report of the action team on global navigation satellite systems (gnss)
report of the action team on global navigation satellite systems (gnss)
report of the action team on global navigation satellite systems (gnss)
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GPS Predictable Accuracy64. The specific capabilities provided by SPS are established by DoD and DOT and arepublished in <str<strong>on</strong>g>the</str<strong>on</strong>g> Global Positi<strong>on</strong>ing System Standard Positi<strong>on</strong>ing Service Performance Standard(formerly known as <str<strong>on</strong>g>the</str<strong>on</strong>g> SPS Signal Specificati<strong>on</strong>, Ref. 7) available through <str<strong>on</strong>g>the</str<strong>on</strong>g> USCG Navigati<strong>on</strong>Informati<strong>on</strong> Service. The figures are 95% accurate and express <str<strong>on</strong>g>the</str<strong>on</strong>g> value <str<strong>on</strong>g>of</str<strong>on</strong>g> two standarddeviati<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> radial error from <str<strong>on</strong>g>the</str<strong>on</strong>g> actual antenna positi<strong>on</strong> to an ensemble <str<strong>on</strong>g>of</str<strong>on</strong>g> positi<strong>on</strong> estimatesmade under specified <strong>satellite</strong> elevati<strong>on</strong> angle (five degrees) and PDOP (less than six) c<strong>on</strong>diti<strong>on</strong>s.For horiz<strong>on</strong>tal accuracy figures, 95% is <str<strong>on</strong>g>the</str<strong>on</strong>g> equivalent <str<strong>on</strong>g>of</str<strong>on</strong>g> 2drms (two-distance root-meansquared),or twice <str<strong>on</strong>g>the</str<strong>on</strong>g> radial error standard deviati<strong>on</strong>. For vertical and time errors, 95% is <str<strong>on</strong>g>the</str<strong>on</strong>g>value <str<strong>on</strong>g>of</str<strong>on</strong>g> two-standard deviati<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> vertical error or time error.65. Receiver manufacturers may use o<str<strong>on</strong>g>the</str<strong>on</strong>g>r accuracy measurements. Root-mean-square(RMS) error is <str<strong>on</strong>g>the</str<strong>on</strong>g> value <str<strong>on</strong>g>of</str<strong>on</strong>g> <strong>on</strong>e standard deviati<strong>on</strong> (68%) <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> error in <strong>on</strong>e, two or threedimensi<strong>on</strong>s.66. Circular Error Probable (CEP) is <str<strong>on</strong>g>the</str<strong>on</strong>g> value <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> radius <str<strong>on</strong>g>of</str<strong>on</strong>g> a circle, centred at <str<strong>on</strong>g>the</str<strong>on</strong>g> actualpositi<strong>on</strong> that c<strong>on</strong>tains 50% <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> positi<strong>on</strong> estimates. Spherical Error Probable (SEP) is <str<strong>on</strong>g>the</str<strong>on</strong>g>spherical equivalent <str<strong>on</strong>g>of</str<strong>on</strong>g> CEP, which is <str<strong>on</strong>g>the</str<strong>on</strong>g> radius <str<strong>on</strong>g>of</str<strong>on</strong>g> a sphere, centred at <str<strong>on</strong>g>the</str<strong>on</strong>g> actual positi<strong>on</strong> thatc<strong>on</strong>tains 50% <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> three dimensi<strong>on</strong> positi<strong>on</strong> estimates. As opposed to 2drms, drms, or RMSfigures, CEP and SEP are not affected by large blunder errors making <str<strong>on</strong>g>the</str<strong>on</strong>g>m an overly optimisticaccuracy measure.67. Some receiver specificati<strong>on</strong> sheets list horiz<strong>on</strong>tal accuracy in RMS or CEP and withoutSelective Availability, making those receivers appear more accurate than those specified by moreresp<strong>on</strong>sible vendors using more c<strong>on</strong>servative error measures.GPS <strong>satellite</strong> signals68. The SVs transmit two microwave carrier signals. The L1 frequency (1575.42 MHz)carries <str<strong>on</strong>g>the</str<strong>on</strong>g> navigati<strong>on</strong> message and <str<strong>on</strong>g>the</str<strong>on</strong>g> SPS code signals. The L2 (1227.60 MHz) and L1frequencies (1575.42 MHz) are used to measure <str<strong>on</strong>g>the</str<strong>on</strong>g> i<strong>on</strong>ospheric delay by PPS equipped receivers.Three binary codes shift <str<strong>on</strong>g>the</str<strong>on</strong>g> L1 and/or L2 carrier phase. The C/A Code (Coarse Acquisiti<strong>on</strong>)modulates <str<strong>on</strong>g>the</str<strong>on</strong>g> L1 carrier phase. The C/A code is a repeating 1 MHz Pseudo Random Noise(PRN) Code. This noise-like code modulates <str<strong>on</strong>g>the</str<strong>on</strong>g> L1 carrier signal, "spreading" <str<strong>on</strong>g>the</str<strong>on</strong>g> spectrum overa 1 MHz bandwidth.69. The C/A code repeats every 1023 bits (<strong>on</strong>e millisec<strong>on</strong>d). There is a different C/A codePRN for each SV. Their PRN number, <str<strong>on</strong>g>the</str<strong>on</strong>g> unique identifier for each pseudo-random-noise code,<str<strong>on</strong>g>of</str<strong>on</strong>g>ten identifies GPS <strong>satellite</strong>s. The C/A code that modulates <str<strong>on</strong>g>the</str<strong>on</strong>g> L1 carrier is <str<strong>on</strong>g>the</str<strong>on</strong>g> basis for <str<strong>on</strong>g>the</str<strong>on</strong>g>civil SPS. The P-Code (Precise) modulates both <str<strong>on</strong>g>the</str<strong>on</strong>g> L1 and L2 carrier phases. The P-Code is avery l<strong>on</strong>g (seven days) 10 MHz PRN code. In <str<strong>on</strong>g>the</str<strong>on</strong>g> Anti-Spo<str<strong>on</strong>g>of</str<strong>on</strong>g>ing (AS) mode <str<strong>on</strong>g>of</str<strong>on</strong>g> operati<strong>on</strong>, <str<strong>on</strong>g>the</str<strong>on</strong>g> P-Code is encrypted into <str<strong>on</strong>g>the</str<strong>on</strong>g> Y-Code.70. The encrypted Y-Code requires a classified AS Module for each receiver channel and isfor use <strong>on</strong>ly by authorised users with cryptographic keys. The P (Y)-Code is <str<strong>on</strong>g>the</str<strong>on</strong>g> basis for <str<strong>on</strong>g>the</str<strong>on</strong>g>PPS. The Navigati<strong>on</strong> Message also modulates <str<strong>on</strong>g>the</str<strong>on</strong>g> L1-C/A code signal. The Navigati<strong>on</strong> Messageis a 50 Hz signal c<strong>on</strong>sisting <str<strong>on</strong>g>of</str<strong>on</strong>g> data bits that describe <str<strong>on</strong>g>the</str<strong>on</strong>g> GPS <strong>satellite</strong> orbits, clock correcti<strong>on</strong>s,and o<str<strong>on</strong>g>the</str<strong>on</strong>g>r system parameters.18