APRS Recommendations

Beaconing Rate

These values were agreed to at the 2008 NZART Conference (Dunedin).

Note that the times below are approximate, you should use the stated value as a guide and in fact use a value a few minutes either side. This is to avoid constant collisions with others who might otherwise beacon at the exact same time as your station.

  • Homes: 60 minutes approximately.
  • Weather Stations: 15 minutes approximately.
  • Trackers: usually defined by the tracker itself. As a rough guide, often enough to give a reasonable track, but not so often as to hog the channel.
  • Objects: This depends on the Object. A repeater or hamfest venue could be once an hour, whereas something related to an urgent situation (washed out bridge, crashed aircraft, AREC/SAR event) should be a lot quicker.

Deviation

As with voice transmission, too much or too little deviation is not ideal. AX.25 packet using AFSK at 1200 baud (i.e. APRS on VHF or UHF) needs pre-emphasis at the transmitter and de-emphasis at the receiver just like voice mode does, and for the same reason (triangular noise spectrum of raw demodulated FM).

More details.

Digipeaters

DWAIT
This is a parameter hidden away in the TNC (or equivalent) configuration. It might be alternatively be referred to by the combination of PPERSISTENCE and SLOTTIME. With traditional BBS oriented packet, the idea is for users to politely share the channel, however with APRS being a one-to-many scheme it is to our advantage for beacons to spread out rapidly through the network and it uses up less timeslots overall if digis immediately repeat what they hear, and FM Capture Effect is used to overcome what one might think would have been interference.

For an APRS digipeater, DWAIT=0 is the correct value, alternatively SLOTTIME=1 and PPERSISTENCE=255 are correct values where those parameters are used instead.

Frequencies

Frequencies on 2m are mainly 144.575 MHz and 144.650 MHz. Some years ago when 144.650 MHz was used for BBS packet use, it was decided to use 144.575MHz for APRS to avoid conflict. More recently (2008) BBS packet use has declined markedly and it turns out that very few made the shift to 144.575 MHz anyway, so it has been proposed that APRS use 144.650MHz as only a small number of digis would have to go through the administrative process and crystal purchases neccesary as compared to the far greater number of existing 144.650 MHz digis changing to 144.575 MHz. As at 3rd June 2008, frequencies are still being debated.

iGates

Transmitter Power
What power does an iGate transmitter need to be using when it is gating beacons of all NZ from the internet to RF? Perhaps it only needs enough to be only just heard by the nearest hilltop digipeater, so that RF users including distant trackers are still able to be heard by the digi and not frequently denied access due to their weaker signals being overwhelmed by very strong and very frequent signals from the iGate.

Paths

Paths in New Zealand are WIDE2-2 for fixed stations and WIDE1-1,WIDE2-2 for mobile trackers, if in a rural area WIDE3-3 if WIDE2-2 doesn't succeed, in accordance with the worldwide New Paradigm.

Txdelay

This is the time delay needed between the PTT line being asserted, and the transmitter to have developed power on the desired frequency, adequate for others to receive you properly. Some handheld radios need around 300 milliseconds, some radios designed for data use might need only 10 or 20 milliseconds.

Note that the units used in the configuration program might not be milliseconds, often they are 10's of milliseconds, so a value of "20" might correspond to 200 milliseconds. In the case of a tracker, the units might be some weird number related to a microcontroller's timer.

In practice it is wise to determine the best value of Txdelay. Too short and you won't be received properly by other stations, and too long will be wasteful of precious on-air time.

To find the right value, start with a small value, say 50mS, and gradually increase it in 50mS steps until you are just being heard, then add another 50mS to cater for receivers with a slow squelch. That is the basic idea anyway.

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