GPS & Electronic Navigation Study Guide
GPS fundamentals, chartplotter operation, electronic chart types, AIS integration, radar overlay, NMEA networks, VHF DSC, and GPS failure backup procedures. Everything tested on the USCG OUPV and Master license exams.
1. GPS Fundamentals
The Global Positioning System (GPS) is a U.S. Department of Defense satellite constellation of 24+ operational satellites in six orbital planes at roughly 20,200 km altitude. A GPS receiver needs signals from at least 4 satellites for a 3D fix (3 satellites give a 2D position only). The receiver measures the time-of-flight of radio signals from each satellite to compute pseudoranges, then solves simultaneous equations to determine latitude, longitude, altitude, and time.
Satellite Geometry and DOP
Accuracy depends not just on signal quality but on the geometric spread of visible satellites. When satellites are clustered in one part of the sky, position uncertainty is amplified — measured by the Dilution of Precision (DOP) factor.
| DOP Value | Rating |
|---|---|
| 1 | Ideal |
| 1–2 | Excellent |
| 2–5 | Good |
| 5–10 | Moderate |
| 10–20 | Fair |
| >20 | Poor |
GPS Accuracy History — Key Exam Fact
Chart Datum Glossary
| Term | Definition |
|---|---|
| WGS-84 | World Geodetic System 1984 — the datum GPS uses for all position output. Almost all modern charts are referenced to WGS-84 or NAD-83. |
| NAD-83 | North American Datum 1983 — essentially identical to WGS-84 for most practical purposes (within 1 meter in CONUS). |
| NAD-27 | Older datum for U.S. charts. Offset from WGS-84 by 10-200 meters depending on location — must apply correction when plotting GPS on NAD-27 charts. |
| HDOP | Horizontal Dilution of Precision — geometric quality of horizontal GPS fix. HDOP 1-2 = excellent; 2-4 = good; 4-8 = moderate; above 8 = poor. |
| PDOP | Position Dilution of Precision — includes vertical dimension. Used for 3D fix quality assessment. |
| VDOP | Vertical Dilution of Precision — altitude accuracy. GPS vertical accuracy is typically 2-3x worse than horizontal. |
| SA (Selective Availability) | Intentional GPS signal degradation by DOD. Removed May 1, 2000. Civilian accuracy improved from ~100m CEP to ~15m CEP. |
| CEP | Circular Error Probable — the radius within which 50% of positions will fall. Modern GPS CEP: 3-5 meters without augmentation. |
| WAAS | Wide Area Augmentation System — U.S. FAA ground reference stations broadcast corrections, improving GPS to <3m accuracy. Available throughout continental U.S. |
| DGPS | Differential GPS — USCG maintains coast guard radio beacons broadcasting GPS corrections. Accuracy ~1-3m near beacon stations. |
2. Chartplotter Operation
A chartplotter combines a GPS receiver with an electronic chart display. Understanding the terminology is essential for both the exam and safe navigation.
| Term | Definition |
|---|---|
| Waypoint | A saved geographic position (lat/lon) stored in the chartplotter. Named and numbered for recall in routes. |
| Route | An ordered sequence of waypoints. The plotter calculates course and distance between each leg. |
| Track | The actual path your vessel has traveled, recorded at set intervals (position history breadcrumbs). |
| COG | Course Over Ground — actual direction of travel relative to the earth, measured by GPS. Differs from heading when current or leeway is present. |
| SOG | Speed Over Ground — actual speed relative to the earth, from GPS. Differs from speed through water when current is present. |
| XTE | Cross-Track Error — perpendicular distance off the planned route track. Port = negative; starboard = positive on most plotters. |
| BTW | Bearing to Waypoint — direction from current position to next waypoint, in degrees true or magnetic. |
| DTW | Distance to Waypoint — great-circle or rhumb-line distance remaining to next waypoint. |
| ETE | Estimated Time Enroute — time to reach next waypoint at current SOG. |
| VMG | Velocity Made Good — component of SOG directed toward the waypoint. Sailing term when sailing upwind at angles. |
| TTG | Time to Go — similar to ETE, time to reach destination. |
| Heading | Direction the bow is pointing, from compass. Not the same as COG when wind or current pushes the vessel sideways. |
COG vs. Heading — Critical Distinction
- Direction the bow is pointing
- Measured by compass
- Subject to variation and deviation
- Used for steering and collision avoidance bearings
- Does not reflect current or leeway effect
- Actual direction of movement over earth
- Measured by GPS from position changes
- Always referenced to True North
- Includes effect of current and leeway
- Used for track plotting and ETA calculations
SOG vs. Speed Through Water
- Computed from rate of position change by GPS
- Includes effect of any current
- Use for: ETA, fuel range, chart plotting
- Can be zero even with engine running if current equals boat speed
- Measured by paddlewheel, impeller, or Doppler log
- Reflects actual boat speed in the water mass
- Use for: fuel burn, sailing polar data, engine performance
- Current = SOG minus STW
3. Electronic Chart Types
The exam distinguishes between official electronic chart formats and proprietary chartplotter maps. Understanding the legal status and technical differences is testable material.
