+m^2 Posted September 28, 2004 Share Posted September 28, 2004 ...I've had the dropsies with my Garmin Summit a couple of times (one time it decided to release itself from the bike mount). The unit still displays and functions, but I'm starting to question the accuracy. What are some ideas for checking gps accuracy? Thanks, John Quote Link to comment
+LthrWrk Posted September 28, 2004 Share Posted September 28, 2004 Somewhere I read a tip about calibrating a GPSr. I got to a website for NOAA. http://www.ngs.noaa.gov Search for NGS Data Sheets.. sorry I dont have the direct link to take you to them.. but it is searchable by State, County.. then you can select locations within the county list. Soon as I get my snail mail transit gps stuff it willbe one of the first day things to do. Charles Mt Vernon, WA PS.. add a lanyard to your GPS so if the holder or you get the dropsy it might not hit the ground. Quote Link to comment
+fly46 Posted September 28, 2004 Share Posted September 28, 2004 If you know someone with a gps, check your unit off of theirs. You should be less than 30 feet off of theirs. Quote Link to comment
+Robespierre Posted September 28, 2004 Share Posted September 28, 2004 go hunt your own caches....or someone elses that you've already found. Quote Link to comment
+ironman114 Posted September 28, 2004 Share Posted September 28, 2004 Here is the linkfor theNGS Data sheets. Find your state then county, click gps only and stability A. Also check to see what datum they used when posting their coordinates. Quote Link to comment
+blindleader Posted September 28, 2004 Share Posted September 28, 2004 I feel like a broken record. Your GPSR doesn't have any accuracy to question because it isn't a measuring device; It's a computer and a radio receiver. Either it works or it doesn't. Any accuracy check you can think of will be meaningless in reference to your unit. That's not to say that the way you use it won't affect the accuracy of your results, but dropping it - no. Quote Link to comment
+EraSeek Posted September 28, 2004 Share Posted September 28, 2004 (edited) I feel like a broken record. Your GPSR doesn't have any accuracy to question because it isn't a measuring device; It's a computer and a radio receiver. Either it works or it doesn't. Any accuracy check you can think of will be meaningless in reference to your unit. That's not to say that the way you use it won't affect the accuracy of your results, but dropping it - no. "Each satellite is equipped with an accurate clock to let it broadcast signals coupled with a precise time message. The ground unit receives the satellite signal, which travels at the speed of light. Even at this speed, the signal takes a measurable amount of time to reach the receiver. The difference between the time the signal is sent and the time it is received, multiplied by the speed of light, enables the receiver to calculate the distance to the satellite. To measure precise latitude, longitude, and altitude, the receiver measures the time it took for the signals from four separate satellites to get to the receiver. " ~The Areospace Corporation But, no, I don't think your accuracy is affected by dropping it. And yes, units have been tested and shown to have slightly varing accuracys. To say a GPSr is just a computer and a radio reciever but doesn't measure anything is like saying a watch is metal, plastic, and pent-up energy, but doesn't measure anything. Edited September 28, 2004 by EraSeek Quote Link to comment
+MedicP1 Posted September 30, 2004 Share Posted September 30, 2004 "Each satellite is equipped with an accurate clock to let it broadcast signals coupled with a precise time message. The ground unit receives the satellite signal, which travels at the speed of light. Even at this speed, the signal takes a measurable amount of time to reach the receiver. The difference between the time the signal is sent and the time it is received, multiplied by the speed of light, enables the receiver to calculate the distance to the satellite.~The Areospace Corporation How can a signal/radio frequency travel at the 'speed of light'? I have seen this in a number of postings. Isn't it actually that the signal travels at the speed of that particular frequency and that figure is used internally in the GPSr as the multipication factor/constant. If it were indeed a light signal then the GPSr wouldn't work at night or indoors/under cover. Or are they refering to the speed of light in a 'radio spectrum' kind of reference. Just curious, not trying to ruffle any feathers. Quote Link to comment
+tirediron Posted September 30, 2004 Share Posted September 30, 2004 "Each satellite is equipped with an accurate clock to let it broadcast signals coupled with a precise time message. The ground unit receives the satellite signal, which travels at the speed of light. Even at this speed, the signal takes a measurable amount of time to reach the receiver. The difference between the time the signal is sent and the time it is received, multiplied by the speed of light, enables the receiver to calculate the distance to the satellite.~The Areospace Corporation How can a signal/radio frequency travel at the 'speed of light'? I have seen this in a number of postings. Isn't it actually that the signal travels at the speed of that particular frequency and that figure is used internally in the GPSr as the multipication factor/constant. If it were indeed a light signal then the GPSr wouldn't work at night or indoors/under cover. Or are they refering to the speed of light in a 'radio spectrum' kind of reference. Just curious, not trying to ruffle any feathers. What they SHOULD say is that signals "Travel at a speed which is aproximately that of light", ie radio waves travel aproximately 186,000 miles/second. If one is to be excruciatingly accurate, this speed is different for every frequency, but the variation is so slight that its of no consequence to you or I. Well I agree that it's unlikely a GPSr's accuracy would be affected, it is possible. The whole thing is based on measurement, and if in the course of dropping your unit, the resistance in a particular circuit is raised slightly because it was poorly soldered or whatever, then yes, it could affect the accuracy! Quote Link to comment
