Home Current News News Archive Shop/Advertise Ridecamp Classified Events Learn/AERC
Endurance.Net Home Ridecamp Archives
ridecamp@endurance.net
[Archives Index]   [Date Index]   [Thread Index]   [Author Index]   [Subject Index]

Re: [Norton AntiSpam] Re: [RC] [RC] Garmin Forerunner 301 - Truman Prevatt

GPS units are very poor with altitude - on the other hand the GPS system has become the prime source for navigation information in military reconnaissance aircraft. Many Low Earth Orbit satellites use GPS as a prime source of navigation. GPS also works well in the open ocean. I just finished a project where we showed that by using multiple GPS units on an aircraft and processing the output of each unit to produce a fix - we can do better than the INS system on board. Three years ago I worked with Air Force Research Labs in NM on a free flight LEO satellite that derived all it's nav and timing from GPS. We were able to locate radio frequency emitters on the ground to amazing accuracy from this navigation data supplied by GPS.

The reasons for this seeming dichotomy is on the ground in a tree canopy or non-flat terrain you do not have connectivity with a sufficient number of satellites where as in the air you do. You also have some issues with multipath where you are connected to a reflected path rather than the direct path - especially with the birds that are low on the horizon. Barometric altitude is actually inferior to GPS altitude if you have sufficient spacecraft coverage.

While you might get better results by taking your lat/long to a map program - you suffer from an error known as "layover." Layover is the result of fixing a location on a two dimension grid that is above or below the grid. The receiver measures the range from multiple spacecraft to itself. When the altitude is in error this forces a layover error on the lat/long measurement. Depending on the geometry - the layover error can actually be larger than the altitude error. The layover shows up as a bias error rather than a random error - that is you true location my not be within the error ellipse size calculated by your device. Layover cannot be compensated for after the fact because to do so would require the altitude error - which is unknown. This is an unavoidable fact of life of the use of GPS for instantaneous location measurements on the ground. That problem goes away in many of the survey units where you can set them to take a long term average and leave them in the same location to do that over several hours. Doing that will give you amazing accuracy - 10 meters - in both lat/long and altitude. When DGPS is used or you have a military unit that uses the good code - you can get down to sub meter. That's the good news - the bad news they have to stay in the same location for a couple hours.

The bottom line is a GPS receiver is no better than the number of spacecraft it has connectivity to. The more the better. When you are on the surface of the earth it's difficult to get sufficient connectivity to enjoy the accuracy the GPS system supports unless you are willing to stand still and endurance riders generally aren't willing to stand still ;-) . GPS is fine for hacking around - however, I get very nervous when I hear a ride has been measured by GPS (unless it is done correctly with a survey unit). I just get ready to ride about 5 to 10 percent further than advertised.

Truman

April wrote:
Hi, Marlene,

I don't bother with altitude readings from a GPS unit that doesn't have a barometric altimeter built in. When I upload my data to MotionBased.com, I use their known elevation data rather than the unit elevation data and get more reliable data after-the-fact.

--


“He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you.” Friedrich Nietzsche, Beyond Good and Evil


=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=


Ridecamp is a service of Endurance Net, http://www.endurance.net.
Information, Policy, Disclaimer: http://www.endurance.net/Ridecamp
Subscribe/Unsubscribe http://www.endurance.net/ridecamp/logon.asp

Ride Long and Ride Safe!!

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=


Replies
[RC] Garmin Forerunner 301, Marlene Moss
Re: [RC] [RC] Garmin Forerunner 301, April