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The differential GPS chips use L1 and L2C to calculate the ionospheric interference.

With the new GPS satellites, the new band L5C is being deployed, and I am wondering about the following things:

  1. Will L1 and L5C be used as new type of differential ionospheric error correction? Could L1 use both L2C and L5C as a kind of double correction at the same time? (Would it even be necessary or make sense, or are they still testing this and I’m getting ahead of myself)

  2. Separately, could L1/L5 produce superior differentiation to L1/L2?

  3. I’ve been reading documentation and cannot seem to grasp what the new nav data rate for L5C signal is (per each SV). Is it still the same 50Hz of L1 and L2, or is it 25Hz, or is it 100Hz? Whats throwing me off is the following line: “The 50 bit/s data is coded in a rate 1/2 convolution coder”

  4. What update rate do the SBAS SV’s transmit? Is it also 50Hz?

Jack Shephard
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The L1/L5 frequency pair can be used to compensate for ionospheric delay much in the same way as L1/L2. The wider spacing of L1/L5 as compared to L1/L2 will not enhance accuracy significantly.

In theory, if using all of L1/L2/L5, second order ionospheric effects can be eliminated (see here). We will see if this gives superior correction when manufacturers adopt this (or a similar) method (I'm sceptic).

The quadrature part of L5 is used as a pilot and does not carry data modulation. A convolutional coder expands the net data rate of 50bit/s into 100 symbols/s (to shape the power spectral density of the signal in space). These are phase shift modulated onto the inphase part of L5. Net rate is 50bit/s.

WAAS and EGNOS send 250 net bits/s in 500 symbols/sec. The relevant documentation (MOPS DO-229) is behind paywalls, I cannot link it here.

BTW: The term "differential" does not apply here, it is used when signals from at least two different antennas contribute to the solution (either independent receivers or antenna arrays).

Andreas
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  • Thank you for the explanation. Triple frequency seems insane (in lab) will see how it turns out when more capable SV’s get deployed. I’ll try to get that article through school about WAAS. As per differential, doesn’t L1 and L2 come from different antennas considering those are two different signals? – Jack Shephard Aug 28 '18 at 10:26
  • @JackShephard For GNSS-antennas, one common phase center for all frequencies (and elevations) is needed. The antenna receives signals as if the lines of sight would all end in one point, for all frequencies and for all SV elevations. For differential, you have different antennae with different phase centers, either rigidly coupled on a vehicle or moving independent, and **geometry between phase centers** plays a role in your position solution. – Andreas Aug 28 '18 at 18:05
  • @JackShephard MOPS DO-229 is mostly about aviation, not sure if it's worth the effort. Read [IS-GPS-200](https://www.gps.gov/technical/icwg/IS-GPS-200H.pdf) instead. The datasheet of [Zarlinks GP2021](http://pdf.datasheetcatalog.com/datasheet/zarlinksemiconductor/zarlink_GP2021_JAN_01.pdf) is also an interesting read, even if you are not planning to use this one. – Andreas Aug 28 '18 at 18:21