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Listening to the SA GRN

How it all works

The SA GRN offers users a combination of voice, data, and paging services to an area covering over 95% of South Australia's population. To maintain coverage over such a wide area a lot of frequencies and transmitter sites are required.

The trunking network uses a number of different voice and data frequencies at each site. As a user activates their press to talk button to transmit on a talkgroup the network controller (computer) allocates them an available voice channel from the site they are affiliated with, a voice channel is also allocated at any other sites where users on that talkgroup are located.

All sites have one frequency that constantly sends and receives information to and from radios - this is called the control channel. When switched on the radios transmit their ID number to the site on the control channel and tell it which talkgroup it is tuned to, this is called affiliation, the control channel also tells the radios which voice frequency to transmit and receive on.

 Diagram 1 shows radio 1 affiliated with the Mt Lofty site, he wants to talk to radio 2 who is affiliated with the Adelaide site, they are both on talkgroup A. Radio 1 pushes his PTT button and is instantly allocated a voice channel from the Mt Lofty site, at the same time radio 2 is allocated a voice frequency from the Adelaide site so he can hear the transmission from radio 1. Anyone else with a radio on talkgroup A will also have a voice channel allocated from whatever site they are affiliated with and will therefore be able to hear radio 1.


GRN Radio Network Diagram

Diagram 1

When radio 1 is finished transmitting the voice channel becomes inactive and another channel is allocated when radio 2 activates his PTT button to reply.  The same thing is happening with radio 3 and 4 but because they are on a different talkgroup to radio 1 and 2 they cannot hear or talk to them.

If a site had five frequencies allocated to it one would be used as the control channel and the other four would be voice channels. If the number of users wanting to transmit is greater than the number of voice channels available then they will get a 'busy' tone on their radio and will have to wait for someone to stop transmitting before a frequency is available for them to use. This is why the busier sites on the GRN have the most frequencies allocated, whilst many of the rural sites with lower activity levels can have a minimum of four voice channels.

The SAGRN transmitter output frequencies are split up into two separate groups, the first are between 412.475 mhz & 413.4625 mhz. The second are between 415.1125 mhz & 418.0625 mhz. There is also 3 frequencies located on 414.200, 414.2125 and 414.225 mhz which are licensed as transmitter output frequencies but at this stage appear to be unused.

All of the control channels are located in the lower group of frequencies with most of the voice channels located in the upper group, however some sites do have 1 or 2 voice channels in the lower portion of frequencies. Each site also has a backup control channel so that if interference or a problem is found with the primary control channel the site will immediately change to the backup one.

Here are some of the ways that you can listen to the SA GRN.

1) Conventional Scanner

It is possible to use an ordinary scanner to monitor the GRN, however it does present some problems. The main difficulty is because the transmitters change frequencies after each transmission you need to program your scanner with all of the voice channels for the sites you want to listen to.
It is best not to use the 'delay' feature as this will only slow down the amount of time it takes to get to the next frequency.

One of the major drawbacks with using a normal scanner though is that you will hear all users on the network, not just the ones you are interested in. So you might be listening to the fire service responding to an incident one minute, and then the Mile End bus depot discussing a problem the next.

Some agencies are also using digital radios which when monitored on a scanner sound like unintelligible data, this is very annoying and will prevent you from listening to other users. This is one of the major drawbacks in trying to use a normal scanner on a trunking network. As well as this the SA GRN transmits an unmodulated carrier on each voice channel every minute to test transmitter components, although it only lasts half a second it is quite frustrating and will lock your scanner onto voice channels unnecessarily.

Using a conventional scanner may be possible on rural sites with low activity levels, but is not really practical on the busy metropolitan ones.

2) Trunk tracking Scanner

The trunk tracking scanners are similar in appearance to a conventional scanner, but allow you to track specific users on trunking systems. By searching within trunking systems you are able to find the talkgroups that agencies are using, you can then enter them into a scan bank and listen to only those users, similar to what you would do using a normal scanner on non trunking frequencies. Whilst searching you also have the ability to lock out users that are of no interest (including digital talkgroups). The trunk trackers are also able to operate as normal scanners and you can actually use it to listen to trunking and conventional users at the same time.

Currently Uniden have a couple of trunking scanners available in Australia. The UBC245 portable trunking scanner released in the year 2000 does not work correctly on the SA-GRN due to a unique configuration setting on the system. DO NOT PURCHASE A UBC245 TO MONITOR THE SA-GRN.

This problem was rectified with the UBC780XLT mobile trunking scanner that was released in April 2001, this is an excellent scanner and works very well on the SA-GRN.  The PRO 96 portable and Pro 2096  Base / Mobile trunking scanners have also been found to work well on the SA-GRN, both scanners only require the user to enter the control channel frequency to monitor GRN traffic, thereby leaving more channels available in the scanner for other frequencies. The Uniden UBC246T has also been successful on the SA-GRN, it also has the ability to automatically lock on to any nearby radio transmissions with its 'Close Call' function. And the recently released UBCT-8 and UBCT-9 also works well on the SAGRN.

Uniden UBC 780xlt Trunk tracker

Uniden UBC 780xlt Trunk tracker

Trunking Software

Before trying to operate this software it is advisable that you read the documentation

This software when set up correctly will allow you to watch talkgroups and radios as they become active and move around the SAGRN. When used in conjunction with a computer controlled scanner you can actually follow users as they change from frequency to frequency, similar to what a trunk tracking scanner does. The advantage of this software is that it allows you to see all of the talkgroups that are active at once and will allow you to prioritise the users you want to listen to.

To get the Trunker software to work correctly you need a small hardware interface that goes from a scanner monitoring the control channel data, to the COM port of your computer. These interfaces are commonly called data slicers and can be made cheaply and easily or bought in Adelaide from D.J.M. Electronics who are located at Unit 1, 543 Churchill road Kilburn, telephone 8349 6308, cost per unit is $25.

Although Trunker is excellent software it is designed primarily to work with 800 mhz trunking systems, the SAGRN however operates in the 400 mhz range so it is difficult to get it to work correctly on 400mhz Motorola Smart-zone Omnilink systems like the SAGRN.

The problems with Trunker have been overcome with some redesigning of the software by a local person to fix the problems and get a version that runs correctly on the South Australian and New South Wales Government Radio Networks.

Austrunk 1.4a

Austrunk is based upon the latest version of Trunker but features many enhancements to allow it to show greater information on the Smartzone Omni-Link Motorola systems found in Australia.

Austrunk features

* Automatic frequency detection of both the S.A. GRN and N.S.W. GRN when locking onto sites.
* Correct displaying of trunking diagnostics and channel status.
* Displays the correct site alias.
* Correct control channel and alternate control channel display.
* Lists neighbouring sites with frequency and channel ID, and displays them on screen.

Austrunk version 1.4 is available for download here, however it is advisable that you also download Trunker 3.1 and read the documentation before trying to use the Austrunk software. This program can be difficult to get running for newcomers and people with little electronics or scanning experience, if you have any difficulties try joining the Trunker yahoo-group available at


Austrunk operating on the Trott Park GRN site

Austrunk operating on the Trott Park GRN site

Austrunk operating on the Mt Lofty GRN site

Austrunk operating on the Mt Lofty GRN site

See Also:
Scanning Software

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