Locking to satellites, tracking them and taking accurate measurements are the main function of a receiver. The merits that distinguishes a receiver from others is how well it can track satellites under all environmental conditions and dynamics, and the type and quality of measured data.
Tracking satellites in an open field with no interfering signal and no partial obstruction is not much of a challenge. The challenge comes when there are interfering signals that partially mask satellite signals or there are partial blockage that reduces the amount of signal energy that reach the receiver.
The problem is that more interfering signals appear every day from new communication systems. The interfering signals also appear from power lines, radar systems and equipment operated by police cars and ambulances. More each day applications of GPS receivers expand to include urban areas where the GPS receiver is subject to more of such interference and partial obstructions.
In tracking satellite's carrier signals we have to track three main dynamics:
The motion of the satellite is the most smooth compared to the other two. It can be predicted with the accuracy of few meters per day. Even if we don't track the satellite for an hour and only rely on prediction, we will not be off by more than one meter.
The motion of the receiver is unpredictable. The behavior of the clock of the receiver can also be unpredictable and abrupt, especially when it is subjected to shock, vibration, and temperature changes. The change in the behavior of the receiver clock happens even when the receiver is not moving and, for example, it is sitting on a survey point.
All of the above three dynamics add together and result in a relative dynamic between the receiver and the satellite. And it is this total dynamic that the receiver must track.
There is a direct relationship between the ability to track a satellite and its signal strength. Consider the following analogy: You can easily observe a bright star in a clear night. If the brightness of the star decreases and starts to move randomly(forgive the moving star) you may have difficulty tracking it. Imagine that you are observing a weak star with a binocular. You may have no difficulty tracking its smooth motion. Now imagine you are following the same star with a binocular but this time you are sitting in a four wheel drive driven by a teenager in a bumpy road. Now you must track not only the motion of the star but also the motion of the car. If some imaginary instrument could track the motion of the car and compensate for it, you again will have no problem to follow the smooth motion of the star, even if the star becomes dimmer or if you drive under a tree which partially blocks your view. Even if you momentarily loose it you can quickly find it again when the obstruction disappears because you can predict the smooth motion of the star.
The concept of Common Tracking is to track the carrier dynamics of the receiver (and its oscillator) separate from tracking the carrier of each individual satellite. To track the dynamics of the receiver (and its oscillator) we combine the strength of all satellites together. Knowing this, we compensate for the dynamics of the receiver and its oscillator and then we track the carrier of each satellite separately based on the signal strength of the satellite. By this revolutionary technique (patent pending) we put ourselves back to the case of observing stars on a stable ground. We can track satellites even under heavy foliage. The more satellites we track the more signals we have to add together and track the dynamics of the receiver and its clock. The satellites "cooperate" together to perform this difficult task. Then we need the strength of each satellite separately to track only its own smooth and highly predictable motion.
The power of Common Tracking is not only in tracking low signal satellites (e.g. under foliage or low elevation angles) but also under high dynamic and interfering signals (that effectively reduces signal-to-noise ratio). In summary with Common Tracking we achieve the following advantages: