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VHF

What is Smart Beaconing ?

SmartBeaconing™ is an algorithm developed by the makers of the HamHUD for dynamically choosing when and/or how often to send position updates.

The main goal of SmartBeaconing is to talk only when there's something useful to say. It's the simplest form of data compression: Data Volume = Information Volume (as opposed to "Data > Information"). It minimizes the amount transmitted while simultaneously maximizing the value of what does get transmitted.

In short, the faster you're moving the more frequently it will send out position data. It also knows to send out position updates when you chage direction by a significantly.

Configuration

When configuring SmartBeaconing functionality, there are a few parameters to consider:

  • Slow Speed
    This is the speed below which you're effectively still.
  • Slow Rate
    This is how often your unit transmits when it's still (or nearly still - any speed below "Slow Speed").
    20 minute intervals (1200 seconds) is plenty. If you are not moving, your beacon time is adjusted to be similar to a fixed station. There is no need to repeat "I'm still in the same place!" too often!
  • Fast Speed
    This is the speed above which you will transmit at the "Fast Rate".
  • Fast Rate
    This is how often your unit transmits when it's moving at any speed greater than "Fast Speed".
    60 seconds is the fastest recommended beacon rate for mobiles on VHF (120 seconds is better)

Travelling at speeds in between the "slow" and "fast" speeds will result in a beacon interval proportional to your speed. For example, if you're travelling at half your "fast speed", then you'll transmit half as often as your "fast rate" specifies. 

The following parameters make up the functionality known as "Corner Pegging".   Corner Pegging ensures that even travelling at low speeds, directional changes will cause a beacon to be sent (For example if you are in a 4x4 in the outback - you may be travelling quite slowly but changing direction will result in a beacon)

  • Minimum Turn Angle
    This is the angle by which you must change heading by in order to transmit a beacon.  
  • Turn Slope
    This is a calculation of turn angle versus speed  (angle/speed)
  • Minimum Turn Time
    This is how often your unit transmits when it's continuously changing direction.  If you are driving in a 360 degree circle, this limits your beacons to only 1 in the given interval - recommended setting here is the same as the minimum beacon interval for the network - i.e.  60 seconds is the fastest recommended beacon rate for VHF (120 seconds is better)

 

 

APRS RF Bandwidth @ 1200 baud

Here is an explanation of APRS and the effects of channel traffic on it's performance. (Bandwidth)

In this example a mobile stations transmission rate is set to 30 sec.
(Not recommended in practice)

Packets are transmitted at a maximum speed of 1200 bits per second.

So assuming an APRS posit (position packet) is 50 bytes in size from a non compressed tracker or IGate (IGates decompress compressed packets before retransmission) the packet will consists of 400 bits of data plus (from memory) 296 bits for ax.25 protocol overheads.

Total number of packet bits 696

The time taken to transmit a packet is DataTime + TXDelay +TXTail In this case 0.58 + 0.35 +0.04 = 0.97 Sec ~ 1 Second per packet.

This packet is transmitted 3 times assuming the longest mobile path of WIDE1-1,WIDE2-2. So in effect this one packet is taking up 3 seconds of RF time or available bandwidth.

Therefore you could theoretically have a maximum of 20 (60sec / 3) mobile stations transmitting one posit every minute or 10 mobile stations transmitting one posit every 30 seconds before all available bandwidth would be used. ( No transmissions from home stations or objects.)

In practice and due to the nature of AX25 (as a CSMA protocol with hidden transmitters etc.) the best bandwidth you could expect for maximum efficiency (90% of txed data delivered from point A to point B) is 30% to 40% of the total bandwidth. So in reality you could have 4 mobile stations transmitting posits through 2 hops (digipeaters) every 30 seconds before you start creating packet collisions and loosing data on the network. Or you could have 24 stations transmitting through 2 hops every 3 minutes. This again does not include home stations or objects.

This is how we get the figure of 100 packets (of 80 bytes) per 10 min as an average for the VK network. (equals 33% of the bandwidth)

As you can see packet radio is not all that efficient especially with a large number of station or digis on a network. As APRS uses a the non-connected (UI) feature of ax.25 it is possible to get away with using a bit more of the bandwidth (approx. 50%) with reasonable efficiency (maybe 70%). This would be considered the Peek Traffic Load of the RF network.

