One other aspect of raw signal recording lies outside the receiver testing topic, but is of interest for GNSS signal monitoring. With a simulator, the user has control over the power of the received satellite signal, whereas in the recorder the entire signal-to-noise ratio observed at the point of reception has been recorded, and the user can only control the amplitude of the entire noise plus signal. Equally, it is not really possible with a simulator to recreate a particular physical environment made up of many reflected signals, jammers, manmade noise, and moving scenery. For instance, it is not possible with a record/replay device to manipulate individual satellite signals, nor to introduce specific errors in the radio signals. None of the characteristics of the record/replay device replace the functionality of the simulator in fact, both are valid tools for development and testing. In both cases the signal is created by generating radio frequency (RF) carriers and modulating them according to the GNSS signal formats.Ĭontrast of Two Approaches. In both cases, the starting point is digital data, on the one hand recorded in the field, on the other hand calculated by mathematical algorithms using the scenario specified in the simulator. Replaying the signals is in some ways similar to generating simulated signals. In fact, many manufacturers have dedicated field recording teams who send the files back to the engineers interested in the signal environments. This allows file distribution between equipments, and a split between making the recording in the field and replaying it in the laboratory. Once we have our wanted signals reduced to some form of digital representation, we can simply store and retrieve them at will, handling the recordings as simple, if somewhat large, data files. All we need is some hardware and a processor that can handle the data rates. Even the GPS L1-only low bandwidth C/A code requires at least 2 Mbytes per single second of recording, or more than 100 Mbytes per minute.įortunately, with digital storage technology advances, we can now make use of higher storage capacities (1 TByte of storage is readily available at reasonable cost) and also higher write/read bandwidths (100 MBytes per second is realistic). The basic problems are the amount of data to be stored in real time and the ability to recover it in real time. Early devices were of necessity restricted in the signals they could handle and store, limited both by budget and available technologies. Some receiver manufacturers developed their own capability to do this. In real terms this means a device must receive the radio signals from the GNSS satellites, reduce them to a form suitable for storage, and then recreate signals from the stored data in a manner that makes them look completely real to any receiver under test or development. To tackle this problem, GNSS signal record-and-replay capability is gaining acceptance as a practical tool for recording a signal environment at a single point in time and replaying at will. Furthermore, developers would like to see for themselves any such anomalies and try to understand and correct them, but it is not always desirable or practical (and certainly not economical) to put development engineers in locations scattered all over the globe. Therefore, proper comparisons can only be made by assessing all competing receivers in the same trial, and any performance anomalies seen cannot necessarily be tracked down by returning to the same location at some point in the future. These environments, however, are time-varying, and thus not repeatable in the true sense. This allows investigation of conditions difficult to simulate, such as multiple reflections and interferers. However, this approach is becoming more complex as the number of GNSS signals and their reception environments increase.Īnother way of testing receivers is through field trials. Simulators have traditionally demonstrated performance and repeatability in the laboratory environment, and this approach remains the only option for planned signals not yet broadcast from space. The authors also discuss the benefits of the different test approaches to the development and characterization of various GNSS receiver types.Īs new GNSS systems become available, and users take receivers to ever more challenging environments, the need for repetitive and repeatable testing during development grows ever stronger. The design and verification of a new class of portable wideband record-and-playback system considers the relative merits and limitations of both simulator and record/replay approaches.
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