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Technical Specifications

APRS Protocol Technical Details

Modulation and Data Rate

  • Modulation Type: AFSK (Audio Frequency Shift Keying)
  • Data Rate: 1200 baud (North America, most regions)
  • Alternative Rate: 9600 baud (some areas, experimental)
  • Frequency Shift: ±1000 Hz
  • Mark Frequency: 1200 Hz (logic 1)
  • Space Frequency: 2200 Hz (logic 0)
  • Audio Level: 3000 mV p-p typical

Radio Specifications

VHF Band Parameters (Most Common)

  • Frequency: 144-146 MHz (2-meter band)
  • Channel Spacing: 12.5 kHz or 25 kHz
  • Deviation: ±5 kHz
  • Modulation: FM (F3E)
  • RF Power: 1W - 50W typical
  • Antenna: Vertical polarization

UHF Band Parameters (Alternative)

  • Frequency: 430-440 MHz (70-cm band)
  • Less common: Regional use only
  • Same modulation: As VHF

Packet Structure

AX.25 Frame Format

APRS uses AX.25 protocol (amateur radio adaptation of X.25):

Flag | Address | Control | PID | Information | FCS | Flag
0x7E | 112-560 bits | 8 bits | 8 bits | Variable | 16 bits | 0x7E

Detailed breakdown:

  1. Flag: 0x7E (01111110) - Frame delimiter
  2. Address Field:
    • Destination: 56 bits (7 bytes)
    • Source: 56 bits (7 bytes)
    • Digipeater path: 0-8 entries × 56 bits
  3. Control: 0x03 (UI frame)
  4. PID: 0xF0 (No layer 3 protocol)
  5. Information: APRS data payload
  6. FCS: 16-bit CRC checksum
  7. Flag: 0x7E (frame end)

APRS Data Formats

Position without timestamp:

!DDMM.mmN/DDDMM.mmW$

Position with timestamp:

@DDHHMM/DDMM.mmN/DDDMM.mmW$

Compressed position:

!/YYYYXXXX$csT

Mic-E format:

(Encoded in destination address)

Position Precision

Format Latitude Precision Longitude Precision Example
Standard ±0.01' (~18m) ±0.01' (~18m) 4901.23N
Compressed ±0.3m ±0.3m Base-91 encoded
Decimal ±0.00001° (~1m) ±0.00001° (~1m) Extensions

Beacon Intervals

Smart Beaconing

Adaptive beacon rates based on movement:

# Pseudocode for smart beaconing
if speed == 0:
    interval = SLOW_RATE  # 30 minutes
elif speed < SLOW_SPEED:
    interval = SLOW_RATE  # 30 minutes
elif speed > FAST_SPEED:
    interval = FAST_RATE  # 1 minute
else:
    # Linear interpolation
    interval = calculate_proportional(speed)

# Also beacon on heading change
if heading_change > TURN_MIN:
    beacon_now()
    reset_timer()

Recommended Settings:

Parameter Value Description
FAST_SPEED 60 mph High-speed threshold
SLOW_SPEED 5 mph Low-speed threshold
FAST_RATE 120s High-speed interval
SLOW_RATE 1800s Low-speed interval
TURN_MIN 30° Minimum turn angle
TURN_TIME 60s Turn beacon delay

Digipeater Paths

Path Syntax

VIA call-SSID,call-SSID,...

Common Paths

Path Description Use Case
WIDE1-1 First-level digipeater Fill-in digi
WIDE2-1 Wide area coverage Mobile stations
WIDE1-1,WIDE2-1 Recommended path General use
WIDE2-2 Two hops Long distance
WIDE3-3 Three hops NOT RECOMMENDED
ARISS ISS digipeater Space communications

Path Processing

Example of path decrementation:

Original:  SOURCE>DEST,WIDE1-1,WIDE2-2:data
After Hop 1: SOURCE>DEST,DIGI1*,WIDE2-2:data
After Hop 2: SOURCE>DEST,DIGI1,DIGI2*,WIDE2-1:data
After Hop 3: SOURCE>DEST,DIGI1,DIGI2,DIGI3*,WIDE2-0:data (DONE)

Symbol Table

Primary Symbol Table /

Common symbols:

Code Symbol Description
/O Circle Base/Home
/' Small Aircraft Aviation
/> Car Automobile
/k Truck Truck/Van
/s Ship Maritime
/y Sailboat Sailing vessel
/u Bus Bus/RV
/- House Fixed station
/^ Large Aircraft Heavy aircraft

Alternate Symbol Table \

Code Symbol Description
\n Red Cross Emergency
\j Workzone Construction
\s Satellite Space station

Weather Data Format

Complete Weather Report

!4903.50N/07201.75W_090/001g005t077r000p000P000h50b10120

Field Breakdown:

Field Code Description Unit
Wind Direction 090 Direction (3 digits) Degrees
Wind Speed /001 Sustained wind MPH
Gust g005 Peak gust MPH
Temperature t077 Temperature °F
Rain (1h) r000 Last hour 1/100 inch
Rain (24h) p000 Last 24 hours 1/100 inch
Rain (midnight) P000 Since midnight 1/100 inch
Humidity h50 Relative humidity %
Pressure b10120 Barometric 1/10 mbar

Optional Weather Fields

Code Description Unit
L Luminosity W/m²
l Luminosity 1000 W/m²
s Snow 1/100 inch
# Raw rain counter Count

