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

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:

FAST_SPEED

SLOW_SPEED

FAST_RATE

SLOW_RATE

TURN_MIN

TURN_TIME

Digipeater Paths

Path Syntax

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

Common Paths

WIDE1-1

WIDE2-1

WIDE1-1,WIDE2-1

WIDE2-2

WIDE3-3

ARISS

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:

/O

/'

/>

/k

/s

/y

/u

/-

/^

Alternate Symbol Table

\

\n

\j

\s

Weather Data Format

Complete Weather Report

!4903.50N/07201.75W_090/001g005t077r000p000P000h50b10120

Field Breakdown:

090

/001

g005

t077

r000

p000

P000

h50

b10120

Optional Weather Fields

L

l

s

#

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

r/lat/lon/dist

r/34.5/-117.5/100

p/prefix

p/W6

b/call1/call2

b/WB4APR/N0CALL

o/obj1/obj2

o/BALLOON

t/poimqstunw

t/p

s/pri/alt/over

s//&

d/digi1/digi2

d/WIDE

a/latN/lonW/latS/lonE

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

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

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