APRS For HF - SARCNET

Automatic Position Reporting System for HF

APRS Network

The Automatic Position Reporting System (APRS) network comprises:

RF User - Equipped with a base station transceiver and APRS Terminal Node Controller (TNC)
Portable User - Equipped with an APRS-compatible Hand-Held radio
Mobile User - Equipped with a mobile transceiver and APRS TNC
Weather Station - Conneted via the Ineternet
Fill-In Digipeater - Provides extra coverage for a Wide Repeater
Wide Digipeater - Wide area node
IGate - Equipped with a base station transceiver, APRS TNC and Internet connection
APRS Database and Web Server - APRS Internet Service (APRS-IS)
Internet User - Mobile Device with APRS app connected to the Internet

The APRS Network

APRS Description

The APRS defines:

Stations, for broadcasting location, telemetry data, text over both RF and Internet networks
Nodes, which are identified by Callsign-SSID format

Each node can store and forward messages

Messages, which are unnumbered AX.25 datagrams

The message “Path” contains a routing list

Digipeaters modify the Path to avert the “ping-pong” effect

APRS Secondary Station ID (SSID)

Each APRS node uses a callsign with a SSID suffix, identifying its purpose:

-0 (No SSID) Home Station
-1 Digipeater, or Home Station running a Fill-In Digi,
-2 Digipeater [#2] on 70CM
-3 Digipeater [#3]
-4 HF to VHF Gateway
-5 IGate (Dedicated system, not home station)
-6 Operation via Satellite
-7 Kenwood D7 Handheld
-8 Secondary Mobile usage (also Maritime vessels) (VHF / UHF)
-9 Mobiles (VHF / UHF)
-10 Operation via Internet Only (No RF capability)
-11 is for APRStouch-tone users (also high altitude balloons)
-12 Portable Units such as Laptops, Camp Sites etc
-13
-14 Interstate Truckers
-15 Operation via HF

APRS Paths

The APRS path is a routing list for the message: VK3ABC-1, VK3XYZ-1, etc
Can use callsign aliases for special nodes: WIDE1-1, WIDE2-1
The SSID of aliases sets the number of hops allowed
The path is modified by each digipeater
The SSID is decremented in each hop, but -0 is omitted.
An asterisk (*) signifies when a callsign or alias was used
Example:
WIDE1-1, WIDE2-2 (Original path)
<callsign1>*, WIDE2-2 (First hop)
<callsign1>, <callsign2>, WIDE2-1* (Second hop)
<callsign1>, <callsign2>, <callsign3>* (Third hop)

APRS Rules for HF

There are special rules for APRS used on HF, primarily because HF, unlike VHF/UHF, presents a very wide area network.

No Digipeating on HF!
Most receiving Gateways will route to APRS-IS
So, Path = WIDE1-1 is sufficient
Use 300 Baud, 200Hz FSK, 1600Hz/1800Hz
For SSB must be spot on frequency
LSB & USB both work because AX.25 is NRZI
Avoid any ALC action on TX to limit distortion
Beacon time ≥ 10 minutes: Permits up to 70 users
Good luck – Because APRS has no ACK or FEC

APRS Frequencies and Gateways

There are defined frequencies used for APRS. However the APRS stations may change over time.

40m 7048.600kHz LSB

VK2IO-4 Sydney Primary Net Station (also 7.047.17 LSB)
VK7HSE-4 Kingston Secondary Net Station

30m 10147.600kHz USB

VK1IAN-1 Canberra RX Only
VK2ACW-1 Orange
VK2HL-1 Sydney Part Time RX Only
VK2IO-4 Sydney Offline
VK2OL-5 Sydney Full Time RX Only
VK2ZSZ-4 Queanbeyan Full Time RX Only
VK2ZZM-5 Sydney Full Time RX Only
VK3MY-4 Melbourne Primary Net station
VK4AB-4 Ipswich Full Time
VK4DMI-4 Pine Rivers Secondary Net station
VK4MQ-5 Toowoomba Full Time RX Only
VK4RM-4 Mackay Full Time
VK4UN-4 Tewantin Full Time
VK5RR-4 Waikerie
VK6LD-4 Perth Full Time
VK7ZRO-4 Bothwell Full Time
VK8KMD-10 Alice Springs
ZL3GR-4 Tauranga RX Only

20m 14103.00kHz LSB

VK2IO-4 Sydney Offline

APRS TNC Pin Assignments

We use an Argent Data Systems T3 Mini TNC board. This is an excellent device with a good configuration tool. It has a wide variety of input/ouput pins, suitable for communications, control and telemetry.

Argent Data Systems T3 Mini TNC

Pin Assignments:

A1-A4 Analog inputs 1-4 (max voltage 4xVdd)
1W Dallas 1-Wire data bus
SQL Squelch input
AUX Auxiliary I/O pin
GND Ground
RST Reset (active low)
B-RX Serial port B input
B-TX Serial port B output
A-RX Serial port A input
A-TX Serial port A output
CFG Configuration select
AOUT Audio output (to radio)
AIN Audio input (from radio)
GRN Green LED output for external LED
RED Red LED output for external LED
PTT Push-to-talk (open collector)
5V Regulator output or Vdd input, 5v max, 2.8v min
VIN Unregulated supply voltage, 6v to 28v
GP1-GP12 General purpose digital input/output pins

TNC Configuration Tool

Argent Data Systems T3 Mini TNC board comes with OpenTracker - a versatile configuration tool.

OpenTracker TNC Configuration Tool

APRS TNC Wiring

The Argent Data Systems T3 Mini TNC board connects directly to a Serial TTL GPS Receiver. We use solderable enamelled copper wire to interconnect such devices.

Argent Data Systems T3 Mini TNC - Ublox NEO-6M GPS Receiver - GPS Patch Antenna

A 5V regulator and two audio line transformers complete the design for connecting the TNC to a radio transceiver.

Pictorial Schematic Diagram

APRS TNC Assembly

The components are assembled into a Hammond H9004 ABS Enclosure. The transformers and regulator are glued to the base. A sleeved grommet is used for the CAT-5 cable.

APRS TNC Assembly 1

The completed APRS TNC dongle needs to have a clear view of the sky. It will starts transmitting when a 3D GPS fix is valid.

APRS TNC Assembly 2

APRS Display

The resulting APRS data, relayed to the Internet by an IGate, is easily displayed on the aprs.fi website.

APRS Station Info

By querying the aprs.fi website additional station information can be displayed.

References