GPS World, September 2016

How Four Frequencies Help When the Ionosphere Is Disturbed 50 GPS WORLD WWW GPSWORLD COM SEPTEMBER 2016 WITH RICHARD B LANGLEY GALILEO CYCLE SLIP DETECTION By Laura Van de Vyvere and René Warnant T he availability of data offered in the Galileo GNSS Open Service on four carrier frequencies opens the way to new multi frequency solutions for civil users In the research reported in this article we focused on one of the consequences of signal tracking loss the appearance of cycle slips and how the use of the four frequencies can help in their detection Cycle slip detection is a key issue for high precision positioning applications Any users in need of determining a precise and reliable position must be aware of the potential presence of cycle slips in their data since they compromise data quality Traditionally two carrier frequencies were used for positioning for instance the GPS L1 and L2 frequencies More recently three carrier positioning has allowed enhanced precision and accuracy Though using a third carrier frequency has allowed us to partially solve the cycle slip detection issue existing procedures are still lacking in some aspects One of todays main challenges is cycle slip detection under high ionospheric activity which is why we focused on this specific case study And since the use of three frequencies helps to improve reliable cycle slip detection might not the use of an additional fourth frequency further improve detection capability Since Galileo supplies four frequencies in its Open Service we thought we might be able to improve cycle slip detection algorithm performance once more Framework In this article a new quad frequency cycleslip detection algorithm is introduced seemingly an unexplored track in the literature until now The algorithm uses undifferenced carrier phase observations from a singlestation static receiver First developed for post processing the algorithm also has been adapted to real time applications This algorithm aims to improve cycle slip detection under high ionospheric activity CYCLE SLIPS Though code pseudorange measurements are commonly used for standard positioning any precise positioning application needs to use carrier phase measurements due to their better quality Unfortunately the latter are potentially subject to cycle slips generating a constant bias in data and if undetected and uncorrected impacting the inferred positioning Carrier phase measurements are made by observing the beat phase that is the difference between the received carrier from the satellite and a receiver generated replica At the first observation epoch only the fractional part of this beat phase can be measured but the integer offset between the satellite signal and the receivers replica is unknown This integer number of cycles is called the initial phase ambiguity and remains constant during the observation period The carrier phase observable between a satellite i and a receiver p in meters is given by the following equation 1 where the subscript f k indicates the term dependency on the frequency and Φ on the carrier phase observable G is the geometric term that is a function of the geometric range between the receiver and the tracked satellite the tropospheric delay and satellite and receiver clock bias I is the ionospheric delay M is the multipath error HW stands for satellite and receiver hardware delays c is the vacuum speed of light N is the initial phase ambiguity and ε is the random error also called phase noise At the first observation epoch an integer counter is initialized and as the tracking goes on it is incremented by one cycle whenever the beat phase changes from 2π to 0 If the receiver even briefly loses track on the signal the counting is suspended and an integer number of cycles is lost This loss can result from various causes signal obstruction rapid change in the carrier phase observable and so on In the observation equation the cycle slip will appear as a change in the value of the initial phase ambiguity Thus a onecycle slip will involve a phase measurement shift of about 20 centimeters equal to the carrier wavelength depending on the affected carrier frequency The cycle slip size can be any value from one to thousands of cycles Ionospheric delay is the only term that could possibly be confused with a small cycle slip Indeed during an ionospheric perturbation event this delay variation between two observation epochs spaced at 30 second intervals say often reaches 20 centimeters the size of a one cycle slip in the phase measurement or more The ionosphere activity has two main consequences Firstly as mentioned before slips can be hidden in observation noise including ionospheric variability and not detected