Inside GNSS Media & Research

SEP-OCT 2018

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Page 62 of 67 S E P T E M B E R / O C T O B E R 2 0 1 8 Inside GNSS 63 the Dual-FE (D-FE2), with cable length of approximately 50 meters. Both FE are connected to a PC for IF sample record- ing. PC1 records the IF sample stream of the S-FE and PC2 records both IF sam- ple streams of the D-FE. Both FE record with a sampling rate of 200 MHz/s and 2 bits per sample (real valued IF sam- pling). e recorded signals of the D-FE are clock and time synchronized and can be used in post processing without additional clock synchronization effort. However, comparing the record of the S-FE with the record of the D-FE2 is only possible if the FE clocks are syn- chronized. e clock synchronization is described below. Also a time offset synchronization is needed. is is cur- rently done by tracking the GPS SIS and determining the PC time offsets to the GPS time. e transmitted UAV signal is with- out secondary code and therefore with- out any long range timing information. To overcome this lack of information, the Hardsync option in the MuSNAT is used for the pseudorange determination. In Hardsync mode, the code ambiguity is resolved under the assumption of a vanishing measured receiver clock error, a vanishing satellite clock error, and with geometric distance much smaller than the code period. This procedure can be used and is uncritical as we are only interested in the pseudorange dif- ferences and therefore all constant time offsets are canceled, as mentioned ear- lier. Front-end Clock Synchronization The clock synchronization between the GNSS receivers is another crucial element in this testbed because of the absence of an atomic clock in the UAV transmitter. e simplest means of syn- chronization is to use a coaxial cable in between the two receivers, but a clock synchronization between two build- ings (and in a later phase of the project between multiple buildings of the Uni- versity campus) clearly needs a long distance synchronization tool. We have chosen the so-called "White Rabbit Proj- ect" for this task. An additional advan- tage of such synchronization devices is, that they support clock synchronization in addition to time s y nc h ron i z at ion, which provides us a GNSS synchroniza- tion with all devices of our setup. W h i t e R a b b i t (WR) is a col lab- orat ive project of CERN, GSI Helm- ho lt z C e nt re for Heavy Ion Research, and other partners from universities and industry. e hardware design as well as the source code are publicly avail- able (Additional Resources). Our ver- sion is a COTS product of the Spanish company Seven Solutions S.L. with the name WR-LEN. WR-LEN provides sub- nanosecond accuracy via fiber connec- tions over 80 kilometers of length. us this approach will allow us to later use multiple ground stations distributed even kilometers away from each other. In our setup (see Figure 3) we used three WR-LENs. ey were connected in a daisy chain (master, slave No. 1, and slave No. 2), in which the master was driven by a GNSS receiver with the PPS and 10 megahertz clock signal. e front-ends were connected with slave No. 1 and No. 2, respectively. One of the front-ends has two phase-aligned inputs, which gives us the possibility to compare the test results with the WR-synchroni- zation, i.e., a "perfect" synchronization. The longest fiber connection we used was 250 meters long with most of the coils still on the cable reel. e accuracy of the WR-LENs were measured in our laboratory. erefore, the virtual bench with a customized application program was used to mea- sure the time difference between the clocks of the WR-LENs. Figure 4 shows the histogram of two WR-LENs over a measurement time of approximately 36 minutes. e standard deviation of the clock jitter was 9.4 picoseconds. During the flight tests, we observed deviation between 14 and 16.6 picoseconds. is can perhaps be explained by stronger temperature conditions and/or a higher uncertainty of the measurement setup, but the results are still in the expected range. The brochure of the WR-LEN of Seven Solutions is showing similar results with the same deviation and a maximum time interval error (MTIE) of ±45 picoseconds. The WR concept demonstrated even better results with deviations of five to six picoseconds, but also, that there is a temperature effect on these systems of approximately four picoseconds per one degree Celsius. (See Additional Resources for both of these systems.) Positioning and Ranging Verification For positioning and ranging verifica- tion, a multistation is used (again, see Additional Resources). The multista- tion operates with an electronic distance measurement (EDM) unit to compute the slope distance from the device to a reflector (prism). e distance is calcu- lated by comparing the electromagnetic wave transmitted from the instrument to the one that is reflected back to the instrument. For the position accuracy specification, the distance to a fixed target was measured for five minutes. e distance between the multistation and the target was 20.127 meters. In the five minutes, a standard deviation of the measured distance of 93 μm could be observed. A distance bias was not deter- mined, as constant distance offsets are canceled during the delta range process- ing. e multistation is able to acquire a target (reflector) up to 450 meters away and track it in locking mode up to 250 meters while the target is moving. is characteristic is very important as we want to track the UAV when it is f ly- ing. e maximum speed that the lock mode supports is 14 m/s. As mentioned before, the idea of using the multista- tion is to measure accurately the posi- tion of the three antennas with respect FIGURE 4 Normalized offset between two WR-LENs Normalized offset [psec] Pr obability densit y 0 –10 –20 –30 –40 –50 10 20 30 40 50 σ = 9.4 psec 0.12 0.1 0.08 0.06 0.04 0.02 0

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