Inside Unmanned Systems

OCT-NOV 2018

Inside Unmanned Systems provides actionable business intelligence to decision-makers and influencers operating within the global UAS community. Features include analysis of key technologies, policy/regulatory developments and new product design.

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34  October/November 2018 unmanned systems inside jamming performance really in the LNA. Is that going to stop compressing sooner than you would expect? And how does it handle multipath?" Additionally, the receiver tracking quality is absolutely critical whether you're using single frequency or dual fre- quency. Other things to consider, Riley added, are using the pseudorange from RTK directly or indirectly, the car- rier tracking that is the fundamental measurement for RTK, and knowing how good your position engine is and being aware the receiver may have some biases. Mapping and GIS type accuracies start the process off—either autonomous or just receiving signals from the GNSS satellites using the pseudorange or the code phase, and that provides something in between the meter to five meter level performance depending on the quality of the GNSS receiver and the state of the atmosphere, Riley ex- plained. This can be improved some by using either a local differential or SBAS such as WAAS or EGNOS (this is a system where corrections are provided over a satellite link to try and correct some of these effects). "This can deliver sub meter accuracy, but for many ap- plications you want to go to even more accurate and that's where RTK comes in," said Riley, whose presentation illus- trated how to use the pseudorange to effectively position with the accuracy of the measurement you're getting from the carrier phase. "The problem with L1 RTK is because this 19 centimeter (wavelength) and because the pseudorange can be on the order of a meter or more error you have a lot of search can- didates," he said. "So it's very easy to get the wrong integer or you have to search and wait for a long time to get the correct integer with L2." Using dual frequency solves this problem, so then you have a lot less search candidates. It's much easier to actu- ally identify what the real candidate is, it's more reliable and you get it quicker, according to Riley. DUAL CHANNEL RTK BENEFITS The webinar featured a slide showcasing critical consid- erations/post-process versus real-time; base setup/lo- cation: L1 only RTK requires relatively short baselines; dual band can handle longer baselines (10km+); PPP—no customer base required; corrections over internet or MSS satellite link; near RTK performance (e.g. Trimble's RTX 2cm 2D); antenna placement-masking during banking; EMI Code versus carrier performance; and radio link. This summer Trimble announced that its Trimble RTX GNSS corrections technology can now achieve horizontal accuracies of better than two centimeters. With precise point positioning (PPP) services, the cus- tomer doesn't need a base station. For example they can get RTK-type accuracies using a correction stream that the service provider (for example Trimble's RTX service) delivers over the internet or over a satellite link. Riley adds that a lot of systems today are using L1, L2 and L5 so Trimble is actually using triple bands in a lot of the newer products. He showed off the company's pro- gression of offerings, starting with a single antenna GNSS with only dual band, and explaining INS systems, anten- nas and IMUs that allow you to do attitude, capped off by dual band, dual antenna systems that let you get near instantaneous heading of the platform and can be coupled with an IMU. The small systems would fit on to a small to moderate size UAS. Integration tools and information on setting up anten- nas also are key to the success of performance, as well as mitigation of interference, some of which is due to poor shielding of the electronics on the UAS. Loewen's MicroPilot, which since 2004 has been the world's leading manufacturer of professional autopilots for UAS, f lies a wide range of drones from fixed-wing multi- rotor helicopters to transitioning vehicles. Loewen talked about some of the advantages of RTK from the point of view of the f light control system integrated into a typical UAS to how the advantages of RTK will help improve the performance of drones. All RTK systems need some source of corrections, Loewen said, and one way you can get those corrections is to have a base unit next to your ground control station. That base unit knows where it is, so it can figure out the error for all the different satellites. It's listening to the same satellites as your GNSS system inside your UAS. It sends those connections to the system in the UAS usually over the same datalink you might use for your commanding control. Now one disadvantage of this is the RTK base needs to know exactly where it is located so it can calculate the corrections, he explained. So you must have that at a surveyed location, but another possibility is you can connect to an RTK correction service and that RTK correction service will, via the internet, receive these WEBINAR RECAP » DUAL CHANNEL RTK

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