VTS Equipment and Technology 4 — Questions and Answers
Question 1: What technology is used in a VTS tidal monitoring system to measure water depth in real time?
- Acoustic Doppler Current Profiler (ADCP) and pressure transducer gauges (Correct answer)
- Satellite altimetry only
- Radar cross-section measurement
- Inertial navigation sensors
Correct answer: Acoustic Doppler Current Profiler (ADCP) and pressure transducer gauges
VTS tide gauges typically use acoustic sensors, pressure transducers, or float-based systems, while ADCPs also measure current velocity at multiple depths.
Question 2: A VTS operator observes significant 'blind sectors' on the primary radar display. What is the most common cause?
- Insufficient radar transmitter power
- Physical obstructions such as buildings or terrain blocking the antenna beam (Correct answer)
- Excessive sea clutter suppression settings
- Incorrect chart overlay calibration
Correct answer: Physical obstructions such as buildings or terrain blocking the antenna beam
Blind sectors occur when physical structures block portions of the radar beam, creating areas with no radar coverage that must be compensated with overlapping radar coverage or other sensors.
Question 3: Which ITU frequency band is allocated for AIS transmissions?
- MF (300 kHz – 3 MHz)
- HF (3–30 MHz)
- VHF (156–174 MHz) specifically channels 87B and 88B (Correct answer)
- UHF (300 MHz – 3 GHz)
Correct answer: VHF (156–174 MHz) specifically channels 87B and 88B
AIS operates on VHF maritime channels 87B (161.975 MHz) and 88B (162.025 MHz) using TDMA technology.
Question 4: What is the purpose of a Differential GPS (DGPS) beacon station used in conjunction with VTS operations?
- To provide enhanced GPS accuracy by broadcasting correction signals to nearby receivers (Correct answer)
- To replace GPS when satellite signals are unavailable
- To track vessel movements without requiring onboard equipment
- To synchronize VTS radar with UTC time
Correct answer: To provide enhanced GPS accuracy by broadcasting correction signals to nearby receivers
DGPS stations broadcast correction signals derived from a fixed reference receiver, reducing GPS position errors to sub-meter accuracy for nearby vessels.
Question 5: In a digital VHF recording system used by VTS, what is the primary reason for time-stamping all recorded communications?
- To comply with FCC licensing requirements for broadcast stations
- To enable accurate reconstruction of the sequence of events during incident investigations (Correct answer)
- To measure vessel transit times automatically
- To synchronize audio with CCTV footage for billing purposes
Correct answer: To enable accurate reconstruction of the sequence of events during incident investigations
Time-stamped recordings allow investigators to reconstruct the precise timeline of communications during incidents, collisions, or near-misses.
Question 6: What is 'Long Range Identification and Tracking' (LRIT) and how does it differ from AIS?
- LRIT uses satellite transmission to report vessel positions globally every 6 hours, while AIS is VHF-range only (Correct answer)
- LRIT is a VHF system for coastal tracking; AIS uses satellite
- LRIT and AIS are identical systems with different brand names
- LRIT tracks cargo containers; AIS tracks vessel hulls
Correct answer: LRIT uses satellite transmission to report vessel positions globally every 6 hours, while AIS is VHF-range only
LRIT uses satellite links to report SOLAS vessel positions to flag states at defined intervals globally, while AIS is a VHF broadcast system with a range of roughly 20–40 nautical miles.
Question 7: A VTS operator needs to verify that a vessel's AIS position matches its radar position. What tolerance is generally acceptable for a stationary vessel?
- Within 0.1 nautical miles
- Within 10 nautical miles
- Within approximately 10–30 meters, depending on GPS accuracy (Correct answer)
- Exact pixel-perfect overlay is required
Correct answer: Within approximately 10–30 meters, depending on GPS accuracy
Standard GPS accuracy is typically 5–15 meters, so a position discrepancy of 10–30 meters for a stationary vessel is generally within acceptable tolerances.
VTS Equipment and Technology 4
What technology is used in a VTS tidal monitoring system to measure water depth in real time?