Net14 The World Wide APRS Net

With the title Automatic Position Reporting System it is perhaps not surprising that the main interest seems to be the tracking of stations that are moving on land, water and in the air. But APRS does offer a communications tool with some advantages over other modes:-

1. A common frequency is employed.

2. With stations sending beacons at regular intervals one can see at a glance what stations are QRV without the need to tune across the bands listening out.

3. Personal keyboard messages can be exchanged either in real time or left if the operator is not at the keyboard at that time.

4. The exchange of messages can be quite relaxed time wise allowing the use of translation software and therefore communication with stations in their native tongue.

5. Messages of general interest, called Bulletins, may be sent by any station for reception by all stations.

6. Stations can act as digipeaters that potentially enables a world wide net.

Net14-20m APRS

Objective: The establishment of a World Wide HF APRS network primarily for the exchange of messages between licensed radio amateurs in real time. Considerations of tracking, propagation study etc are secondary. It is not intended that it should be a fixed Net with nodes like Packet but one where members are QRV when it suits them.

Method: HF APRS stations in the best locations with digipeaters enabled with an agreed setup.

Mode: To conform to the general HF APRS/Packet practice, i.e. 300 baud, SSB. PSK has its attractions for DX working but errors that are acceptable with normal one to one keyboard communication create problems with APRS For example an error in the beacon's callsign or locator results in a ghost station appearing on maps.

Frequency: 20 metres would seem to offer the best prospect for DX working and we will be operating on 14.103 MHz (Dial reading, LSB with KAM tones). This will put the sidebands 200Hz above the Beacon segment. If your modem uses PK-232 tones, as does AGWPE, set your dial reading to 14.103,510 MHz. Please ensure that your station is on frequency and does not drift.

Station Setup

Station Icon: To identify your station as part of Net14 it is suggested that you use a SSID of -14.

Location: Enter your Longitude and Latitude as required by your software. You can opt for an IARU locator and leave the Longitude and Latitude boxes blank and enter your IARU locator in the Beacon comment, between square brackets thus:- [IP91XR]. This will result in a grid symbol being displayed on receiving stations maps. See Background Information & Tips below.

Unproto address: Enter APRS,WIDEn-N. A maximum n of 3 is probably about as high as it's worth using

Beacon comment: Keep it brief.

Beacon interval: The beacon interval needs to be adjusted against conditions. 15 minutes would seem a good starting point.

Digipeater Setup

Enable digi: Enable your digi unless there is another station close by doing a better job than your station.

Digi Routes: If your software has this facility for crossband digipeating please only enable inband.

UI-View Messages & Ping: While UI-View offers some attractive features, for messages and PING in particular, these can only be used between stations using UI-View. Therefore if you message a non-UI-View station ensure the APRS box is checked.

Exclude Stations: If your software has a facility to exclude stations and an audio announce stations facility you may wish to exclude your own station so you are only alerted by other stations being received.

Status Text: If your software provides for Status Text you may wish to use it to convey some information about yourself or your station. But keep it brief and set the interval longer than your beacon since it does not alert other stations to your presence.

Modem: The recommended modem is the Soundmodem written by Andrei, UZ7HO. It is easy to set up and has a number of useful features including multiple receivers to deal with stations that are off frequency and filters to allow unwanted frames to be blocked from your APRS application. These filters are setup in the ini file by the operator to their own preferences. Blocked frames will appear green in the Soundmodem Monitor.

The "ExcludeCallsigns=" statement is used to block stations by callsign with multiple callsigns separated by commas. This may be used, for example, to block stations that crossband from 2m by inserting their alias. This will stop your digi from adding to the clutter.

The "ExcludeAPRSFrmType=" statement will block frames by the type of data. The type of data in the frame is indicated by the first character in the Information field of the frame, which for example is "}" in the case of Third-party traffic (IGated traffic). The ASCII code of the character is entered, which in the case of "}" is 125. Multiple entries are again separated by commas. For a full table of the codes in the Information Field visit www.aprs.org/doc/APRS101.PDF and you will find APRS DATA IN THE AX.25 INFORMATION FIELD on page 17. To convert these codes to their ASCII code visit www.asciitable.com . Another way for operators to make their choices is to check the uz7ho Monitor and note frames that they would rather not digi or see in their APRS application. The first character after the time marker on a new line is the one to note. Just check that on the ASCII table.

The uz7ho program and handbook may be downloaded at http://uz7.ho.ua/modem_beta/ this is not a link.

Net14-30m APRS

The operating parameters for the channel will be the same as that for Net14, except for the frequency of course, and are laid out in full on the Project Setup page. The main points being as follows:-

Frequency: 10.147,6 MHz SSB/USB. This is the dial setting for KAM tones. For PK-232 tones (AGWPE subtract 510Hz).

Baud Rate: As with Net14 and general HF practice the baud rate is 300bd. Please remember that at 300bd a frame will take 4 times as long to transmit as the 1200bd used on 2m. So keep beacons and messages as short as possible and adjust your beacon interval against conditions. 15 minutes seems a good starting point.

Digipeaters: If there is a digi in your area doing a better job than your station can then leave it to that station. Please, please, please do not crossband 2m because it will clutter up the channel to no good purpose. Also crossband from 30m can be a no no because of the presence of crossbanded 2m stations. Particularly vehicles transmitting their position every few minutes. And please no Internet to RF Gates but RF to Internet by all means.

SSIDs: We use -14 on Net14 to indicate 20m operation, suggest -15 for 10m and -10 for 30m. Why use these SSIDs? Because it allows the station's originating frequency to be identified and highlight stations that have possibly been crossbanded from 2m.

