Overview

ChroGPS Dash is universally compatible with all GPS/GNSS receivers supported by GPSd . The dashboard communicates with GPSd via its JSON protocol, which acts as a universal translator for dozens of different GPS chip protocols.

Popular Compatible GPS Chips

How Compatibility Works

GPSd as Universal Translator

flowchart LR
    subgraph rcvr ["GPS Receivers"]
        A1["u-blox
UBX binary"]
        A2["Quectel
MTK binary"]
        A3["Trimble
TSIP"]
        A4["Generic
NMEA text"]
    end
    B["gpsd
universal translator"]
    C["JSON
standard output"]
    D["ChroGPS Dash
Shows satellites!"]
    A1 --> B
    A2 --> B
    A3 --> B
    A4 --> B
    B --> C
    C --> D
  

Field Normalization

ChroGPS Dash handles different field naming conventions:

• PRN or svid → Satellite identifier
• ss or snr or cn0 → Signal strength
• el or elevation → Elevation angle
• az or azimuth → Azimuth angle
• gnssid → Constellation identifier
• sigid → Signal/frequency identifier (for multi-band)

Constellation Detection (Enhanced Three-Tier Fallback)

ChroGPS Dash uses intelligent constellation detection that handles both NMEA and u-blox numbering schemes:

flowchart LR
    P1["Priority 1: gnssid (dual-scheme support)
Tries NMEA 4.10+ mapping first (2=GLONASS, 3=Galileo)
Validates against PRN range
Falls back to u-blox mapping if mismatch (6=GLONASS, 2=Galileo)"]
    P2["Priority 2: PRN/SVID Ranges (standard ranges)
GPS: 1-32 • SBAS: 33-64, 120-158
GLONASS: 65-96 • etc."]
    P3["Priority 3: Talker ID (NMEA only, legacy)
GP=GPS • GL=GLONASS • GA=Galileo • etc."]
    P1 -->|"gnssid unavailable or invalid"| P2
    P2 -->|"ambiguous"| P3
  

Multi-Band Receiver Support

Modern dual/tri-band receivers (L1+L5, L1+L2+L5) report the same satellite on multiple frequencies. ChroGPS Dash handles this intelligently:

Backend Storage:

• Tracks each signal separately: PRN_gnssid_sigid
• Example: GPS PRN 10 on L1 = 10_0_0, on L5I = 10_0_6
• Preserves all signal data in cache

Frontend Display:

• Aggregates signals by PRN
• Shows strongest signal per satellite
• One dot per satellite on skyview
• No duplicate PRNs in tables

Supported Signal Types:

• GPS: L1C/A, L2C, L5
• GLONASS: L1OF, L2OF
• Galileo: E1, E5a, E5b
• BeiDou: B1, B2, B3
• QZSS: L1, L2, L5

Satellite Caching System

ChroGPS Dash maintains a persistent satellite cache to show ALL satellites even when gpsd sends them in batches:

How it works:

• Cache location: /var/tmp/cgpsd-sat-cache.json
• TTL: 30 seconds per satellite
• Accumulates satellites across refreshes
• Individual satellite expiration (not whole cache)
• Automatically filters out stale satellites

Benefits:

• See 25+ satellites instead of just 15-17
• Satellites persist across page refreshes
• No missing satellites from batched gpsd messages

GPS Data Performance Cache

ChroGPS Dash caches the full live gpsd response to avoid blocking on the gpsd socket on every AJAX request. Multi-constellation receivers send each constellation’s data (GPS, GLONASS, Galileo, BeiDou, QZSS, SBAS, NavIC, etc.) as separate messages roughly 1 second apart, making a complete cold read take 3-5 seconds. The cache eliminates this overhead on most requests.

How it works:

• Cache location: /var/tmp/cgpsd-gps-data-cache.json
• TTL: 10 seconds
• Written after each cold gpsd read; subsequent requests within the TTL are served instantly
• Results in a repeating pattern of several fast AJAX refreshes (~200 ms) followed by one slower cold-read refresh (~3-5 s)

What is cached:

• Live satellite data (SNR, elevation, azimuth, used flags)
• GPS position, fix mode, and DOP values
• GPSd version and device info

Ghost Satellite Filtering

ChroGPS Dash filters out “ghost” satellites that GPS receivers report from almanac data but aren’t actually visible:

Filtered when ALL of:

• Elevation = 0°
• Azimuth = 0°
• Signal strength = 0 dBHz
• Not used in position fix

Example: QZSS satellites (PRN 193-197) visible only in Japan/Asia will be filtered out for users in other locations.

Multi-GNSS Support

Automatically detects and displays:

• 🇺🇸 GPS (USA)
• 🇷🇺 GLONASS (Russia)
• 🇪🇺 Galileo (European Union)
• 🇨🇳 BeiDou (China)
• 🇯🇵 QZSS (Japan)
• 🇮🇳 NavIC (India)
• 🛰️ SBAS (WAAS/EGNOS/MSAS/GAGAN)

Troubleshooting

Running into wrong constellation colors, missing or duplicate satellites, or no position fix? Those are covered in the GPS & Satellite Data Issues section of the Troubleshooting guide , which has dedicated entries for each:

• Wrong constellation colors
• Missing satellites
• Duplicate satellites on a multi-band receiver
• Satellites showing but no position fix

Protocol Details

Protocols Supported by GPSd (and thus ChroGPS Dash):

Testing Your GPS

curl -fssL https://chro.gpsda.sh/gpsd-test | bash

Technical Details

Conclusion