GNSS & GPS Fundamentals
GNSS is the umbrella term. GPS is one constellation among many. More satellites in view = faster lock and better precision under tree cover.
What is GNSS?
Global Navigation Satellite System — a generic term for any satellite constellation that provides positioning, navigation, and timing (PNT) services. A modern receiver tracks multiple constellations simultaneously for improved availability.
Frequencies & RTK
Consumer GNSS receivers use L1 at 1575.42 MHz. Newer dual-band receivers add L2/L5 for better signal through foliage, inside buildings, and in urban canyons.
1575.42 MHz — civilian standard. Universal compatibility.
Dual-band — better multipath rejection. Superior under canopy.
Real-Time Kinematic — sub-meter to ~1 cm precision using base station corrections (local or network RTK).
Popular Marine Chartplotters
Display size, resolution, touchscreen capability, internal GPS, sonar support, and network protocol — the top chartplotters compared side by side.
| Model | Display Size | Resolution | Touchscreen | Internal GPS | Sonar Compatibility | Network Protocol | Transducer Support | Power (W) | Price (USD) |
|---|---|---|---|---|---|---|---|---|---|
| Garmin GPSMAP 8412 | 12 in | 1280 × 800 | Multi-touch | Yes (GPS+GLONASS) | CHIRP, SideVü, ClearVü | NMEA 2000, Garmin Marine Network | GT56 UHD, GT34 UHD, GT23, GT20 | ~35 W | $2,399 |
| Garmin GPSMAP 743xsv | 7 in | 800 × 480 | Multi-touch + Keys | Yes | CHIRP, SideVü, ClearVü, LiveScope | NMEA 2000 | GT24 UHD, GT22, GT20 | ~22 W | $999 |
| Lowrance HDS PRO 12 | 12 in | 1920 × 1080 | Multi-touch + Keys | Yes (GPS+Galileo) | CHIRP, StructureScan 3D, ActiveTarget, ActiveImaging | NMEA 2000, Ethernet | ActiveTarget 2, HDI, Totalscan, StructureScan 3D | ~38 W | $2,199 |
| Simrad NSS12 evo3S | 12 in | 1920 × 1080 | Multi-touch + Keys | Yes | CHIRP, StructureScan 3D, ActiveImaging, HALO radar | NMEA 2000, Simrad Net, Ethernet | ActiveImaging HD, StructureScan 3D, HALO pulse compression radar | ~38 W | $2,199 |
| Raymarine Axiom+ 12 | 12 in | 1280 × 800 | Optical bonded touch | Yes (GPS/GLONASS/Galileo) | CHIRP-DownVision, SideVision, RealVision 3D | SeaTalk ng, NMEA 2000, NMEA 0183 | RV-200, RV-320 RealVision 3D, Axiom CV/CVX transducers | ~28 W | $1,999 |
| Humminbird SOLIX 12 CHIRP MEGA SI+ | 12 in | 1280 × 800 | Multi-touch + Keys | Yes | CHIRP, MEGA Down Imaging+, MEGA Side Imaging+, MEGA 360 | Ethernet, NMEA 2000, Humminbird Network | MEGA 360 Imaging, MEGA LIVE Imaging, HELIX G4N transducers | ~30 W | $1,799 |
Fishfinder & Sonar Specifications
CHIRP vs single-frequency sonar. Frequency tradeoffs for deep water vs shallow water. And the proprietary imaging technologies from each major brand.
CHIRP Sonar
Instead of pinging on a single frequency, CHIRP sweeps across a frequency range — producing far better target separation and image clarity at all depths.
Low Frequency = Depth
50 kHz / 83 kHz traditional single-frequency transducers are designed for deep saltwater — but have relatively poor detail near the surface.
High Frequency = Detail
455 kHz / 800 kHz CHIRP imaging delivers stunning detail in shallow water. Ideal for structure-fishing — but maxes out around 200–300 ft.
