Simulation Pipelines

Simulation Pipelines

This page shows the high-level flow of a FERS run. The diagrams are meant to help users understand what happens to a scenario, why different radar modes produce different output files, and where common settings take effect.

Whole Run

flowchart TD
    A[Scenario XML] --> B[Validate XML structure]
    B --> C[Load waveforms, antennas, timing, platforms, and targets]
    C --> D[Build simulation world]
    D --> E[Create transmit and receive events]
    E --> F[Run event-driven simulation]
    F --> G[Finalize receiver signals]
    G --> H{Runtime output backend}
    H --> J[Write HDF5 result files]
    H --> K[Emit VITA 49.2 UDP streams]
    D --> I[Optional KML export]

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Key user settings:

• XML structure comes from XML Schema Reference.
• <rate>, <oversample>, schedules, platform motion, and radar modes affect event generation and signal rendering.
• Receiver names determine HDF5 output file names.
• Runtime CLI/API configuration selects HDF5 or VITA 49.2 UDP output; .fersxml does not select network transport.

UI Scenario Workflow

flowchart TD
    A[Scenario Builder] --> B[Edit global parameters, assets, platforms, and components]
    B --> C[3D viewport previews geometry, pointing, paths, patterns, and RF links]
    C --> D[Timeline previews motion over simulation time]
    B --> E[Export .fersxml]
    B --> F[Simulation Run view]
    F --> G[Run simulation]
    F --> H[Generate KML]
    G --> I[Write HDF5 files]
    G --> J[Show output metadata]
    J --> K[Optional metadata JSON export]

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The UI preview helps catch setup mistakes before a full run, but the generated HDF5 files are the simulation output. Use external analysis tools for signal processing and plots.

Scenario Loading

flowchart TD
    A[Main .fersxml file] --> B[Expand include files]
    B --> C[Validate DTD and XSD]
    C --> D[Read global parameters]
    D --> E[Read waveforms]
    E --> F[Read timing sources]
    F --> G[Read antennas]
    G --> H[Read platforms and motion]
    H --> I[Attach transmitters, receivers, monostatics, and targets]
    I --> J[Resolve name references]
    J --> K[Ready to run]

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What can fail here:

• XML elements are in the wrong order.
• A required parameter is missing.
• A referenced waveform, timing source, or antenna name does not exist.
• A waveform file, antenna file, or RCS file cannot be opened.
• A radar mode does not match its waveform type.

Event-Driven Simulation

FERS does not simply render every object independently from start to finish. It builds a time-ordered set of events and processes them over the simulation interval.

flowchart TD
    A[Start simulation time] --> B[Schedule initial transmitter and receiver events]
    B --> C{Next event time}
    C --> D[Advance platform positions and rotations]
    D --> E[Process continuous streaming signals up to event time]
    E --> F[Handle event]
    F --> G{More events before end time?}
    G -- yes --> C
    G -- no --> H[Flush remaining streaming and IF data]
    H --> I[Finish output files]

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Events include:

• Pulsed transmitter pulse starts.
• Pulsed receiver window starts and ends.
• CW/FMCW/SFCW transmitter streaming starts and ends.
• CW/FMCW/SFCW receiver streaming starts and ends.

Schedules control when these events are created.

SFCW Streaming Pipeline

Stepped-frequency continuous-wave radar uses the same streaming event path as CW and raw FMCW, but the active RF frequency changes at each configured step.

flowchart TD
    A[Stepped-frequency waveform] --> B[Active schedule period starts]
    B --> C[Create one SFCW streaming source]
    C --> D[For each receiver sample and path, compute retarded transmit time]
    D --> E[Select active RF step from dwell/step timing]
    E --> F[Use step RF for phase, wavelength, antenna gain, and path power]
    F --> G[Write I_data and Q_data]
    G --> H[Store SFCW metadata and emitted step counts]

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Important SFCW settings:

Setting	Effect
<step_size>	RF spacing between consecutive dwells. Its magnitude controls unambiguous range.
<step_count>	Number of frequency steps in one sweep. Together with abs(step_size), it controls effective bandwidth.
<dwell_time>	Active transmit time inside each step period.
<step_period>	Time from one step start to the next. Larger values introduce silent gaps.
<sweep_count>	Optional finite number of sweeps per active schedule period.

An SFCW receiver remains active throughout its scheduled streaming interval, including transmitter gaps between dwells. Each path is evaluated at retarded transmit time receive_time - propagation_delay. A sample taken during a transmitter gap can therefore contain a delayed echo emitted during an earlier dwell. FERS does not automatically blank the receiver from SFCW transmitter dwell timing; apply intentional receive gating during analysis or with an explicit receiver schedule.

For hardware whose transmitter and receiver share a retuned oscillator, post-processing time gates approximate synchronized blanking. FERS does not model a frequency-selective receiver front end that rejects an earlier dwell's delayed RF after retuning.