+blindleader Posted September 30, 2004 Share Posted September 30, 2004 (edited) How can a signal/radio frequency travel at the 'speed of light'?Because the signal is light. I won't even try to answer the rest of your post. This is not a good forum to learn basic physics. The web is full of educational sites for all levels. How stuff works sometimes has good stuff. I don't mean to be short, but this kind of post is sure to generate a raft of posts containing a mix of facts and nonsense. edit: Wow. I don't type fast enough. Before I could even hit send, my prophecy was fulfilled. Edited September 30, 2004 by blindleader Quote Link to comment
+Thot Posted September 30, 2004 Share Posted September 30, 2004 (edited) How can a signal/radio frequency travel at the 'speed of light'?Because the signal is light. Like you say this isn’t the place to discuss this topic, but I don’t agree that radio frequency radiation is light. Visible light (which, combined with adjacent frequencies, is what we normally call light) is only a small frequency zone in the entire range of the electromagnetic spectrum. All electromagnetic radiation travels at the same speed. That is, the entire magnetic spectrum (X-rays, microwaves, radio waves, light, etc.) travels at the same speed, thus it all travels at the same speed as light, so it travels "at the speed of light." But, light is only one small part of the electromagnetic spectrum. http://imagine.gsfc.nasa.gov/docs/science/...emspectrum.html Edited September 30, 2004 by Thot Quote Link to comment
+KG7JE Posted October 1, 2004 Share Posted October 1, 2004 the entire magnetic spectrum (X-rays, microwaves, radio waves, light, etc.) travels at the same speed, Not precisely true. That is why signals tend to smear together (or apart) over long distances. Also why fiber optics have to have regenerators every so many kilometers. And that is why a prism works so good at spreading light into the various colors. But, for GPS signals the variation from the low frequency to the high frequency is accounted for in the mathematical model. And they must also take into consideration other quantum effects. However, for the majority of us it is simple enough to say that all electromagnetic radiation (of which radio and light make up just a small part) travels at the same speed in a vacuum. It slows down in the atmosphere and travels slower yet in water. Absolutely crawls in glass (comparatively speaking) Quote Link to comment
+Thot Posted October 1, 2004 Share Posted October 1, 2004 (edited) the entire magnetic spectrum (X-rays, microwaves, radio waves, light, etc.) travels at the same speed, Not precisely true. That is why . . . Gosh, how do we strop this already technical thread from becoming more and more submerged in the minutia? I was trying to answer the questions about light, the speed of light, seeing the signal at night if it’s “light” and the remark that radio frequencies “are” light. Of course, at the granular level, you’re right, light travels very slightly slower in air than in a vacuum and, in turn, each frequency in the spectrum travels at an ever so slightly different speed than the others resulting in “dispersion” of the spectrum. It’s for this reason, I believe GPSs transmit two different signals at two different frequencies and high precision GPS receivers (not our handheld units) look at the difference in the arrival time of the two frequencies in order to make the correction. But, for GPS signals the variation from the low frequency to the high frequency is accounted for in the mathematical model. To use your phrase, "not precisely true." Our handheld units aren't concerned with the variation from low to high frequency as they only receive a single frequency. And, they make a simple standardized first order (linear) arithmetic correction -- not a mathematical model in the normal meaning of the term. And they must also take into consideration other quantum effects. Err . . . GPSrs? I don’t think so. Edited October 1, 2004 by Thot Quote Link to comment
+Anonymous' Posted October 1, 2004 Share Posted October 1, 2004 I've dropped my GPS a few times and it doesn't seem any different. Quote Link to comment
moopgroop Posted October 1, 2004 Share Posted October 1, 2004 I just like to play hide and seek with my toy! Quote Link to comment
+Tread-n-Lightly Posted October 2, 2004 Share Posted October 2, 2004 (edited) So if you were driving your car at the speed of light and turned your headlights on, what would happen??? W5TTX Edited October 2, 2004 by w5ttx Quote Link to comment
+m^2 Posted October 6, 2004 Author Share Posted October 6, 2004 Great thoughts and information! I talked with a Garmin tech and he related that '...if it finds the satellites, it works...' Thanks, John Quote Link to comment
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