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PRN133 ranging now useable for SoL, at non precision approach level
Up front, I apologize for nit-picking. I agree with all the posts
about modernized signals (L2C, L5, L1C). They have all been designed to correct various problems experienced with C/A code. Meanwhile, the number of C/A codes is being expanded to 209, or about 40% of the balanced C/A codes. Many or most of these may never actually be broadcast, but they are defined. From Spilker's paper in the Summer 1978 special issue of the ION Journal (alias "Vol. I Red Book"): Spilker, J. J., "SIGNAL STRUCTURE AND PERFORMANCE CHARACTERISTICS (SPACE SEGMENT)", NAVIGATION, Vol. 25, No. 2, Summer 1978, pp. 121-146. http://www.ion.org/search/view_abstr...?jp=j&idno=680 "Thus the advantage of the Gold codes is not simply a low cross- correlation between all members of the family but that there are a large number of codes all of similar good properties. .. . . Fig. 2-13 shows the cumulative probability of various cross- correlation interference levels for the GPS C/A code for various doppler shifts from fd = 0 to + 5 kHz. Note that the 4 kHz doppler gives the worst cross-correlation sidelobe over this range; however, the other doppler shifts give similar results. These cumulative averages are formed by averaging results for alI 1023 of the Gold codes of period 1023 in the GPS family. All possible code time offsets are considered for each doppler offset and ail possible pairs of codes in this family. . . . Peak Cross-correlation (any doppler shift) -21.6 dB Peak Cross-correlation (zero doppler) -23.8 dB Probability of worst case or near worst case cross-correlation 0.25 " Couldn't find a free on-line copy of that paper, but you can browse the Vol. I "Blue Book" via Google Books: Global positioning system: theory and applications, Volume 1, Bradford W. Parkinson and James J. Spilker (eds.), AIAA, 1996, ISBN: 156347106X, 9781563471063 http://books.google.com/books?id=lvI1a5J_4ewC pg. 99: "The number of balanced Gold codes is . . . 513 . . . ." pg. 102, Table 9 The table shows that the cross-correlation of any pair of Gold codes is 1/1023 with probability 3/4, 63/1023 with probability 1/8, and 65/1023 with probability 1/8. Taking 20*Log10 of those values gives worst case cross-correlation of -23.8 dB, and best case of -30 dB. Again, these theoretical cross-correlation peaks are valid only for zero Doppler. As Spilker notes in his 1978 ION paper, the worst case Doppler raises that peak about 2.2 dB. IS-GPS-200E shows C/A code generation in Figs. 3-8 through 3-10, and Table 3-I. http://www.gps.gov/technical/icwg/ http://www.gps.gov/technical/icwg/IS-GPS-200E.pdf The table and Fig. 3-10 show a "2-tap" mechanization, in which 2 stages of the G2 shift register are tapped and added modulo 2 to the output of the G1 register. Since the G2 register has 10 stages, there are 45 ways ("10 choose 2") to do this. Table 3-1 shows only 36 unique tap pairs. PRNs 34 and 37 are identical on C/A code (though unique on P-Y code). The reason is that the other 9 tap pairs produce unbalanced codes, i.e., the number of 0s and 1s differ by more than 1. Note also that the selection of a pair of stages is equivalent to a time delay of the G2 sequence. The newly defined C/A codes are defined on the basis of G2 time delay, so that information has been added to Table 3-I for consistency. C/A codes can easily be generated in a spreadsheet, which shows that the other 9 2-tap codes are unbalanced. IS-GPS-200E defines 173 new C/A codes, for a total of 209 unique ones, or 210 total. "6.3.6.1 Additional C/A-code PRN sequences. The PRN C/A-code is described in Section 3.2.1.3 and 36 legacy C/A-code sequences are assigned by SV-ID number in Table 3-I. An additional set of 173 C/A- code PRN sequences are selected and assigned with PRN numbers in this section as shown in Table 6-I. Among the 173 additional sequences; PRN numbers 38 through 63 are reserved for future GPS SVs; PRN numbers 64 through 119 are reserved for future Ground Based Augmentation System (GBAS) and other augmentation systems; PRN numbers 120 through 158 are reserved for SBAS; and PRN numbers 159 through 210 are reserved for other Global Navigation Satellite System (GNSS) applications." This information will evntually migrate from Sec. 6 to Sec. 3. The implication is that someone has gone through the tedious effort of examining roughly 125,000 (513 choose 2) pairs of codes for undesirable cross-correlation properties, and selected the 173 "best" in some sense for the PRN expansion. |
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