Getting back to GPS poor specs. Again I believe if the military was going to develope a navigational system and spend billions on it I would think they would design a system that would be better and not just work on the fringe.
The system as developed has proved to be quite reliable. It doesn't "just work on the fringe". Why do you think it does? Because you read some S/N spec and jumped to conclusions on a topic you apparently know little about?
GPS accuracy is affected by a number of factors, including satellite positions, noise in the radio signal, atmospheric conditions, and natural barriers to the signal.
If you have fewer satellites (but not less than four), and the ones with good signals are in a limited area in the sky, the computed position will be less accurate. This is well understood. In most cases with as few as four satellites even with unfavorable geometry, your location is still "good enough" for most purposes.
Noise can create an error between 1 to 10 meters and results from static or interference from something near the receiver or something on the same frequency.
Even assuming this is correct, and I'm not sure it is, so? 10-meter accuracy is still pretty darn good. Can you cite the source for this information?
Objects such a mountains or buildings between the satellite and the receiver can also produce error, sometimes up to 30 meters.
Citation, please.
The most accurate determination of position occurs when the satellite and receiver have a clear view of each other and no other objects interfere.
No argument there.
That to me means wide open spaces.
To me, it means any location with a clear line of sight to at least four satellites at a time is usually adequate. 360° horizon-to horizon and a dozen satellites may help get
somewhat better accuracy, but is not required for basic positioning.
Obviously, mountains and clouds can not be controlled or moved, nor can interference and blockage from buildings always be prevented. These factors then, will affect GPS accuracy. To overcome or get around these factors, other technology, AGPS, DGPS, and WAAS, has been developed to aid in determining an accurate location. These bandages, if you will, are all ground based.
WAAS is not ground based. It's satellite-assisted DGPS. See your own description later.
Assisted GPS (AGPS/A-GPS)
AGPS (Assisted Global Positioning System) is a system that assists conventional GPS when reception of the radio signal from the satellite is poor or non-existent (line of sight is blocked). the A-GPS gains information via a wireless network,on cell towers, to relay the satellite information to the receiver. With this assistance, the GPS doesn't have to calculate the satellite's orbit, which shortens initialization time, and increases battery life.
AGPS is typically used in cell phones to provide location information when a cellular signal is available and GPS is compromised. Apparently it is now mandated for phones sold in the USA to provide a redundant source for location information for emergency services. It does require a cell signal, but, since it uses a different RF wavelength and much stronger signals, it
might work when conventional GPS doesn't, like inside buildings. The cost of providing this data is added to your phone rate, or may be considered data use.
Differential GPS
To further increase reliability of GPS, DGPS (Differential Global Positioning System) technology was developed.
DGPS is used to improve the accuracy and precision of GPS, not reliability.
Like the AGPS, the DGPS uses a fixed GPS location (such as a cell tower) to send information to the GPS receiver. DGPS, however, looks at both the satellite and the fixed location adjusts for any difference between the two, (I'm not sure why it needs to look at the satellite information) and then sends that information to the receiver. DGPS is particularly helpful when atmospheric conditions interfere with reception.
That's partly right. DGPS uses a receiver at a known location and calculates the error in the pseudorange (essentially, the distance from satellite to receiver) calculated for each GPS satellite it can see based on its known location. Corrections for these errors can be transmitted to other nearby DGPS-enabled receivers and applied to the pseudoranges at the DGPS receiver's location, or saved and applied to the collected data from a suitable receiver in post processing.
Because DGPS only provides a
correction to the range to each satellite, the DGPS receiver still, of course, has to acquire the GPS data itself.
WAAS
There are a few things that cause GPS not to be perfectly accurate and interference prone. The charged particles of the ionosphere, and water vapor of the troposphere slow the signal slightly. Multipath, ephemeris errors, and the atomic clocks on the satellites themselves can also contribute to the inaccuracies.
Throughout the continental U.S. are 25 "reference stations." These are GPS receivers that are placed at points that have been very accurately surveyed. These reference stations receive the same GPS signals as the moving receivers. The difference is instead of using the signal's travel time to calculate position, the reference stations use their known position to calculate timing. Because the reference stations know exactly where they are, they can figure out what the travel time of the signals should be. They then compare the calculated times with the actual times. The difference is an "error correction" factor.
Yep. Basic DGPS so far...
The error information is sent to two master stations (one on the U.S. east coast, one on the west coast) which calculate correction algorithms and assess the integrity of the system. A correction message is uplinked to the two WAAS satellites via a ground uplink system.
The two satellites are in geostationary orbits. These satellites then transmit the correction information back down to the GPS user on the GPS frequency. The GPS receiver then decodes this information and applies it to its calculated position to significantly improve the accuracy.
So instead of relaying the DGPS corrections directly to nearby receivers, it relays the corrections to geostationary satellites that can provide them to WAAS-equipped receivers over a
Wide Area (thus,
Wide-Area Augmentation System).
Did you catch that? It uses land based reference stations.
Sure. It's quite clever, actually. It uses land-based reference stations to make small corrections to the signals received by mobile receivers, improving their accuracy from a couple dozen meters to typically less than ten.
Why bother with satellites at all and just use the north star?
Several reasons. Can you see the North Star in daylight or when it's cloudy? How about when it's screened by trees, buildings, or other obstructions even if the rest of the sky is clear? How can you tell your longitude from the North Star? How accurately can you tell your latitude from the North Star? How convenient would it be to gather the measurements?
I know GPS works out to sea. We all know that. But I also know both military and commercial planes can be equipped with GPS transmitters broadcasting information. Even ships at sea can do it too.
Maybe they
could be, but
are they? How are these ships and planes going to know their own locations accurately? Can you provide a reference that demonstrates that this has ever been done?
I know my friends like Rayzor and Alpha20mega are going to call me out for all this and explain why i'm so wrong, so be it.
We aim to please. We also don't like blatantly wrong information to stand unchallenged.
But I honestly believe GPS can be done and probably better without satellites involved.
Do you have anything other than idle speculation to back up that belief?
I see you posted again with something that may address this question after I started this response. I haven't read that post in detail, but did note this paragraph:
Ground-based systems may seem like a good alternative, but there are pitfalls. One design would involve hundreds of thousands of transmitters, which would be ridiculously difficult to build and maintain. Another possibility is differential GPS, which currently sees use in some maritime and nautical settings, but it’s also subject to jamming like satellite GPS. The other is LORAN, a radio-based system that originally saw use in World War II and has seen some renewed interest today thanks to its resistance to jamming, but its accuracy was never very good.
Other systems are certainly possible, but are they
practical?