Telemetry

Analog/Digital Channels

T#SSS,A1,A2,A3,A4,A5,D1D2D3D4D5D6D7D8,COMMENT
  • SSS: Sequence number (000-999)
  • A1-A5: Analog values (000-255)
  • D1-D8: Digital bits (0 or 1)

Telemetry Equations

Define conversion formulas:

EQNS.a1,b1,c1,a2,b2,c2,...,a5,b5,c5

Value = a × raw² + b × raw + c

Example for battery voltage:

EQNS.0,0.05,0
# Voltage = 0 × raw² + 0.05 × raw + 0
# If raw = 120, then voltage = 6.0V

Message Format

Direct Message

:ADDRESSEE:Message text{MSGID
  • ADDRESSEE: 9 characters (padded with spaces)
  • Message: Up to 67 characters
  • MSGID: Optional 1-5 characters for acknowledgment

Example:

:WB4APR   :Testing APRS message{001

Acknowledgment

:SENDER   :ack001

Rejection

:SENDER   :rej001

Bulletins and Announcements

Bulletin

:BLNn     :Bulletin text
  • n is bulletin number (0-9 or A-Z)

Announcement

:BLNnABCD :Announcement text
  • ABCD is group identifier

Object Format

;OBJECTNAM*DDHHMM/DDMM.mmN/DDDMM.mmW$COMMENT
  • OBJECTNAM: 9 characters (padded)
  • * = live, _ = killed
  • Position and timestamp

Status Messages

>Status text

Maximum 62 characters

APRS-IS Connection

Login String

user CALLSIGN-SSID pass PASSCODE vers SOFTWARENAME VERSION filter FILTER

Example:

user WB4APR-9 pass 23515 vers WebAPRS 2.0.0 filter r/38.00/-97.00/500

Filter Syntax

Filter Description Example
r/lat/lon/dist Range r/34.5/-117.5/100
p/prefix Prefix p/W6
b/call1/call2 Budlist b/WB4APR/N0CALL
o/obj1/obj2 Objects o/BALLOON
t/poimqstunw Type t/p (position)
s/pri/alt/over Symbol s//&
d/digi1/digi2 Digipeater d/WIDE
a/latN/lonW/latS/lonE Area a/34/-118/32/-116

Performance Metrics

Network Capacity

  • Channel capacity: ~1200 bps
  • Effective throughput: ~800-900 bps (due to overhead)
  • Simultaneous users: Depends on beacon interval
  • Recommended density: <50 mobile stations per frequency

Range Expectations

Environment Handheld (5W) Mobile (50W) Base (50W + antenna)
Urban 2-5 miles 10-20 miles 20-40 miles
Suburban 5-10 miles 20-40 miles 40-80 miles
Rural/Open 10-20 miles 40-80 miles 80-150+ miles
Elevated 20-40 miles 80-120 miles 150-300+ miles

Propagation Factors

  • Line of sight: Primary limitation on VHF
  • Antenna height: Critical for range
  • Terrain: Hills block signals
  • Buildings: Urban attenuation
  • Weather: Minimal effect on VHF FM
  • Seasonal: Foliage affects propagation

Timing and Synchronization

Collision Avoidance

  • CSMA (Carrier Sense Multiple Access): Listen before transmit
  • Random delay: Prevent synchronized collisions
  • Exponential backoff: Retry strategy

Network Delay

  • RF propagation: Negligible (~1 μs/mile)
  • Digipeater delay: 100-500 ms per hop
  • IGate processing: 100-1000 ms
  • APRS-IS latency: 100-2000 ms global
  • Total delay: 1-5 seconds typical

Error Detection and Correction

  • FCS: 16-bit CRC in AX.25
  • Detection capability: 99.998% error detection
  • No FEC: No forward error correction
  • Retry mechanism: Supported for messages
  • Best effort: Position reports not retried

Security Considerations

Authentication

  • Passcode: Weak security (validation only)
  • No encryption: All data transmitted in clear
  • Amateur radio: Self-policing community
  • License verification: Via license database

Abuse Prevention

  • Rate limiting: APRS-IS server limits
  • Flood protection: Automatic filtering
  • Monitoring: Community oversight
  • Enforcement: Through licensing authority

Hardware Interface Specifications

Audio Interface

Input Specifications

  • Input impedance: 10kΩ typical
  • Input level: 50-500 mV RMS
  • Frequency response: 300 Hz - 3 kHz
  • SNR required: >20 dB for reliable decode

Output Specifications

  • Output impedance: 600Ω typical
  • Output level: 100-3000 mV p-p
  • THD: <5%
  • Pre-emphasis: 6 dB/octave (optional)

Serial Interface (TNC)

  • Interface: RS-232 or USB
  • Baud rate: 9600, 19200, 38400, 57600
  • Data bits: 8
  • Parity: None
  • Stop bits: 1
  • Flow control: RTS/CTS or none

GPS Interface

NMEA 0183

  • Sentences: GGA, RMC, GLL
  • Baud rate: 4800 or 9600
  • Update rate: 1 Hz typical
  • Format: ASCII text

Example NMEA sentence:

$GPGGA,123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M,46.9,M,,*47

Power Requirements

Device Type Voltage Current (RX) Current (TX) Peak
Handheld 7.4V 100-200 mA 1.5-2 A 2 A
Mobile 13.8V 200-500 mA 5-12 A 15 A
Tracker 12V 50-100 mA 1-2 A 2.5 A
IGate 12V 300-500 mA N/A 1 A

Document Version: 2.0 Last Updated: December 2025 Maintained by: APRS Community