The importance of repeatability in GNSS receiver testing
In any testing regime at any stage in the lifecycle of a GNSS receiver repeatability of tests is essential. Regardless of whether you are designing a receiver from scratch, comparing different receiver designs, developing new applications, integrating receivers into end equipment or production testing finished kit, if you are not able to run exactly the same test (or test sequence) time and time again then the results of the tests will not be conclusive.
In development work, for example, the ability to repeat a test sequence after improving a design provides evidence as to whether the rework has been successful. When comparing different receivers, it goes without saying that the comparisons are meaningless if the devices are not tested on a “level playing field”. And in production testing it is essential that every single unit tested is assessed against exactly the same set of key performance indicators.
This requirement for repeatability is sufficient in itself to rule out any form of “live-sky” testing using real-world GNSS signals. For while, on the one hand, it is easy enough to ensure that the test rig is located in exactly the same position for each test, the relative positions of the satellites used for the tests will never be repeated in the real world. In this respect, live-sky testing can never be repeatable.
One alternative is to capture live satellite signals using a record and playback system (RPS), such as the Spirent GSS6400. This approach allows an actual environment to be captured and replayed at will, complete with real-life propagation, fades and interference. And, because the original test scenario has been recorded on the GSS6400, each time it is played back the conditions of the original recording will be repeated exactly.
However, for the maximum in repeatability (and control), using an RF constellation simulator equipped with suitable software offers the user the ability to create both simple and complex test scenarios and to replay them at will.
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