If the load went to around 60% of the bandwidth you would find that only one in 5 or 6 of the txed packets would get through one hop. (In our mobile example, txing one posit every 30 sec in this network would mean that a posit would be rxed by the receiving station one digipeater away on average every 2.5 to 3 minutes. A receiving station two digipeaters away would rx one posit every 5 to 6 minutes)

Note that all these figures are approximations to give you an understanding of how a packet network could perform. So much of the practical side of the systems is dependent on local conditions.

A few more thoughts:

At a tx rate of 30 sec you will get an updated position every 0.83 Kms when traveling at 100 kph, at 3min its 5kms per update. What is the resolution and accuracy of the maps and equipment you are using? I Km per pixel, 500 mtrs per pixel or 1 mtr per pixel.

 

Local Information Initiative

APRS Local Information

APRS was originally developed for much more than simply tracking objects and stations. The ability to pass relevent local information and alerts to travellers through a region is an ideal use of APRS, given the APRS radios and accessories that are available today. Some examples are:

Kenwood TH-D7
Kenwood TM-D700
Kenwood TM-D710

HamHUD

This link details the local information initiative as devised by Bob Bruninga WB4APR. Here in Australia we are using several of these ideas to provide local information for travellers (and of course locals!)

It is very important for all users of APRS to understand the details and purpose of this local info system, and the most important aspect of this system is the word LOCAL. That is, the packet announcing the information is seen DIRECT ONLY - in other words the information can only be seen in the local DIRECT coverage area of the repeater or event being advertised.

Done right, these objects are a great asset to the Traveller/Mobile APRS user with no impact on the network. Done wrong, these objects are just more QRM and SPAM.

Tiny Web Pages

Tiny Web Pages refers to the format of a message to fit neatly on the display of a Kenwood D7, D700 or D710 radio. They can be used to display to users of these radios information on everything such as WX, Satellites, DX, Traffic Problems, Nets, Meetings and so forth. Some examples:

TWP

Recommended repeater frequencies

Within each WIDE digipeater footprint, one or two local voice repeaters are advertised. This information is transmitted by the digipeater itself, and is NOT digipeated via any other station, and not relayed to the internet by the IGate. This ensures that the information transmitted is only seen by the stations in range of that digipeater, and therefore is targetted to the relevent area.

The beacon will advertise the frequency, offset and CTCSS tone of the repeater. This information is presented according to the APRS Frequency Reporting (AFRS) specification, and provides information at a glance incuding frequency, offset, CTCSS Tone, Range and any regular nets conducted on the repeater.

The information transmitted flashes on the front panel for 10 seconds when the packet is received.

1 .. 1

If the driver does not happen to see this display, then the Repeater will show up as an object on the radio's STATION list as shown below on a D700:

D700 Object List

 

IRLP / Echolink frequencies and status

In a similar concept to the Recommended Repeater Frequencies, the current operational status of any participating IRLP node within the local zone is advertised. Since the IRLP node creates the APRS object, it is transmitted to RF by the local IGate. Like the Voice Repeater information, the IRLP node will advertise its frequency, offset and CTCSS tone, as well as its current status (LINK CLEAR, CONNECTED TO STNxxxx, OFFLINE)

The information transmitted flashes on the front panel for 10 seconds when the packet is received.

D700 IRLP display...1

As with the repeater objects, if the driver does not happen to see this display, then the IRLP node will show up as an object on the radio's STATION list as shown below on a D700:

D700 object list

 

APRS Digipeaters 101

Why Digipeating is required

"Digipeater" is short for "Digital Repeater"; a repeater for packet data rather than voice. Unlike the standard voice repeater that receives on one frequency and retransmits what it hears simultaneously on another frequency, the usual digipeater is a single frequency device. It receives a packet of data, stores it in internal memory and then a moment later retransmits it on the SAME frequency.

Digpeating is much more critical to APRS than to conventional packet because APRS heavily involves packet data transmission to and from moving vehicles. Traditional packet was overwhelmingly used between fixed locations, typically with better and higher antennas.

Signal levels that you may consider adequate on voice probably WON'T be adequate on packet, because data transmission is an all-or-nothing proposition. ALL of a packet has to be received PERFECTLY to recover ANY data from it. The kind of noisy, scratchy, not-completely-noise-free, operation so many people inflict on voice repeaters, especially with underpowered handhelds, JUST WONT WORK on data transmissions. A pop, a momentary burst of white noise, flutter, or multipath-induced phase distortion that you don't even notice on voice WILL be fatal to a packet transmission.