Queryable objects, scalable, auto-update, ECDIS-compliant, smaller file size
Requires compatible ECDIS or certified plotter; older plotters may not support
Official — can replace paper charts on SOLAS vessels when used in ECDIS
Identical appearance to familiar paper chart, easy to understand
Pixelates when zoomed, no queryable objects, must update entire file
Supplemental only — not a legal substitute for paper charts
Highly optimized for chartplotters, frequently updated, includes POI data
Not meeting S-57 standard; not ECDIS-compliant; tied to specific hardware
Supplemental — treated same as RNC for legal purposes
S-57 Standard and ECDIS
The S-57 standard (IHO Special Publication 57) defines the data format for ENC charts. Objects are encoded as points, lines, and areas with attributes that describe depth, hazards, light characteristics, and restricted areas. A certified ECDIS system can display and interrogate these objects.
ECDIS (Electronic Chart Display and Information System) meeting IEC 61174 certification can legally replace paper charts aboard SOLAS vessels (international voyages). ECDIS must use ENCs, receive continuous updates via AVCS (Admiralty Vector Chart Service) or equivalent, and meet strict alarm and safety contour requirements.
A recreational chartplotter — even a high-end model — is NOT an ECDIS and does NOT satisfy paper chart carriage requirements for regulated commercial vessels. For OUPV vessels, the requirement is generally to carry paper charts or equivalent aboard. Always verify current USCG requirements for your vessel class.
4. Chart Datum and GPS Offset
One of the most dangerous but overlooked sources of navigational error is plotting a GPS fix (referenced to WGS-84) on a chart that uses a different horizontal datum without applying the correction.
How to Check a Chart's Datum
- Look at the title block — datum is usually stated: "Horizontal Datum: NAD-83 (WGS-84)" or "North American Datum of 1927."
- Look for a datum note in the chart margin, usually near the latitude scale or title block.
- If the chart says "NAD-83 (WGS-84)" or just "WGS-84," no correction is needed for GPS positions.
- If the chart says "NAD-27," apply the datum shift correction shown in the note, or change your GPS receiver output to NAD-27 if the receiver supports it.
Applying the Datum Shift
NOAA charts that use NAD-27 include a datum shift note that gives the correction in arc-minutes: e.g., "Satellite-derived positions should be moved 0.06' southward and 0.13' westward to agree with this chart."
Apply the correction mathematically to the GPS lat/lon before plotting. A correction of 0.06' ≈ 370 feet. Near rocks or shoals, this can be fatal.
5. AIS Integration with Chartplotter
AIS (Automatic Identification System) transponders and receivers feed target data to the chartplotter, displaying vessel identity, position, COG, SOG, and predicted tracks.
Closest Point of Approach — predicted minimum distance between your vessel and a target if both maintain current course and speed.
Time to Closest Point of Approach — time in minutes until CPA is reached.
The line extending from an AIS target showing predicted future position based on SOG and COG.
A user-defined range ring or sector around your vessel. An alarm sounds when an AIS target enters the zone.
Mandatory on SOLAS vessels (ships 300+ GT international, all passenger vessels). Transmits every 2-10 seconds underway.
