# Do GPS satellites even exist?



## blujeenz (22/5/18)

I dont think so, my findings as follows:

Pretty much the same numbered "sats" visible all the time, numbered 1~32.
My device receives on a frequency of 16.3Mhz instead of the official sat freq of 1.5Ghz or 1.2Ghz.

Official frequency use according to Wiki.(land based radio HF)


https://en.wikipedia.org/wiki/Radio_spectrum#By_frequency

Some back story.
I was trying to disable the startup warning screen on my Garmin Nuvi 40 and hit up Google to see how to do it.



Turns out I need to HEX edit the firmware bin file, but I could'nt find any indication of which bits to edit.
In the process I discovered some hidden menu's, press and hold on the signal strength icon gives the detailed sat page, while press and hold on the battery icon reveals the diagnostic pages.
No damage resulted from me fiddling in these menu's, it still works just fine.

Main menu with the 2 areas to press and hold.



The sat detail page, this one snapped with a cam on macro mode.



These are from screenprint function on the Nuvi 40.
Location deleted for privacy. 






Pressing top right bat icon results in this menu, where the operating frequency can be seen, 16.368Mhz.



The above menu also detects charging, batt volts and a host of other points of interest.

What I'd like to know from other users in SA is, does your device also have the same 1~32 sat numbers?
My understanding is that each sat has its own unique ID and it should have a visibility cycle, but if the GPS device just assigns a number on 1st come 1st served basis, it could well indicate ground based transmitters.
I'm in Cape Town, for what its worth.

The other thing I'd like to know is, if your devices also operate on the 16Mhz frequency?

Reactions: Like 1 | Informative 2


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## craigb (22/5/18)

If I recall correctly, GPS satellites are generally geostationary. They do drift a bit, but yes, your going to see the same ones most of the time. Their locations are fairly predictable and many apps allow you to download data files the contain the expected locations of the satellites to speed up your lock on. 

I think that is an internal chip frequency not broadcast frequency.

Reactions: Like 1 | Agree 1 | Informative 1


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## blujeenz (22/5/18)

craigb said:


> I think that is an internal chip frequency not broadcast frequency.


My understanding is that GPS devices are receive only and thus that freq should be an operating Rx freq and not necessarily the CPU speed, but more users posting their stats could shed light on my theories.


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## craigb (22/5/18)

blujeenz said:


> My understanding is that GPS devices are receive only and thus that freq should be an operating Rx freq and not necessarily the CPU speed, but more users posting their stats could shed light on my theories.



Correct, the GPS chip is receive only, but it has a controller that processes the signals it receives, and that chip is probably operating at that frequency, checking the broadcast frequency at that rate and processing the inputs.

Reactions: Informative 1


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## Silver (22/5/18)

Fascinating stuff @blujeenz and @craigb

Reactions: Agree 1


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## Steyn777 (22/5/18)

What many people do not realize is that Google Maps, for instance, uses cell tower triangulation. As recent as 2009 Google released a video demonstrating that GPS was NOT required in determining ones location.

Reactions: Like 1 | Agree 2 | Winner 1


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## blujeenz (22/5/18)

Steyn777 said:


> What many people do not realize is that Google Maps, for instance, uses cell tower triangulation. As recent as 2009 Google released a video demonstrating that GPS was NOT required in determining ones location.



Yes, fully agree, but back in the day there were no cell towers to calc from, ie the first Garmin device was a panel mounted marine device made in 1990.

The main issue I'm driving at is this one.
Taken from Garmins website.



> *The GPS Satellite System*
> The 31 satellites that currently make up the GPS space segment are orbiting the earth about 12,000 miles above us. These satellites are constantly moving, making two complete orbits in less than 24 hours. They travel at speeds of roughly 7,000 miles an hour. Small rocket boosters keep each satellite flying on the correct path.



The sat numbers on my screen dont disappear twice a day and nor does my device listen on the "official" freq.


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## Raindance (22/5/18)

Do these GPS devices not function based on pinging the satellites and then triangulating position based on the time lapse of the echo? I can not figure out how they would determine position from just receiving a signal. Not arguing, just wondering.
Also, what is on the next menu @blujeenz .

