Can you trust the plastic jesus?

Fishplay · 06-26-2003, 07:30 PM

Last Friday on the return to NPBAR my gps errored.

I was about 4 miles out and it started issuing incorrect steerage messages. I could see the bridge and knew I was on the correct coarse.

As I neared a boat crossed my bow. By this time if you knew where to look and looked closely you could see the jetty tips.

I held my coarse and the plastic jesus continued to display a northerly steerage message. I pointed this out to Keta and told him I thought the other boat was correcting in accordance to the false gps heading.

We were at the bouy line and were still being told to steer North. About the time the Gps came back on track I noticed the boat that had crossed my bow make a sharp correction. He was off coarse at this point by nearly a half mile headed at the light house.

This is the first time I have ever seen the GPS error!

Now I'm thinking I may be a little to trusting in the thing. I wish I had had the Loran set to navigate also so I could have checked the two against each other.

Anyone else experience this last weekend; or ever???

The compass was reading correctly and I was running off it the whole time. Had it been foggy and I not been using the compass together with the GPS this could have run us into trouble.

From now on I will be using reduntant navigational aids.

GPS, Loran, compass and maybe even backup GPS.

Kinda spooky [img]graemlins/eek13.gif[/img]

Mark Mc · 06-26-2003, 07:43 PM

Good lesson there Fishplay; good for you for believing your compass......

Comment: The course a GPS gives you (even when working properly) is based on points you have already covered, like driving down the street by looking in the rearview mirror. It does not tell you where you are actually headed, real time. Try going straight for a while, then stop the boat & let her spin a little and see how easily it gets confused.

[ 06-26-2003, 09:08 PM: Message edited by: Mark Mc ]

Miss B Haven · 06-26-2003, 08:11 PM

It's Saddam's WMD, he's trying to destroy the sport fish fleet off the Oregon Coast. I have this on 4th hand knowledge from GWB.

Seriousley, haven't had that one happen yet. I have however had my brain tell me that my electronics were way screwed up. Thank goodness I listened to my electronics. They were right, my brain was wrong (low clouds/patchy fog situation). The guy with me was 75 years old (and boating all his life) and said the same thing: believe the electronics and the compass, forget the brain. :shocked: :whazzup:

WaterDog · 06-26-2003, 08:36 PM

On saturday I brought my "plastic Jesus" with me and marked the #1 buoy as we headed out so we had a back up nav system. When we headed home Wayne used the loran (set point #6 - rockpile ?)to come back and I sat in back and played with my gps since I don't often get to use it. It looked to me like whatever that point was we were about a half a mile south of the course my gps was showing. As we got closer tho' everything seemed to line up and my gps was right on.

Fishplay · 06-26-2003, 08:36 PM

Hey Mel, I too had that happen when I first started using GPS. I was fishing bouy 10 in heavy fog and just knew that damn GPS was lying.

When the fog cleared we were fishing about 5 miles from where I thought we were. :shocked:

Just like all my relationships. First one learns to trust and about the time it turns to worship

the bomb drops. :grin:

Fishplay · 06-26-2003, 08:43 PM

WD
Waypoint six is the sparkplug. With loran you can track off a little because it is much slower to update than GPS.

Also Wayne has one leg shorter than the other and given a big enough distance could actually navigate in circles!

WaterDog · 06-26-2003, 08:47 PM

Quote:

Wayne has one leg shorter than the other and given a big enough distance could actually navigate in circles!

<font size="2" face="verdana,arial,helv">Well shoot!!! That explains a few things!!! :grin:

crabbait · 06-26-2003, 10:12 PM

For the pups who have not had much GPS experience, Gps works like this: There are satellites in orbit scattered around the sky. They are all equipped with an atomic clock and they all transmit the time continuosly. The GPS receiver gets these transmissions. The receiver knows the location of these satellites and the constant speed of radio signals. It compares the difference in the time that the GPS receiver gets the various time signals and is then able to pinpoint the only location on the globe that will receive those signals with that time delay.

Accuray is affected by two things. First, the relative location of the satellites that the receiver is picking up (must have at least three). If the satellites are close together and, worse yet, close to overhead, the accuracy suffers. The more satellites that are received and the further apart and lower to the horizon they are, the better the accuracy of the unit. Locating your antenna with a good unobstructed view of the entire horizon increases accuracy.

Second, the military is able to introduce error into the GPS system to stifle attempts to use GPS to naviagate offensive weapons against us. In the first years of GPS, this was common. The military stopped the erroneous input in June 1998 (if I remember correctly) but again started to mess with the signals during our latest little set to. I don't know when they will stop or how much error they have introduced. It used to be in the neighborhood of 50-100yds.