Voluntary/recreational transponder. Transmits every 30 seconds. Lower power (2W vs 12.5W for Class A).
Activated (responding), sleeping (present but not tracked), dangerous (CPA/TCPA alarm), lost (stopped transmitting).
Not all vessels have AIS. Small fishing boats, kayaks, and some Navy vessels do not transmit. Never rely solely on AIS for collision avoidance.
AIS Data Fields on Chartplotter
| Field | Source |
|---|---|
| MMSI | AIS transponder config |
| Vessel Name / Call Sign | AIS transponder config |
| COG | Target vessel GPS |
| SOG | Target vessel GPS |
| Heading (HDG) | Target vessel compass |
| ROT | Target vessel gyro/GPS |
| Position | Target vessel GPS |
| Navigational Status | AIS transponder config |
| Destination / ETA | AIS transponder config |
| Draught | AIS transponder config |
6. Radar Integration with Chartplotter
Modern marine electronics allow radar overlay on a chartplotter display, combining the radar echo image with the electronic chart. This aids in identifying radar contacts relative to charted hazards.
Radar Overlay Requirements
- Chartplotter and radar must be from same manufacturer or NMEA 2000 compatible
- Radar must provide heading data (gyro or compass stabilized) for accurate overlay
- Without heading input, radar image rotates with vessel heading — overlay becomes meaningless
- Position must be current — stale GPS fix causes overlay misalignment
- Chart scale and radar range must be comparable for useful display
MARPA Targets
MARPA (Mini Automatic Radar Plotting Aid) allows the operator to manually acquire radar targets. The system then tracks the target and computes CPA and TCPA — similar to AIS but using the radar return rather than AIS data.
- Works on targets that do NOT have AIS
- Requires accurate heading input (compass stabilization)
- Target must be manually acquired (click on radar echo)
- Less precise than AIS tracking; dependent on radar returns
- Typical capacity: 10-20 MARPA targets simultaneously
Radar Display Modes
| Mode | Description |
|---|---|
| Head Up (HU) | Vessel's heading always at top of display. Display rotates as vessel turns. |
| North Up (NU) | North always at top. Vessel symbol rotates. Chart-aligned. |
| Course Up (CU) | Planned course always at top. Display rotates when course changes. |
| Relative Motion (RM) | Own vessel stationary at center; all other targets shown relative to own motion. |
| True Motion (TM) | All vessels move on display according to their actual ground speed and course. |
7. NMEA 0183 and NMEA 2000 Networks
NMEA (National Marine Electronics Association) standards define how marine electronics communicate. NMEA 0183 is the older serial protocol. NMEA 2000 is the modern network standard used on most current vessels.
NMEA 0183
- Speed: 4,800 baud default (some devices support 38,400)
- Topology: One talker, multiple listeners (serial bus). Daisy-chain.
- Wiring: Two-wire (TX+, TX- for talker; connect to RX+, RX- on listener)
- Sentence format: $TTSSS,data*checksum CR/LF
- Max sentence length: 82 characters
- Limitation: Only one talker per network without a multiplexer
- Still used: Many older instruments, some VHF radios, simple autopilots
NMEA 2000
- Speed: 250 kbps — much faster than NMEA 0183
- Topology: True network — multiple talkers and listeners on same backbone
- Wiring: 5-wire backbone (power+, power-, CAN-H, CAN-L, shield)
- Based on: CAN bus (Controller Area Network) — same as automotive
- Messages called: PGNs (Parameter Group Numbers)
- Advantage: Plug-and-play; all devices share data bidirectionally
- Requires: Proper terminator resistors at each end of backbone
Key NMEA 0183 Sentences
| Sentence | Data Provided |
|---|---|
| $GPGGA | GPS Fix: lat, lon, altitude, HDOP, fix quality, number of satellites |
| $GPRMC | Recommended Minimum: position, SOG, COG, date/time, magnetic variation |
| $GPGLL | Geographic Latitude/Longitude: lat, lon, time, status |
| $GPVTG | Track Made Good: COG true, COG magnetic, SOG in knots, SOG in km/h |
| $HEHDT | Heading True: compass true heading (HE = heading device) |
| $HEHDM | Heading Magnetic: compass magnetic heading |
| $VWVHW | Speed Through Water: heading and boat speed from paddlewheel/impeller |
| $SDDPT | Depth: depth below transducer, offset to keel |
| $WIMWV | Wind Speed and Angle: apparent or true, speed, angle to bow |
| $GPGSV | GPS Satellites in View: satellite PRNs, elevation, azimuth, SNR |
| $GPGSA | GPS DOP and Active Satellites: fix mode, satellites used, PDOP, HDOP, VDOP |
| $SDDBT | Depth Below Transducer: depth in feet, fathoms, meters |
NMEA 0183 Talker IDs
8. VHF DSC — Digital Selective Calling
DSC (Digital Selective Calling) is the digital signaling system built into modern VHF marine radios. It allows vessels to send automated distress alerts, make individual calls, and perform position polling — all via Channel 70.