Would like to participate but my Garmin was recycled recently. Now have a Tom Tom, same crap, different name.

Regards


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## Steyn777 (22/5/18)

blujeenz said:


> Yes, fully agree, but back in the day there were no cell towers to calc from, ie the first Garmin device was a panel mounted marine device made in 1990.
> 
> The main issue I'm driving at is this one.
> Taken from Garmins website.
> ...


They won't of they are not really there. If your GPS is using a different system as to what you believe it does, then that info is basically a "nice to have but bs" thing

Reactions: Agree 1


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## Salamander (22/5/18)

@Raindance
The GPS system works by measuring the time it takes for a signal to reach the receiver from each satelite. The receiver "knows" where each satelite is and can therefore work out by some very powerful magic where it is.

Reactions: Like 1 | Agree 1 | Funny 1 | Informative 1


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## Raindance (22/5/18)

Salamander said:


> @Raindance
> The GPS system works by measuring the time it takes for a signal to reach the receiver from each satelite. The receiver "knows" where each satelite is and can therefore work out by some very powerful magic where it is.


Thanks @Salamander, very powerful magic indeed, will have to get me some! Lol.

Regards

Reactions: Like 1 | Agree 1 | Funny 1


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## Silver (22/5/18)

The thing is I need a GPS in my mods because almost every other day one goes missing and I cant find it!

Reactions: Funny 5 | Can relate 1


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## blujeenz (22/5/18)

Raindance said:


> Do these GPS devices not function based on pinging the satellites and then triangulating position based on the time lapse of the echo? I can not figure out how they would determine position from just receiving a signal. Not arguing, just wondering.
> Also, what is on the next menu @blujeenz .
> 
> Would like to participate but my Garmin was recycled recently. Now have a Tom Tom, same crap, different name.
> ...


From what I've read, sat's are broadcast only but there is a time signal in the broadcast stream and like @Steyn777 say's, the device should know where the sat position is ie hard coded.
I'm curious to know if pressing the same icons in your device yields similar info, I wasnt necessarily polling Garmin users only.
Whats next? hard to say seeing as I dont want to destroy my Nuvi 40 by prying off the RF shields. 

@craigb no CPU's found inside, just RF shielded modules, although it doesnt mean that Garmin didnt incorporate them into a "sat module".
[The SS mesh that the arrow is pointing at seems to be some kind of antenna, its a loop with a piece of foam inside to keep its shape.]
The G241 item is some kind of ceramic.





*Correction, the G241 item is the antenna, looks the same as an antenna on a Nuvi3xx/6xx as described*. https://www.instructables.com/id/How-to-open-a-Garmin-nuvi-3xx-or-6xx-power-butto/


https://www.instructables.com/id/How-to-open-a-Garmin-nuvi-3xx-or-6xx-power-butto/


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## Raindance (22/5/18)

Lets see, be back soon.

Regards


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## Salamander (22/5/18)

I got the same menus on my Nuvi 51. The sat signal one is actually listed in the help file. There isn't a CPU in the device - it's all built into the GPS module.

Reactions: Thanks 1


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## Raindance (22/5/18)

Ok, TomTom is raindance proof, no shortcuts I could find.

However found the G241 to be a processor running at 16Mhz.


Think that explains the listed speed.

Regards


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## blujeenz (22/5/18)

Raindance said:


> Ok, TomTom is raindance proof, no shortcuts I could find.
> 
> However found the G241 to be a processor running at 16Mhz.
> View attachment 132705
> ...


Possible, but Im inclined not to think so due to the different format.
I've edited my post to reflect the G241 as an antenna based on external info.


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## Salamander (22/5/18)

Correct. Most of the antennas appear to be ceramic blocks. Look for a small chip that has Ublox on it. That is the most common GPS receiver - for DIY projects anyway


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## blujeenz (22/5/18)

Salamander said:


> Correct. Most of the antennas appear to be ceramic blocks. Look for a small chip that has Ublox on it. That is the most common GPS receiver - for DIY projects anyway


Couldn't see anything like that on my pcb but it may be under the soldered shielding.


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