Proprietary Imaging Technologies
MEGA Down Imaging+, MEGA Side Imaging+, MEGA 360 — high-frequency (1.2 MHz+) imaging for extreme detail. MEGA 360 provides 360-degree view around the boat.
StructureScan 3D, ActiveTarget, ActiveImaging — real-time forward-looking sonar for trolling. 3D rendering of underwater structure and fish.
SideVü, ClearVü, LiveScope — LiveScope provides real-time scanning sonar with perspective and forward modes. UHD transducers (Ultra High-Definition) at 2 kW power.
Transducer Specifications
The transducer is what actually generates the sonar signal. Frequency range, power output, mount type, and beam width are the four parameters that matter most.
| Transducer | Frequency Range (kHz) | Mount Type | Max Depth (ft) | Beam Width | Power Output | Cable Length | Price (USD) |
|---|---|---|---|---|---|---|---|
| Garmin GT56UHD-TM | 130-210 / 455-800 / 1000-1200 UHD | Transom-mount | 250 ft (UHD) / 500 ft (CHIRP) | 43° / 16° (SideVü) | 2 kW RMS | 20 ft | $649 |
| Lowrance ActiveTarget 2 | 520-620 (High CHIRP) | Transom-mount / trolling motor | 200 ft (live view) | 20° (forward) / 130° (down) | Active module | 20 ft | $1,699 (kit) |
| Humminbird MEGA 360 | 800-1200 kHz CHIRP | Jackplate / transom | 125 ft (360 view) | 360° continuous / 125 ft range | Integrated controller | 25 ft | $1,499 |
| Airmar B175HW | 1000-2000 kHz (Wide CHIRP) | Through-hull | 300 ft | 25° (wide beam) | 1 kW | 30 ft | $1,195 |
| Simrad StructureScan 3D | 455-800 / 800-1300 kHz | Transom-mount | 300 ft (3D view) | 270° total scan | Active module | 20 ft | $1,299 (kit) |
Handheld GPS & Watches
Hiking, hunting, and backcountry GPS receivers. Multi-constellation support, battery life, and ruggedness are the key specs.
| Model | GPS Constellations | Battery Life (GPS mode) | Display Type | Water Resistance | Weight (g) | Price (USD) |
|---|---|---|---|---|---|---|
| Garmin GPSMAP 67 | GPS, GLONASS, Galileo, BeiDou, QZSS | 180 hours (AA NiMH) | 3" transflective color, 240×400 | IPX7 (1 m / 30 min) | 230 g | $449 |
| Garmin inReach Explorer 2.0 | GPS + GLONASS (positioning), Iridium (satellite messaging/SOS) | 100 hours (10-min track) | 2.3" transflective color | IPX7 | 260 g | $499 (subscription required) |
| Garmin Fenix 7X Pro | Multi-band GPS, GLONASS, Galileo, BeiDou, QZSS | 89 hours (GPS); 136 hours (multi-band) | 1.4" MIP transflective, 280×280 | 10 ATM | 89 g | $999 |
| COROS Vertix 2 | Dual-band GPS, GLONASS, Galileo, BeiDou, QZSS | 90 hours (full GPS); 50 days smartwatch | 1.45" transflective color, 280×280 | 15 ATM | 87 g | $699 |
| Suunto Vertical | GPS, GLONASS, Galileo, QZSS, BeiDou (dual-band) | 60 hours (best GPS); 500 hours (tour) | 1.4" transflective MIP, 280×280 | 10 ATM | 96 g (Ti version) | $899 |
Marine VHF Radios
Integrated GPS, DSC (Digital Selective Calling), NMEA 2000 networking, and AIS receiver support are the features that distinguish modern VHF radios.
