Why GPS compass for 500 GT vessels only

GPS compass offers several advantages over gyro compass. Then why GPS Compass (the heading device) is allowed to be installed in the vessels upto 500 GT only and not for bigger vessels above 500 GT?

Can you explain to me what advantages you think a GPS Compass has over a gyro…?Because GPS Compasses much less accurate, the errors aren’t consistent, and they’re subject interference from overhead obstructions and selective availability…

Seems like the gyro is still the winner here.

I’d expect a GPS compass to be significantly cheaper which should make them attractive for smaller vessels. However, relying on external satellite constellation more susceptible to external interference makes the vessel significantly less self-sufficient when it comes to navigation. In case of larger vessels, the consequences of GPS compass malfunction may be more severe - a <500 GT vessel leaks significantly less fuel when run aground than a bigger ship. Keep in mind that you can buy GPS jammers from eBay…

A top satellite compass costs about $4,000. It has no moving parts and does not require much power of maintence. It is usually powered by a 12 volt battery in the wheelhouse that is charged by a Const’s volt.

A sat compass is cheap enough that a boat could afford two of them. But they are so reliable that I’ve never seen more than one installed. The only problems I’ve ever seen were with the wiring and interface wiring to other devices.

A USCG approved gyro is about $25,000 to start. My cellphone has s gyro. No reason for an adequate small vessel gyro to cost over $2,000

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Tugsailor, I doubt your cellphone has a gyrocompass - it likely has a magnetic compass, backed by a GPS to tell you what you COG is. And this is the GPS compass’ main limitation - if you aren’t in motion, it isn’t accurate at all.
Try moving a magnet around it and find out for sure…

Cell phones commonly have a 3 axis piezo gyro installed. This is the same tech as the so called “rate gyro” integrated into most fluxgate- and satellite compasses. It gives you a very high rate (khz range) source of attitude data, but does not provide a stable heading reference.

I was just looking at this:Introduction to MEMS gyroscopes


MEMS gyroscopes can measure angular velocity. Digital cameras use gyroscopes to detect hand rotation for image stabilization. A yaw rate gyroscope can be used in cars to activate the electronic stability control (ESC) brake system to prevent accidents from happening when the car is making a sharp turn. And a roll gyroscope can be used to activate airbags when a rollover condition happens.

A yaw rate gyroscope can be used in cars to measure the orientation to keep the car moving on a digital map when GPS signal is lost. This is called car dead-reckoning backup system.

Swing meters wtf?- from this thread I recall that pushboat swing meters uses mechanical gyroscopes but I’m not sure.

The GPS compass I have on board is part of the DP DGPS. It consists of two GPS antenna’s and calculates the heading using the difference in position of the tow antenna’s.
No need to make speed over ground…

I am most familiar with the $4000 Furuno SaT Compass. It has a small “radome” with little “porcupine quills” sticking out of it to keep birds from sitting and shitting on it. I think it has 3 antenna inside the ‘radome’.

I have heard about other systems that have two separate antennas placed more fore and aft.

You have the citation for that requirement? Nav safety regs. 33 CFR 164.35 “Equipment: All vessels” only mentions gyrocompass. Is it SOLAS? Just curious. Thanks!

I am no expert on this topic, and so feel free to educate me. My comment is anecdotal.

I’ve sailed on supply boats and tugs outfitted with gyrocompasses, and on tugs with GPS compasses. My concern with gyrocompasses on supply boats and tugs was, how does one determine gyro error? Smaller vessels generally won’t have a gyro repeater/alidade to shoot either azimuth of sun/star or to shoot a transit bearing to correct for gyro error. Not to mention the lack of proper maintenance. I never had an auditor ask about gyro error. Is this an audit item on >500 ton vessels? I imagine that in the case of an accident, the USCG is going to ask any vessel outfitted with a gyro what the error was.

To me the installation of GPS compasses on smaller vessels makes sense The GPS compass (when the vessel is moving) is highly accurate. They’re much less expensive to purchase and the overall cost of ownership is negligible.

That’s a bit of a simplification. It measures phase angle difference for each satellite across multiple antennae, thus determining the relative bearing. The process has a lot of similarities with how it determines its position by measuring phase angle between different satellites, but it requires different hardware. If this could be done in software by simply comparing positions, GPS compasses would not be so expensive.

