silly question about gps

kito · Jan 27, 2014

I haven't installed my chartplotter yet so I'm not an authority but would changing the setting "north up" or "course up" make a difference?

WayneH · Jan 27, 2014

I first thought "What a silly question!" then I had a second thought, and then a third thought and finally a fourth thought. So I took some aspirin for my headache and took a nap.

When I woke up, I think I had a thought. I don't believe it makes a difference if your GPS is mounted sideways. Unless it has an internal compass. My Ipad does that. Shows the position and COG and heading which is always different from the COG. The Garmin 12XL does not have an internal compass that I know of so you shouldn't have a problem with that.

In our two trips from Freeport to Pensacola, I only run across two vessels like the barge you described. Both of those were also 180° off their actual course. With the Houston Ship Channel, Bolivar marshaling yards, Port Arthur and its marshaling yards and New Orleans, I think we got a very representative sampling of vessels.

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Ron20324 · Jan 27, 2014

Seems to me there has been talk about this 180 problem a few times. I've not had my second coffee yet, but it IIRC the antenna leads need to be reversed- or something like that. I'm sure someone will chime in- or do a search.

Gunni · Jan 27, 2014

This is a common problem with the barge pushers and small operator tugs. The AIS arrow is moving backwards. These are the dump-truck drivers of the sea. I assume they had their brother Duffus install the required AIS system to save money. All the same, you can see their location and relative direction of travel. Bottom line, it is an internal setting in the sender configuration.

Joe · Jan 27, 2014

Don't they drive on the left hand side of the road over there? Look, the gps can "face" any direction... it doesn't use a compass, it uses a series of fixes from the satellite.. to show movement and direction.. How else could it show your position relative to fixed nav aids.... Assuming other vessels are okay, then the problem is with the AIS transmission from the sender... get on the radio and ask him if he can see you, and then ask what direction he's travelling. Ask him to look at the ais screen and identify his own vessel and check the data for his own safety.

ifmdalvey · Jan 27, 2014

Wayne is nearly correct with his response.
The GPS receiver does not have a compass and therefore orientation does not matter.
The GPS works out your speed, COG by taking fixes very rapidly, you do not see this happening, it then joins the dots of those fix positions and from the distance and direction traveled in a know period of time works out all the solutions.
Remember, it is not heading you are presented with but track, an important difference.

Benny17441 · Jan 27, 2014

It is good enough that the AIS alerts you to the presence and ID of a vessel; not hard look up and tell which direction is headed.

Brian D · Jan 27, 2014

Maybe a setting in the other guys system that is used based off of antiquated equipment is the cause. Maybe his system require a reverse data stream that is being transmitted OTA inadvertently. Who knows. Go ask the skipper and when you do, bring a beer.

Jackdaw · Jan 27, 2014

ifmdalvey said:
Wayne is nearly correct with his response.

The GPS works out your speed, COG by taking fixes very rapidly, you do not see this happening, it then joins the dots of those fix positions and from the distance and direction traveled in a know period of time works out all the solutions.
Remember, it is not heading you are presented with but track, an important difference.

It does this for course, but not speed. Speed is a first-order calculation taken directly from Doppler data.

Bill Roosa · Jan 27, 2014

I'd have to disagree with Jackdaw. GPS does not use doppler data. The atomic clock on all GPSs' sats are synced. The GPS transmits a time signal. The transmitted time is not the time of arrival at the boat gps antenna. In fact all the transmitted times for all the sats are not the times of arrival. Since the gps unit on the boat has the sat orbital parameters for all the sats (ephemeris) It knows where the sat was when it transmitted its time signal...... and a math problem similar to the one you do when reducing three star sightings yields a location of the receiving antenna.
and speed is just a D=R*T solved thusly --> R=(position1-position2)/T or two positions yield a distance between. You divide by time and get a speed. Which is why you don't want to put a GPS antenna on the masthead or up high on a pole. It moves around so much in ways not associated with the boat course that you get really crappy location data.

heritage · Jan 27, 2014

Jackdaw said:
It does this for course, but not speed. Speed is a first-order calculation taken directly from Doppler data.