With APRS, the problem is more prevalent than with conventional (connected) packet because it operates in a non-connected mode. With traditional packet, a station receiving a defective packet will automatically send a request for retransmission to the sending station, or the sending station will automatically retry if the receiving station doesn't acknowledge in a reasonable time. With APRS there is no ACK/NAK (Negative Acknowledgement) handshaking process. The sending station broadcasts packets at intervals and "hopes" the receiving station(s) get them error-free. The receiving station ignores the packet if it is defective in anyway.

Signals to/from mobile units can and do fluctuate in strength by 15-20 dB as the mobile moves over even a short distance. For reliable data transmission, you must have good signal strength over the intended path. The signal should be enough that even with a 20dB drop, the signal will remain noiseless and hard quieted.

Remember that APRS will performance will vary with local conditions an weather. For instance in Melbourne you can use a 10 watt transmitter and 1/2 wave ground plain antenna and have an 80% success rate with your beacons. In Sydney the situation is much different due to the hilly terrain. You would need more power and a different antenna, probability a 1/4 wave to increase your success rate in the valleys.

APRS Digipeater usage

To increase the reliability of transmission from mobiles (i.e. likelihood that a packet will "get through"), APRS uses two categories of digipeaters:

  • "WIDE Area" digipeaters placed in stretegic locations (typically hilltops, the tall buildings, water towers, etc); i.e. similar to the placement one would choose for a voice repeater. WIDE Area digipeater installations are generally installed as part of a co-ordinated approche with the involvment of a state APRS Co-Ordinator. This type of Digipeater responds to the alias call sign of "WIDEn-N".
  • "Fill-In" digipeaters in areas where mobile stations have poor access to a WIDE Area digipeaters (typically home stations running an APRS client with digipeating enabled). This type traditionally responded to the alias call sign "RELAY" although "RELAY" has been phased out and the alias of "WIDE1-1" is now used.

 

How APRS paths are used

PATH settings determine what kind and how many digipeaters will be used to deliver your packets to their destination. Typically the "destination" will be either other stations listening on RF, or an I-Gate that will receive your packet on RF and transfer it into the APRS Internet Servers (APRS-IS).

A transmission path of "WIDE1-1, WIDE2-1" is requesting the helping hand of nearby cooperating home stations or WIDE Area digipeater as the first step into the APRS network.

As in conventional packet, each digipeater in the chain "crosses off" the call sign it responded to. This example shows results as a user tries to use three wide area digipeaters in succession. The path string will change like this as the packet propagates from digi to digi. Note that these advanced paths require that the "call sign" actually be changed by each digi that processes it. This process of "call sign substitution" is unique to APRS and requires special APRS awareness in TNCs.

By placing two WIDEn-N statements in series in the path, you allow a simple home station "Fill-In digipeater" to "relay" the first hop while leaving the second n-N hop(s) for "real" WIDEn-N digis to properly process and decrement.
The example below shows the life of a packet that has been digipeated in this way:

WIDE1-1,WIDE2-2 (as the user transmitted it)
WIDE1*,WIDE2-2   (as a home fill-in digi or the first high-level digi transmitted it)
WIDE1*,digicall1*,WIDE2-1 (after first WIDE digpeat)
WIDE1*,digicall1*,digicall2*,WIDE2* (after second WIDE digipeat)
  • In areas without home station fill-in digipeaters, a "real" WIDEn-N digi will act on the first hop and decrement it to zero (WIDE1-0) which shows on-the-air as " WIDE1* " . By contrast a "non APRS Aware" home station will retransmit the packet as " WIDE1-1* "; i.e. not N-n decremented but still marked as used.
  • The next digi to hear the packet will act on the second hop WIDE2-2 and transmit it decremented to WIDE2-1.
  • The third digi, if any, will transmit the packet decremented to to WIDE2-0 . (actually shows as "WIDE2*" ).
    No further digipeating will occur.

Because all APRS digipeaters use the same generic call signs, the re-transmission process can happen in several geographic directions simultaneously if several more digipeaters are within range of the one transmitting. A widening circle of digipeats involving more and more digis on each hop will spread outward from the user in all directions. This phenomenon, known as UI flooding, is sharply different from the directed linear sequence of digis, each identified by a unique call sign, used in traditional connected packet.

