Recording and Replay of GNSS RF Signals for Multiple Constellations and Frequency Bands

S. Hickling, T. Haddrell

Abstract:	The need for record and playback of real life GNSS signals is increasing as new constellations come on stream and GNSS receivers are expected to operate in ever more challenging environments. This paper will discuss the design and verification of a new class of portable wideband record and playback system and examine how the design choices are informed by the relative merits and limitations of both simulator and record/replay approaches. Further, the paper will discuss the benefits to the development and characterisation of the different test approaches for various GNSS receiver types. Traditionally, simulators have been used for generating repeatable GNSS signals, and have become a sophisticated means for the development and performance testing of advanced receivers. There are some situations, however, that are difficult to model, either because the signal dynamics are too variable, or the user environment is not predictable or stable in signal terms. Many purchasers of GNSS technology now include “real life” trials in their acceptance testing, and so the ability to record the signal environment of a specified test route or environment is useful in repeatable pre-testing of the scenario in a laboratory situation. Once a scenario is captured, it may be replayed at will, and is portable amongst compatible devices. Users can build up a library of such files, enabling engineers to check out receiver improvements in a variety of locations without ever leaving the bench. Recording and replaying RF signals introduces some losses of its own, and of course the signals can only be replayed exactly as received, only amplitude control being available as a variable parameter to the operator. This is not, however, a limitation of simulators, which can individually manipulate satellite signals and artificially introduce errors in the received signals, all of which are under the operator’s full control. And so there is an interesting discussion as to which form of device any particular development strategy should need. Another use of raw signal recording is to look for events that disrupt the “live” received signal. In this mode, recording takes place continuously purely for providing a data set for event analysis. Examples of this type of application include atmospheric scintillation, other space weather effects, local interference and deliberate jamming. Replaying the recorded RF signals enable researchers to analyse the effects of the disruption after the event, and allow multiple replays of the event into different types of receiver. Relevant recordings can be shared with development engineers to enable improvements for later designs. The current state of the art provides for expensive instrument standard recorders, or more modest portable units enabling recording of single frequency bands (eg, just L1). The paper will describe the specification and design of a new class of the portable unit with ability to extend over three bands simultaneously. This extends the application of record/playback to military and survey type receivers, and offer flexibility in the use of the available channels and recoding bandwidth. The paper describes the work undertaken to secure the specification against customer needs, and the challenges that needed to be overcome in order to generate a design for the product and realise the final unit. We start with the basic architecture and portability requirements, and then look at the technologies involved in the implementation. Key to the operation is the frequency standard adopted, which directly drives the fidelity of the reconstructed GNSS signals. We also examine some of the other technical challenges, such as preventing the overlap of adjacent bands (and their associated noise contribution) when allocating close together signals for recording, and dealing with inter- constellation and inter-frequency offsets when replaying multiple signals form essentially separate data streams. The paper also describes the built in ability to record digital data, such as GNSS assistance or differential data, synchronously with the RF signals, for replaying into what looks like a “real time” data stream consistent with the recorded signals. This can also be used for telemetry, sensor data, video and event recording. We describe how this is attained with minimal disturbance to the recorded RF. After briefly reviewing the operation of the device from the user point of view, the paper looks next at the available assignments of frequencies, bands and sample rates for the recording channels available. The latter sections of the paper will describe the testing of prototype devices and the results from various combinations of RF recording (bands, bandwidths, constellations) and then the replaying of these files into various types of GNSS receiver. Recording and replaying RF signals can never be lossless, and we focus on the causes and mitigation of the various losses encountered in the downconversion, analogue to digital conversion, sampling rates, recording bandwidth limitations and then the reverse process of digital to analogue conversion and up-conversion to the original RF frequencies. We will present the early field results and discuss the capabilities and limitations of the device. Finally we review the concept and compromises made to see if further developments or improvements are possible, in line with the ever increasing demands from customers for improved receiver technology.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	1907 - 1918
Cite this article:	Hickling, S., Haddrell, T., "Recording and Replay of GNSS RF Signals for Multiple Constellations and Frequency Bands," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 1907-1918.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In