We recently became interested in the Automatic Packet Reporting System that is used over Amateur Radio. This system is primarily used on the 2m band to report position, weather, emergency information, telemetry and other data over a shared RF channel. The traffic on the Amateur Radio APRS network originates from a variety of sources including mobile VHF radios (FM and DSTAR), HTs, Weather Stations, Personal Computers and more recently, smart phones. APRS uses a combination of the Internet and the 2m (and 70cm) radio bands to transport position and other information over RF to gateway ports to the Internet. The data is consolidated and displayed on sites like aprs.fi.
We decided to set up an APRS station in our shack so that we could learn about APRS and its applications. We selected a Kenwood TM-D710A Transceiver for our APRS base radio. Kenwood is a leader in APRS technology and they incorporate the necessary AX.25 Terminal Network Control (TNC) in several of their radios including the TM-D710A. We added an AvMap G6 GPS to the base radio to provide a local display of the APRS station information that is received over the air. We also made use of our Diamond X300NA antenna to test our APRS station. This antenna is up about 50 ft and provides a decent level of gain (6.5 dB) for APRS work. While this antenna has a fairly low pattern designed for repeater access work, it turned out to perform well during the initial testing of our APRS station.
The first step in getting the station on the air was to program the TM-D710A as an APRS Digipeater. A Digipeater listens to the shared APRS radio channel (the 2m APRS channel in the United States is on 144.390 MHz FM). When an AX.25 APRS packet is heard, the Digipeater’s TNC decodes the packet, displays it on the local radio (and an attached display or PC if available) and then decides whether to retransmit the pack so that other stations further away from the source can also receive it. APRS has used a number of protocols for Digipeating since its inception. The current protocol is called WIDEn-N. The WIDEn-N protocol uses a Time To Live mechanism to ensure that APRS packets are only propagated a limited number of hops before they are discarded. It also provides tracing so that the source of an APRS packet and route that it has taken can be determined. Kenwood has a good document which explains APRS Digipeating and how to set up the TM-D710A to operate as a Digipeater. The following picture shows our APRS station operating in Digipeater mode and provides an example of APRS packets as they are received and displayed. The AvMap G6 GPS is a nice accessory for an APRS station as it displays the received APRS packet information on a map display. In a mobile APRS setup, the AvMap can also provide “dead reckoning” navigation based to an APRS station that is also mobile. The AvMap will calculate an intercept path been its location and a target APRS station accounting for the speed and direction that both stations are moving in.
APRS packets contain a good bit of information about the station originating the packet. In addition to the GPS coordinates where the packet was originated, APRS packets may contain telemetry or other information from the originator. The picture below shows a packet from a weather station connected to an APRS transceiver. As you can see, the current weather conditions at the source are contained in the packet and can be displayed on the Kenwood TM-D710A.
The APRS system also provides for the transmission of short messages and eMail to and from APRS radios and client devices such as PCs or smartphones. The following picture shows an example of a short message sent to our APRS base station from one of our iPhones. APRS provides for Secondary Station Identifiers (SSIDs) which allows multiple devices owned or operated by a single call sign to be separately identified. In our case, we have the following APRS devices setup so far:
- AB1OC-10 – our APRS base station
- AB1OC-7 – an APRS HT
- AB1OC-9 – Fred’s iPhone running an OpenAPRS client (search for “OpenAPRS” in the iTunes Store)
- AB1QB-9 – Anita’s iPhone running the OpenAPRS client
To complete our APRS system, we also purchased a Kenwood TM-D72A HT. This unit also has a built-in AX.25 TNC and a GPS making it an ideal tactical APRS station for emergency and other public service work.
The second step in setting up our APRS station was to create an APRS Internet Gateway or IGate. An IGate is an APRS radio node that is attached to an Internet connected computer for the purpose of getting APRS packets on an off the Internet. In most APRS networks, an IGate node is the last step in the radio path between and APRS client device like a mobile radio and the Internet. Once the APRS packets find their way to an IGate, the IGate is responsible for routing the APRS packets using the Automatic Packet Reporting System-Internet Service protocols to an APRS-IS tier 2 server. The Tier 2 Servers are typically deployed regionally and handle the process of distributing the world-wide load of APRS packets to the IGates. The packet processing and routing of the Tier 2 Servers is coordinated by a set of Tier 1 APRS-IS Servers. This two-level structure is required to efficiently and reliably handle the massive load of APRS packets that are routed world-wide. Other non-radio oriented services which use APRS protocols such as Citizen Weather Observer Program (CWOP) and OpenAPRS can also interface to the APRS-IS Servers.
There are several APRS clients that provide IGate functionality. We selected the APRSISCE/32 software provided by KJ4ERJ to implement our IGate. We chose APRSISCE/32 because it has an active developer and support community, is feature rich, provides a good user interface including nice map displays, and is compatible with the Windows 7 platforms that we use. Other popular choices include UIView, WinAPRS and several other programs. APRSISCE/32 has a good Wiki that explains the program and how to set it up. The Wiki plus the APRSISCE Yahoo Group were all we needed to get our IGate up and running. There is also a user manual for the software that is available here.
The APRSISCE/32 Software combines APRS packet information received by the APRS base radio with packets routed over the internet from the APRS-IS servers and combines this information on a map display. APRSISCE/32 can also interface with the CWOP servers to obtain and display weather station information as well. The software includes IGate functionality which will intercept packets on the RF side of the network and route them to the Internet. It can also selectively route packets on the Internet side of the APRS network over the air. Obviously, one must be careful to be selective about routing APRS-IS Internet packets over the air as a single poorly configured IGate can completely overload a regional APRS network! In our case, we choose to route position information from our two iPhones (AB1OC-9, and AB1QB-9) to the RF side of our IGate. In the future, we plan to route packets from our Weather Station on CWOP once the necessary filtering is available in APRSISCE/32. One of the cool things about APRSISCE/32 is that it can show the RF paths to our IGate in real-time as packets are routed from RF clients to the Internet. The picture above shows the RF path taken from a station in Northern New Hampshire to our APRS Station and it’s associated IGate. The picture below shows the path from a station on Cape Code, MA to our IGate.
As you can see from these screen shots, the APRS system is quite adaptable and can find a path to the Internet-based upon the availability of Digipeaters, IGates and the current RF propagation conditions. This behavior makes APRS a perfect tool for coordinating emergency operations and it is used as part of RACES and ARES activities.
There are a number of interesting applications that are built on the APRS platform. One of these measures VHF propagation in real-time based upon the paths that the APRS stations and Digipeaters use to propagate APRS packets. This information is available in map form here.
Another useful site is the aprs.fi site. This site provides maps and route displays for APRS mobiles, weather stations, Digipeaters, etc. The picture below shows an example of a APRS Mobile Unit’s route display as it moves through our area.
APRS fi can also provide detailed information about the APRS packet traffic associated with a given station. An example of this information is shown for our APRS IGate/Digipeater below.
At this point, we have completed the first phase of our APRS station and its is fully operational. Next we will be installing a dedicated antenna for our APRS station on our tower and we will move our IGate software to an always on PC. We plan to provide a future post on these upgrades.
– Fred (AB1OC)