Research and development projects
GNSS and especially GPS is used within a great number of different applications. The equipment and the methods are varying depending on accuracy requirements. Our research is mainly concentrated on the development of new methods in order to increase the accuracy and precision of the measurements, but we do also carry out studies and system developments using available techniques. You can find a few examples within different areas below.
Time and frequency
A receiver must observe at least four satellites to be able to find its position. Three satellites are used to estimate the three coordinates of a position and one satellite is used to estimate the time difference between the receiver's clock and the clocks in the satellites. The time difference must be calculated since positioning within GNSS is based on time interval measurements. The receiver measures the time it takes for the signal to pass between the satellite and the receiver which requires that the clocks must be synchronised. The distance to the satellite, which is used to calculate the position, is obtained through a multiplication of the time interval with the speed of light. Since the time in the satellites is related to UTC the clock in the receiver or clocks connected to the receiver may also be related and synchronised to UTC.
Time and frequency measurement using code-receivers
Most of the receivers that are used for time and frequency applications have an output with 1-second pulses that are traceable to UTC. These pulses can be used to characterise and calibrate clocks and oscillators. A direct measurement according to this method gives an accuracy of better than 300 ns relative to UTC. The accuracy for a frequency comparison is approximately 1E-12 for an averaging time of several hours. The accuracy is limited by the accuracy of the broadcasted satellite position and clocks as well as the signal delay in the atmosphere.
Sometimes this accuracy is insufficient and there could be a need for comparison of clocks and oscillators against a special national reference. A method called "Common-View" (CV) could be used in this case. CV-measurements involves at least two laboratories that observe the same satellite during the same instant of time. It is very important that the observations are done for one satellite at a time or that it is possible in deferred time to calculate the time difference between the laboratory clock and the clock in each satellite separately. A subtraction of the data collected at each laboratory an estimate of the time difference between the clocks in the different laboratories is obtained. Since the measurements are substracted from each other most of the common errors such as satellite poition and clocks as well as the signal delay in the atmosphere is canceled, partly dependent on the distance between the laboratories. The inaccuracy of the satellite's clock cancels because it is common for the two laboratories. A measurement according to this method gives an accuracy of better than 5 ns relative to each clock. The accuracy for a frequency comparison is approximately 5E-14 for an averaging time of several hours.
The Common-View technique is of great interest for SP since the method is utilized to compare SP's clocks and time scale UTC(SP) relative to UTC and TAI. During the last years the method has been developed through the use of multi-channel receivers and GLONASS-receivers. SP has developed a system, which makes it possible to automatically measure clocks and oscillators relative to UTC(SP) continuously without sending the equipment to SP.
Time and frequency measurement using phase-receivers
Today the methods using the code observable are not accurate and precise enough for characterizing the most delicate clocks and oscillators. To improve the accuracy for time comparison with GNSS one has to make use of more sophisticated receivers that can measure phase changes of the carrier transmitted from the satellites.
The method may give an accuracy of a few tenths of picoseconds and a frequency accuracy of 1E-16 if the measurements are averaged for several hours up to 24 hours. To obtain this accuracy calculations of the satellites' positions and clocks as well as the signal delay in the atmosphere is also required.
The largest source of error comes from environmental temperature variations that affects the signal delay in the equipment. It is also required for absolute time measurements to accurately calibrate the constant time delay in the receiver. A great part of the research within time and frequency measurement with phase-receivers for GPS and GLONASS concerns these error sources. SP also develop methods for calculations in real time (see below).