We were promised a ground based supplement to GPS some years ago. This would send a signal to the satellites giving them a known position for reference that they could verify and adjust to. This is an important piece of the puzzle because precision GPS approaches are scheduled to replace ground based precision approach equipment in the near future. With the "augmented" system, aircraft will be able to make precision approaches to any place on the globe without other equipment.

The other day, we (the FAA) received word that the augmented system was funded and that we could expect the installation of the ground based augmentation to begin soon. What this means to us 'Dogs is that as soon as the installation is complete and the military stops "tweaking" the signals, we will be able to place a lot more trust in the guidance the "Plastic Jesus" provides (depending on mounting location and satellites received).

Even with the introduced error, the GPS will give an accurate enough signal to get within sight or sound of the whistler/bell buoys.

[ 06-28-2003, 02:26 AM: Message edited by: crabbait ]

corrirod · 06-26-2003, 10:46 PM

Fishplay, are you sure the update rate was at the highest setting? That's the only way to explain an error of that proportion. Your accuracy should be less than 30ft. assuming the Feds have increased the error and be as low as 12ft. based on the 1998 settings.

I have never experienced an error like that unless it was a user error. Much like my computer at home. I yell and scream at it but it's usually my fault. :shocked: [img]graemlins/dork.gif[/img]

Fishplay · 06-26-2003, 11:43 PM

I have never changed any settings on my unit. It had worked fine all day.

I know this sounds relative to the Twilight Zone, but it happened just as I say. I'm 99% sure that the other boat running with us was under the same odd influence.

He was running a 1/4 to 1/2 mile south of us all the way from the rockpile, then seemed to cross my path about the same time I discovered the tracking error.

My guess is he was following his plotter blindly while chatting it up in the cabin.

Just something everyone should think about. I encourage the use of redundant navigation systems.

[img]graemlins/idea.gif[/img]

I was running a garmin 45 with the NPBAR set as a Goto and navigating from the highway screen.

This I beleive gives you the shortest path to destination. IE; a srtait line for navigation.

Now I know I was south of that striaght line but by how much I'm not sure. In order to stay on the highway you have to make constant corrections that I don't bother with. I double check my desired heading which we know is aprox 62 degrees and reference it(GPS) occassionaly.

Once you get far enough off the imiginary line the PJ just starts reeding a steerage heading to put you back on coarse.

Normally as the destination is approached you will track back onto the highway. This did not happen Friday.

It is possible that PJ was stuck on the fact that I was off the imaginary line and PJ had lost focus of the destination.

In the past however it has always seemed to correct or recover from this more timely.

In todays world Black Helicopters are just as plausible. :grin:

Chum King · 06-27-2003, 08:30 AM

It sounds to me that your GPS was not locked in, and was simply working from its last known location.

The Garmin 45 is a handheld unit and, if my memory serves me correctly, it is not a 12 parallel reciever. That is going to mean it is somewhat more prone to lose track on the satellites, and that will be further aggrevated if you are inside of a cabin.

I know that I can lose the signal even with my 12 parallel Magellan 315 propped up against the window in the cabin. If you are operating a GPS from within a cabin, you really need an external antenna that can get an adequate "view" of the sky.

Good luck.

DriftR · 06-27-2003, 10:21 AM

My sea chart "Yaquina Head to Columbia River" has a little notice on it that all charts have:
Warning: "The Prudent mariner will not rely solely on any single aid to navigation, particularly on floating aids. See US Coast Guard Light List and U.S. Coast Pilot for details." Off chart #18520. Now what do all those little numbers and lines mean on that chart??????? Hummmmm? GPS, Loran, Radar, Compass, Charts and the knowledge on how they work is always helpful.

blubeast · 06-27-2003, 10:38 AM

Trust it... yes, but use other tools too. While on my first trip to the Ranch last Sat, I pushed one too many buttons on my gps while trying to change the configuration. Thing froze up. Would not turn off, nor function. Not a good thing to happen 32 miles out! Un-plugged the power, tried to re-start it... got a computer type error message ('enter code 4004')... would not go past the first warning screen...great... now what?...

Got out the manual and remembered there was a way to do a Factory Reset... but would lose all waypoints... small price to pay... reset the system and it worked fine... all the way back home.

Lessons learned... #1 re-read the manual and re-learn how to use it before next trip out on the salt. #2 take key way points along on trip in writing. Had the compas as fall-back, but wanted the map from the gps to minimize the trip.

WaterDog · 06-27-2003, 10:52 AM

Quote:

Originally posted by blubeast:
take key way points along on trip in writing. Had the compas as fall-back, but wanted the map from the gps to minimize the trip.

<font size="2" face="verdana,arial,helv">I have a "write in the rain" notebook I keep with my gps for this very reason. I Keep a hard leaded pencil handy and jot down the Waypoint #, name, and anything else I feel is important like if I catch fish there. :grin: