ST. Lawrence County ARES 


                  ( Amateur Radio Emergency Service )


Supporting Emergency Communications and Public Service



( Automatic Position Reporting System )

What is APRS?

After years of amateurs using radio equipment to relay digital information from weather to mail from station to station an Amateur Radio user named Bob Bruninga, WB4APR, integrated the equipment needed to run a streaming signal from a GPS to the Radio Waves, this gave listeners with the proper equipment the ability to know the position of the transmitting station.  This is how APRS was born.

The APRS system uses packet radio as the means of transmitting the data.  Unlike traditional packet operations in which communications occur on a one to one basis, however, APRS uses unconnected packets to transmit data on a one-to-all basis.
An APRS packet may contain station location (latitude and longitude) and station type (home, portable, mobile, digipeater, weather station, etc.) information.  An APRS base station receiving these packets, processes the information contained within them, and displays an appropriate symbol on a map showing the location of the station.  If a station such as a mobile or portable station is in motion, APRS changes the position of that station on the map when it receives a new position packet that indicates a change of location.

The station in  motion could be only a radio, TNC and a laptop computer running APRS.  As the station travels along its route the station operator updates the position of the station on the APRS map and the APRS relays the new position to other APRS stations to update the position if the stations on their APRS maps.  This requires much manual data entry and is fairly operator intensive, if however the station is fed information from a GPS all the data entry is automatically entered, freeing the operator to enter other information of importance or to just sit back and enjoy the ride.

Place an APRS station in an emergency vehicle being used in a disaster area or in the lead car of a parade, and the possibility of using APRS as a public service tool for tactical communications becomes obvious.

In addition to tracking stations in motion, APRS also tracks any object in motion when the position of that object is entered into the system.  For example you can enter the latitude and longitude of a hurricane and the position of the hurricane appears on the map of everyone using APRS on that channel, you can also interface weather monitoring equipment an APRS station to disseminate real-time weather information to other APRS stations.

As with traditional packet radio stations, APRS stations use digipeaters to propagate their transmissions, but unlike traditional packet, APRS stations do not have to specify a digipeater path.  Rather, APRS stations can use generic digipeater paths so that no prior knowledge of the network is needed.  To further propagate APRS, the Internet is an integral part of the system and is used for collecting and transmitting current APRS data on an international and real-time basis.

APRS Components for the Mobile Station

The three main components of a mobile station are the GPS the TNC and the Radio.
Some Radio Manufactures are now producing hand held radios with the GPS and TNC integrated into the unit, allowing for instant transmission to the APRS network.

The GPS requirements for APRS are simple.  The GPS receiver must output data in the format specified by the National Marine Electronics Association (NMEA) standard NMEA-0183.  Most navigational equipment that has data output will match this criteria.
For mobile and portable APRS applications, the size, weight and power requirements of the GPS receiver should be considered.
The smaller the better, especially if the completed unit will be carried by hand.
Although most APRS setups just use a regular GPS with a serial port, some units are built with just a receiver.  These are much smaller without the case, power supply and display.  They are also easier to build into a package with the TNC and Radio.

The TNC or Terminal Node Controller is the coupling between the GPS and the Radio.  It takes the digital signal from the GPS and reformats it to a form that is transmitted via the Radio.  Most major TNC manufactures now support GPS signal input and many of the older models have firmware upgrades to allow for APRS operation.  Some newer manufactures are producing pure GPS ready TNC's which are limited and can not do many of the functions of a regular TNC but have all the functionality needed to interface a GPS directly to the radio, these units are very small and are perfect for hand held stations.

The Radio requirements for mobile APRS are perhaps the easiest.  There are no special radios for this task.  Because the type of radio will depend on the frequency the user intends to use.  For remote travelling applications an HF radio will be best suited but for the typical rural and suburban use VHF radio's will be best suited.  The only requirement is that the radio can be keyed to transmit via the TNC.  Of course as in the GPS and TNC size will matter if this is a hand held unit, so in that case keep it as small and light as possible.

APRS Components in the Field

APRS Digipeaters -  TNC's and Radios are both components of the standard digipeater station.  Although any Base station can be a digipeater and in fact is encouraged to be a digipeater to remove holes in APRS network coverage, sometimes it is to a users or radio club's advantage to setup a digipeater in an area where the user may not wish to have a station but where the radio coverage is extensive.  This system will have a fixed location but will store and forward all received location packets to other stations, it will be however a simple design and as power efficient as possible, perhaps running on solar or battery power.

Most TNC's can be setup to act as a digipeater and once connected to a radio can go about relaying information as long as the power supply lasts and the radio coverage is good.  These stations can also relay other information on themselves such as station weather information and power supply/status information if the proper interfaces have been attached to the unit.  Since they are stationary these stations do not require a GPS, and the location is just programmed into the firmware.

Radio's for a remote station have the same requirement as for mobile/portable stations, the only requirement being that they can be keyed on by the TNC and are efficient enough to be run long hours on batteries.  Remote station radio's should also be rugged and hardened to extremes of temperature since most remote locations are not climate controlled.

APRS Components for the Base Station / Gateway

Base Stations and/or Gateways are backbones of the APRS network.  As with the fixed station digipeaters these stations do not require GPS interfaces as they are at fixed locations and can have their latitude and longitude entered manually.  The Base Stations do however usually have a Personal Computer attached to them, and they can retransmit the received packets to other stations just as a digipeater if configured to do so.  These stations if connected to the internet can also act as a Gateway to send position data for all stations polled by its local TNC to APRS Servers allowing international use of the APRS network from any computer user, even non-amateurs may query the server from an internet connected computer.
Other gateways may exist that use two or more radios of different frequencies to bridge coverage areas.  For these systems each radio is equipped with its own TNC which are either linked to the others TNC's or to a Computer to allow crossing of data between the frequencies.  This allows users to get more station location information than would normally be available with only one local network.  This may be used for getting more information from the local VHF APRS network through a HF gateway to another VHF network which is isolated hundreds of miles away.
The ST. Lawrence County and NNY APRS system.

   Last fall a few members of the Oswegatchie Valley Amateur Radio Club decided to set up an APRS network in the North Country.  Our plan was to install a couple digipeaters in the area and try to get others interested in this mode of communications.  KA2MHZ and WA2NAN were the first two stations on locally and installed two digi's, one at the FINE repeater site, and one in Gouverneur at the Radio Shack store. We settled on using the UIDIGI firmware as it can be run as a stand-alone without the need for a computer.  KA2JXI burned the neccesary EPROMS for the project.  We now have a digi at K2SLC the EOC in Canton, and one at the qth of member KC2HGK in Casterland and Ogdensburg.  We are in the process of putting one in Copenhagen.  Other local stations on board at this time are W2NVW, KA2JXI, KC2HGK and N2MX.  Several others are coming aboard soon.  The ARES -  RACES group is in the process of incorporating APRS into the county Emergency Plan.  If you might be interested in trying APRS contact any of the stations listed for more information.

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Web Pages by Kerry Bickford, WA2NAN1997-2012