The fundamental task of a GNSS receiver is to measure distances to several GNSS satellites and compute receiver coordinates. Distances are measured to satellites by measuring the travel time of signals from the satellites to the heart of the receiver electronics where the received signals are processed. Data used from a base receiver (at a known point) removes common errors in the rover and yields accurate results. The signal path from each satellite to the receiver electronics consists of two parts: 1) the direct path in space from the satellite to the receiver antenna, and 2) from the receiver antenna to the receiver electronics. The first path is unique to each satellite. The second path is common for all satellites, and is where the signal travels through antenna electronics, antenna cable, and to the receiver analog and digital sections. We call the signal travel time through the second path “the receiver bias.” As long as the receiver bias is the same for all satellites, it acts as a component of the receiver clock offset which we solve as the fourth unknown (along with x, y, z). In other words, if the receiver bias is the same for all satellites it does not impact position computations.

    The assumption that the receiver biases are the same for all satellites is true for GPS but not for GLONASS. The reason is that the receiver bias depends on the satellite signal frequency. All GPS satellites transmit on the same frequency so they all create the same receiver bias. GLONASS satellites transmit on different frequencies so each GLONASS satellite generates a different receiver bias. In technical terminology GLONASS satellites cause inter-channel biases which, if not taken into account, can significantly degrade position accuracy. The good news is that all common errors between the base and the rover receivers are cancelled. Therefore, if the magnitudes of the GLONASS inter-channel biases in the base receiver and in the rover receiver are the same, these biases will be cancelled and they will not degrade the position accuracy. In such cases GLONASS satellites act as good as GPS satellites. But this rarely happens. The bad news is that the magnitudes of the inter-channel biases depend not only on the receiver design and its electronic components but also on temperature and slight variations in the electronic components. Even in the best case where the base and the rover receivers are from the same manufacturer and have identical design, components, and manufacturing dates, there is still the issue of temperature and minute component differences. The magnitude of the GLONASS inter-channel biases can prohibit the use of GLONASS satellites for precision applications.

    When the objective is to achieve centimeter and sub-centimeter accuracy, dealing with GLONASS inter-channel biases is not an easy task. This may be the reason why, until recently, most manufacturers had avoided GLONASS for so many years. Currently, some manufacturers simply ignore the GLONASS inter-channel biases and assume that customers use identical receivers as base and rover. In the early years we did the same and since we were the only GPS + GLONASS manufacturer, all receivers had similar inter-channel biases. When the inter-channel biases were noticeable we used GPS + GLONASS to resolve ambiguities (fix the integers) and then ignored GLONASS measurements or significantly deweighted them. Even in such cases we found significant improvement over the GPS-only usage. With some receivers, when the inter-channel biases between the base and the rover become intolerable, the receiver firmware ignores the GLONASS satellites and provides solutions based on GPS satellites only! Dealing with the problem in this manner does not allow the customer to know why his GPS+GLONASS receiver does not show any improvement over GPS-only receivers. Such receivers are still in use. When the receiver firmware cannot isolate the GLONASS satellites with high inter-channel biases it provides inaccurate results. This is a serious problem which causes the user to accept faulty results as the truth. Some manufacturers try to measure the GLONASS inter-channel biases in a sample of preproduction receivers and hardcode these biases into the firmware. This is a positive step forward but by no means can cure the problem because there are still differences between electronic components and their characteristic vary by temperature and time.

    We calibrate all GLONASS inter-channel biases in every receiver continuously and in real-time with an accuracy of ±0.1 millimeter. We have designed and implemented special patent-pending hardware inside the TRIUMPH chip to achieve this. This real-time calibration is done in the background without any impact on the normal use of the receiver. The bottom line is that we have made GLONASS as good as GPS and offer you a lot more satellites to do your job faster and more accurately, even in adverse conditions. Our TRIUMPH receiver has 216 channels to track all GPS, GLONASS, and Galileo signals and to cover your needs now and well into the future. Is your GLONASS as good as GPS? Ask the manufacturer of your receiver how this is handled. It might impact past or future observations. And as you know, many times past observations (e.g. construction) cannot be re-made. GLONASS and GPS, with tens of billions of dollars of investment, is up there for free. Used in combination wisely, they can provide a real advantage and improve your bottom line.