Inside GNSS Media & Research

NOV-DEC 2017

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Split SDR Files Split SDR Files File 1 File 1 File 0 File 0 52 Inside GNSS N O V E M B E R / D E C E M B E R 2 0 1 7 supports all current and future SDR file formats. is also means that the WG must "get it right the first time" since major revisions to the standard would be undesirable and adverse to the goal of w idespread adoption. Hence, we considered the entire space of possible GNSS SDR data collection topologies. Figure 3 illustrates these topologies. Figure 3.a illustrates the simplest data collection topology that can exist. is is when a single swath of RF spectrum (referenced henceforth as a "band") is down-converted and sampled to pro- duce a single data stream. e stream, which may be IF sampled (real samples) or baseband sampled (complex samples) is written to disk as a single file. Figure 3b represents a DCS that writes parts of a single data stream across multiple files. is may be done to reduce the sustained write performance requirement of storage drives (similar to striping mode in RAID systems). Figure 3.c is similar to Figure 3.a, except that the data stream represents more than one RF band. An example of this topology is a direct RF sampling front-end architecture that intentionally aliases multiple bands to fall next to each other at baseband. Some bands may be spectrally inverted as a result of the digi- tal down-conversion process. A DCS may produce multiple data strea ms. Each strea m may contain information from one of many antenna elements (as shown in Figure 3.d, where each stream may also encompass multi- ple bands as in Figure 3.c). Alternatively, a stream may be sampling one of sev- eral bands received by a single wideband antenna, where the multiplexed streams written to file represents channelized samples. Combinations of the above are also possible. Each stream may also be sampled at different rates and bit depths. For example, consider a civilian GPS L1, L2, L5 system. In this case, the L1 and L2 streams may be sampled at rate f 0 and the L5 stream at 10·f 0 (since the L5 sig- nal's null-to-null bandwidth is 10 times wider than L1 C/A and L2C), where f 0 represents the base sample rate. Figure 3.d may also represent how these multi- ple streams are multiplexed into a single lane of packed binary data and written to a single file. Similar to Figure 3.d, Figure 3.e illus- trates a GNSS data stream multiplexed with other data that is written to a single file. is non-GNSS SDR data may be from additional sensors (as shown), and may be written in a proprietary format that may or may not be known. e specification of metadata param- eters for non-GNSS data is outside the scope of the present standardization effort. However, the standard must sup- port adequate information to skip over such non-GNSS data bytes. Since the metadata schema is extensible, it can cover the description of non-GNSS SDR data as needed by specific users. It is important to note that although we use the term "GNSS SDR data" in this article to generally refer to the type of sampled data for which metadata parameters are defined in the standard, the samples need not correspond to GNSS frequency bands. For example, frequency bands containing RF signals of opportunity are supported as long as they can be represented by the stan- dard's metadata parameters. Due to the typically high data rate of GNSS SDRs, some DCSs write data as temporally split files, as illustrated in Figure 3.f. is allows for efficient data management through multiple smaller files compared to a single large file. e metadata for each file must associate the previous and next files in order to represent the sequence. Note that the DCS block shown in Figure 3.f could represent any of those described in STANDARDS FIGURE 2 Standardized Metadata Exchange Collection System (A) GNSS SDR Processor (A) Collection System (B) GNSS SDR Processor (X) GNSS SDR Processor (Y) Collection System (C) GNSS SDR Processor (C) SDR File Metadata Format (A) SDR File Metadata Format (B) SDR File Metadata Format (C) Collection Systems Data/Metadata SDR Processors Sensor FIGURE 3 Fundamental GNSS SDR Data Collection Topologies Data Collection System (DCS) Single SDR File (a) single band, single stream, single file DCS Split SDR Files (f) temporal splitting of files t DCS 0 File 0 (g) spatial splitting of files DCS 1 File 1 Δt DCS 0 File 0 (h) spatial‐temporal splitting DCS 1 File 1 Δt t Single SDR File (c) multi‐band, single stream, single file Bands at RF f DCS Multiplexed SDR File Multi‐stream DCS (d) multi‐stream, single file Multiplexed SDR File Multi‐sensor DCS (e) multi‐stream, single file Sensor DCS (b) single band, single stream, multiple files File 0 File 1

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