Some receivers accept L1, L2, E5b/B2I signals, while others accept L1, L5/E5a/B2a signals. Which one is better? To answer this, it's essential to understand the concept of "chip" and its length. In the standard GPS L1 C/A signal, modulation occurs at a frequency of 1.023 MHz, meaning that the length of one bit of the sequence is approximately 1 microsecond, or about 300 meters. Receivers usually accept signals with an accuracy of up to 1/100 of a chip, which is about 3 meters.
The GPS L2C signal has a standard chip size of 300 meters, while the GPS L5C signal has a chip size ten times smaller (and a modulation of 10.23 MHz). This not only provides reception with an accuracy of 30 cm, but it also helps deal with the urban landscape's curse - multipath. The larger the shift of the reflected signal relative to the chip size, the more distinguishable the signals are for the receiver. The smaller the shift relative to the chip size, the greater the chances that the combined, slightly shifted signal will be received. For example, a shift of the reflected signal by 30 meters is a shift of 1/10 of a chip for L2C and a whole chip for L5C. Of course, 24 satellites currently broadcast the L2C signal, and only 17 satellites broadcast the L5C signal, but this will improve with new launches.
A similar situation exists for BEIDOU, with B1I and B2I signals having a chip frequency of 2.046 MHz, while B1C and B2a signals have a frequency of 10.23 MHz. Additionally, B2I is a dying signal, currently transmitted by 15 satellites, while B2a is transmitted by 29 satellites. As new satellites are launched, the ratio will shift even more.
For GALILEO, nothing changes as E5a and E5b have the same chip frequency of 5.115 MHz. As for GLONASS, its chip frequency is only 0.511 MHz for both G1 and G2, so it's better not to use GLONASS at all when GALILEO and BEIDOU are available.
From the antenna design perspective, the fewer the frequency bands, the simpler (and cheaper) the antenna, and the more interference is filtered out by the filters.
Therefore, L1/L5 receivers are better than L1/L2/E5b receivers.
However, there is a downside to every positive aspect. As of February 2023, GPS L5C is broadcast by only 17 satellites. These signals are somewhat like "Schrodinger's cat." Since these signals are for aviation (and aviation is all about redundancy), it was decided that aviation receivers would not use L5C signals until there are 24 GPS satellites broadcasting L5C (approximately in 2027). As the number of L5C signals is insufficient and relying solely on GPS L5C is not feasible, other systems are necessary. Aviation standards still do not recognize Galileo or Beidou. As a result, L5C signals are considered "unhealthy" and should not be used. On the other hand, GPS L1 C/A signals are considered healthy. Therefore, a small trick is required for receivers when receiving L5C signals: they should consider their health status from L1 C/A signals.
© Eltehs SIA 2023
