GPS World, April 2016

can be used at any stage in GNSS applications The software is available at rtklib com Three receivers were compared the previously discussed high performance GPS single frequency receiver evaluation kit a low cost high performance GPS receiver with RTK functionality and a professional grade multi GNSS multi frequency RTK survey receiver As the low cost receiver is marketed for UAS use it was of interest to see how the receiver compared to the others in a dynamic case The evaluation kit receiver was of interest due to similar receivers often being used on UAS today The professional grade receiver was of interest since it is a highend receiver capable of receiving multiple constellations and frequencies The experiment was performed to simulate some of the conditions that might be experienced on UAS The most approximate test vehicle that was available at the time was a car The receivers were set up to capture GPS signals only The low cost and evaluation kit receivers are only capable of receiving the L1 signal and were set up accordingly The professional grade receiver was set up to capture the L1 L2 and L5 signals A truth reference for the test vehicle was needed for comparison and for this we used a multifrequency receiver with an inertial measurement unit IMU The benefit of the IMU is that it contains gyros and accelerometers that can capture very precise movements at times when GNSS signals might not be available during periods of sky blockage for example However due to the gyros drifting the IMU needs to be updated with GNSS data every few minutes to give an accurate solution The receiver was configured to capture GPS L1 L2 L5 GLONASS L1 L2 and WAAS The GNSS data was then post processed in precise point positioning PPP mode with data from several nearby stations The GNSS PPP data was then smoothed and combined with the IMU data to form a GNSS PPP plus IMU solution It was assumed that the GNSS receiver and IMU gave a correct solution at all times A diagram of the setup can be seen in FIGURE 7 The car with the equipment was driven around the town 46 GPS WORLD WWW GPSWORLD COM APRIL 2016 and campus at the University of Colorado in Boulder The path included a parking lot a wide open area parts of a highway an open area major roads open area with parts covered by trees residential areas with many trees covering the sky and a parking garage with complete sky blockage The parking garage was entered towards the end of the experiment The receiver data was post processed using an RTKLIB setup to process the data as if it was received in real time A multi frequency multi GNSS receiver was set up with a roofmounted antenna at the University of Colorado to collect data for the duration of the experiment and this data was later used as base station data for the RTK calculations The low cost receiver had a hard time regaining a position solution while the evaluation kit receiver did slightly better The professional grade receiver only lost a clear position for about 10 seconds This behavior agrees with expectations the low cost receiver is new and is being updated regularly with new software and the evaluation kit receiver is known for being able to perform well under poor conditions The professional grade receiver has the support of additional GPS signals which could explain why it was the first to regain a good position solution TABLE 2 shows some of the values calculated from the experiment which further confirms that the evaluationkit receiver is able to calculate a position more often than the professional grade receiver but a more inaccurate position In the table availability is defined as how many data points the receiver was able to capture divided by how many would have been captured if the receiver could capture data at all times RTK solution is how often the captured data was sufficient to calculate an RTK solution Fix solution is defined as how often the ambiguities could be resolved out of the available RTK data points and float solution is how often the ambiguities could not be resolved out the available RTK data points The comparison of the results using SPS versus the RTK technique for the evaluation kit receiver is interesting Using RTK increases FIGURE 5 RF spectrum when the antenna is inside the airframe UAS switched off See text concerning y axis scale FIGURE 6 RF spectrum when the antenna is inside the airframe UAS switched on all systems running See text concerning y axis scale