I have a cheaper Simrad HS 70 with only two antennae. Aside from being less accurate, it also doesn’t output useful angle of heel. I’m very happy with it, though. Dealing with fluxgate sensors in steel boats, especially at high latitudes, can only be described as traumatic. This thing outputs an accurate enough heading a few seconds after power on, which is exactly what I need.

I’m not sure I get the problem. You sight along the ship’s centerline same as you would for zeroing the radar’s head-up, call that out to someone reading the gyro bearing, then jog it across to get within limits.

The accuracy of a GPS compass does not improve with movement. In fact, repeatability goes down if you move around.

To me, the greatest advantage of a GPS compass over a gyro is that it’s tolerant to power loss. If you lose power to the gyro in open water, the best reference you have for zeroing it back out is the magnetic compass, which may be not very good at all.


There has been a move towards GPS compasses in vessels trading in the Pacific islands because of the difficulty in getting gyro compasses serviced. Many years ago I was in a small general cargo vessel trading in the Pacific when the gyro failed. We were already without a working radar and the master communicated with head office to inform him of his intention to make daylight landfalls and transits through various reefs. Communication was only possible at night with the ancient HF transceiver.
The Gyro technician arrived from Suva to the small port where we were working cargo. To our surprise he seemed to know nothing about the gyro he was supposed to service but he was a mean guitar player.

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I alway ask if GPS compass on tugs or yachts. There are areas where microwave, telephone links , will temporarily knock gps compass off. It’s a what the hell moment when autopilot goes nuts when crossing microwave link between telco towers.

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The cost of doing a minor service on a gyro is usually more than the $4,000 it costs to buy a sat Compass. A gyro costs about $20,000.

I like having a gyro, sat compass, and magnetic compass, and a switch to select autopilot heading input. However, there are a lot of things other than a gyro that I’d rather see thevowner spend $20,000 on.

Mine consistently makes a ~60 degree turn to starboard every time I go under a bridge.

That can happen with a magnetic Compass too. Hand steering is usually best for bridges.

Large vessel mariners with large crews have the custom of a helmsman always steering by hand in constricted waters. Small vessels with just one person in the wheelhouse are usually best steered by autopilot. That frees the watchstander up to navigate and talk on the radio.

Modern autopilots with “work mode” steer a small vessel much better than a man in most situations, even in restricted waters.

Sound like a hybrid system is needed, combine gps compass with MEMS sensor to catch inadvertent turn as a result of the loss of gps signal with a Kalman filter to smooth the errors.

EDIT: Here it is: VECTORNAV VN-200 GPS Aided INS


The VN-200 is a miniature, high performance GPS-Aided Inertial Navigation System (GPS/INS) that combines MEMS inertial sensors, a high-sensitivity GPS receiver, and advanced Kalman filtering algorithms to provide optimal estimates of position, velocity, and attitude.


Indeed. I’ve already had an accident due to autopilot malfunction, and wouldn’t trust it anywhere within at least a couple of crash stop distances of shore. There are times when the unrealiability of GPS compasses is a major annoyance, though. For example, when working in zero visibility in tight spaces, radar is basically my only heading reference.

The HS-70 already has a solid state gyro for roll compensation etc, and its marketing material makes much of how long it will stay on course after loss of satellite coverage. However, that’s not what’s going on under a bridge. Rather, I read the signal indirectly, so it’s now coming from an unexpected direction. The sensor sees a mismatch between satellite course and inertial course, and decides that the GPS course is the reliable of the two. IMNSHO, it should stop sending course updates when it sees that something is badly awry, but I suppose the engineers at Simrad see things differently.

Fighter jets use INS as their primary position source, and the drift is reportedly minuscule (as in a few meters per sortie). I always assumed that all the movement would exclude usable accuracy at sea, but I’m thrilled to see that I was wrong.

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Seems like Dr. Kalman should be able to handle this problem relatively cheaply.

In addition to weighing INS and GPS data a system maybe could also use map data to see it was under a bridge and weigh the INS data more heavily.

Don’t know if that is practical in an inexpensive system. The hybrid car here remembers trips and adjusts battery/gas accordingly.

Don’t know about motion from the sea vs an aircraft, the auto steering on the ship uses an adaptive system. Course keeping improves over time, in rough seas it will slowly reduce steering errors over maybe 15 or 20 minutes on auto.

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