Curious GPS.....

As I understand it....

Because of security reasons, the military does not let the public have the full capability of the GPS satellites, even though it's still accurate.

The GPS satellites are owned by the United States even though a few countries have development plans of their own GPS satellite so they are not reliant on the U.S. GPS satellites.

As far as your compass reading, depends on the refresh and actual programming. I know GPS directions really drains my phones battery life.

That was called Selective Averaging and that has been disabled for a long time.

Wild guesses:

1) I suspect that your GPS does not have magnetic sensors to detect the direction it is pointing. I'm guessing your GPS uses position coordinates to determine the direction of motion. Thus, when not moving it is a little out of scope to ask it which way it is pointing. Just a guess.

2) I suspect that your (possibly $100 - $200) GPS is the source of the 9 foot change in reported elevation. A very crude speed of light number is about 1 ns/ft for straight line travel. GPS math is very complicated but if your position is out on a hyperbolic curve (mathematically speaking) then 1 ns of timing error could be far more than one foot of position error. Thus, I'll call your GPS good and blame it all at the same time.

The GPS does not rely on just one satellite, but several. I have one on my phone and one hand held that identifies the satellites for me. I've had 12 or 13 satellites providing positional information to me at a time. The longer I stay in one place the more accurate a reading I get. Thus the longer you sat under that tree, the better your position became.

When I get home from work, my GPS will say I'm home, but 30 min. later, it knows I'm in the back corner of my house.

If I recall correctly there are 24 active.

There was an in-depth discussion about GPS some time ago. Many of the same members were involved. Maybe AH can remember the approximate date, I can't.

Smart phones have rendered hand help GPS obsolete.

I have an app that reports my compass heading, speed, street address (very accurately) and other information.

Then there is Device Manager which pinpoints my location on a map, and Life 360 which tells me where members of my family (with smart phones) are anywhere in the USA. Of course Google and others as well.

Did you note just how many Sats you were receiving when this was happening (all the GPS units I ever had except one, could show you just how many were in sight at any one time). If you have less than 4 (if I remember correctly), this sort of "wobble" is quite normal.

In a city with tall buildings, you would be very lucky to get even 2 Sats, but that would still allow say a Tom Tom or a Garmin to get you exactly where you want to go.....less than 2 might make it with a good unit....but less than 1, you are back to reading maps!!

The accuracies are enormously good, but remember that with the first few original GPS

Sats, the internal clock was affected by being "in Space" and they had to be reprogrammed to correct this error.....the builders had completely forgotten the fact that in space travel, time actually runs faster, know since the 1950s I believe.....

All the Astronauts are technically slightly younger than they would be if they had never gone into space.....the closer to the speed of light, the greater the effects.....if I remember correctly....

From Wiki here:-

Global_Positioning_System

Accuracy

GPS time is theoretically accurate to about 14 nanoseconds. However, most receivers lose accuracy in the interpretation of the Sat signals and are only accurate to 100 nanoseconds.

For a 'proper' GPS read, you need 4 satellites.

The first satellite provides the reference time for the GPS unit.

The second satellite provides one 'time of flight' distance, so the unit can no place itself on the surface of a sphere centered around the second satellite.

The third satellite provides another 'time of flight' distance, now the GPS unit can locate itself on the edge of a circle where the two spheres overlap.

The 'time of flight' distance from the fourth satellite narrows the GPS location to one of two points where the circle intersects the third sphere.

The GPS then chooses the location with the lower elevation, as it is unlikely the GPS unit is in orbit ABOVE the satellites.

A fifth satellite will provide confirmation, as well as additional 'fine tuning' of the position, compensating for atmospheric disturbances, building reflections, etc.

You support my "off the cuff" comments with regard to GPS. Thanks.

if you were using your smartphone, you must also remember that the phone has accellerometers and magnetic flux gate devices to generate both an INS independent of and in addition to the GPS satellite constellation as well as a magnetic compass heading.