📐 DAY 13: PATCH TEST (PART 1) – CALIBRATION OVERVIEW & DATA COLLECTION

Why, What, and How to Collect Patch Test Lines for MBES
Instructor: Engr. Rokib Hossain | River Engineering Solutions
🏠 Course Homepage

📖 Today's Outline

❓ Why Do a Patch Test?

Even after careful mechanical alignment and offset measurement, small residual angular errors and time delays remain. The patch test is a dedicated survey that quantifies these errors by sailing over a suitable seabed feature (e.g., a sloping ramp or a small isolated target) in specific patterns. The results are then used to update the alignment and latency values in your MBES acquisition software.

Without a patch test, your multibeam data may have:

  • Systematic depth errors (due to roll or pitch misalignment).
  • Horizontal positioning errors (due to yaw misalignment or latency).
  • Inconsistent depths when sailing in opposite directions.
📌 Key concept: The patch test does not replace mechanical alignment; it refines it. Always perform a patch test after first installation and after any sensor removal or significant vessel modification.
🧩 [Diagram showing misaligned vs corrected swaths]
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🧩 Parameters Determined: Roll, Pitch, Yaw, Latency

The patch test computes four critical corrections:

  • Roll offset – rotation around the vessel's longitudinal axis (port‑starboard). Causes one side of the swath to be too deep and the other too shallow.
  • Pitch offset – rotation around the transverse axis (fore‑aft). Causes depth mismatch when sailing opposite directions.
  • Yaw offset (heading alignment) – rotation around the vertical axis. Shifts outer beams laterally, causing "mustache" patterns.
  • Latency (time delay) – delay between the GNSS time stamp and the sonar ping. Causes a constant along‑track shift that varies with speed.

Each parameter is derived from a specific line pattern (see below). Modern MBES software (e.g., Hypack Patch Test, QINSy Calibration, Teledyne PDS) automates the computation once the data is collected.

🗺️ Choosing a Suitable Test Site

For a reliable patch test, find an area with:

  • Constant water depth of at least 10–15 m (but not too deep to avoid long survey times).
  • A well-defined sloping feature (natural ridge, dredged slope, or a wreck) for roll and pitch analysis. A flat bottom is NOT suitable – you need a change in depth.
  • An isolated small target (e.g., a rock, a concrete block, or a prominent object) for yaw and latency – ideally less than 2 m in diameter.
  • Low vessel traffic and calm weather (waves <0.3 m).
  • Good GNSS coverage and RTK fix throughout the test.

If natural features are not available, some hydrographers deploy a temporary target (e.g., a weighted metal cone) on the seabed.

🗺️ Pro tip: Mark the patch test area on your navigation screen and run a quick pre‑scan to confirm bottom feature visibility before starting the formal test.

📏 Roll Calibration Line

Purpose: Determine the roll offset between the transducer and the motion sensor.

Procedure:

  1. Identify a linear slope that runs perpendicular to the planned line direction (i.e., the slope changes across the swath).
  2. Sail a single line over the slope at slow speed (4–6 knots).
  3. Acquisition software will compare the port and starboard depths; any difference indicates a roll offset.

Note: Some methods use two lines in opposite directions over the same slope to cancel out pitch effects. Check your software's recommendation.

📈 [Diagram of roll line over a sloping seabed]
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📐 Pitch Calibration Line

Purpose: Determine the pitch offset (fore‑aft tilt).

Procedure:

  1. Select a slope that is parallel to the line direction (i.e., depth changes along the line).
  2. Run two lines in opposite directions over the same slope, ideally at the same speed and as close as possible to the same track.
  3. If pitch offset is present, the depth profiles from the two directions will not align – one will show the slope steeper, the other shallower.

Some software also uses a single line with a known feature, but the opposite‑direction method is more robust.

🧭 Yaw (Heading) Calibration Line

Purpose: Determine the angular offset between the transducer's forward direction and the vessel's heading.

Procedure:

  1. Find a small, isolated target (rock, wreck, artificial object).
  2. Run two lines in opposite directions, passing over the target. Keep the vessel speed constant.
  3. In the processed data, the target will appear in two different positions horizontally if yaw offset exists. The offset is calculated from the lateral shift.

Modern software can also use a sloping feature for yaw, but the target method is more accurate.

🎯 Important: Yaw error is the most critical because it directly affects horizontal positioning of outer beams. A 0.5° yaw error at 30 m depth can cause a 0.5 m horizontal error at the edge of the swath.

⏱️ Latency Calibration Line

Purpose: Determine the time delay between the GNSS time stamp and the sonar ping.

Procedure:

  1. Use the same isolated target as for yaw, or a steep slope.
  2. Run two lines in opposite directions at two different speeds (e.g., 4 knots and 8 knots).
  3. Latency manifests as a shift of the target's position proportional to speed. By comparing positions at different speeds, the latency is computed.

Alternatively, use a known feature with a steep edge and compare the leading edge position at different speeds.

