes the types and formats of messages that can be transmitted over the network. The packet format may differ slightly depending on the transmission medium (see below), but the contents and purpose of the packet do not change.

Types of messages:
  1. Beacon (Beacon) - perhaps the most frequently used message. It contains the call sign of the station, its coordinates and a short comment.
  2. A short text message (Message) is an analogue of SMS, contains the call sign of the recipient, the sender and the text of the short message.
  3. Package from weather station (WX) - contains information about the meteorological conditions at a given point (temperature, pressure, humidity, wind strength and direction).
  4. Other message types.


Beacons and text messages are the most commonly used message types. There are a number of types of messages, which can be found in the standard. Among them, for example, a station telemetry report, that is, it is possible to transmit freely determined parameters of the readings of some sensors.

How is the network organized and what parts does it consist of?


First of all, these are stations (mobile and stationary), there are still objects. The call signal (Callsign) and coordinates (QTH) of the station or object are transmitted to the network. Additionally, an icon indicating the destination of the station (car, bus, house, etc.) and a short text comment are transmitted. For example, I want to know where my car is now. Or worse, my wife wants to know where my car is now! The network just performs the function of transmitting information about the coordinates of the station and its other parameters to all those interested. As a rule, stations are displayed on the map on the transmitted coordinates. Another station, being a full member of the network, can send a text message to another station. For example, a taxi dispatcher on a map sees the cars that are closest to the right place and sends a text message with the address where to go.

Why is the system global?


Because in the ideal case, the packets from the station should go to the global Internet. After that, anyone connected to the Internet will be able to get information about the station. For this there are world APRS servers. Thanks to the Internet, the separate parts of the radio network are united in one global network APRS (and the stations can communicate with each other).

But how do packets of information get on the Internet?


There are just a lot of options and a huge scope for amateur radio!

The easiest option is when the station has a direct connection to the Internet (for example, a program on a smartphone + GPS + mobile Internet, or a program on a stationary computer + an Internet connection). In this case, a direct connection to the APRS server is performed and the program directly receives and sends messages to the global APRS network.

But it is boring! Radio amateurs have their own radio broadcast!

Wireless for packet transmission


It is believed that the APRS network has replaced packet radio. I think this is not entirely true. Packet radio is a radio version of the x.25 protocol, which is called AX.25 [ 10 ]. The stack is simple: at the physical level, transceivers transmit and receive a modulated signal, which is converted by modems into information bytes. Bytes are collected in packets. Packages contain everything you need: the source, the receiver (in the amateur ax.25 this is the call sign), the message, the checksum. There are also methods for establishing and maintaining a connection with an acknowledgment (by analogy with TCP). There are packets without acknowledgment of receipt (similar to UDP). Previously, there were a large number of BBS with which it was possible to establish connections through the radio broadcast, receive and send mail. Such BBS with mail exchange functions were even on amateur radio satellites, which could after some time, due to the characteristics of the orbit, fly over any part of the planet, sending mail anywhere.

Over time, the Internet appeared, became available, and closed the messaging needs. The network of packet radio stations began to decrease. But the equipment remained. And this packet equipment can be used even now for transmitting APRS network packets over the air. That is, APRS did not replace the packet, but packet communication, or rather only one type of packet - the UI ax.25 packet (without delivery control) and the radio channel are used as the APRS network message transmission medium.

Thus, packet radio is one way of transmitting an APRS packet from a station to a system. In principle, other methods and modulations of the radio signal, such as BPSK, can be used and are used as the medium for transmitting messages from the APRS network.

Do you need internet at all?


It seems that everything is fine, but suddenly the post-apocalypse, the sanctions, the accident at the provider? Can the system work without the Internet? That part of the system that uses the Internet, of course, will stop working in case of an accident. It will be impossible to find the coordinates of the objects through the browser and the stations directly connected to the world APRS will remain without communication. However, that part of the system that uses the radio channel for communication will be able to work independently.

A well-developed APRS radio network implies the existence of a network of a class of stations - radio repeaters (packet repeaters - digipeaters (DIGI)). Here you can draw an analogy with the base stations of cellular communication. These stations stand high, hear far and receive signals from mobile radio stations from mobile objects, which usually operate on VHF. After receiving an APRS DIGI message, it retransmits it (this is a topic for a separate conversation, because the packet is modified, a DIGI call sign can be added to the path to understand the path, or the packet is dropped under certain conditions).

