The 2m APRS Network

APRS was developed to convey LOCAL real-time information. As such, we are aiming to minimise the amount of NON-LOCAL data that is transmitted on the VHF network. Although it may be nice to see the location of every repeater in the country, getting the data to your display via RF prevents several mobile stations in the local area from accessing the network due to the high collision rate.

To quote the founder of APRS, Bob Bruninga WB4APR:
SUCCESSFUL APRS: The success of your local APRS is not how many stations you see on your maps, nor how far away, but how reliably your mobile or handheld or portable station can communicate with others in the local area. There is a big difference. This fundamental principal should drive everything we do with APRS in our local areas.

RELIABLE APRS: Said another way, the more stations you see above about 60 to 100 or so in typical areas, the more packets you don't see due to collisions and the less reliable your network is for local real-time APRS use.

In New South Wales, APRS operations are generally found on the national frequency of  145.175 MHz.   There may be small pockets of APRS activity on other frequencies in areas that are not yet integrated into the national network. If you are unsure what frequency is used in your area, check with your local APRS group.  

If you really must see data from OUTSIDE the local area, please consider connecting directly to the APRS-IS stream. We are planning to establish a 70cm APRS network that will carry additional traffic, making it available via RF, yet keeping the congestion away from the primary 2m frequency to allow the mobile and portable stations to use APRS as it was intended.

This link is a good description of the RF channel utilisation for a 1200bps APRS channel.

CURRENT 2m DIGIPEATER NETWORK
(Click to enlarge)

VOICE ALERT!

The Voice Alert concept makes use of the fact that the radio in your mobile/portable setup is always going to be monitoring the national APRS frequency. Voice Alert gives all mobile APRS users an instant calling channel to other mobile APRS users, regardless of the voice frequency each station is monitoring. Its also like a radar detector for other mobile APRS operators in simplex range on the open road.

It is ESSENTIAL that this is only implemented by mobiles and portables ONLY, otherwise it defeats the purpose of this application.

The concept is simple - Instead of turning the speaker volume down on your noisy 145.175 channel, simply set CTCSS encode/decode to 91.5Hz. This does 2 things.

1 - It keeps your speaker quiet, however anyone who understands voice alert knows they can contact you by voice by simply calling on 145.175 with a 91.5Hz tone.

2 - Your APRS position transmission will be sent with the 91.5Hz sub-tone, and serve as a radar ping to other Voice Alert stations in range.

Effectively, you will never hear anything on your 145.175 Voice Alert unless another APRS Voice Alert station is in simplex range. This will make sure you will have an opportunity for a voice call and then to QSY to other
channel for a QSO.

On the road you may pass another ham, but you never know it. With Voice Alert, you now will be
aware of each other's presence AND you will have a KNOWN contact frequency.