GPS World, April 2016

INNOVATION INSIGHTS BY RICHARD B LANGLEY WHATS THE WEAKEST THING ABOUT GNSS Literally its the signals The strength of GNSS signals is notoriously low as anyone who has tried to operate a consumer level device inside a steel and concrete building can readily attest Unlike mobile phone signals GNSS signals are too weak to survive the attenuation of walls floors and ceilings and so typically cannot provide a dependable signal indoors for most receivers Even outdoors the signals can be significantly attenuated by dense wet foliage and completely blocked by buildings and other objects The GPS C A code signal generated by the transmitter in a satellite is approximately 27 watts If such a transmitter were operated on Earth it would provide a decent signal even inside a nearby building First responders for example can communicate with each other using portable transceivers with even lower powered transmitters However GPS satellites are about 20000 kilometers away at their closest and the signals they transmit spread out as they travel to the Earth and even with the directivity of the satellite transmitting antenna by the time the signals reach the surface of the Earth their power density is only on the order of 10 13 watts per square meter And thats outdoors This signal is so weak that it is buried in the receivers background noise which is similar to what you hear when you tune an AM radio between stations So how can GPS possibly work with such a weak signal The received signal is actually spread out over several megahertz of radio frequency spectrum by the pseudorandom noise ranging code It is this known noise like code that allows receivers to determine the biased ranges to satellites and from those ranges determine their positions Knowing the code the receiver de spreads the weak received signal concentrating it and lifting it above an acceptably low background noise All is fine and well as long as the received signal density doesnt drop much below the 10 13 watts per square meter level but also the background noise level mustnt rise much above the acceptable level for which the receiver is designed Both of these criteria are reflected in the carrier to noise density ratio or C N 0 of the signal Why might the noise level change The noise comes from the receiver itself as well as from naturally produced electromagnetic radiation from the sky the ground and objects in the receiving antennas vicinity The sky noise includes so called cosmic noise from the sun Milky Way galaxy other discrete cosmic objects and radiation left over from the Big Bang as well as radiation from our atmosphere For the most part the noise from these sources is small but occasionally the sun can have a radio outburst that can significantly increase the noise level at GNSS frequencies and actually overpower the GNSS signals as happened with GPS in December 2006 But the noise level can also be impacted by human made electrical devices in the vicinity of a GNSS receivers antenna This radio frequency interference or RFI can come from devices such as radio transmitters microwave ovens motors relays ignition systems switching power supplies and light dimmers So when siting the antenna of a GNSS receiver or designing a GNSS based navigation system electromagnetic compatibility is an important concern This is particularly true for airborne platforms In this months column we take a look at how RFI can impact GNSS equipment on unmanned aircraft systems and how robustly can the equipment navigate those systems APRIL 2016 WWW GPSWORLD COM GPS WORLD 43 FIGURE 1 Setup to test for GPS RFI FIGURE 2 Inside the UAS including the GPS antenna satellite is providing incorrect or subpar data This requires one additional satellite bringing the minimum number of satellites that have to be in view of the receivers antenna up to six two more than non RAIM GNSS operation However using RAIM requires additional computational power which one might not be able to provide on board a UAS due to size weight and power limitations It has been suggested that a GNSS system coupled with an inertial navigation system INS could be used for UAS navigation A micro electromechanical system MEMS INS would be very small would not require a lot of power and could improve the performance of a UAS navigation system A GNSS plus MEMS INS approach may well be able to provide the robustness needed for UAS However the analysis of such a system is outside the scope of this article Some basic considerations should be taken into account for a UAS GNSS positioning system Integrity should be prioritized over accuracy if the system is used for navigational purposes Low altitude operations could bring on problems of sky blockage The proposed solution to this is to use a receiver capable of using multiple constellations to ensure that as many satellites as possible are in view RADIO FREQUENCY INTERFERENCE Radio frequency interference or RFI is the interference caused by electromagnetic waves interacting with a system they were not intended to interact with A fa