In addition to repeaters, there is also a class of stations - bridges (GATE), they can send a packet from one band to another. For example, if GATE has two transceivers for different bands, then it can transmit a packet from one band to another. Communication range on VHF can reach 50km and by accepting a packet to VHF, GATE can transmit it to HF, where the packet can be transmitted for hundreds of kilometers. Thus, the package "flies" to another city. And if there was a network that covered all the necessary territories with the help of DIGI and connected them to each other, then the Internet would no longer be necessary. But such a decision has financial and technical limitations. Therefore, there is another class of stations - the gates that are connected to the Internet - IGATE. It is through them that the packets from the radio broadcast fall into the global network of APRS.

How do APRS servers work?


Worldwide APRS servers are interconnected and exchange messages that each one of them receives. The clients of these servers are endpoints that connect to the world wide APRS network for exchanging messages, or IGATE stations that send packets from the air to the world network. Other stations, to whatever server they are connected to, will receive this message. There are a lot of messages in the global network, so the servers support a filtering system according to certain parameters.

Network APRS - real-time network!


The package from the station is sent to the radio broadcast, as in a void, without guarantees of delivery. But if the packet is accepted, it will be immediately processed / relayed / sent further, or to the Internet. The received package will immediately be displayed on the world map if you have the appropriate software open. In the properties of the object, you can see how old the data from the object is. Despite the lack of a delivery guarantee, there is a mechanism for acknowledging the receipt of a text message (we send the message N times until we receive an acknowledgment from the addressee).

List of terms and abbreviations




Examples of devices and solutions for working with APRS


1 Fixed PC with Internet connection, without radio, without GPS


Equipment:

Software:

Opportunities that we get:

Comment: this is probably the easiest option that can be.



1.1 Laptop PC with Internet connection, without radio, with GPS


Equipment:

Comment: in this configuration, your coordinates will automatically change over time and other members of the APRS network will be able to see your movements.



1.2 Laptop PC without Internet connection, with transceiver, with GPS


Equipment:

Comment: in this configuration, your coordinates will automatically change over time and information about this will be transmitted to the radio broadcast. If your packets are received on the radio by other members of the APRS network, they will be able to see this information. If packets are received by a station connected to the world wide APRS network (IGATE), then everyone will be able to see the information.



1.3 Fixed PC with Internet connection, with transceiver


Equipment:

Comment: in this configuration, the station can receive, send and process packets both from the world network APRS, and from the radio broadcast, within its radio visibility range. Such a station can operate as IGATE (send received packets from the radio to the world APRS network).

1.4 Stationary PC without Internet connection, with a transceiver


Equipment:

Comment: in this configuration, the station can receive, send and process packets from the radio broadcast within its radio visibility range. Such a station can operate as DIGI (relay packets received from the radio broadcast). If IGATE is within the radio visibility of this station, then its packages can get into the global APRS network (for example, you can use the text message sending service on the EMAIL on the Internet while you are in some deep forest or at sea).

1.4 Fixed PC with Internet connection, with receiver


Equipment:

Comment: in this configuration, the station can operate as IGATE, performing this important function. If you try, you can do without even an amateur receiver, but a USB dongle with SDR technology and properly connect all the programs together.

Remark on how to connect the transceiver to the APRS station


As a rule, transceivers are designed to transmit human voice. The digital signal is transmitted in the same frequency band as speech (as in modems in telephony). To transmit and receive a sound-modulated digital signal, the transceiver may have a separate line-in and output (they do not make any changes to the signal).

But the signal must somehow be modulated and demodulated. With regard to packet communications for operation on HF, AFSK 300 baud is used. This is frequency modulation: one frequency is “1”, the second frequency is “0”. Initially, a modem was used for work. At the reception, he distinguished 0 from 1 using filters, and two frequency generators were switched to transmission. The resulting stream of zeros and ones was converted into packets using special devices - TNC (packet controllers). Thus, the wiring diagram looked like this:

RS232 RX,TX(TTL) RX,TX(Audio)  <-------> TNC <---------->  <------->  <------>  ( )


In due course, separate parts of this chain began to be combined in one device. For example, there are transceivers with a built-in modem; there is a TNC with a built-in modem; I think there are even prefab blocks, TNC with transceiver. Considered a very popular solution based on the program MixW . In this case, the TNC and modem are emulated by MixW, and the transceiver is connected to the sound card of the computer. On the part of APRS software, this program acts as KISS TNC and interacts with it through the COM port. By the way, two COM ports and a null modem wire are not required. Included in the MixW utility is a virtual null modem driver in which you can set pairs of connected virtual ports. The APRS program will consider that it communicates through a hardware COM port with a hardware TNC. In general, the program MixW is very developed, there is even a driver for a network card that will transmit data using packet radio (tested on Windows). It was possible to connect two computers through sound cards and ping. But everything is designed, of course, on the radio.