| Model | Power Output | GPS Integrated | DSC | NMEA 2000 | AIS Receiver | Price (USD) |
|---|---|---|---|---|---|---|
| Standard Horizon GX2400GPS | 25 W / 1 W | Yes (internal) | Class D | Yes | No (AIS model = GX2400B) | $329 |
| Icom M510 PLUS | 25 W / 1 W | Yes | Class D | Yes (NMEA 0183 + 2000) | AIS + GPS built-in | $549 |
| Uniden UM725GBK | 25 W / 1 W | Yes (UM725G model) | Class D | NMEA 0183 only | No | $269 |
DSC (Digital Selective Calling)
The digital distress feature of VHF. When connected to GPS, a DSC distress call transmits your exact position along with the alert. All modern fixed-mount VHF radios are Class D DSC capable. Must be programmed with a 9-digit MMSI number (Maritime Mobile Service Identity).
AIS (Automatic Identification System)
Collision-avoidance system that shows positions of nearby ships equipped with AIS transponders on your chartplotter. Most commercial vessels over 300 GT are required to broadcast AIS. An AIS receiver lets you see them — an AIS transceiver lets you be seen too.
Power Consumption & Battery Runtime
Marine electronics run on 12V DC. Chartplotter + VHF + fishfinder stack up quickly — the battery bank is often the real limiting factor on a day of fishing.
Typical Power Draw
Typical Battery Bank Runtime
Common Marine Electronics Errors
Garmin, Lowrance/Simrad, and Raymarine each use different error messaging. The most common issues are nearly always power or connectivity related.
Garmin Marine
Most likely cause: NMEA 2000 backbone power issue. A disconnected or reversed power/ground connection at the backbone terminator kills the entire NMEA 2000 network.
Troubleshooting: Check red (12V+) and black (ground) wires at both terminators. Measure voltage at backbone — should be 12.0–13.8V DC. Confirm terminator resistors (120 Ω) installed at BOTH ends of the backbone.
Most likely cause: Transducer cable pinched, disconnected, or corroded at the connector. Wet-pluggable connectors still corrode if not greased.
Troubleshooting: Disconnect and inspect transducer connector. Apply dielectric grease. Inspect cable for physical damage. Swap with known-good transducer if available.
Most likely cause: Transducer element failure. Ceramic element cracked from impact or long-term fatigue.
Troubleshooting: Unit beeps continuously or shows "Transducer Fault". Measure resistance across element — should be 500–1,500 Ω. If open or shorted, replace transducer (typically covered under warranty if under 2 years).
Lowrance / Simrad
Most likely cause: Card formatted as NTFS, exFAT, or too large for older units. Corrupted file system from hot-removal.
Troubleshooting: Reformat to FAT32 (max 32 GB on older units, 128 GB on newer). Use SD Card Formatter utility. Maximum recommended size = 32 GB for map cards.
Most likely cause: Corrupted download, bad SD card, or insufficient battery/voltage during flash.
Troubleshooting: Use fresh 4–32 GB SD card formatted FAT32. Re-download update from manufacturer. Ensure engine running or battery charger connected during update. Never interrupt mid-update — bricking is possible.
Most likely cause: Water too deep for frequency being used. Transducer not submerged (trailering mode). Boat speed > 30 mph breaks sound cone. Air bubbles from aerated water.
Troubleshooting: Verify transducer is below waterline. Reduce speed below 30 mph for consistent returns. Switch to lower frequency band for deep water. Verify no debris stuck to face.
Most likely cause: NMEA 2000 backbone connection issue. T-connector loose or backbone terminator removed.
Troubleshooting: Check red/black power wires to backbone terminator. Inspect all T-connectors along backbone. Confirm 120 Ω terminators installed at BOTH ends (not one, not three). Measure continuity.
Raymarine
Most likely cause: Backbone short or power issue. SeaTalk ng is Raymarine's flavor of NMEA 2000 — same physical layer, same rules.
Troubleshooting: Check power injector connections. Measure voltage at backbone. Check for pinched cable near steering/trim. Verify 120 Ω terminators at both ends of backbone run.
Most likely cause: Autopilot heading sensor calibration issue. Fluxgate compass not calibrated or subjected to magnetic interference from nearby wiring.
