The revised IS-GPS-200D specification
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The Revised IS-GPS-200D SpecificationNew Spreading Codes+A Brief Introduction to IS-GPS-800 Spreading CodesNeil Gerein, NovAtel Inc.December 1st, 20061GPS spreading codes have been unchanged for decadesIRN-200D-001IS-GPS-200DPublic releaseICD-GPS-200CICD-GPS-200BICD-GPS-200AICD-GPS-200198319841987199319972004200622New IRN could be a problem for some legacy receivers• New spreading codes defined for C/A, P and L2C• IS-GPS-200D can be found at http://www.navcen.uscg.gov/gps/geninfo/IS-GPS200D.pdf• IRN-200D-001 can be found at https://gps.losangeles.af.mil/engineering/icwg/Docs/IRN-200D-001%207Mar06.pdf33While on the topic of spreading codes …• Some interesting codes for GPSIII L1C• Defined in Draft IS-GPS-800 http://gps.losangeles.af.mil/engineering/icwg/Docs/Draft%20IS-GPS-800%2019Apr06.pdf44IRN-200D-001• Not applicable to Block II/IIA, IIR/IIR-, or IIF• Used for other L1/L2 applications, such as SBAS• New CNAV data on PRNs 36-63PRN #C/APL2 CM/CL36-63Future GPS SVsFuture GPS SVs64-119Future GBASFuture GBAS-120-158SBASOther GNSS-159-210Other GNSSOther GNSSFuture GPS SVsOther GNSS55A Simple Linear Feedback Shift Register(LFSR)0111Time = t0In10011In2 XOR0011011066A Simple Linear Feedback Shift Register(LFSR)101011Time = t1t0In10011In2 XOR0011011077A Simple Linear Feedback Shift Register(LFSR)Timet0t1t2t3t4t5t6t7Input01001110D110100111D211010011D3/Output11101001XOR(D1,D2)01110100In10011In2 XOR001101108LFSR pseudorandom sequences have the property that when XORed with a phaseshifted version of itself the sequence does not change, it only obtains another phase8GPS C/A coder has two LFSRsG112345678910C/A code1.023 MHz12345678910G2All ‘1’s99GPS C/A Code PRN 2 – phase select 3 and 7G112345678910C/A code1.023 MHz12345678910G2All ‘1’sLimited # of phase shifts = limited # of codes1010Use initial load value instead of XOR phase selectTimet0t1t2t3t4t5t6Input0100111D11010011D21101001D3/Output1110100XOR(D1,D2)011101011To get the D3/Output sequence we can initially load the registers with ‘1 1 1’.
Toget the phase select XOR(D1,D2) we can initially load the registers with ‘1 1 0’.11Use initial load value instead of XOR phase selectTimet0t1t2t3t4t5t6Input1010011D11101001D21110100D3/Output01110101212GPS C/A coder with programmable load valueG112345678910C/A code1.023 MHz12345678910G2All ‘1’sThis style of coder must be used for IRN-200D-0011313The P-code sequence is over 38 weeks in length14Each PRN is an individual week of the sequence.14New P-codes are old sequences shifted by 1 to 5 daysPRN 38Day 2Day 3Day 4Day 5Day 6Day 7Day 1PRN 75Day 3Day 4Day 5Day 6Day 7Day 1Day 2PRN 112Day 4Day 5Day 6Day 7Day 1Day 2Day 3Day 5Day 6Day 7Day 1Day 2Day 3Day 4Day 6Day 7Day 1Day 2Day 3Day 4Day 5PRN 149PRN 186Most current ASICs should be able to handle this change1515L2 CM/CL coders are designed for initial load valuesAdditional load values defined in IRN-200D-001Change to ASIC not needed1616IS-GPS-800 : The GPS L1C draft specification• Developed with experts from Japan for QZSS• Data channel ranging code – L1CD• Pilot channel ranging code – L1CP• 1.023 Mbps X 10 millisecond length = 10230 chips• Overlay code applied to L1CP 100 bps x 18 seconds = 1800 bits Each satellite has a unique overlay code Overlay codes for PRNs 1 through 63 – one LFSR Overlay codes for PRNs 64 through 210 – two LFSRs1717A single Legendre sequence is used for all PRNs• L(2k modulo 10223) = 1for k=0,1,…,5110• L(t) = 0otherwise• Evaluating 2k for large k causes difficulty For example, 2500 is a 150 digit number Use L(k2 modulo 10223) instead• Examples L(02 modulo 10223) = L(1) = 1 L(12 modulo 10223) = L(2) = 1 L(22 modulo 10223) = L(4) = 1 L(32 modulo 10223) = L(9) = 1 L(1432 modulo 10223) = L(20449 modulo 10223) = L(3) = 118% Matlab code to generate base Legendre sequenceLegendre = zeros(1,10223);for k = 1:5110index = mod(k^2,10223);Legendre(index + 1) = 1;end18Weil codes are the XOR of L(t) and a shift of L(t)L1C code is Weil code with 0 1 1 0 1 0 0 insertedWeil index(w)Insertion Index(p)‘Memory’ codes1919L1CP Overlay codeRegister taps unique for each PRNS2 register only used for PRN # 64 through 210.2020Impacts of new spreading codes• Some old GPS L1 chip sets may not handle new codes• Software update for new P and L2C codes• Implementation of L1C via ‘memory codes’• Galileo L1 also uses ‘memory codes’ Interoperable Memory is not free Increased ASIC real estate and power consumption2121Questions?22.
