or Connect
EpicSki › The Barking Bear Forums › On the Snow (Skiing Forums) › General Skiing Discussion › Don't tell me how fast you ski based on your car GPS
New Posts  All Forums:Forum Nav:

Don't tell me how fast you ski based on your car GPS - Page 2

post #31 of 46

Ske-Bum is basically right, but I think he is a little too pessimistic about toy GPS units.   Most of them have WAAS correction for operation in the US, which improves the accuracy to about 3 meters.  And the specified 10 meter accuracy is specified as 95%, so its something like a 2 or 3 sigma number, meaning most of the time you are quite a bit better.

I can't find specs on how many channels the receiver in my Garmin eTrex Vista hcx actually uses, but the satellite status page shows it tracking more than 3 -- does it actually use the full constellation, I don't know.

 

I don't know what hand-held military units are like.  Are they any different from consumer handhelds?

The high end GPS receivers built into navigation units play a lot of tricks to get better accuracy and resolution, some even track phase changes as well as time differences.  And for ultimate accuracy, you can do differential gps where the Coast Guard stations rebroadcast correction terms (based on the GPS signals they get at a fixed, surveyed site).

 

I did actually use my GPS on the hill once.  At Solitude we came out of the woods to the back-to-the-base feeder trail at the bottom of Honeycomb canyon, and my directionally-challenged self wasn't sure whether to go left or right.  The pitch at that spot was level enough that I couldn't be sure which way was downhill.  So I got out the GPS and checked.  That was the only time I've actually used it, as opposed to leaving it in my pack and downloading the logs at the end of the day.  (Admittedly, getting out the trail map and thinking for a minute would have given the same result.)

post #32 of 46

MDF, we both use the same exact gps.  I find that mine has better accuracy when it has more than 4 sats (usually about 10).  My accuracy mostly gets down to 7', but when I test it it is better than 7.  Even without WAAS on the accuracy is good.

 

Usually if I remember right the gps needs 4 sats to be working?  With 3 it can only triangulate position which cannot measure elevation?

post #33 of 46

I doubt any hand held GPS has WAAS, as of yet, this is very expensive and requires high processor rates, along with the ability to lock onto one or more of the land based stations. A GPS receiver may be able to TRACK up to 9 sats at once, but it may not have the ability to process all that data, that is chunk of data that needs to be sorted. No receiver can get all the sats at once, since some are on the other side of the earth at any given time. And yes you can get a better position using data off of a land based station that has corrected the data (the basic theory of WAAS), again you are talking money. Now in the future do I expect these issues to be corrected, yes. I would say within 5 yrs that hand held GPS could be down to 3 cm accuracy. However it still isn't going to fix the Z plane. 

 

Pine again GPS doesn't triangulate anything, that is a common misconception. The more sats you have the more accurate you are, again processors are the key, but it still won't fix the Z plane, but it is close. Is it close enough for most people, that depends. However saying you went 75 mph because your GPS said so is silly.  

 

My worry is that people will put too much faith in their GPSs and lose their basic sense of direction. It is already happening actually. 

post #34 of 46



 

Quote:
Originally Posted by Ske-Bum View Post

My worry is that people will put too much faith in their GPSs and lose their basic sense of direction.

 

Don't worry, I never had one.  In college I had a math class in a basement room with an exterior door.  After every class I came out and turned the wrong way.

post #35 of 46

The WAAS system uses ground stations to compute DGPS corrections that are then rebroadcast from a geostationary satellite.

And Garmin handheld units have done WAAS corrections for some time now.

It doesn't help MojoMan's problem, because the system only generates corrections for the U.S.

See for example

http://www8.garmin.com/aboutGPS/waas.html

 

And yes, I know how it works but I will continue to explain it as "triangulate using the differences in distance to the satellites."  Seems a perfectly reasonable extension of the meaning of the term.

 

GPS can do the z-axis, it's not like the satellites are directly overhead.  Sure the GDOP (geometric dilution of precission) is almost always worse for the vertical, but it is not that bad.  (Of course, the altimeter is more accurate, or at least has more resolution.)

post #36 of 46

Providing the altimeter is barometric and the gps does not simply tell you altitude based on sat data.

post #37 of 46


 

Quote:
Originally Posted by Ske-Bum View Post

I doubt any hand held GPS has WAAS, as of yet, this is very expensive and requires high processor rates, along with the ability to lock onto one or more of the land based stations.

 

Huh??   Hand held GPS's with WAAS have been available since somewhere around 2000/2001.  My antique Magellan map 330 is WAAS capable.  Here is a link from 2003 reviewing GPS that includes WAAS as part of the comparison.

 

http://www.pocketgpsworld.com/handheldcomparison.php

post #38 of 46

http://www8.garmin.com/products/gpsmap76s/spec.html

0.05 m/s = 0.18 kph.  Just say'in.

 

As with any device, including radar, you have to be prepared to throw out unreasonable data.  Occasionally I've had my GPS tell me I've been going hundreds of mph, but on the whole it's been pretty accurate.

post #39 of 46


 

Quote:
Originally Posted by MojoMan View Post



 

Quote:
Originally Posted by J2R View Post



Quote:
Originally Posted by MojoMan View Post

The only difference between military and consumer GPS unit is the sophistication of the maps and the durability of the device itself. The military also uses DOD satellites systems that are carefully monitored for reliability, signal strength, and there are numerous backup systems.  

 

Having served in Kuwait I can assure you that military issued GPS can suck about as badly as the consumer devices. GPS relies on passive signals that use triangulation. Any GPS unit can lose coverage during atmohepheric distrubances. They are also prone to sunspots or other fits. They also can spit out  bogus data. No special magic occurs just because the GPS unit is in use by a member of the military. The DOD cannot alter the laws of physics. 

