Why GPS compass for 500 GT vessels only

It makes sense to limit GPS compass to smaller vessels. The mechanical gyros I used have been very reliable, but we still check for error with an azimuth at least daily if possible and check against the magnetic after every course change.

On a larger vessel heading information is too critical to rely on a single source.

Maybe larger vessels could benefit from a GPS compass for additional redundancy, the current cross-checks depend upon the crew taking positive action.

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What is this position jump of which you speak?

When the “tracks” function is left on at the pier I can see the position jumping around. I assume it’s noise, reflected signals as you said.

From reading threads on here about DP my understanding is that the GPS input is filtered for errors.

An example is that during a discussion about anemometers it was mentioned that DP drill rigs use wind speed / direction data to help determine if a change in position is GPS data error or actual change in position.

I don’t have a lot of practical experence, just what I’ve read.

in the world of DP ( that I work in) gps can be the most accurate sensor, (close to 50mm offshore these days) but everyone knows its the most unreliable and never to be trusted.

All GPS signals are kalman filtered other wise live data will jump so much you would never read the dial, but thats separate from all the other issues with a weak signal being sent from outer space. There is a reason you pay independent companies for corrections but that still doesnt help when you shadow the antenna at a rig or you get a multi path reflection.

Your DP system then takes those independent signals and applies another kalman filter that compares several to try to decide which one is telling the truth but that only works when you have 3 online.
INS would solve that for a single GPS, but thats a $50k+ box

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GPS is not perfect. If I leave my plotter on in the slip for a week and leave the track on, I will get a fuzzball centered on the actual location of the antenna. Most of the fuzz will be within about 30-50 feet of the actual location. COG and SOG derived from this random jumping around will be all over the place if there is no damping. Most displays are damped and mask the underlying issue*.
If I understand a GPS compass correctly, it is not using sequential positions at all, so this random jumping does not disturb it.

  • I can see this on my phone. The plotter holds a rock steady COG and SOG while the iPhone nav program is jumpy.

the fur ball for corrected gps can be under 300mm, last rig I was on would hold inside 300mm on a calm day easily with 50mm dgps

Where is the DGPS signal for that coming from? AFAIK the DGPS beacon system is either offline or about to be.

I believe that the dept of defense was diluting the civilian side of the gps signal and dgps was to compensate for that near ports when more precision was needed. When DoD allowed full gps for all users no need for dgps.

As I understand it, there (at least) two ways to use the GPS system. The primary way most of our consumer-grade GPS appliances (cellfone, car, yacht nav system, etc.) operate is using the doppler shift of the GPS signals to compute a position - this has the advantage of being very fast, leading to only tiny delays in depicted location, and the “jitter” of position errors can be filtered out fairly well.
The second mode is, in my experience, called “survey mode”, using the actual timing offsets of the satellites and since SA (selective availability) was turned off, can result in very accurate position info - at the expense of significant computational load, which takes computational capability (which costs power) and time . This method is capable of measuring things like continental drift and land mass motion (millimeters per year) though there must be significant filtering taking place for sub-millimeter measurements.
For something like a car or a speedboat, the “fast with jitter” mode makes the most sense - but the slower and more accurate method may be more practical for applications (like ship heading or rig position) where high accuracy is desired.

There may well be other technology in use here - I haven’t kept up - - but it is good to know that there is more than one way to derive position info from the GPS constellation (including the use of GLONASS and the other sat systems up there).

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I think there might be DIY DPGS setups. You might set up one “survey mode” base station that broadcasts corrections to the mobile units running around nearby.

Here is a good article, it’s from Ocean Navigator - 2003. GPS compass

Any interruption in receipt of satellite signals, such as might occur when a vessel passes under a bridge, would cause a dropout of heading information. Furuno has addressed this problem by incorporating a three-axis, solid-state gyro assembly that will continue to provide precise true heading information for up to 30 seconds after the interruption of satellite reception and progressively degrading heading information for several minutes.

Surveying GPS often involves timing the work to coincide with good satellite pass coverage. It also relies on averaging over time. Over an hour or so, a survey gps on a tripod in a fixed position will “cook in” to an area the size of a dime.

Often a base station is set up at a nearby official geographic benchmark. The base station communicates by radio with on site fixed stations, and the RTK (real time kinetic) roving unit that the “rod man” carries a top a pole.

The raw field data is further corrected by computer processing.

differential signals from 3rd party providers, Cnav, Fugro, Trimble, etc
Except in the GoM ( where they get away with iala becaons) all DP vessels are required to have the receivers and subscribe to them as the rules say must use corrected GPS due to the issues with std gps

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<500GT is not SOLAS

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I don’t think any DP vessels use beacons. In the GoM the DGPS units (usually only one) can use WAAS corrections though.

You’re thinking of Selective Availability, which was discontinued in May 2000. Your ship’s GPS is almost definitely a DGPS, operating either on local beacons or WAAS satellite signals.

plenty use IALA as the equipment is readily available, operators too tight to pay for corrections.
I have had plenty of conversations where the guys complain that moving around the rig is a risk on gps as they mask the shore iala stations.
How far offshore does waas cover?
Can you point me to any supplier that makes a box to take waas data for a classed vessel?

This is the first I’ve heard of it.

The entire GoM. Farther than the beacon signals.

Trimble SPS351. I don’t remember what make and model DGPS the Chouest boats used but all of them had one that used WAAS.

Every DP vessel I was ever on used one WAAS DGPS and one paid provider.

WAAS uses 38 ground-based reference stations but the corrections are ultimately transmitted to geostationary satellites every few seconds.