Background Information & Tips on HF Working

These notes assume that the reader already has some experience with APRS on the VHF band.

Operating APRS on the HF bands is different in a number of respects to operating on VHF. On the VHF band APRS employs FM modulation while on HF, with the exception of some activity on 10m where FM is used, SSB is used. Not SSB as we normally think of it with phone but with just two tones. These are generated by a TNC or a computer program and results in two corresponding sideband frequencies when SSB is selected on the transceiver. The tones are always 200Hz apart but their actual frequency is dependent on the standard used, of which there are two: -

KAM 1600Hz & 1800Hz i.e. centred on 1700Hz

PK-232 2110Hz & 2310Hz i.e. centred on 2210Hz

Where a carrier (dial) frequency is quoted it is normally for KAM tones and therefore when PK232 tones are used it is necessary to make an adjustment to the carrier frequency of 510Hz. With APRS it is normal to use LSB on most of the HF bands so if PK-232 is used the carrier should be increased by 510Hz.

On HF the tuning is very critical, 50Hz off frequency you will not decode 300bd APRS signals. If your setup does not provide a tuning aid, such as that provided by the AGWPE program, load a program, such as a PSK program, that does. If your setup uses KAM tones centre the displayed signal on 1700Hz, for PK tones centre it on 2210Hz. The 300bd APRS signal is about 300Hz wide. The received signals may not be on frequency as far as you are concerned but you have to go along with the crowd or sit there in isolation.

On VHF/FM APRS has a baud rate of 1200 whereas on HF, again with the exception of some SSB activity on the 10m band, 300 baud is used to keep within accepted digimode practice. This means of course that the transmission of data will be four times slower and the possibility of collisions will be correspondingly greater. In the case of VHF/FM there is the capture effect where normally the stronger signal will prevail and the weaker one is lost. This is not the case with HF/SSB, any serious QRM will result in both signals being corrupted and not being decoded.

Unlike VHF, where paths between stations tend to be consistent, HF communication is very much dependant on conditions and can vary from non-existent to 1000 miles+. These conditions can change in a matter of minutes and there is the usual QRM and QRN problems that one experiences with other modes on HF. While the ear and brain may allow phone or CW communication under quite adverse conditions any corruption to the APRS frame will result in it not being decoded.

So how might we maximise our chances with APRS on HF? As noted above the chances of collisions are four times higher with 300 baud. If the length of the frame is kept to a minimum this will reduce the opportunity for a collision. Compare the two frames below. They both convey the essential information on the station's callsign, location and operator's name and yet one is twice as long as the other. Other information can be communicated if messages are exchanged or information can be made available in the Station Info if the station's software has that facility.

G0JXN-14>APU254,WIDE3-3
=5143.11N\00001.71W-Cheshunt Herts.Op Jim. www.thisismywebsite.com {UIV32}

G0JXN-14>APU254,WIDE3-3
=5143.11N\00001.71W-Op Jim {UIV32}

Do you really need to give your address, website etc? After all your location will be shown on the receiving station's map. Another way of shortening the frame is to use an IARU Locator instead of longitude/latitude. This is achieved by inserting it between square brackets at the beginning of the Beacon comment. This does not of course give a precise location.

G0JXN-14>APU254,WIDE3-3
[IP91XR]Op Jim

NB: IARU 1 has adopted the New n-N Paradigm (New Style Digi) wherein WIDEn-N has replaced TRACEn-N and it is necessary to amend the UI-View .ini file. The information on www.apritch.myby.co.uk/uiview_newn-n.htm (this is not a link) is specific for VHF operation in your own country. For HF operation you need to make the following changes:-

DIGI_ENABLED=TRUE
UI_ONLY=TRUE
ALIAS_SUBSTITUTION=TRUE
DUPE_SUPPESS_SECONDS=25
ALIAS=your-call
UIFLOOD=TRACE
UITRACE=WIDE
WIDEN-N=FALSE
TRACEN-N=TRUE
SUBST_ALIAS=your-call

Adjust the interval of your beacon according to conditions. If they are poor you may wish to reduce it to a minimum but if they are good be a responsible operator and don't clutter up the channel unnecessarily.

Also avoid congesting the channel by keeping message lines as short as possible and of course use the Q code and abbreviations. If you can determine the path to the station you are messaging define that rather than using WIDEn-N. It will avoid cluttering up the whole Net searching for a path.

The reception of an ACK this can be a problem on HF. Once you have established communication with another station don't worry too much about getting ACKs. If you keep sending repeats of a line, which the other station may already have on their screen anyhow, you will just clutter up the channel unnecessarily. If you get any gaps in the received lines simply prefix your next message line with the missing line number(s). So if lines 9 and 11 are missing, on your next message you would send thus: -
09,11?Normal message.

Unlike some keyboard modes there is no urgency in responding to messages. If you are not at the keyboard when a message downloads you can answer later if the sender is still on frequency. If a message exchange is in real time you can break off to deal with a domestic matter knowing that the frequency will not be taken over in your absence. If you have translation software you can communicate in other languages. But not with abbreviations of course.

Unless there is another digipeater locally, doing a better job than you can, have your digipeater enabled. When it comes to crossband digipeating the subject is contentious. I am personally against this. In the years that I have been playing with APRS on HF I can rarely communicate with the 2m stations I see scattered all over Europe. Many are mobiles, sending out their position information about every minute, and they couldn't communicate with me unless they parked up anyhow. It just serves to clutter up the channel.

Internet Gateways give great pleasure to many but this Net is intended to be RF only.

I have no desire to create conflict but I will not digipeat any station that crossbands or stations that are crossbanded as they will defeat the objective of the net.