DSC Distress Call — Step by Step
- Ensure GPS is connected and has a fix (position will be transmitted automatically)
- Open the red cover on the DSC DISTRESS button (or lift guard)
- Press and hold DISTRESS button for 5 seconds
- Select nature of distress if prompted: Sinking, Fire, Flooding, Collision, Grounding, Piracy, Abandon Ship, Man Overboard, EPIRB activation, Undesignated
- Radio automatically transmits on CH 70: MMSI, position, time, distress nature — 5 transmissions in ~30 seconds
- Radio switches to CH 16 for voice distress call
- Make voice MAYDAY call on CH 16: MAYDAY MAYDAY MAYDAY, vessel name, MMSI, position, nature of distress, persons aboard, vessel description, assistance needed
- Monitor CH 16 for acknowledgment from Coast Guard or other vessel
Vessel Radio Licensing Requirements
| Situation | FCC License Required? |
|---|---|
| U.S. vessel, domestic waters only | No (voluntary) |
| U.S. vessel, traveling to foreign port or crossing international waters | YES — Ship Station License |
| Commercial passenger vessel, domestic | YES — Ship Station License |
| EPIRB use (406 MHz) | Registered with NOAA — not FCC licensed separately |
| Handheld VHF on vessel already licensed | Covered by vessel license |
9. Satellite Communications and HF Radio
For offshore passages beyond VHF range, mariners use SSB (HF) radio, satellite communications, and NAVTEX for weather and distress communications.
SSB (Single Sideband) HF Radio
- Frequency bands: 2-30 MHz (HF bands: 4, 6, 8, 12, 16, 22 MHz)
- 2182 kHz: International distress and calling frequency (HF)
- Range: 100-3,000+ NM depending on frequency and propagation
- Propagation: Ionospheric skip — lower frequencies work at night; higher frequencies work during day
- Weather: NOAA marine weather broadcasts on SSB
- WX fax: SSB can receive radiofax (HF fax) weather charts via a TNC or computer
- License: Operator requires GROL (General Radiotelephone Operator License) for commercial use
NAVTEX
NAVTEX (Navigational Telex) is an automated maritime safety information broadcast system operating on 518 kHz (international, English) and 490 kHz (national language) and 4209.5 kHz (HF for extended range). Broadcasts include:
- Navigational warnings (drifting hazards, light outages)
- Meteorological warnings and forecasts
- SAR (Search and Rescue) information
- NOTAM (Notice to Airmen if relevant to aviation/maritime)
- Received by dedicated NAVTEX receiver or SSB with decoder
- Range: ~200-400 NM from transmitter on 518 kHz
Iridium Satellite Communications
- 66 LEO (Low Earth Orbit) satellites provide global voice, SMS, and data
- Works at both poles — covers entire earth including Arctic and Antarctic
- Iridium GO! and similar devices provide WiFi hotspot for satellite internet
- Iridium 9555 handset: voice calls anywhere, global
- Used for GMDSS compliance on vessels where INMARSAT satellite coverage is limited
- Data rates: 2.4-10 kbps (voice-grade), not suitable for large file transfers
INMARSAT
- Geostationary satellites — coverage to approximately 70° N/S latitude
- INMARSAT-C: used for GMDSS messaging, SafetyNET broadcasts, telex
- Fleet One, Fleet Broadband: higher-speed data for larger commercial vessels
- SafetyNET: INMARSAT broadcast of MSI (Maritime Safety Information) to subscribed vessels
- Does NOT cover poles — Iridium required for extreme latitudes
Weather Fax (Radiofax) Reception
NOAA broadcasts high-resolution weather map facsimiles on HF radio frequencies (3357 kHz, 8503 kHz, 12789 kHz, and others). A vessel with SSB and a fax decoder (hardware or software) can receive synoptic weather charts, wave analysis charts, and tropical weather analysis.
