NOF (Next Objective Fix) – Navigation Glossary

Introduction to Position Fixing: Definitions and Context

Position fixing is the process of determining the precise current location of a vessel or aircraft, usually in geographic coordinates, and plotting it on a chart. This is a fundamental practice in both marine and aviation navigation for ensuring safe passage, collision avoidance, and effective route management. Techniques for position fixing range from traditional visual or celestial methods to modern electronic systems like GPS and radar.

In marine navigation, position fixing is essential for confirming location relative to hazards, ports, and waypoints, and for timing arrivals. In aviation, it is crucial for maintaining flight paths, ensuring obstacle clearance, and accurate time estimates.

The process of position fixing fits into objective-based navigation—planning a route as a series of checkpoints or “fixes.” Each fix is a known position that allows the navigator to verify and, if necessary, correct their track, thus minimizing navigational errors. This is especially important in areas with unreliable GPS, signal loss, or limited navigational aids.

Regardless of method, all fixes must be promptly plotted, clearly labeled with time, and checked against previous fixes and dead reckoning (DR) positions. This cycle of fixing and monitoring forms the backbone of safe navigation.

What is the Next Objective Fix (NOF)?

Definition

The Next Objective Fix (NOF) is the specifically planned next position along a vessel’s or aircraft’s intended route where a reliable fix will be obtained, using one or more navigational methods. Unlike arbitrary or opportunistic fixes, the NOF is deliberately selected in the passage plan to ensure proactive verification of position and route adherence. It serves as a critical checkpoint for navigation safety.

Purpose and Importance

• Verification: Provides a scheduled opportunity to confirm that the vessel or aircraft remains on course, cross-checking against previous fixes and DR estimates.
• Course Correction: By comparing the actual fix at NOF with the expected position, navigators can quickly correct for drift, current, wind, or instrument error.
• Safety: Early detection of navigational hazards and prevention of accidental incursion into restricted or dangerous areas.
• Planning Structure: Introduces discipline and predictability, which is vital when GPS or real-time systems may be unavailable, jammed, or spoofed.

Usage Context

In marine navigation, NOFs are used for regular position checks, near waypoints, hazards, or high-traffic zones. In aviation, NOFs are applied during en-route phases and approach procedures to maintain prescribed flight paths and comply with air traffic regulations.

Organizations such as the IMO and ICAO recommend or require the establishment of NOFs in passage planning, especially in high-risk areas or where navigation aids are limited.

Key Related Terms and Concepts

Fix

A fix is a position determined by the simultaneous intersection of two or more independent lines of position (LOPs). The more and the better-spread the LOPs, the more reliable the fix. Fixes are plotted and labeled with exact time, and form the basis for subsequent navigation.

Line of Position (LOP)

An LOP is a line or arc along which the vessel or aircraft’s position is known to lie, based on a particular observation (e.g., compass bearing, radar range, celestial sight). Two or more LOPs intersect to provide a fix. LOPs are used in both marine and aviation navigation.

Dead Reckoning (DR)

Dead Reckoning is the process of estimating current or future position using previous fix, course steered, speed, and elapsed time—without reference to external observations. Errors in DR accumulate over time, so it must be recalibrated with fixes.

Estimated Position (EP)

An EP is derived from incomplete information—typically a single LOP combined with a DR position, or aided by other data such as soundings or drift estimates. EPs are less reliable than fixes and used only until a better fix can be obtained.

Running Fix

A running fix is used when only one navigation aid is available at a time. Bearings or ranges are taken at different times, and the earlier LOP is advanced to the time of the later observation. The intersection provides the running fix.

Course and Speed (Knots)

Course is the intended direction, in degrees, relative to true or magnetic north. Speed is measured in knots (nautical miles per hour). Both are essential for DR and for calculating fix intervals and times of arrival.

Time Interval

Time interval is the period between navigation events (fixes, DR plots, EPs). The interval is chosen based on speed, risk, and proximity to hazards. Shorter intervals are used near dangers; longer intervals in open, low-risk areas.

Advancing LOPs

Advancing LOPs means moving a previously obtained LOP forward in time along the course by the distance traveled. This is key for running fixes.

Plotting LOPs

Plotting LOPs involves drawing lines on the chart based on observations. The intersection of two or more LOPs forms a fix. Accuracy in plotting and instrument reading is critical.