Note that the APRS RF network is designed to limit the number of digipeaters any pack will pass through to a maximim of 3. ie WIDE1-1,WIDE2-2


Today's recommended universal path settings under the "New Paradigm" are:

* WIDE1-1, WIDE2-2 (Will produce three hops and will take advantage of home fill-in digis. Use in rural areas with low APRS activity only.)
* WIDE1-1, WIDE2-1 (Will produce two hops and will take advantage of home fill-in digis. Use in busy urban and suburban areas.)

 

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.

 

Australian VHF APRS Guidelines

The configurations provided here are the recommendations for best performance across the entire Australian VHF APRS network

Mobile Stations

MOBILE stations should use the following configuration guidelines.

  • DO set your beacon rate to a MINIMUM of 60 seconds. (The preferred rate is 120 seconds for mobiles)
    Why? 
    Because the APRS frequency is a SHARED resource. Beaconing too frequently "hogs" the frequency so that nobody else can get in. If you have SmartBeaconing available in your tracker, USE IT.   SmartBeaconing adjusts your beacon interval based on your speed - the faster you are travelling, the more frequent your beacons.
  • DO set your path to WIDE1-1,WIDE2-1. This will ensure your APRS setup will work throughout Australia.
    Note that in some areas WIDE1-1,WIDE2-2 may be required - check your state APRS site for details.
  • DON'T use any of the following paths - RELAY, WIDE, TRACE, TRACEn-n.
    All of these digipeater aliases are obsolete, and are being progressively phased out across the network

     
  • DON'T set a mobile station up as a digipeater of any kind.

Home / Fixed Stations

Generally, the following guidelines apply to a fixed/home station.

  • DO set your beacon rate to a MINIMUM of 30 minutes.
    Why? 
    Because the APRS frequency is a SHARED resource. Beaconing too frequently "hogs" the frequency so that nobody else can get in. Besides - your house isn't moving, so there is no need to refresh your position as often as a mobile station does.

  • DO check your state page to find out the path to use as a fixed station
    Why?
    Each state has implemented s alightly different methodology for fixed station beacon paths, in order to optimise the network availablility for everyone. 
  • DON'T use any of the following paths - RELAY, WIDE, TRACE, TRACEn-n.
    All of these digipeater aliases are obsolete, and are being progressively phased out across the network

     
  • DON'T use excessive transmission power. 10 watts to 20 watts is more than enough.
  • DON'T put up a WIDE digipeater unless you truly have wide coverage and there are no other WIDEs in your area, and unless you plan to let it run 24 hours a day, 7 days a week.
    Why?
    The APRS network infrastructure needs to be carefully co-ordinated to ensure it remains effective.
    Each state has a co-ordinator or co-ordinating group that has overall visability of the network, and can determine whether digipeaters will help or degrade the existing network. WIDE area digipeaters also operate a dedicated TNC running the UIDIGI firmware. If you think you meet these conditions, please contact your local region co-ordinator before setting up the digipeater.

  • DON'T set your station up as a Fill-In digipeater (WIDE1-1) if there is already one close by. If you think there is a requirement for a Fill-In digipeater, please contact your local region co-ordinator before setting up the digipeater.
  • DON'T set your station up as an IGATE (i.e. DO NOT gate data from the Internet to RF). This will only create traffic loops and problems on the channel.    
    If there is no IGate serving your local area (i.e within 2-3 digipeater hops), or there is no APRS activity at all where you are, please contact your local region co-ordinator so we can integrate your area into the network effectively.

Please refer to your state APRS site for the correct configurations in your area.

National 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.

For these reasons, the 2m network is generally configured to maximise the availability for mobile/portable stations. The amount of data transmitted from the internet to the VHF network is minimal, so as to allow the maximum amount of airtime for mobile trackers to send thier data into the network.

In some areas a 70cm APRS network has been established that carries 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.

Frequency

In Australia, a single national 2m frequency is used for APRS:    145.175 MHz

There may still be small pockets of APRS activity on other frequencies in areas that are not yet integrated into the national network, however these areas are very small and isolated. 

VHF Path Settings

For Mobile stations, the path of  APRS v WIDE1-1,WIDE2-1 is used nationally (even Internationally).

For Fixed stations, a path of APRS v WIDE2-1 is generally recommended, however each state may apply slightly different practises. Please check your local state pages for further information.

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