Troubleshooting: Perform a full dockside calibration (Deviation table — rotate boat through 360° at slow speed). Verify no new high-current DC wiring within 1m of fluxgate. Check Rudder Reference Unit (RRU) connection for loose pins.
NMEA Protocols Explained
NMEA 0183, NMEA 2000, and Signal K are the three marine data networking standards you'll encounter. They're not interchangeable — understanding the difference matters for installation planning.
The original marine data protocol — an ASCII-based serial format. A single talker (e.g. GPS) can drive multiple listeners (e.g. chartplotter, VHF, AIS).
Limitations: Slow (4800 baud). One talker per network run. No device discovery. Wires must be a twisted shielded pair connected A(+) / B(-), with shield grounded at ONE end only (not both — that creates a ground loop).
The modern standard. Based on CAN (Controller Area Network) — the same physical layer used in cars. Multi-drop network where any device can talk to any other device. Device auto-discovery and plug-and-play.
Installation rules: Requires dedicated 12V backbone power at both terminators. 120 Ω resistor terminator at each end of the backbone (exactly two, never one or three). T-connectors drop devices off the backbone. Maximum cable length — 100m backbone, 6m drop cables.
The emerging open-source standard. JSON-based data protocol that normalizes NMEA 0183 and NMEA 2000 data into a unified, web-friendly format.
Why it matters: Enables dashboards, logging, and mobile apps from cheap hardware (Raspberry Pi + Signal K server). Extensible with plug-ins. Backward-compatible with existing NMEA devices via gateways.
Antenna Placement Guidelines
Antenna placement is 80% of the battle. A $500 antenna in a bad location performs worse than a $50 antenna in a good location. The rules are simple — follow them.
GPS / GNSS Antenna
- Sky view: Must have unobstructed view of the sky in all directions. A metal deck or bimini above = no satellite lock.
- Keep clear of: Radar beams, VHF antenna fields, fluorescent lighting, high-current DC wiring within 1m.
- Cable length: Short as possible. Every 3dB of cable loss halves the signal. RG-8X minimum for runs over 5m; RG-213 for runs over 15m.
VHF / AIS Antenna
- Height is everything: VHF is line-of-sight. Each foot of height adds ~1.22 miles of range. Minimum 10 ft above deck for usable range.
- Separation: Keep at least 3 ft (1m) horizontal separation between VHF and GPS/GNSS antennas — transmit signal desensitizes GPS receiver.
- Ground plane: Fiberglass whip antennas need a ground plane (metal) or are tuned specifically for fiberglass mounts.
Radar Scanner
- Unobstructed 360°: Anything blocking the radar beam creates a blind spot on the display. Cabin bulkheads, masts, davits — all bad.
- Mounting height: Higher = better long-range detection, but worse close-in coverage (due to beam elevation). 8–15 ft above waterline typical.
- Cable length: Radar cable is critical. RG-8X minimum for up to 25 ft; RG-213 required for longer runs. Never splice radar coax — replace whole run.
Data Cable Routing Rules
Never run data cables parallel to power cables. DC power wiring creates a magnetic field that induces noise in signal cables. Cross at 90° when unavoidable.
Use twisted-shielded cable for NMEA 0183. The shield protects against engine and alternator noise. Ground shield at ONE end only — typically at the talker (GPS) end.
Environmental Ratings & Display Specs
Marine electronics live in a harsh environment — vibration, salt spray, UV, and temperature extremes. The rating tells you whether a unit is actually seaworthy or just "water resistant" marketing.
IP Rating Reference
Display Readability (nits)
Temperature & Environmental Specs to Verify
Operating temperature: Marine units should be rated at minimum −20°C to +55°C (−4°F to +131°F). Automotive/consumer units rated 0–40°C will fail in open-boat use.
Humidity: Should be rated at 95% RH non-condensing. Condensation inside the case shortens life dramatically — look for units with Gore-Tex style pressure equalization vents.