Pseudo-Noise (PN) Ranging Systems - CCSDS

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CCSDS INFORMATIONAL REPORT CONCERNING PSEUDO-NOISE RANGING SYSTEMS Table 2-1: Component PN Sequences Code Component Length Chip Sequence C1 2 1, -1 C2 7 1, 1, 1, -1, -1, 1, -1 C3 11 1, 1, 1, -1, -1, -1, 1, -1, 1, 1, -1 C4 15 1, 1, 1, 1, -1, -1, -1, 1, -1, -1, 1, 1, -1, 1 -1 C5 19 1, 1, 1, 1,-1, 1, -1, 1, -1, -1, -1, -1, 1, 1, -1, 1, 1, -1, -1 C6 23 1, 1, 1, 1, 1, -1, 1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, -1, 1, -1, -1, - 1, -1 Each component sequence is placed in a circular shift register with length equal to the component length and clocked at the chip rate. The T4B composite code is formed from the combination of the shift register outputs using the following formula: C = sign( 4C1 + C2 − C3 − C4 + C5 − C6 ) The output of each shift register is fed back to the input, such that each component repeats itself with period equal to the component length. Figure 2-2 shows a functional block diagram of the T4B PN code generation. Because of the sign function, the value of the composite code C in the formula above can be interpreted as being determined by votes from the six component sequences (the negative sign simply means that the component sequence is inverted). C1 is multiplied by four, and thus has four ‘votes’, while the other five components only have one vote. Since the C1 component is the range clock component, the T4B code has a relatively strong clock component. where the combined sequence is C = sign(4C1+ C2 − C3 − C4 + C5 − C6) Figure 2-2: T4B PN Code Generation CCSDS 414.0-G-1 Page 2-4 March 2010
CCSDS INFORMATIONAL REPORT CONCERNING PSEUDO-NOISE RANGING SYSTEMS 2.2.3 T2B PN CODE GENERATION The component sequences used for the T2B code are identical to those used for T4B. The combination logic to form the T2B composite code is given by: C = sign( 2C1 + C2 − C3 − C4 + C5 − C6 ) The combination logic is identical to that used to generate the T4B code, except the C1 component is weighted only by a factor of two (i.e., two votes). Thus this code has a weaker range clock component. Figure 2-3 shows a block diagram of the T2B PN code generation. where the combined sequence is C = sign(2C1 + C2 − C3 − C4 + C5 − C6) 2.2.4 CODE PROPERTIES 2.2.4.1 Code Length Figure 2-3: T2B PN Code Generation The lengths of the component sequences for T4B and T2B are all relatively prime, so the composite code will have a period equal to the product of the component lengths. Since the component sequences are identical for both codes, the composite code length L for the T4B and T2B codes is: L = 2 × 7× 11× 15× 19× 23 = 1, 009, 470 chips CCSDS 414.0-G-1 Page 2-5 March 2010
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Pseudo-Noise (PN) Ranging Systems - CCSDS

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