The State of GNSS Receiver Tracking Channels & Satellite Constellations in Orbit

The number of channels in GNSS receivers seems to be going up and up: 672, 555, 1408.... but what does it all mean when there are only about 120 operational navigation satellites in orbit? Is it a case of “the more, the better?”

Let’s take a look at what’s really happening between space and Earth, and explore why there are so many receiver channels with relatively few satellites in orbit.

In the early days of GPS receivers, there was a direct correlation between the number of satellite tracking channels a receiver had and the number of satellites that receiver was tracking. For example, a Trimble 4000S receiver had just 6 channels, where each channel tracked the L1 signal on a single satellite. At the time, GPS test satellites were only broadcasting the L1 Coarse Acquisition, hence the name “L1C/A” code intended for civilian use.

This single L1 signal from Satellite A uses one of the receiver’s satellite tracking channels.

With the launch of GPS production satellites during the late 1980s and 1990s, GPS receiver technology advanced in both functionality and miniaturization. Trimble survey receivers included proprietary technology that enabled them to track the military L2 signal broadcast by GPS satellites. Each GPS satellite broadcasts on the L1 and L2 bands. The “L” in L1 and L2 refers to the designation used for L-band microwave radio signals in the range of 1-2 GHz (=1000-2000MHz). 

In this scenario, the signals received from Satellite B occupy two receiver channels.

GNSS satellite signals consist of ranging codes and data messages that are carried by the carrier waves.

As a side note, we started using the term “GNSS,”  abbreviated from “Global Navigation Satellite System,” to describe the network of navigation satellite constellations that are in orbit rather than just using the term “GPS,” since GPS was the name given to the US-owned constellation of navigation satellites. With the additions of the Russian GLONASS system, Chinese BeiDou and Japanese QZSS systems coming online, satellite-based navigation was far more extensive than just utilizing the American GPS constellation and “GNSS” became the more precise term.

The ranging codes noted above enable receivers to calculate range (distance) from the satellite to the receiver antenna. In the case of modernized GPS satellites, the L1 carrier signal is modulated (modified) by the Coarse Acquisition (C/A-), Civil (C-), and military-grade precise encrypted (P/Y-) code ranging signals, i.e. L1C/A, L1C and L1P/Y. While the L2 carrier is modulated by two code types, leading to L2C and L2P/Y. GPS satellites also broadcast two ranging codes on the L5 band: L5I and L5Q.

A dedicated receiver channel is needed to track each respective satellite signal. That is, if the receiver is to track L1C, L1C/A, L2C and L5I and L5Q, then it will need to allocate 5 channels for each GPS satellite.

Trimble GNSS receivers are able to track signals from multiple satellite constellations. Where each satellite constellation broadcasts on multiple carrier frequency bands, with various ranging codes. The signal structure of the Japanese QZSS service is intentionally compatible with GPS, however, generally all other Global Navigation Satellite Systems have unique signal structures. The table below summarizes the GNSS signal structures.

Typically, between 40 and 60 GNSS satellites will be visible above the horizon of a user anywhere on Earth. Assuming that there are 5 signals to be tracked per satellite, then this would require, say, 200 to 300 channels.

However, some of the constellations (Galileo, for example) have more complex range codes. Essentially, the range code is the code of zeros and ones that allows the receiver to determine the time and distance traveled of that signal between the satellite and the receiver. Different range codes are used for the different carrier signals, for example, the range code for the L1C signal is different to the range code for the L2C signal.

Complex, right? 

That’s at the satellite end. At the receiver end, the receiver needs to be able to generate replicas of the codes broadcast by each satellite in order to track them. Because of the radically different codes used by the different constellations, it is necessary to devote a certain number of channels to specific satellite signals. What that means is that not all receiver satellite tracking channels can be agnostic and some need to be devoted to specific code types.

So now we can start to understand why a single GNSS receiver needs so many satellite tracking channels. Many of the channels are generic, that is, they will track similar signals, such as some of the GPS signals and some of the Beidou signals. But, some of the channels are set aside to receive specific code signals (such as from some of the Galileo satellites).

The figure below shows an example of how many receiver tracking channels could potentially be utilized when tracking just 5 individual satellites.

So is it a case of “the more, the better”? …perhaps not really.

Of course, it’s not just about arbitrarily tracking satellites. The algorithms in the receiver that decipher the signals, mitigate multipath, and the speed at which it converges those signals to define your position—as well as your correction source—are all also of vital importance.

What you really want to be assured of is that your receiver is capable of tracking all of the currently available satellite signals, as the more signals you’re tracking, the better the opportunity for your receiver to work to its maximum potential. Ultimately, tracking all available satellite signals increases your chances of getting a fixed position and getting your job done as efficiently and as accurately as possible.
 
The table below shows a list of the different satellite constellations, their number of satellites, number of signals and their orbit patterns.

Source: gssc.esa.int