doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Global Positioning System

That's pretty good Chuck. Now I guess I can't buy one for my new truck....darn.

BTW, Raimey seems to know waaaay too much about GPS to be a casual user.

Definitely comparable to his German gun knowledge!

I saw that Amazon has good prices for the mentioned Garmins. There is a new etrex
H version said to work very well in forests and canyons for USD88.50 plus shipping.

JC

Actually it is not. The satellites use microwave transmitters, around 1.2276 GHz, which is close to the radio frequency of hydrogen as defined by radio astronomers.

Microwave propagation is affected by several factors. One major inhibitor to propagation is absorbed by water. So in very heavy canopy during spring and summer there can be problems. Leaves after all have a heavy concentration of water. In fall and winter the reception will improve.

It is not much different than when some one using a satellite dish to view television losses signal during a heavy rain or snow storm. The microwaves are absorbed to the point the signal to noise ratio becomes unusable. With older gps units in the past I have experienced this both in Manitoba and northern Wisconsin. The angle of reception through the canopy is maximized. I assumed at the time that the signal loss was exceeding 20 db or greater in comparision to more southern locations lacking such heavy canopy and a higher angle of reception.

The systems Raimey uses are ultra accurate and capable of accuracy to within an inch or better, no were near the more gross consumer grade handheld accuracy. There has been improvement in recent years with the chip set, increased sensitivity. However, even the best commercial system can be impacted by absorption. I believe, but am not certain that only the military units can overcome this.

I have in the past done work in microwave frequencies up to and including 10 GHz. For a time, this was being considered for ultra wide band data transmission, fiber won the battle due in part to path loss issues.

Many handheld units use dual-conversion with a low-IF architecture. While this is conducive to preventing microchip architecture induced noise, it does restrict gain to about 95 dB and internal noise figures near 8 db. With low signal densities per mm^2 that 8 db does make a difference if path loss is increased by absorption.

Which to buy? The best you can afford. Your use will grow as you become familiar with the benefits.

Pete

Jerry Lape:
Would you please email me at italianstallion@cox.net
Jim

Chuck H: I meant that as a positive! The NUVI is so easy to use, and so accurate, thats its amazing. BUY ONE. On my old Martin Marietta briefcase is a sticky that says "Global Positioning Satellite System" Worked that project for a good while. We have come a long way since then. I used a plain old ETREX on my boat in Mexico and never needed anything more. I dunno what PeteM said....sounds like a Parker guy...Where have you been Chuck?

I bought a Garmin 45 when they were introduced about 15 years ago. Small handheld with a flip up external antenna, I did experience some canopy problems from time to time so about 12 years ago I bought a Garmin 12, same size but internal antenna and utilized 12 channels. Never had a problem with any canopy here in the NE at any time of year and I still use it today. No mapping, just coordinates, accuracy usually ranges around 10 yds. It still functions perfectly except it thinks it's July all year round. Time of day is always correct. Garmin no longer supports it so I can live with the day being off.

I still use the 45 while kayaking on LI Sound and as a backup for the Garmin GPS/Chartplotter on my fishing boat.

And I always carry a map and compass on land and water. I will practice coming back into port at night by compass to be ready for the day the GPS craps out.

Garmin makes the best units but they know it and are an arrogant company, they never admit to a problem. They'll replace a unit but rarely admit they have a production problem. A few years back they had a chartplotter, Model 198. Great unit for the boat except they had designed in a pressure equalizing valve that allowed moisture to enter the unit. The screens would fog up internally until the unit warmed up enough to dissipate it. I went through 3 of them, then they model was gone and replaced with a similar model sans valve.

Chuck H:

I guess I should know a little about receivers as I wrestle with them almost every day.

Jaycee:

As I've said before on the German stuff, I just have good sources, one being this board and I try to look at a lot of examples.

But back to the GPS on the topic of knowns and unknows, you need good data from a separate satellite for each unknown: Long., Latitude and height. Actually height is a demon all by itself that requires a specific constellation configuration for good values. We're spinning say at 1000 miles per hour and it's a complex, dynamic quantum system that looks as if it would never work. At any rate, due to the wavelength of light being near the nanometer range, there's a clock error, which of course is time dependent. So you need good data from 4 birds to get X, Y and Z. If you just have 4 you probably will be getting poor data from one. So more than 4 sources is always better and there are precise orbits which will predict the max number of birds for a given time. Actually in some cases, like on a body of water, X,Y position can be obtained from 3 satellites. Has anyone purchased or used the Rhinos(or seen the advertisment)?

Kind Regards,

Raimey
rse

A few minutes ago, when we sat down for breakfast, I noticed my wife had her Garmin NUVI out of her car, on the dining room table. She entered the daughters address in Boulder and said "I wonder how its going to route me". Well says I, you'll have to wait to find out till you get outside. It wont work in the center of a 2 story house, because of canopies, Mghz, Atomic seconds and waves and stuff...I know cuz thats what the xperts said! She pushed GO and it instantly showed her that she had to go out the driveway, down the road 1.2 miles and turn right...It also showed a full signal strength........

...go figure!!

JC