Reception requires: SSB receiver, fax software (such as Fax-on-PC or dedicated hardware decoder), and an antenna suited to HF frequencies. NOAA publishes the schedule of transmissions (frequency, time, chart type) on their website and in H.O. 118. Weather fax remains valuable where satellite internet is unavailable or too expensive.
10. GPS Failure Modes and Backup Navigation
GPS is not infallible. A competent navigator maintains paper chart and compass skills and knows how to navigate without GPS.
GPS Failure Modes
Hardware or software fault in the GPS unit itself. Solution: carry a backup handheld GPS.
Damaged or corroded antenna connector. GPS will show poor reception or no fix. Check antenna cable and connector.
GPS units need reliable 12V power. Carry a battery-powered backup. Some units have internal backup batteries for short outages.
Tall masts, canyon-like anchorages, or antenna placement can block satellite signals. Ensure antenna has clear 360° sky view.
Deliberate interference with GPS signals. Common near military installations, ports, and conflict zones. Signs: sudden loss of fix, erratic positions, high DOP.
False GPS signals broadcast to mislead receivers. Vessel position may appear to be elsewhere. Verify against radar, depth, and visual bearings.
Dead Reckoning Procedure
- Start from the last confirmed fix (GPS, visual, or radar range/bearing)
- Note the time of last fix precisely
- Maintain consistent compass course — correct for variation and deviation
- Monitor speed through water (paddlewheel) or use engine RPM table
- Calculate: Distance = Speed x Time
- Plot from last fix: apply true course and distance to get DR position
- Update DR position at regular intervals (every 15-30 minutes minimum)
- Cross-check DR against depth soundings, radar ranges, visual bearings
- Account for tidal current: obtain set and drift from tidal atlas or pilot chart
- Label DR positions on chart: time and method (DR vs. fix vs. EP)
Position Types — Know the Difference
11. Electronic Logbook Requirements
A vessel logbook (deck log) is a legal document. Understanding what must be logged and in what format is required for the captain's license exam and professional practice.
Required Log Entries (U.S. Commercial)
- Date, time, and position at departure and arrival
- Position fixes at regular intervals (typically every watch)
- Course and speed
- Weather conditions (wind, sea state, visibility)
- Compass deviation corrections applied
- Any vessel malfunctions, casualties, or incidents
- Persons aboard (POB) at departure
- Fuel on board (FOB) and consumption
- Safety drills conducted (abandon ship, fire, MOB)
- Officer of the watch and crew on duty
Electronic vs. Paper Log
USCG regulations (46 CFR) allow electronic logbooks but impose the same evidentiary requirements as paper logs: entries must be made contemporaneously, original entries must not be deleted (only corrections with notation of original), and the log must be available for USCG inspection.
Many chartplotter software packages include a logging function that auto-records position, COG, SOG, and time at set intervals. This creates a track log useful as a supplement to the deck log but does not replace required manual entries for observations and conditions.