Doppler data?? From where are you getting doppler data? Not from a typical GPS. The GPS simple takes fixes and calculates the speed over ground between those fixes.

All U Get · Jan 28, 2014

Well I'm thinking of our travels and seeing the same thing happening with boats we are following. For a brief time their vector will reverse and point toward us but I can visually see they are maintaining their course. Maybe the vessels have a lot of metals aboard. I do know that we saw the same with our friends Cape Dory in certain situations. Would being south of the equator make a difference while installing? If we're at anchor everything spins in different directions.

All U Get

Jackdaw · Jan 28, 2014

Bill Roosa said:
I'd have to disagree with Jackdaw. GPS does not use doppler data. The atomic clock on all GPSs' sats are synced. The GPS transmits a time signal. The transmitted time is not the time of arrival at the boat gps antenna. In fact all the transmitted times for all the sats are not the times of arrival. Since the gps unit on the boat has the sat orbital parameters for all the sats (ephemeris) It knows where the sat was when it transmitted its time signal...... and a math problem similar to the one you do when reducing three star sightings yields a location of the receiving antenna.
and speed is just a D=R*T solved thusly --> R=(position1-position2)/T or two positions yield a distance between. You divide by time and get a speed. Which is why you don't want to put a GPS antenna on the masthead or up high on a pole. It moves around so much in ways not associated with the boat course that you get really crappy location data.

Bill, well you (and Heritage) can disagree, but you'd be wrong. ;^)

Doppler is integral to how GPS operates, SOG is just one of them.

First is uses Doppler to actually find and listen to the satellites. Remember that they are not geo-synchronous. So even as you sit still, they Doppler-shift in and out, and the frequency that the receiver finds the carrier signal to for each unique bird constantly changes, even though all broadcast on the same base frequency. This is why older GPS chips takes so long to cold-start.

Once fixed, it uses the Doppler delta of the carrier (how much the frequency has shifted vs expected of standing still) to calculate Speed over Ground. All modern units have this basic capability.

To read more, check out this link and read section 3.5.5 - Carrier Doppler Measurement.

http://books.google.com/books?id=6P7UNphJ1z8C&pg=PA92

While it may sound like it, I'm not some GPS Idiot Savant; I've been in the GPS/Navigation business a long time. Magellan GPS launched its last true marine nav unit (FX324 Map Color) while I was the Director of Technology there.

Brian D · Jan 28, 2014

Have to agree with Jackdaw, but not 100%. However, Doppler must be accounted for during start up and during operations. As he noted, the carrier frequency of the satellite is shifting and start high at acquisition and ends low at loss of signal. The GPS terminal must track this shift or the low data rate messages will be lost. I believe ephemeris data of each bird is transmitted during the messages so the GPS receiver knows where to look, when to look, and how much shift (mathematically) the receiver must adjust per channel.

For those who are not familiar with Doppler, it is how a tone appears high in frequency when the sound source is approaching you, then as it moves away, the tone gets lower in frequency. Doppler is also used in light to determine if a star is moving away from earth or towards it (Red Shift / Blue Shift). Doppler is in everything.

Now, TOD (Time of Day) is also important. All the components of the GPS signal work in tangent. Not individual parts, but the sum of all the parts.

Just so you know, I have been working with GEOs, MEOs,and LEOs for many years. Not GPS system mind you, but the theory is all the same. A satellite in orbit exhibits characteristics based on it altitude and velocity.

So, back to the OP, I am unsure of why a vessel would appear to moving backwards. Maybe this is a fundamental flaw in AIS.

This has been a great read. Love it.

Jackdaw · Jan 28, 2014

Brian D said:
Have to agree with Jackdaw, but not 100%. .

I missed the part your disagreed with.

Brian D · Jan 29, 2014

jackdaw, timing is also an integral part of GPS operations. Without it, the ground base GPS will not be in sync with the space based vehicle. There just isn't one source for determining VTG. The part I was not 100% is agreeing was the part where you left out the clocks. As I said before, they all work together. If one fails, the cascading affect will bring down the entire system (not falling out of the sky).

Since you failed to mention it I assumed you had left it out on purpose.