During the development of EISCAT_3D, a Large Aperture Array Radar (LAAR), with direct sampling at each antenna element and constituted of up to 16.000 antenna elements, intended for atmospheric research, the need for a highly accurate timing system was recognized. This paper describes the method and test results of a GNSS based timing system on a 300 m scale formed on L1-only GNSS receivers.Simulations have shown that over a distance of 300 m the maximum allowed total timing jitter is 160 ps. This timing jitter is composed of jitter from the clock distribution, local oscillator, ADC and movement of the antenna phase center due to weather conditions. A reasonable assumption is that at most a third of the total jitter is generated in the clock distribution system, i.e. 50 ps. Such accuracy is impossible to achieve with the traditionally often used non-calibrated cable-based clock distribution system, even heating of clock distribution cables can alter the length of the cables to the extent that too large errors are generated, thus the choice to use a GNSS-based clock distribution system that is unaffected by such effects. Other benefits of building a GNSS timing system include lower cost due to reduced amount of coaxial cable throughout the array and the need for building a continuous cable length calibration system that ensures timing accuracy of the distributed clock system to the necessary levels. By dividing the LAAR into small sub-arrays of 9 elements each, the maximum length of the cables distributing the clock is reduced to 4.5 m which is short enough to be calibrated by length approximation only, assuming that the clock distributed to each sub-array is known. Inserting a GNSS receiver at all of these sub-arrays, to provide a clock reference that is unaffected by changing conditions over the array, each sub-array is now timed to the specified accuracy.In general, a GNSS L1-receiver is rated to produce a clock with an error of less than 50 ns, which is about 1000 times too high. However, unique conditions apply to this GNSS timing system that improvs the accuracy, such as:- A local system, i.e. the maximum distance between two GNSS antennas are 300 m which infer all significant atmospheric errors in this application to be common over the array.- A common highly accurate reference clock is distributed to all receivers, which removes the clock drift errors between the receivers.- Software based selection of satellites used for the timing solution to exclude timing errors from different matrices in the position and timing calculations.- All receivers are stationary which allow long integration times, up to 30 min because the time constant of the cable length change in the reference clock distribution is in that order of magnitudes, to improve accuracy- Phase measurements from one satellite only is sufficient to calculate the timing error between the sub-arrays since the relative position of each receiver is known. - No integer ambiguity solution is necessary, again, since the relative position of the receivers is known and the absolute time difference between the receivers is insignificant, only the phase of the distributed clock is important.Satisfying these conditions decreases the clock error from the GNSS receivers sufficiently to reach the necessary levels of accuracy.Each sub-array contains a Voltage Controlled Oscillator (VCO) in which the distributed clock is reproduced and distributed through a Delay Locked Loop (DLL) to the local GNSS receiver, the radar ADCs and a signal injection system located as close to the radar antennas as possible to calibrate the analogue signal path of the system. The purpose of the DLL is to adjust the phase of the reference clock to be equal throughout the array. This is achieved by creating a closed loop feedback from the GNSS receiver to the DLL and adjusting the phase according to the phase differences in the received satellite signals in respect to a reference GNSS receiver. This reference receiver is a high-end receiver which is used in conjunction with purpose specific software to produce information sent to each of the sub-array receivers necessary to calculate the expected phase of local VCO clock. Compared to the actual phase of the VCO, the DLL can now make the necessary adjustments to the reference clock. The information sent is; satellites to use, Doppler-shift, tracking chip and expected phase. This information allow the sub-array receivers to only be capable of tracking a low number of satellites, no more than 6, and using the tracked phase differences to calculate the expected phase of the local VCO. Thus, full capability receivers are not needed, but instead a Digital Signal Processor (DSP) is used with a GNSS RF-frontend to control the DLL and the Automatic Gain Control (AGC) of the RF-frontend.Test measurements have been performed in a real environment during windy winter conditions, clear weather at -10 C and wind speeds up to 20 m/s in gusts, with three antennas placed randomly, but precisely surveyed, at about 5 m distance from each other placed on a rooftop to simulate the conditions in the EISCAT_3D LAAR. IF data from the antennas were collected during a one hour measurement and then post-processed to calculate the expected phase differences between the antennas. These phase differences provide a direct measurement of the accuracy levels attainable. The test results show that when integrating over 15 min, a total clock distribution jitter of less than 50 ps is achievable with simple calculations that can be implemented into a DSP.
2008. 112-116 p.
International Technical Meeting of the Satellite Division of the Institute of Navigation : 16/09/2008 - 19/09/2008