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[on:]

I got a GPS module from some outfit in China off ebay for something like four dollars, still haven't unwrapped it from stat bag.. No reason to think it's anything other than fine.  I got a great deal, but it's reasonably easy to find them for around ten bucks or so I would imagine (thinking dealextreme or whatever).

I was reading and waiting for Crossroads to chime in.. So there's obviously a reason.. So what is it?  I am talking about VORTAC, a radio beacon network that back in the stone age was used via signal triangulation off the frequency/tower chart to give pretty dang accurate positioning.  I am supposing it's a resolution issue to use actual VORTAC, but (I really don't know) would low power radio triangulation work within the range of reasonable open air lower power beacons?  GPS for large scale with finer granularity coming from triangulating off of local ground beacons of known position...

Per request, here's a thread...

Just a discussion of navigation methods and hardware, and the shortcomings of GPS.

[Reply #1 on:]

It could certainly be fine, maybe more than just fine.  However, $4 doesn't tell me anything.  GPS modules come in different flavors and what you're looking for is the amount of channels it has.  A $4 12 channel unit, versus $4 50 channel unit is a huge difference.  A 50 channels unit won't be $4.

Triangulation is a great way to give accurate positioning, assuming you also have good line of sight between all three points.  Otherwise you're left with only 2 or worse, just one signal again.  However, at the time, it worked.  Nowadays, you try to implement that in a metro area and you'll realize really fast that it doesn't work.  In came GPS because from above, tall buildings don't look much different than a single floor ranch house.

There are many ways to augment a GPS signal, and lately a lot of companies are using open networks to do that.  Google Maps on mobile devices can and does that.  You can tell it to use your phone's GPS, or if turned off, it will use whatever open network it can find, including telephone networks.  Telephone networks is a great way to triangulate, this is how emergency personnel can pin point where a cell call is coming from.  Not only are there powerful repeater antennas covering large areas, but you also have repeater cell towers for say a neighborhood, and when you get down in a city, several tall buildings will also have repeaters on them since the signal can't go through buildings, it just bounces off of them.  So the repeaters are used to send the signal out to the taller antennas.  Use that in a backwards fashion, and you would be able to locate a cell phone within a block radius.  Last time I dialed 911 on my cell phone, the dispatch knew whereabouts I was before I gave her an accurate description of the area (I was on a bike path on the outskirts of town and she mentioned the closest intersection which was a half a mile away.)

But, when was the last time you heard of that network being open to your average Joe?  We are in a sense lucky that we have GPS.  How accurate it is however, depends on a lot of other things.  GPS does nothing for altitude though ... just longitude and latitude.  Barometric sensors can help, but that too changes by location as well as the atmosphere.

[Reply #2 on:]

I agree that GPS is not very accurate at the moment you want it on a moving object.  It is off by up to 100 ft.  Neither is it very good at summing the distance you walked or drove during the day.  It can be off by 1-3% depending on how often it's updated and the path of your track and speed.  There is no easy solution for this while walking.  In a car it's called an odometer!  If you sample more often, you get all kinds of zig zag curves when you're walking perfectly straight.  This is where the extra 3% comes from.  If you sample less often while you're hiking up a mountain, all the switchbacks get truncated.  There should be some algorithm that cleans the data later, but this is difficult.  It takes a human hours to do it each day while looking at a map overlay.  GPS is also not good for total altitude gain and loss.  I was walking on a level path by a lake.  It calculated about 300ft up and down per hour.  I have not found a better solution.  It is not so great for recording a track while paragliding.  When you combine it with pressure then it's much improved.  It works well for Geocaching, especially since they turned off encryption.  I carried a quality GPS every day at Burningman.  All day walking.  I thought I'd at least get a checkerboard pattern showing the streets.  They are as far apart as a city block.  No such luck.  Just a mess of random lines.  Really I wasn't that drunk!

On the other hand, it works great when you keep it in one position for an hour!  It is repeatable from week to week and gives you a relative accuracy of about 5 feet.  Loran was also good for that...

Using Wifi to locate my iPhone in Argentina did not go so well.  Some GPS units have a firmware bug and cannot work south of the equator.  Most of the Wifi signals I used were from routers that were moved from the USA.  I verified a few by asking, and those were not so recently moved.  While walking around BA i think I visited each of the 50 states in the USA according to Google Maps!

The GPS on the older iPhones are horrendous...

GPS is the only way I know to accurately calculate lean angles on a racebike!
Do not even say gyros, it costs 100x as much for one good enough.

[Reply #3 on:]

How often you sample GPS position is a classic "fractal" geometry problem.

The example is usually a coastline.
Take a decent sized island, say Australia. 
Use a 10km ruler to measure the length of the coast, then use a 1km ruler, a 100m ruler, a 1m ruler.
Each time you you will get a larger and larger distance.

I use an application called "MyTracks" on my tablet when I go walking.
The default setting is to take a GPS "fix" every 5m.
Because I go for a walk along the same forest path pretty much every day, I've tried 1,2,3,4,5,10,100 metre intervals.
The difference in total distance covered is amazing!

Under the settings menu there is a GPS accuracy option, the default is 200m, an "excellent GPS" - 10m, a "poor GPS" - 5000m.

The other thing I've noticed when riding my bike into Echunga, is up to a point the faster you are moving, the better the accuracy becomes.

When I've been in the passenger seat of a car, over about 30km/h, the track can be as much as 10-20m off to either side of the road!
Turning on the cell tower option brings that down to around 3-5m!

Still it's better than LORAN or that other thing that used to be around.

When I've had my Arduino on the dash of the car displaying the output from an EM-406 and an MPL115A1 barometer the speed and altitude are around the same as my tablet.
Speed in particular from a GPS seems pretty good and tends to be close to the cars speedometer and odometer.

From the experiments I've been doing it would seem that GPS by itself is ok, but when you get data from another sensor things get better.
As always the more data you have to work with the more accurate position, altitude and speed get.

[Reply #4 on:]

I am talking about VORTAC, a radio beacon network that back in the stone age was used via signal triangulation off the frequency/tower chart to give pretty dang accurate positioning.  I am supposing it's a resolution issue to use actual VORTAC, but (I really don't know) would low power radio triangulation work within the range of reasonable open air lower power beacons?  GPS for large scale with finer granularity coming from triangulating off of local ground beacons of known position...

Actually, VOR/VORTAC isn't as old as you make it sound. Its in active use today around the world. The older technology is Non-Directional Beacon (NDB) which got its start during WWII; to guide bombers to their target, and then later as a means to find their way home. An NDB is basically an AM transmitter.

VORTACs are VORs with Distance Measuring Equipment (DME) built in, which is a bit of a misnomer. Its just another transmitter. The DME is actually in the plane. With two or more VORs triangulation is easily possible. The only time a VOR/VORTAC becomes unreliable is when you're flying directly over one or when its at the fringes of its range. In either case, it may appear you're rapidly jumping from radial to radial. If you plan to use DME, do remember the distance calculated is the slant distance, not spherical distance. So if you're using a GPS, expect these distances will never match (and that topological features can effect the distance) without some additional math.

NDBs are now considered deprecated but are still available across the US. VORs are still in commonly used today. Expect VORs to be around for at least several more decades.