What Makes a GNSS Signal the IGS Analysis Center Workshop 2-6 June 2008, Miami Beach Larry Young (not an expert, just a user) Jet Propulsion Lab

Agenda Current and planned GNSS signals Carrier frequency impact Code rate impact (Code vs semi-codeless) BPSK vs BOC Data vs pilot

Current and planned signals GPS –C/A, P(Y), M, [L1C planned] at L1 ( MHz) –L2C, P(Y), M at L2 ( MHz) –Planned L5 ( MHz) GLONASS –Signals corresponding to C/A, P(Y), near L1 (FDMA) –Signal corresponding to P(Y), near L2 (FDMA) –Helpful change to CDMMA is planned GALILEO –Rich selection of at least 6 signals at or near L1, L5 (called E5a and E5b), and E6 ( MHz) –We hope all signals will be made available to science users Beidou, etc not treated here

Carrier frequency impact At least two frequencies are required for accurate work to remove delay/phase advance from the ionosphere. For example, an ion-free combination for pseudorange is Pc = [1+(F 2 2 )]/ (F 1 2 -F 2 2 )*P1 - F 2 2 / (F 1 2 -F 2 2 )*P2 In order to reduce the magnification of measurement errors in P1 and P2 by this linear combination, the factor F 2 2 / (F 1 2 -F 2 2 ) should be as small as possible. Carrier phase trilaning: Form dual widelanes with the ambiguities resolved, then form an ionosphere-free linear combination of those. The table (next vg) compares use of L2 vs E6 for the third frequency At least one of us favors investigation of a much higher frequency, near 5115 MHz for example, to form very precise carrier phase observables with very small ionospheric effects, enabling compact actively steered arrays that produce multiple beams toward the satellites and null forming toward multipath sources.

Trilane frequency comparison (Good choice, ESA!)

Code rate impact (Code vs semi-codeless) Advantages to higher chip rates –For a given ratio of signal bandwidth to chip rate, the errors due to system noise are inversely proportional to the chip rate. –Multipath errors are less, in some cases, for higher chip rates. (See next VG that shows identical MP for small delays.) –We recommend that studies be performed to see to what extent multipath from greater distances affects the estimated parameters at IGS sites.

Same MP error for delays normally encountered (needs study)

Code rate impact (Code vs semi- codeless) continued

Pilot signals, and BPSK vs BOC Many new codes are “pilots” with no data bits, allowing long coherent integrations for acquisition and tracking of weak signals. BOC and MBOC signals will be exciting to exploit –Better precision –Lower MP (for long delays) –+…

Pseudorange MP for 10 MC/s BPSK vs BOC(10,5) -10 dB MP, 32 ns E-L lag spacing

Carrier MP for 10 MC/s BPSK vs BOC(10,5) -10 dB MP, 32 ns E-L lag spacing

Conclusion The variety of carrier frequencies will allow new approaches, such as trilaning, to be developed The new codes, especially high-rate BOC codes such as the E5a and E5b pair will offer unprecedented pseudorange precision There is still plenty of room for improvement!