airfoilsguy
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GPS Broadcast Or Not

Wed Sep 05, 2007 11:46 pm

Quick and easy question. GPS devices do they broadcast anything up to the sats? I was under the impression that they are receivers only but I have read in many places that they also broadcast some kind of signal. Do they really broadcast anything or is this just conspiracy hocus pocus? I am talking about true GPS receivers not North-star and the likes.

[Edited 2007-09-05 17:12:08]
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David L
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RE: GPS Broadcast Or Not

Thu Sep 06, 2007 12:11 am

They certainly don't transmit deliberately to the satellites (or any other target, for that matter). However, as with most electrical and electronic devices, they do emit a certain amount of EM signals "by accident" when in use. I can't say specifically why or how much - I'm sure someone else will.
 
airfoilsguy
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RE: GPS Broadcast Or Not

Thu Sep 06, 2007 12:15 am

Quoting David L (Reply 1):
They certainly don't transmit deliberately to the satellites

That is what I was looking for. I know almost all unshielded electrical devices broadcast noise. I have herd more then once that "they know where you are" because the sats receive signals from your GPS and relay it to some place or another.
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David L
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RE: GPS Broadcast Or Not

Thu Sep 06, 2007 12:55 am

Quoting Airfoilsguy (Reply 2):
I have herd more then once that "they know where you are" because the sats receive signals from your GPS and relay it to some place or another.

No, the satellites tell the receiver where they are and include, I assume, some timing information that allows the receiver to work out where it is.

There is Differential GPS but I'm pretty sure that involves the receiver receiving additional information from a fixed ground station (position known accurately), which is used to correct some of the uncertainty present from using the satellites alone.
 
FredT
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RE: GPS Broadcast Or Not

Thu Sep 06, 2007 5:08 am

GPS satellites have a highly accurate clock onboard. They broadcast the time. A GPS receiver receives this time signal.

The GPS receiver also knows the trajectories of the satellites. Updated information on the satellite trajectories is also received from the satellites.

If the GPS receiver knows the exact time, it will know the distance to each received satellite through comparing the exact time with the time information received. It will also know the location of each satellite, through knowing the satellite trajectory, and thus will be able to triangulate its own location.

To get the exact time, a fourth satellite is needed. With four satellites, the GPS receiver will be able to make a reasonable assumption of the current time which it will then adjust until it finds a time which makes the information available make sense. Four unknowns, x, y, z and the time, and four satellites providing data to help it find these unknowns.

The precision of the GPS system does however for many applications leave something to wish for, especially if you are not one of the parties trusted by the US DoD to be allowed to have receivers which are capable of receiving the information on the encrypted second frequency transmitted on by the satellites. The error will however vary little with location and only slowly over time.

Thus, if you have a GPS receiver at an exactly known location, you know what the error is in that area at that time. If you transmit this error to a second GPS receiver at an unknown location in the same area, the error can be applied as a correction and, along with more accurate timing techniques such as carrier phase, centimeter precision can be achieved. This is what is called Differenial GPS, DGPS.
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KELPkid
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RE: GPS Broadcast Or Not

Thu Sep 06, 2007 6:16 am

Quoting David L (Reply 3):
No, the satellites tell the receiver where they are and include, I assume, some timing information that allows the receiver to work out where it is.

Nay, the GPS satellites merely tell the world below what time it is, and which satellite number it is  Wink They also slowly broadcast the almanac data, which takes 12.5 minutes to broadcast in it's entirety...the almanac gives the GPS receiver the data for the current orbits the GPS constellation is in (IIRC, there are four different orbits, each orbit has six satellites in it...).

Quoting FredT (Reply 4):
The GPS receiver also knows the trajectories of the satellites. Updated information on the satellite trajectories is also received from the satellites.

Yup, from the almanac data above...  checkmark 

If you've ever wondered why it takes a handheld GPS a few minutes (~10) to get it's bearings when you've just put fresh batteries in it, it's because if it doesn't store the almanac info. in non-volatile memory, then it has to listen for a complete almanac broadcast  Smile
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David L
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RE: GPS Broadcast Or Not

Thu Sep 06, 2007 8:34 pm

Quoting KELPkid (Reply 5):
Nay, the GPS satellites merely tell the world below what time it is

The receiver knows where the satellites are, or at thinks it thinks it does - some even show you. Is that based purely on initial programming without any updating over the years?

Quoting KELPkid (Reply 5):
Yup, from the almanac data above...

If you've ever wondered why it takes a handheld GPS a few minutes (~10) to get it's bearings when you've just put fresh batteries in it, it's because if it doesn't store the almanac info. in non-volatile memory, then it has to listen for a complete almanac broadcast

So do the satellites broadcast information about their position or not? I'm not sure how useful the timing data would be if the receiver didn't know where it came from.
 
KELPkid
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RE: GPS Broadcast Or Not

Fri Sep 07, 2007 12:34 am

Quoting David L (Reply 6):
The receiver knows where the satellites are, or at thinks it thinks it does - some even show you. Is that based purely on initial programming without any updating over the years?

Well, according to wikipedia: http://en.wikipedia.org/wiki/GPS :

"Each GPS satellite continuously broadcasts a Navigation Message at 50 bit/s giving the ephemeris (transmitted in the first part of the message) and an almanac (later part of the message). The ephemeris data gives the satellite's own precise orbit and is output over 18 seconds, repeating every 30 seconds. The ephemeris is updated every 2 hours and is generally valid for 4 hours, with provisions for 6 hour time-outs."

So, I guess I must now eat my foot  footinmouth  As Starlion says, Tech/Ops is a learning experience for all.

I am quite familiar with pulling down the time from GPS, having had programmed military simulations many moons ago that used the time signal fom GPS for timing (they always wondered why us geeks in the trailer had a GPS dome, when the trailer wasn't going anywhere  Wink ). BTW, the hardware does the work for you, unless you're a GPS chip designer, you usually don't have to worry about the raw signal data.

Quoting David L (Reply 6):
So do the satellites broadcast information about their position or not? I'm not sure how useful the timing data would be if the receiver didn't know where it came from.

However, moving on in the same Wikipedia article:

"The coordinates are calculated according to the World Geodetic System WGS84 coordinate system. To calculate its position, a receiver needs to know the precise time. The satellites are equipped with extremely accurate atomic clocks, and the receiver uses an internal crystal oscillator-based clock that is continually compensated for by using the signals from the satellites.

The receiver identifies each satellite's signal by its distinct C/A code pattern, then measures the time delay for each satellite. To do this, the receiver produces an identical C/A sequence using the same seed number as the satellite. By lining up the two sequences, the receiver can measure the delay and calculate the distance to the satellite, called the pseudorange[12]."

So, it is using differences in time as well as the ephermis data...I thought until a few minutes ago that the system used the almanac data plus the time differences from all the received signals to compute where in the heck you are on the surface of the Earth  scratchchin 
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David L
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RE: GPS Broadcast Or Not

Fri Sep 07, 2007 9:35 pm

Quoting KELPkid (Reply 7):
So, it is using differences in time as well as the ephermis data

I was pretty sure some sort of position and timing information was broadcast - I just didn't know exactly how. Thanks for the info.

Anyway... the main point is that the receivers only receive and calculate. They don't transmit actively.  Smile

Quoting KELPkid (Reply 7):
As Starlion says, Tech/Ops is a learning experience for all.

I'll second that.  biggrin 

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