There are transceivers specially designed for operation in the APRS network, which independently perform certain functions.



Examples of devices and solutions for working with APRS (second series)



2 Smartphone


Equipment:

Software:

Comment: in this configuration, the station is always in your pocket, and the map can be displayed as a person (and not a car).



2.1 Smartphone + Transceiver


Actually, I don’t know the details, but the photo speaks for itself.



3 APRS tracker


Yes, yes, this is exactly what is being sold under the name “satellite signaling”. A tracker is an automatic device that either simply records the coordinates and time, or, plus, it can transmit beacons on the air. Interaction with the user in the course of work is not supposed. Trackers put in the car, run on balloons .

www.byonics.com/tinytrak



4 Internet connected router


If the goal is to simply mark your station on the world map at fixed coordinates, then driving around the clock for this personal computer is not very convenient. First, not every computer works around the clock; secondly, you can forget to run the program; thirdly, the program will always hang out in the tray; fourthly, after each reinstallation of windows, it is necessary to re-configure everything. And in general, chasing a PC for the sake of sending one packet is too “fat”. For such purposes, a simple device operating 24/7 with low power consumption is better suited. Router - the most suitable option.

On a router with installed OpenWRT firmware, you can run the APRX client and you will always be on the map. I did this on my TP-Link TL-MR3020 router .

5 PDAs with Windows Mobile


There is a client for PDAs - APRS / CE .



How to get into the network APRS?


Any ham with a callsign can connect to the APRS radio amateur network and use this network for amateur radio purposes. If you are not a radio amateur and do not want to become one, or you want to use the technology to build your network, system, and for your personal or commercial purposes, then you will need to correct some points.

How to raise your APRS server for your system? Is it possible to use radio broadcast if there is no amateur call sign? Which transceivers can be used and how to connect to them? All this, as well as many other technical issues (for example, message formats, range at radio frequencies, types of modulations) - this is already a lot for one article, and besides, I am tired too :) If the topic is interesting, we will continue it .

In the meantime, all the best wishes for de UA3MQJ 73!

List of used sources


1. http://aprs.qrz.ru/ - Site APRS Russia.
2. https://ru.wikipedia.org/wiki/APRS - information on Wikipedia
3. https://code.google.com/p/qaprs/ - my experimental QAPRS (Qt4) project for a stationary PC.
4. https://code.google.com/p/japrs/ - my jAPRS experimental project (JAVA ME) for a mobile phone without GPS.
5. http://www.aprs.org/doc/APRS101.PDF - PDF file - description of the APRS standard.
6. http://xastir.org/
7. http://forum.qrz.ru/27-aprs.html - APRS thread on the QRZ.RU forums
8. APRS thread on the CQHAM.RU forums.
9. UI-View - APRS client for PC
10. Protocol Description AX.25 (PDF)
11. http://www.qslnet.de/member/rz3tw/ap_st/ap_st.htm - APRS - what is it?
12. http://www.mixw.net/index_eng.php - site of the program for working with digital modes of communication MixW.
13. APRSdroid - APRS for Android
14. www.byonics.com/tinytrak
15. Client for Windows mobile
16. http://hamcmw.qrz.ru/pr/index.html - Packet Radio and APRS

Important: APRS technology is given in the article for informational purposes only. It was developed by radio amateurs, radio amateur callsigns are used to identify stations, the global network of APRSs is radio amateur servers, and radio frequencies used for communication are used on the basis of permits issued. You are not allowed to use amateur radio servers and radio frequency bands if you are not a radio amateur. However, the technology itself and the message format can be used in your projects using your servers. You can even use the radio if you bought the frequency, or use the frequency bands, power and modulation types that are allowed by the SCR to use without registration.

Source: https://habr.com/ru/post/249901/

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