GEOG 862
GPS and GNSS for Geospatial Professionals

New Signal Availability and Some Benefits

GPS Signal Availability
Carrier Signal Block IIR Block IIR-M Block IIF
L1 P/Y ⦿ ⦿ ⦿
L2 P/Y ⦿ ⦿ ⦿
L1 CA ⦿ ⦿ ⦿
L2 L2C ⦿ ⦿
L1 M ⦿ ⦿
L2 M ⦿ ⦿
L5 Civil ⦿
Source: GPS for Land Surveyors

In the table, you see that the Block IIR-M satellites have the addition of the M-Code and the L2C to the legacy signals available from the Block IIR satellites. The Block IIF satellites IIFs have all the previous codes and L5. 

Ionospheric Bias

Concerning the effect of the ionosphere—as you know, ionospheric delay is inversely proportional to frequency of the signal squared. So it is that L2’s atmospheric bias is about 65% larger than L1, and it follows that the bias for L5 is the worst of the three at 79% larger than L1. L1 exhibits the least delay, as it has the highest frequency of the three.

Correlation Protection

Where a receiver is in an environment where it collects some satellite signals that are quite strong and others that are weak, such as inside buildings or places where the sky is obstructed, correlation protection is vital. The slow chipping rate, short code length, and low power of L1 C/A means it has the lowest correlation protection of the three frequencies L1, L2, and L5. That means that a strong signal from one satellite can cross correlate with the codes a receiver uses to track other satellites. In other words, the strong signal will actually block collection of the weak signals. To avoid this, the receiver is forced to test every single signal so to avoid incorrectly tracking the strong signal it does not want instead of a weak signal that it does. This problem is much reduced with L2. It has a longer code length and higher power than L1. It is also reduced with L5 as compared to L1. L5 has a longer code length, much higher power, and a much faster chipping rate than L1. In short, both of the civilian codes on L2 and L5 have much better cross correlation protection and better narrowband interference protection than L1, but L5 is best of them all.