Electronic – 66 GPS channels for 22 satellites – why the factor of 3


Modern GPS chips can use signals from many satellites at the same time to obtain a solution. While typically 4 are necessary, a larger number helps improve the solution with some types of errors.

As an example for this question, the Adafruit Ultimate GPS uses a GlobalTop FGPMMOPA6H chip. The Module is said to have 66 channels and be able to track 22 satellites simultaneously.

My basic understanding of (at least the coarse acquisition stage) of GPS signal processing is that the antenna signal is amplified, possibly shifted in frequency and demodulated. All satellites broadcast in the same frequency band, but the signal from each is modulated by a unique Gold code. The signals are identified by correlation – the chip decides which satellites might possibly be above the horizon and loads those Gold codes into the correlators.

There are about 32 GPS satellites, so 22 is more than enough from the surface of the earth (space is a different matter).

But why are there 66 channels for 22 possible satellites?

Reading this answer "You need one channel, per frequency, per satellite." it looks like the factor of three (66/22) might come from the ability to track L1, L2 and L5 signals, but I am not sure that's what is happening here. However this answer The number of channels inside a Navigation receiver is definitely more than a marketing gag suggests that L5 requires two correlators by itself.

Why 66/22?

Best Answer

Adafruit Ultimate GPS is in no way representative of all GPS receivers out there. Many good receivers have more channels than that; 256 is common in high-end devices. Note that 256 is not divisible by 3, so the factor of 3 you're seeing is a red herring.

Also, you rarely get a signal from 22 satellites, and it's not necessary to have a fix. Having a signal from 7-10 satellites is quite common in real-world conditions.