Methods of Obtaining a Fix

GPS Fix

A GPS fix is calculated from signals received from multiple satellites, providing high accuracy (often <10m). The navigator plots the latitude and longitude from the receiver onto the chart and labels it with the time and “GPS Fix.”

Best Practices:

• Cross-check GPS with other methods, especially near hazards or in degraded signal environments.
• Record DOP and satellite status.
• In aviation, monitor RAIM status.

Limitations:
GPS can be degraded by interference, jamming, spoofing, or atmospheric conditions.

Visual Fix

A visual fix uses compass bearings to two or more charted objects (e.g., lighthouses, buoys) simultaneously. LOPs are plotted from these objects; their intersection is the fix.

Best Practices:

• Choose aids with a wide angular spread (ideally 60°–120° apart).
• Correct all bearings for magnetic error.

Limitations:
Not possible in poor visibility or when suitable aids are lacking.

Dead Reckoning Position

A DR position is estimated from the last fix using course, speed, and time.

Best Practices:

• Update DR after each fix, course, or speed change, and at regular intervals.
• Use DR as a baseline to compare with subsequent fixes.

Limitations:
Errors accumulate over time if not corrected with external observations.

Running Fix

A running fix is obtained by taking a bearing (or range) to a single object at two different times, advancing the earlier LOP based on course and speed, then finding the intersection.

Best Practices:

• Record times and bearings precisely.
• Use when only one aid is visible.

Limitations:
Accuracy depends on correct time, course, and speed.

Estimated Position (EP)

An EP is used when only a single LOP and a DR are available (or similar limited information).

Best Practices:

• Seek a true fix as soon as possible after plotting an EP.

Limitations:
Lower reliability; should not be used near hazards.

NOF: Procedures, Calculation, and Plotting

When and Why to Use NOF

• Regular Intervals: Set NOFs at intervals based on speed, proximity to hazards, and desired accuracy.
• Critical Points: Plan NOFs at waypoints, before entering restricted or hazardous areas.
• Monitoring Progress: NOFs provide reference points for verifying position and updating passage plans.

Step-by-Step NOF Procedure

1. Determine Fix Interval:
   Calculate safe time/distance between fixes.
   Example: Closest Hazard (NM) / Speed (knots) = Fix Interval (hours).

2. Identify NOF Location:
   Mark the next NOF on the chart, considering navigational aids and GPS coverage.

3. Advance DR Plot:
   From the last fix, plot DR positions at each interval up to the NOF, labeling each with time, course, and speed.

4. Prepare for NOF:
   Identify available fixing methods (visual, radar, GPS, soundings) at the NOF.

5. Obtain and Plot the Fix at NOF:
   Use the planned method(s) to obtain a fix at the NOF, plot it, compare with DR and EP, and update the navigation log.

6. Adjust as Necessary:
   If deviation is detected, correct course/speed and update the passage plan.

7. Repeat the Cycle:
   Continue to the next NOF as per passage plan, maintaining regular verification and adjustment.

Example: Marine NOF Passage

• Vessel speed: 8 knots
• Closest hazard: 2 NM
• Fix interval: 2 NM / 8 knots = 0.25 hr (15 min)
• NOFs scheduled every 15 minutes or at each waypoint
• At each NOF, use GPS, then cross-check with radar or visual bearings

Example: Aviation NOF Application

• Aircraft: On a cross-country segment
• NOFs scheduled at VOR radials, DME distances, or GPS waypoints
• At each NOF, confirm position with available NAVAIDs or GPS
• Update ETA and check for route deviations

Best Practices and Recommendations

• Always schedule NOFs with sufficient margin for error, especially near hazards.
• Use multiple fixing methods when possible for cross-verification.
• Maintain clear, up-to-date navigation logs and chart annotations.
• Train regularly in traditional navigation skills as backup to electronic solutions.
• Plan contingencies for times when a fix cannot be obtained at a scheduled NOF.

Conclusion

The Next Objective Fix (NOF) is a vital concept in safe navigation for both marine and aviation operations. By deliberately planning where and how to obtain the next reliable fix, navigators can maintain situational awareness, minimize risk, and comply with regulatory standards. Whether using GPS, radar, visual, or traditional methods, the disciplined use of NOFs ensures that no matter the circumstances, the navigator always knows where they are—and how to stay safe.

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