 

I may be a little behind the times here, but my understanding was that consumer devices used L1 GPS signals. L2 signals are broadcast on a different frequency, at a higher power (thus allowing greater accuracy), and are intended for military use and are encrypted so cannot be used in consumer devices such as we are talking about.
 


You are talking about 2-way GPS tracking and communication systems. Passive GPS navigation relies on passive signals coming from a satellite grid that allows the GPS device to use triangulation to place the user on a grid. There is no active communication involved and no information that anyone could make use of.  There is no data to encrypt. The signal itself is nothing but dead noise--a pulse of EM radiation sent out by a satellite. 

 

Anyone with a GPS unit operating on the same pulse frequency can use the signal to triangulate their position. It's not top secret stuff.  It is totally benign.


They used to deliberately degrade the 'civilian' GPS signal so it couldn't be used as a missile guidance system.  Military receivers could get full accuracy.  The degradation was disabled back in 2000, so now all GPS receivers get the signals with full accuracy.  It wasn't 'encrypted', but before 2000, consumer GPS systems were only accurate down to about 50-100m.

 

http://en.wikipedia.org/wiki/Selective_Availability#Selective_availability

post #40 of 46
Quote:
Originally Posted by brownie_bear View Post


 

Quote:
Originally Posted by Ske-Bum View Post

I doubt any hand held GPS has WAAS, as of yet, this is very expensive and requires high processor rates, along with the ability to lock onto one or more of the land based stations.

 

Huh??   Hand held GPS's with WAAS have been available since somewhere around 2000/2001.  My antique Magellan map 330 is WAAS capable.  Here is a link from 2003 reviewing GPS that includes WAAS as part of the comparison.

 

http://www.pocketgpsworld.com/handheldcomparison.php


I stand corrected. However I will put out three words; Line of sight. If it doesn't have line of sight with a WAAS station it ain't picking it up and therefore isn't using it. 

post #41 of 46

If you really want to have fun with your GPS hang it by the window of your plane during takeoff (the flight attendants won't see because they are seated too!) Put it in running mode and watch it calculate your caloric consumption.  I have an old Garmin Forerunner which has simple software and does not know to adjust for ridiculous speeds.  It does your ego good to see 100 calories consumed per minute!

post #42 of 46
Quote:
Originally Posted by Yuki View Post

You want to know how fast you generally ski all the doo-dah day?

 

Take a look at how fast you are driving as you start out of the lot.  Weird but I always found myself doing like 36 miles an hour and then it hit me (insert smacking sound), that I was (visually) going at about the speed stuff seemed to go by skiing.

 

I have more confidence in the GPS today than I had years ago but then the highway gradient is much less than a ski run.  In the car they seem to agree pretty much with the speedo now but a some years back they sucked for even the horizontal axis.

 

They were not even legal for navigation via the USCG back then because there was a built in but variable error (so they could not be used for targeting).  Some days it was 30' and other days 2 miles.  A guy put a major league cruise ship up on the rocks off Grand Banks because he was required to use Loran with GPS only as a "consulting tool".  Oooops.  My buddy ran his boat to fishing grounds with the GPS linked to autopilot but had to shut it off coming in the inlets.

 

Heck, the first version killed a guy and his wife in CT.  In the fog they followed the directions right to a ferry slip.  They went splash in the night.  Seems the ferry left the dock. nonono2.gif


Bolded part works surprisingly well. 

post #43 of 46

since my road bike has a speedometer feature in the computer, and very accurate, I have an idea of speed on the hill based on how speed feels on the bike, where speed like skiing are the normal pace for riding, 25 to 45 mph mostly. works pretty well as the wind feeling and sound is similar. ^^^

post #44 of 46

My Delorme PN40(?) handheld has a save feature that will plot your speed at every user defined set distance.  It's pretty easy to tell if the accuracy is in the general ballpark.  If it says I'm going 30 mph and then 70 mph 20 feet later, the mistake is obvious.

 

That said, I took it out on my yearly trip to Jackson Hole 3 years ago to see how fast I generally skied.  Jackson on a groomer weekday is wide open and empty so you can really let it rip on a lot of runs without fear of collision.  With very wide radius rr track turns, I found that I would generally peak around 50 mph +/- for a very short time period.  This was pure carving with no skidding.  Felt very fast.  I also took a couple of straightline shots down some short steep groomers and got roughly the same result.  I weigh around 150 lbs and was wearing a typical somewhat bulky ski jacket.  I could easily see adding another 10 mph with more aero clothing and a tuck.  I'm guessing anything faster at my weight would require a really steep hill, full tuck and full aero suit.

 

From what I recall, up to around 35 mph on a groomer didn't seem very excessive and is probably cruising speed for many.  Fairly deadly in a collision.

post #45 of 46

I had someone tell me they went XX speed on a certain slope, that happens to be used for timed racing events, when I questioned the validity of his speed he yelled at me "you need to learn math"...I did not reply. then I blew by him while laying out some turns on my fatties with fritchis. how fast I make laps and how many I make in a day has more meaning for me. 

post #46 of 46

My equation for calculating true skiing speed based on GPS:

 

y (speed) = GPS reading + 10% if male + .5% per day since reading was made + 8% if trees are reaaally whizzing by.

 

If unit reads less than 45 mph when you're trying hard, discard number as instrumental error. If unit reads more than 10 mph over your self-estimate of speed, save number to show to buddies.

 

When in car and being flashed by state trooper, all readouts over speed limit are instrumental error.  

New Posts  All Forums:Forum Nav:
  Return Home
  Back to Forum: General Skiing Discussion
EpicSki › The Barking Bear Forums › On the Snow (Skiing Forums) › General Skiing Discussion › Don't tell me how fast you ski based on your car GPS