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Advantages And Disadvantages Of A GPS

Tue Aug 07, 2012 3:15 pm

From the book i'm reading one of the advantages is;
"High-redundant satellite capacity system"?
Don't really understand what this mean can anyone help?  
 
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Jetlagged
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Advantages And Disadvantages Of A GPS

Tue Aug 07, 2012 3:37 pm

Roughly translated that means there are more satellites in orbit than the minimum needed. So even if one or more became unusable the GPS would still be able to calculate position to the same accuracy. In practice you need to have four satellites in view to get an accurate GPS position. There may be between six and ten actually in view at any one time, i.e. up to six more than the minimum.
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nomadd22
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Advantages And Disadvantages Of A GPS

Tue Aug 07, 2012 3:40 pm

The description is in poor english so it's a little bit of a guess, but it priobably means that loss or blockage of some satellite signals won't greatly affect the accuracy. Many devices are also equipped with tiny gyros to keep heading information flowing iif the GPS signals are completely lost for some reason.
It could also be referring to a system with backup receivers and power sources so there's no single source of failure.
GPS is used in many types of systems, so the redundancy can be referring to several aspects of whatever system it's referring to.

[Edited 2012-08-07 08:42:17]
Anon
 
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Advantages And Disadvantages Of A GPS

Tue Aug 07, 2012 4:25 pm

Thanks for help guys, it makes sense now
cheers
 
Northwest727
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RE: Advantages And Disadvantages Of A GPS

Tue Aug 07, 2012 10:18 pm

At least in the USA, RAIM (Receiver Autonomous Integrity Monitoring) is required to to verify the signals when using a GPS as a primary source of aviation for IFR. In a basic sense, RAIM is the GPS' ability to detect a corrupt GPS signal and stop using that bad signal...and if the GPS can't do this, alert the pilot/crew that the GPS is unreliable. In order for the GPS to be able to to do this, multiple satellites must be in view (I believe you need at least 6 satellites for RAIM, 5 in order to get the GPS to work accurately-correct me if I am wrong).

When you throw in WAAS (Wide Area Augmentation System), the system gets even more redundant, and more accurate. I believe you do not even need RAIM if you have WAAS, but again, correct me if I am wrong.
 
N243NW
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RE: Advantages And Disadvantages Of A GPS

Wed Aug 08, 2012 12:29 am

Quoting Northwest727 (Reply 4):
(I believe you need at least 6 satellites for RAIM, 5 in order to get the GPS to work accurately-correct me if I am wrong)

Not quite. RAIM requires at least 5 satellites. If you have 6 satellites, you have the ability to identify a faulty satellite and "throw it out" from the GPS position calculations.
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DiamondFlyer
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RE: Advantages And Disadvantages Of A GPS

Wed Aug 08, 2012 5:14 am

Quoting N243NW (Reply 5):
Not quite. RAIM requires at least 5 satellites. If you have 6 satellites, you have the ability to identify a faulty satellite and "throw it out" from the GPS position calculations.

IIRC, you only need 4, and 5 to throw one out, if you have a Baro-aided GPS system.

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zeke
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RE: Advantages And Disadvantages Of A GPS

Wed Aug 08, 2012 11:49 am

Quoting DiamondFlyer (Reply 6):
IIRC, you only need 4, and 5 to throw one out, if you have a Baro-aided GPS system.

Correct, in a very basic description of how the system works, a GPS satellite transmits a time signal, and also a list of orbits the satellites (bit like a street directory, except they are always moving in a predictable orbit planes).

The receiver gets the different transmissions from each satellite in view, and knows which one sent what time signal. It also has in its memory a list of where they are in space.

Knowing where a satellite is in space, and the amount of time it takes to get the signal from the satellite, gives the receiver a sphere around the satellite where the aircraft must be, The distance from the satellite is basic maths, d=speed of light/time, bit like a constant DME from a ground based DME.

Do this calculation 4 times over, the position is the intersection of the 4 spheres. It is solving 4 simultaneous equations, for x,y,z, and time.

Baro-aiding achieves additional redundancy as the receiver puts a pseudo satellite extending from the center of the earth through the aircraft, and solves one of the equations, Z is known, so only 3 satellites are needed to solve for x,y, and time. The receiver can then compare the position generated by the 4 satellites, and the various combinations of 3 satellites and the baro-aiding. if an error exists between the calculated positions, they can discount the bad satellite from further position calculations.

This is very simplistic overview.
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N243NW
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RE: Advantages And Disadvantages Of A GPS

Thu Aug 09, 2012 12:22 am

Quoting DiamondFlyer (Reply 6):
IIRC, you only need 4, and 5 to throw one out, if you have a Baro-aided GPS system.

My apologies. I forgot that the satellite requirement decreases by one if accurate barometric altitude is used.
B-52s don't take off. They scare the ground away.

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