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| insert here APRS interface running on an eeePC701 |
| APRS interface running on an eeePC701 |
HF APRS in connected mode
APRS?
Yes, APRS POSIT update via PSKmail is only 1 click away (use the POS. button).
The only thing you have to do is input your local position (latitude, longitude) into the 'options' form and click the POS. button. The position will be updated via moc.udnif|tisop#moc.udnif|tisop. via email, but will not enter the APRS backbone.
HF APRS in unproto mode
What is a local station on HF
The problem with APRS on HF is that the range of the servers is unpredictable, and frequency planning is a nightmare. When a mobile station sends a message, a POSIT etc… all servers which hear the mobile station feel responsible for transporting the packet, unless strong source routing is used.And the area covered is the whole world (in principle, depending on propagation). As a matter of fact, I am often too close to the server to maintain a stable connection, and I have to use one which is further away!!
PSKmail tries to solve this problem by letting the servers talk to each other, and negotiate which one takes responsibility. The servers not responsible then just drop the packets.
The servers are all connected to the APRS backbone. As soon as a client connects a server or sends a LINK packet to it, it is added to a link table at this server. The server owning the link sends a short message over the backbone that it is in charge now, and other servers drop this particular client from their link tables.
This way there is no ambiguity who serves whom, and RF traffic is reduced to the bare minimum. It is also the basis for a simple level 3 routing scheme. The problem with gating directed message packets from internet to HF has herewith been solved… There is no reason why the gateway can't be bi-directional!!
PSKmail-0.1.9 used aprsd to connect to aprs, but in first tests it turned out that it would be much easier to build the connection into the program itself. In PSKmail-0.1.10 this has been implemented.
When connected to aprs, the servers send a time/position message so the health of the server pool can be checked constantly via findu.com.
Linking up with the server
There are 2 ways to tell the server you want it to service your client:
1.Use connected mode to send email messages
2.Use unproto mode and send a link packet: <SOH>00uPA0R><PI4TUE xxxx<EOT>
The link packet can be send from the keyboard (Ctrl-L).
The server will respond with <SOH>00uPI4TUE<>PA0R xxxx<EOT> and refresh the link list.
The server will now keep your call in the list for a limited duration, unless you generate some traffic now and again. A posit or a regular message is enough to keep the link status. Once you get out of range you send a link packet to a different server, and you are in business again….
Starting with server version 0.2.0 the server will acknowledge APRS messages with 'QSL', so you know the message has come through without error.
Protocol
Why is PSKmail not using the aprs protocol on HF?
The aprs protocol was designed to work with VHF digipeaters. Therefore it uses long headers which are neither suitable nor necessary for use on HF. In the HF arena digipeating is useless, as the area covered by a station is immense (remember we have CW QRP qso's with ZL…). Moreover, the HF environment is completely different from VHF/UHF. QRM and QRN knock out bits from our information packages, rendering them relatively useless. The TOR system (the commercial mother of AMTOR) used packets optimized for conditions on HF. They are short, so chance of being hit by QRN is minimized.
PSKmail tries to find a compromise between info throughput, TX power, bandwidth and environmental conditions. Header information is left to a minimum, the server takes care of protocol conversion to APRS (TM), so interoperability is possible. The payload (the message) is in aprs format to keep conversion to a minimum.
e.g. rrrrrrrrrr<SOH>00uPA0R:26 !5122.44NI00520.17E-PSKmail-0.1.10805E<EOT>
You will recognize the aprs posit message with the characteristic coding… The only overhead added is the preamble (rrrrrrrrrrr) to make sync easier, the protocol header (00u), the call of the originator (PA0R), and the PSKmail service used (:26). As a post-amble a 16-bit CRC is added (805E) to enable a check if the packet is received ok.
In future, the CRC will be used to repair small packet errors, making the system even more robust.
Just think about it. A position message for a stationary object will have the same CRC code every time…. In such an extreme case you actually need only to receive the CRC correctly to completely rebuild the message (you have a chance of 1:32000 this does not work).
Does it work?
It does. The first interoperability test has taken place between PA0R and K6ACJ-7. PA0R was on 10148 kHz, linked to the PI4TUE server in Eindhoven. K6ACJ used a D7 (FM dual bander), connected to the aprs network.
How many users on 1 qrg?
I don't know, not many. When a station is connected to the server, the others are locked out. But the servers can easily be staggered in frequency (every 200 Hz a server)… Messages take relatively long (6 characters per second), which is a factor of 4 slower than 300 Bd packet. On an HF packet channel information throughput grinds to a halt when more than 4 stations are active. You can do your own calculations; YMMV. We will have to try this in practise, and work according to the flexible response principle.
Aprs messaging operation with PSKmail
Auto-posit
PSKmail will send a posit packet to the server every xx minutes (30 minutes for a stationary station, 5 or 10 minutes for a mobile). This is automatic. The position message will only be picked up by the server you are linked with. The server will ack the posit message with 'QSL' in case there is no error.
The APRS menu has a switch for killing the posit beacon (default is on), in case you want to chat in TTY mode.
Station-to-station messages
To send a message to a station via aprs, Just input the call and the message into the input field of the client an hit ENTER:
PE1FTV Test message…
That's all.
When the message is heard by the server you are linked with, it will be input into the aprs system. The packet will be acked by the server with a link package, so you know it has arrived at the server. End-to-end acknowledgement of the message is built in.
Via the APRS menu you can get the list of the last 10 lines of the messages at findu.com.
This way you can use pskmail as an APRS message box.
Group messages
Group messages is being tested at the moment. To send a message to the group, write group address and message in the input field and hit return. Contrary to station-to-station messages, group messages are not acked.
Instant email
To send a short (one-line) email to a station in unconnected mode, just input the email address and the message into the input field of the client and hit ENTER:
gro.tasma|r0ap#gro.tasma|r0ap Test message…
That's all.
When the message is heard by the server you are linked with, it will be input into the email system. The packet will be acked by the server with a link package, so you know it has arrived at the server, and if the email address is valid it will immediately be delivered into pa0r's mailbox..
Example of an HF aprs 'qso'
A qso between PA0R and PE1FTV, 16 December 2005. The distance between the stations is only 3.2 km (less than 2 miles).
PE1FTV is at work, operating his station on 144.800 MHz (the PA0 aprs frequency) via an internet link…
PA0R is at home, using a laptop and the FT897 (20 Watts), operating on 10.148 MHz via the SM0RWO gateway in Stockholm (2000 km). Some of the qso was knocked out by a pactor2 station appearing on frequency…. and going away…
09:58-PE1FTV-ja, prachtig Rein, goede morgen{15
09:59-PE1FTV-Loopt best vlot{16
10:01-PE1FTV-aha .. vandaar ..{17
10:04-PE1FTV-same to you, tot werkens en 73!{19
10:05-PE1FTV-[AA] Sorry … out of office .. 73, Ad{20
Here is the log (from gMFSK) of this short 'good morning' qso….
insert here Screenshot-2.png
APRSD installation
It is not necessary anymore to install aprsd with the server, PSKmail uses its own connection to the internet.
Project queue
- Server sends group of received posits every xx minutes (from memory) to keep the qrg clear for messaging
- Possibility to download list of messages in connected mode (POP for aprs messages) (DONE)
- Possibility to get list of posits near you in connected mode (POP for relevant posits)
- Add aprs protocol receive ??
- Telnet port to connect Xastir (an aprs map and UI application) to the PSKmail client
All this so you won't have to run your RX the whole day during a trip….
// from old_wiki 15/12/2017//