Pulsed Radar Pipeline

Pulsed radar is organized around transmitted pulses and receiver windows.

flowchart TD
    A[Pulsed waveform file] --> B[Transmitter emits pulse at PRF]
    B --> C[Direct path calculation if enabled]
    B --> D[Target reflection calculation]
    D --> E[Range delay, Doppler, antenna gain, RCS, and propagation loss]
    C --> F[Receiver inbox]
    E --> F
    F --> G[Receiver window opens]
    G --> H[Render I/Q samples for window]
    H --> I[Add timing effects and receiver noise]
    I --> J[Normalize and optional ADC quantization]
    J --> K[Write chunk_N_I and chunk_N_Q datasets]

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Important pulsed settings:

Setting	Effect
Transmitter <prf>	Controls how often pulses are emitted.
Receiver <prf>	Controls how often receive windows are opened.
<window_skip>	Delays the start of each receive window.
<window_length>	Controls the duration and sample count of each receive window.
<simSamplingRate>	Controls geometry interpolation used in pulsed response generation.
<adc_bits>	Enables final quantization.

Pulsed output is written as one I/Q dataset pair per receiver window.

CW Streaming Pipeline

CW output is streamed over the receiver's active intervals. <cw_from_file> supplies a finite complex envelope sampled at <rate>; each active transmitter segment starts at the first HDF5 sample and stops at the file end without wrapping. Generated <cw/> remains a continuous unit-envelope tone.

flowchart TD
    A[CW HDF5 waveform or generated tone] --> B[Active transmitter interval]
    B --> C[For each receiver sample]
    C --> D[Evaluate file envelope or generated tone at retarded transmit time]
    D --> E[Update platform geometry]
    E --> F[Calculate direct path if enabled]
    E --> G[Calculate target reflections]
    F --> H[Sum received complex sample]
    G --> H
    H --> I[Apply timing effects and noise]
    I --> J[Normalize and optional ADC quantization]
    J --> K[Write I_data and Q_data]

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Important CW settings:

Setting	Effect
<rate>	Output sample rate.
<cw_from_file>	Finite HDF5 complex envelope; no implicit repetition.
<cw/>	Generated continuous tone.
Schedules	Active transmit and receive intervals.
Antenna patterns and platform rotation	Directional gain over time.
nodirect	Removes direct transmitter-to-receiver path.
nopropagationloss	Removes path-loss scaling for debugging.

FMCW Pipeline Without Built-In Dechirp

Use dechirp_mode="none" when you want FERS to write the received FMCW signal and do the dechirp in your own analysis script.

flowchart TD
    A[FMCW HDF5 waveform or generated chirp] --> B[Active transmitter interval]
    B --> C[Generate received raw baseband samples]
    C --> D[Apply delays, Doppler, gain, RCS, and path loss]
    D --> E[Sum direct and reflected paths]
    E --> F[Add timing effects and receiver noise]
    F --> G[Normalize and optional ADC quantization]
    G --> H[Write raw I_data and Q_data]
    H --> I[External analysis performs dechirp]

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Use this mode when:

• You want full control over dechirping.
• You are testing a custom processing chain.
• You want to compare FERS output with another FMCW processor.

For <fmcw_from_file>, raw output preserves the supplied finite complex envelope after propagation. FERS does not infer an analytic chirp bandwidth, direction, period, or repetition from the samples.

FMCW Pipeline With Built-In Dechirp

Use dechirp_mode="physical" or dechirp_mode="ideal" when you want FERS to write IF output directly.

flowchart TD
    A[FMCW HDF5 waveform or generated chirp] --> B[Received raw baseband sample]
    C[Matching complex dechirp reference] --> D[Mix received signal with reference]
    B --> D
    D --> E[Low-pass IF filtering when if_sample_rate is configured]
    E --> F[Resample to receiver-local if_sample_rate when configured]
    F --> G[Add applicable receiver effects]
    G --> H[Normalize and write I_data and Q_data]

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If if_sample_rate is omitted, dechirped FMCW is written as legacy full-rate IF output at <rate> * <oversample>.

For <fmcw_from_file>, the dechirp reference uses the same finite complex samples, including their amplitude and phase modulation. Reference playback ends at the file boundary and does not synthesize chirp repetition.

Reference choices:

Reference	Use when
attached	Monostatic radar dechirps using its own waveform.
transmitter	Bistatic receiver dechirps against a named transmitter.
custom	Receiver dechirps against a named waveform.

Dechirp modes:

Mode	Meaning
physical	Includes timing-source effects in the reference relationship.
ideal	Uses an idealized reference for cleaner beat-signal analysis.

Output Finalization

Finalization is where FERS turns simulated receiver voltage samples into the selected runtime output backend. HDF5 remains the default. VITA 49.2 UDP uses the same processed receiver samples but scales them against a fixed full-scale value for int16 IQ packets.

flowchart TD
    A[Rendered receiver samples] --> B[Add receiver thermal noise]
    B --> C[Apply timing phase effects]
    C --> D[Find fullscale value]
    D --> E{adc_bits > 0?}
    E -- yes --> F[Quantize samples]
    E -- no --> G[Normalize floating-point samples]
    F --> H{Output mode}
    G --> H
    H --> I[Write HDF5 datasets and attributes]
    H --> J[Emit VITA Signal Data and Context packets]

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Important output behavior:

• Stored samples are normalized.
• Use the HDF5 fullscale attribute to reconstruct physical I/Q values.
• Pulsed outputs write chunk datasets.
• CW and FMCW outputs write I_data and Q_data.
• Metadata attributes describe the receiver mode and sampling settings.
• VITA output uses configured fixed full-scale; clipping is reported through VITA stream statistics and packet indicators.