12. GPS Antenna, Power, and Maintenance
GPS Antenna Placement
- Clear, unobstructed 360° sky view — no mast, boom, or VHF antenna directly overhead
- Minimize cable run length to reduce signal loss
- Keep away from radar scanner (interference and physical damage risk)
- Active antenna requires power on the coax cable (+5V DC) from receiver
- Check connector for corrosion annually — salt air degrades connections
Power Requirements
- GPS receivers: typically 0.5-2A at 12V DC
- Chartplotter: 1-4A at 12V depending on screen size and backlight
- Radar: 4-15A peak at 12V (magnetron type); FMCW solid-state radars draw less
- AIS transponder: 0.5-2A receive; up to 4A transmit
- Fuse each device at the panel with the correct fuse rating
- Voltage drop: use appropriately sized wire for cable runs
Waterproofing and Environment
- Chartplotters: IPX5 or IPX7 rating typical for cockpit-mounted units
- Use dielectric grease on all connectors exposed to the elements
- Heat can destroy LCD screens — avoid direct solar exposure when possible
- Rinse salt water off displays with fresh water regularly
- Store handheld GPS in a dry bag when not in use
- Inspect transducer connection annually for marine growth and damage
Battery Backup and Redundancy
A professional captain never relies on a single electronic navigation system. The minimum redundancy plan includes:
- Primary: installed chartplotter with dedicated GPS antenna
- Secondary: portable handheld GPS (waterproof, with own batteries)
- Tertiary: paper charts, compass, and dividers for the route area
- Backup power: separate battery or power bank for electronics
- Handheld VHF (separate from installed radio) in ditch bag
- EPIRB: registered, tested, hydrostatic release checked
- PLB (Personal Locator Beacon) for each crew member on offshore passages
- Offline chart app on tablet as additional backup
13. Practice Problems with Solutions
Work through these problems the way the USCG exam expects: apply the concept, show your reasoning, then check the answer. Expand each problem to see the solution.
1Your chartplotter shows XTE = 0.15 NM R. What does this mean and what action do you take?
XTE 0.15 NM R (Right) means you are 0.15 nautical miles to the right (starboard) of your planned track line. To return to track, alter course to port until XTE decreases to zero, then resume your original course. The actual correction course depends on the angle between your current heading and the bearing to the nearest point on the track.
2Your GPS shows position 36° 25.4' N, 075° 52.1' W. The chart you are using is referenced to NAD-27. A note in the chart margin says 'NAD-27 to WGS-84: shift 0.12' S, 0.18' E.' Where do you plot the fix?
Your GPS outputs WGS-84 positions. The chart uses NAD-27. To plot the GPS fix correctly on a NAD-27 chart, you apply the datum shift in reverse: move the plotted position 0.12' north and 0.18' west from the raw GPS readout. Alternatively, set your GPS to output NAD-27 positions if the receiver supports datum selection. Always check chart datum before plotting any GPS fix.
3Your chartplotter shows an AIS target with CPA = 0.3 NM and TCPA = 8 minutes. COLREGs applies. What do you consider first?
A CPA of 0.3 NM is close and a TCPA of 8 minutes means action time is limited. First, visually confirm the target and assess the situation under Rule 7 (risk of collision) and Rule 8 (action to avoid). Determine the aspect (bearing from the other vessel to you) to establish which vessel is give-way. AIS provides COG and SOG but does not replace a proper visual watch. Take early and substantial action (Rule 8) if you are give-way — do not wait for CPA to shrink further.
4Your GPS fails 20 miles offshore. Last known position was 10 minutes ago. Your heading has been 045°T and SOG was 8 knots. Where are you now?
Distance traveled in 10 minutes at 8 knots: (8 knots x 10/60 hours) = 1.33 NM. Plot from last known fix: apply a course of 045°T, distance 1.33 NM. This is your DR (dead reckoning) position. Now cross-check: use depth sounder against charted depths, take visual bearings to any charted landmarks, and monitor VHF for any Coast Guard broadcasts that could help confirm position. Switch to paper chart and compass navigation.
5You are setting up an NMEA 0183 network connecting a GPS, chartplotter, VHF radio, and autopilot. The GPS outputs at 4,800 baud. The autopilot requires NMEA heading data. What sentence does the autopilot need and from which device?
The autopilot needs heading data, but GPS provides COG (Course Over Ground via $GPRMC or $GPVTG), not magnetic compass heading. If the autopilot is following a GPS track, COG from $GPRMC is sufficient. However, for proper compass steering, a magnetic compass with NMEA output provides $HEHDM or $HEHDT sentences. In an NMEA 0183 network, the GPS is the talker on a bus; all devices share one baud rate (4,800 standard). Connect GPS TX to the NMEA IN of each device (chartplotter, VHF, autopilot) in a daisy-chain or star topology.