⏱️ [Graph showing target shift vs speed for latency calculation]
(Replace with your own image)

📊 Collecting Data – Best Practices

  • Quality control: Monitor real‑time coverage and ensure RTK fix (FIX, not float) throughout. Record motion data (heave, roll, pitch) and SVP.
  • Speed and heading: Maintain constant speed and heading during each line. Avoid turning during the critical portion.
  • Logging: Log raw data in the acquisition software. Do not apply any temporary patch test corrections while collecting – we want raw data.
  • Repeat runs: For each parameter, collect at least two good lines to verify consistency.
  • Environmental conditions: Avoid times of strong thermoclines or high noise. Calm weather gives cleaner data.
✅ Pro tip: Before starting the patch test, run a short test line and examine the water column display to ensure no excessive noise or bubble interference.

🔗 External Resources & Tools

Refer to these official guides for patch test procedures:

ResourceDescriptionLink
Hypack Patch Test ManualStep‑by‑step for collecting and processing patch test linesHypack Help
Teledyne RESON Patch Test GuideDetailed PDF with line patterns and analysisTeledyne RESON
QINSy Calibration ModuleCalibration manual for roll, pitch, yaw, latencyQPS QINSy
NOAA Patch Test ProceduresField procedures for MBES calibrationNOAA PDF (Chapter 6)
🔗 Authority linking: Hypack, Teledyne, QPS, and NOAA are trusted references.

📋 Homework & Fieldwork Checklists

📘 Day 13 – Homework Checklist

  • ✅ Read the entire Day 13 post.
  • ✅ Watch a video demonstration of a patch test (search "MBES patch test data collection").
  • ✅ Draw a schematic showing the four line types (roll, pitch, yaw, latency) and the seabed feature used for each.
  • ✅ Write a short paragraph explaining why a flat seabed cannot be used for roll calibration.
  • ✅ Share this post with #Hydrography #PatchTest.

🛠️ Fieldwork Readiness Checklist – Patch Test Data Collection

  • ✅ Before leaving port: verify that the patch test site is clear of traffic, has suitable seabed features, and weather forecast is calm.
  • ✅ On site: conduct a quick line to confirm bottom feature visibility and noise levels.
  • ✅ For roll: select a slope that runs perpendicular to the line. Sail at constant speed and log data.
  • ✅ For pitch: select a slope parallel to the line; run two lines in opposite directions over the same slope.
  • ✅ For yaw: identify a small isolated target; run two lines in opposite directions passing over the target.
  • ✅ For latency: use the same target and run two lines at different speeds (e.g., 4 and 8 knots) in the same direction.
  • ✅ After each line, check that the data was recorded correctly and that GNSS fix remained RTK fixed.
  • ✅ Backup all raw data immediately after finishing the patch test.
🖨️ Pro tip: Use the Print / Download PDF button to keep these checklists in your field binder.

❓ Frequently Asked Questions

🔹 How long does a patch test take? ▼
Collecting the data typically takes 1‑2 hours, depending on the distance to the test site and number of lines. Processing (Day 14) adds another 1‑2 hours. Plan a half day for the entire patch test.
🔹 Can I perform a patch test in shallow water (e.g., 5 m)? ▼
Yes, but the resolution of angular errors is lower because the lever arm effect scales with depth. Deeper water (10‑20 m) is preferred for better sensitivity. However, many shallow‑water systems work well in 5‑10 m as long as a good slope exists.
🔹 Do I need to redo the patch test if I change the SVP? ▼
No, SVP affects ray‑bending but not the physical alignment offsets. However, if you apply a wrong SVP, the patch test results may be biased. Always use a good SVP during the patch test.
🔹 What if my patch test area has no natural feature? ▼
You can deploy an artificial target (e.g., a concrete block with a flat top, or a metal cone). Ensure it is placed securely and its position is measured. Some teams use a pre‑surveyed rock or a wreck from the chart.
🔹 Can I use a single‑beam echosounder for patch test? ▼
No, patch test is specific to multibeam because it uses the swath geometry. Single‑beam has different calibration procedures (e.g., bar check).

✅ Action Items & Internal Linking

🎯 Your Tasks for Today

  • 📌 Step 1: Bookmark the Course Homepage and the Hypack patch test guide.
  • 📌 Step 2: Download the checklist PDF using the button at the top.
  • 📌 Step 3: Using Google Earth, identify a potential patch test site near your area (look for underwater slopes or isolated wrecks).
  • 📌 Step 4: Leave a comment on this post: "The most challenging part of patch test data collection for me is ____."
  • 📌 Step 5: Proceed to Day 14: Patch Test (Part 2) – Processing & Applying Corrections using the link below.
◀ Day 12: Transducer Mounting & Alignment Day 14: Patch Test (Part 2) ▶
🔗 Internal linking note: Day 12 and Day 14 are correctly linked. This builds a strong internal network for SEO.

👨‍🔧 Engr. Rokib Hossain | River Engineering Solutions

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