GPS Antenna(s)

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Wife and I built/tested GPS satellites at Rockwell International in Anaheim and Seal Beach in the late 80's.
They said "some day everyone will have a device in their pocket with GPS".
We were like "sure, whatever you say".
It was all Air Force money then, with no indication it would go civilian. I tuned and tested RF antennas. The frequencies were all classified then. Now they're published in magazines.
 
Larry N. said:
That's interesting, Dan. Every reference I've found shows that four satellites are needed for altitude, three to get horizontal location. And a presentation a number of years ago by the GPS labs at Colo. Univ. at Boulder showed the same. My experience indicates the same. On receivers where I looked at the number of satellites as the receiver searched, no position was shown until 3 satellites were in view, and it took four to get altitude.

At Finding Latitude and Longitude with GPS - How GPS works they say, "The system needs at least 3 satellite to obtain a latitude and longitude fix. Four satellites allows your height to be determined as well and the more satellites that can be received, this higher the accuracy of the results."
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I am going from memory which is a bad thing these days - LOL...

When you think about it it makes sense. 2 satellites could be somewhere here on earth or 180 degrees on the opposite side of the satellites. From a nano second difference the satellite and GPS have no idea.

As Wasssilaguy said my Garmin 295 would show you the satellites that it was pinging.
 
DonTom said:
My atomic clock gets its signal from WWV. Has nothing to do with satellites. It's an HF signal from Ft. Collins, Colorado. Could that be what yours is also?

-Don- Reno, MV
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Possibly, I don't really know. The connection icon on the screen looks kinda like a satellite ground station, similar to the radio telescopes south of Bishop.
I know that when I change the batteries in the thing, I have to lay it on the dash of the motor home for a day or so until it resets itself. After that I can hang it back over the door and it works fine, even to the point of the connection icon staying on.
 
OK - It's time to google this sh&t for a refresher...

So the satellites carry atomic clocks on board. The equation is based on time of transmission vs. receipt of transmission. As mathematically stated you only need 3 satellites for 3D positioning.

The 4th satellite only provides a reference time so the ground GPS doesn't need a clock - whew...

The tricky part is all the math required to allow for delays in signal based on satellite position vs. the atmosphere the signal has to pass through.

Pretty complex stuff.

1660738859509.png
 
DonTom said:
My atomic clock gets its signal from WWV. Has nothing to do with satellites. It's an HF signal from Ft. Collins, Colorado. Could that be what yours is also?

-Don- Reno, MV
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It's definitely HF. I have three units like that, two in my basement where satellite couldn't possibly reach, but they work fine. Note that WWVB in the US broadcasts on 60kHz, which is the source for the misleadingly-named "atomic" clock.
 
Larry N. said:
It's definitely HF. I have three units like that, two in my basement where satellite couldn't possibly reach, but they work fine. Note that WWVB in the US broadcasts on 60kHz, which is the source for the misleadingly-named "atomic" clock.
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The time signals broadcast by WWVB from Ft Collins, CO originate from the cesium atomic time standard clock maintained by the National Institute of Standards and Technology in Boulder, CO. That clock and a similar one in France are considered the most accurate clocks in the world.
 
NY_Dutch said:
The time signals broadcast by WWVB from Ft Collins, CO originate from the cesium atomic time standard clock maintained by the National Institute of Standards and Technology in Boulder, CO. That clock and a similar one in France are considered the most accurate clocks in the world.
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That's true, Dutch, and it's extremely accurate, just as the regular WWV broadcasts are very accurate, but the advertising seems (to me at least) to imply that the device you buy is an atomic clock, rather than just being fed by a signal derived from one, even though that's not possible for most of those devices, especially a consumer device.
 
You can get a GPS clock, telecom industry uses them all the time.
When we built them, the satellites had rubidium clocks, would lose a couple nano-seconds every day but are corrected by ground based cesium standards. Newer satellites I believe have cesium on board, but are still corrected regularly by the ground based standard.
 
All of the cell companies use GPS to sync time across their systems. This makes your phone clock accurate. We won't even get into the discussion of leap seconds.
 
Had the pleasure of learning from one of the guys who sat on the think tank where the GPS concept was originally hatched (supposedly). Very interesting man named Dr. Love, can't recall his first name. Had also witnessed the sinking of the Bismarck, from another ship he was stationed on.
 
Ex-Calif said:
The 4th satellite only provides a reference time so the ground GPS doesn't need a clock - whew...

The tricky part is all the math required to allow for delays in signal based on satellite position vs. the atmosphere the signal has to pass through.

Pretty complex stuff.

View attachment 156710
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This is true but misleading. The whole system is based on time. This means you get nothing without the time sync. The first satellite syncs the time then 2 more for lat/long, and the 4th for altitude.

See the graphics here:


These graphics would be better if the globes were of equal size, because they are, and if the earth was shown between the satellites once you add 2 or more. For example, even with 4 satellites you are still in a 3d sphere of error of 10 meters. Ground GPS as stated before can be more accurate by using earth topography to put you on the face of the earth solving altitude.
 
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Bobtop46 said:
All of the cell companies use GPS to sync time across their systems. This makes your phone clock accurate. We won't even get into the discussion of leap seconds.
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It's not just cell companies. In the late '80s/early '90s AT&T was redesigning the phone network for many parts (eventually all) to be based on GPS time, critical for syncing many newer parts of the network, and especially so for some of the new stuff that was to come out. Although I wasn't directly involved, one of my peers was heavily involved, so I had occasional updates on the progress. But since I retired in '98 I lost those updates.
 
Larry N. said:
It's not just cell companies. In the late '80s/early '90s AT&T was redesigning the phone network for many parts (eventually all) to be based on GPS time, critical for syncing many newer parts of the network, and especially so for some of the new stuff that was to come out. Although I wasn't directly involved, one of my peers was heavily involved, so I had occasional updates on the progress. But since I retired in '98 I lost those updates.
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That is correct. All timing used to be slaved from a master clock located in a hardened underground ATT facility in Hillsboro, Missouri. Central standard time was referred to by all entities as "network time".
If a central office lost connection to that reference, internal clocks in the local gear provide timing until the reference signal is restored. The slipping that occurred due to this mismatch could often be heard as audible clicks on a voice call. The longer the local gear ran without the reference, the further it strayed, and the clicking got worse.
 
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The atomic clock uses a frequency generated by the vibration of a Cesium crystal under stimulation. This crystal is contained in a volume that is strictly environmentally controlled and used to generate the base frequency clock, this is typically called the L.O. (Local Oscillator). This L.O. is then multiplied and divided and filtered to get to the two desired frequencies used for the GPS transmitters. All GPS satellites use these same frequencies. The new frequencies are directly related to the L.O. in phase, this is referred to as being coherent.

Every phase is measured from 0 to 360 degrees, and we know when 0 begins (coherency) and you measure phase shifts, then you can accurately calculate distance and speed. Compensation for the medium these frequencies travel through are applied by very sophisticated techniques so the accuracy for geolocation is very precise. It's so precise that the DOD forced the GPS providers to induce errors for commercial use.

We used the same principles for RADAR during my working years. Pretty amazing stuff that is pure physics and mathematics.
 
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