6What is the difference between SOG and speed through water, and when does the difference matter most?
SOG (Speed Over Ground) is your actual speed relative to the earth, measured by GPS. Speed through water is measured by a paddlewheel or impeller sensor and reflects motion through the water mass. The difference equals the current. In a 2-knot following current, a boat doing 6 knots through water shows SOG of 8 knots. In a head current, the opposite occurs. Speed through water matters for calculating tidal stream effects, fuel burn, and engine loading. SOG matters for ETA calculations and chart plotting.
Frequently Asked Questions
What is HDOP and why does it matter for GPS accuracy?
HDOP (Horizontal Dilution of Precision) measures the geometric quality of GPS satellite positions in the horizontal plane. A low HDOP (1-2) means satellites are well spread across the sky, giving better position accuracy. An HDOP above 4 is considered poor. PDOP (Position DOP) includes the vertical dimension. On the USCG exam, remember: lower DOP = better accuracy. SA (Selective Availability) was removed in May 2000, improving civilian GPS from ~100m to ~15m accuracy.
What is the difference between ENC and RNC electronic charts?
ENC (Electronic Navigational Chart) is a vector-format chart meeting the S-57 standard. Objects are stored as individual data points that can be queried, zoomed without pixelation, and automatically updated. RNC (Raster Navigational Chart) is a scanned image of a paper chart — it pixelates when zoomed in and cannot be queried. ECDIS (Electronic Chart Display and Information System) uses ENCs and can legally replace paper charts on SOLAS vessels. A recreational chartplotter typically displays RNCs or a proprietary vector format.
How does chart datum affect GPS position on older charts?
GPS positions are referenced to WGS-84 datum. Many older U.S. coastal charts use NAD-83, which is nearly identical to WGS-84 (within a meter in most areas). However, some older charts use NAD-27, which can differ from WGS-84 by 10-200 meters depending on location. Always check the note in the chart title block. If the chart says NAD-27, apply the datum shift correction before plotting a GPS fix, or the plotted position will be offset from the actual position.
What is XTE on a chartplotter and how do you use it?
XTE (Cross-Track Error) is the perpendicular distance your vessel has deviated from the planned track line between two waypoints, shown in nautical miles or tenths. A positive XTE means you are to the right of your track (starboard); negative means left (port). To return to track, steer toward the track until XTE reaches zero, then resume the original course. BTW (Bearing to Waypoint) and VMG (Velocity Made Good) are related displays on the same screen.
What is MMSI and how do you register a DSC radio?
MMSI (Maritime Mobile Service Identity) is a unique 9-digit number assigned to a vessel's VHF DSC radio. U.S. vessels register through BoatUS, Sea Tow, or directly with the FCC (if licensed). The MMSI must be programmed into the DSC radio before it can send or receive individual calls or participate in automatic distress alerting. For a distress call, the radio transmits the MMSI, GPS position, time, and nature of distress on Channel 70 automatically when you press and hold the DISTRESS button for 5 seconds.
What are the main NMEA 0183 sentences a navigator should know?
Key NMEA 0183 sentences: GGA (GPS fix, position, altitude, HDOP), RMC (recommended minimum data: position, SOG, COG, date), GLL (geographic position: latitude/longitude), VTG (course and speed over ground), HDT or HDM (true or magnetic heading from compass), VHW (speed through water), DPT (depth), MWV (wind speed and angle). NMEA 0183 runs at 4,800 baud by default (some newer at 38,400). Sentences begin with $ and end with a checksum after *.
What backup navigation methods must a captain use if GPS fails?
If GPS fails, use dead reckoning: start from last known position, apply compass course (corrected for variation and deviation), boat speed (from paddlewheel or engine RPM table), and elapsed time to estimate current position. Cross-check against depth soundings on the chart and visual bearings to charted objects. Paper charts, compass, and dividers are essential backup tools. GPS jamming and spoofing are real threats near ports and military areas. LORAN-C was the predecessor backup system; it was shut down in 2010. Celestial navigation remains the ultimate backup for offshore passages.
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