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Designing a Private APRS System

Most non-commercial users of APRS use the existing network of digipeaters and Internet gateways. But if you're tracking a large number of stations with frequent updates, or using APRS commercially, you'll need your own system.

Argent Data Systems manufactures hardware but does not provide installations. Still, building your own Automatic Vehicle Location (AVL) system is well within the reach of a typical land mobile radio shop.

Why APRS?

For commercial users, APRS's main benefit is that it's open:

  • All APRS devices and software interoperate.
  • Most commercial AVL systems are closed and vendor-locked.
  • APRS hardware is flexible and inexpensive.
  • Ideal for low-budget or technically savvy organizations.

APRS Data on the Air

APRS commonly uses 1200 baud AFSK. It works with standard voice radios, but packets sound like short tone bursts (~0.5 seconds), which can be annoying if sharing a voice channel.

Modes of Operation

1. CSMA/CA (Carrier Sense Multiple Access)

  • Each tracker transmits on a fixed or variable interval.
  • Transmissions only occur if the channel is clear.
  • Works well on dedicated channels with <25 trackers.
  • Utilization ~20% of available airtime due to possible collisions.

📊 Example:
0.5s per transmission × 24 trackers × 1/minute = ~100% usage

2. Time-Slotted Transmission

  • Each tracker transmits at a unique GPS-synced time.
  • Near 100% utilization, no collisions.
  • Argent devices support half-second slots (up to 120/minute).

⚠️ Downsides: - Trackers must be manually configured. - Not ideal if devices roam between digipeater zones.

3. Mic Encoder (Mic-E) Mode

  • APRS bursts are transmitted at the end of voice transmissions.
  • Minimal impact to voice users.
  • Harder to wire; no auto-updates unless the user is talking.
  • Polling is possible, but adds complexity.

Radio Interfacing

Trackers typically connect to:

  • Speaker output (receive audio)
  • Mic input (transmit audio)
  • PTT line (push-to-talk)

Some radios allow CTCSS-based dual-function PTT or channel switching.

Trackers and GPS units require power — usually shared with the radio.

Digipeaters

A digipeater is a simplex (same-frequency) digital repeater.

  • Extends APRS range significantly.
  • Each digipeater doubles airtime use.
  • Keep hops ≤ 2–3 for efficiency.

Deployment is simple: just a radio, antenna, and TNC (e.g. Tracker2).

🛰️ Cross-band or duplex digipeaters are possible but more complex.

If your area is small (e.g., racetrack), no digipeaters may be needed.

Base Station

Goal: display tracked units on a PC.

  • Basic setup: scanner + PC + soundmodem + Xastir.
  • Hardware TNC (e.g. Tracker2) simplifies interfacing and allows transmit features like messaging or polling.

Multi-site systems can be linked via the Internet or a private TCP/IP network using a hub server.

Mobile Terminals

Some users need in-vehicle displays of nearby stations.

Options:

  • Mobile PC with full APRS mapping
  • Mapping GPS with waypoint updates from a tracker
  • Garmin FMI GPS + Tracker2 for two-way messaging

What Argent Data Systems Can Provide

  • Off-the-shelf APRS trackers
  • Custom hardware (custom PCB layout, specific connectors)
  • Custom firmware (special features or interfaces)

💡 Customizations may be available at low or no extra cost depending on order size.