NetSim Astra Q&A

Q1. What is NetSim Astra, and what problem does it solve?

NetSim Astra is a web-based tool for satellite RF coverage planning. A single configuration holds the constellation, the RF parameters, and the analysis settings. One run does all of these steps:

• Propagates the constellation with SGP4
• Computes an ITU-R link budget at every grid point and every time step
• Renders the coverage heatmap on a 3D globe
• Produces an access and gap report
• Writes per-sample CSV files

Astra computes C/N₀, C/N, G/T, E_b/N₀, BER and power flux density for every sample of every run, without additional configuration. Its scope is this workflow only. Astra does not do orbit design, attitude and sensor modelling, waveform generation, or packet-level protocol simulation.

Q2. We already use Ansys STK. What does Astra do differently?

STK is a mission-engineering environment. Orbit mechanics is the reference frame, and the analyst builds a communications study around it:

• Attach a Transmitter object to each transmitting asset and a Receiver object to each receiving asset.
• Set the grid with a CoverageDefinition.
• Point the Grid Constraint Options at the receiver.
• Define an Access Constraint Figure of Merit that references that constraint.

The analyst then exports the result report by report.

In Astra, the same study has three steps: select the constellation, set the RF parameters and the region of interest, and run. One pass produces the coverage heatmap, the access and gap report, and the per-sample CSV files. The files contain C/N₀, C/N, G/T, BER and PFD at every grid point. Both tools can produce communications-quality coverage. In STK, the analyst assembles it from objects, constraints and figures of merit. In Astra, it is the default output of one run.

There are two modelling differences:

• Clutter. STK documents an ITU-R propagation chain (P.618 rain, P.676 gases, P.840 clouds, scintillation), terrain propagation through TIREM, and an urban propagation model. It does not document a land-cover clutter model, and its published model list has no ITU-R P.2108 implementation. Astra classifies every grid point against ESA WorldCover. It applies ITU-R P.2108-1 to built-up land and ITU-R P.833-10 to tree cover.
• Regulatory screening. STK exposes flux density, PFD and spectral PFD as receiver constraints. It does not document an ITU Radio Regulations Article 21 elevation-dependent limit mask. Astra applies that limit mask and reports a PFD margin at every grid point.

Q3. We already use MATLAB and the Satellite Communications Toolbox. What does Astra do differently?

MATLAB is a programming environment. In the coverage workflow, the analyst writes code for each step:

• The satelliteScenario
• The satellites (walkerStar or a TLE file obtained outside MATLAB), with a transmitter on each satellite
• The region grid (shapefile, projection and point-in-polygon filter, with the Mapping Toolbox)
• A loop that creates a groundStation and receiver at every grid point
• The field-of-view and link loop, which calls sigstrength for each link, and the p618PropagationLosses call for each point
• The contour and geoplot code
• The post-processing of accessIntervals tables into coverage and gap statistics

MathWorks' Coverage Maps for Satellite Constellation example defines its helper functions inside the example script. The toolbox does not ship them. They include the link loop (satcoverage), the footprint geometry (satfov) and the contour gridding (contourDataGrid). A live TLE fetch by category, and the area coverage, revisit and response-time statistics, have no toolbox function and are also user code. If a team adopts the workflow, these functions become part of its codebase, and the team must maintain them across MATLAB releases.

The coverage map itself has no GUI path in MATLAB. The current release has no coverage app, coverage function or Live Editor task for it. The Satellite Link Budget Analyzer app analyses explicitly defined point-to-point links (an uplink, a downlink or a crosslink), not a coverage grid. The satellite scenario viewer displays a scenario that a script already built. In Astra, the three configuration tabs write one config.json, and the headless CLI reads that same file. The coverage map is a product feature, not user code.

Q4. Can we see the capabilities side by side?

The table below compares the three tools for one workflow: producing and validating an RF coverage map over a region. It does not assess the products outside that workflow. A mission-design or waveform-design task would need different rows. The table does not compare licensing and packaging. Each vendor states these, and they change between releases.

Q9. Which constellation sources does Astra support?

Astra supports four sources, and each one works with every analysis mode:

• Synthetic Walker. Walker Delta or Walker Star, with configurable planes, satellites per plane, altitude, and inclination. It supports up to 100 planes and 360 satellites per plane, at altitudes up to 36,000 km, which covers LEO through GEO.
• Live TLEs. There are three input routes. The first is a constellation group from the CelesTrak or N2YO categories (Starlink, OneWeb, GNSS and the GEO operators). The second is a pasted or uploaded TLE list of up to 500 records. The third is a list of up to 500 NORAD IDs or satellite names, which Astra matches against the active catalogue. Live runs poll at a configurable interval.
• Single fixed satellite. A stationary satellite at any position, for antenna-pattern testing and deterministic single-timestep runs.
• GEO and IGSO. Manual entry, INSAT and GSAT presets, or live TLEs. GEO adds the multibeam CINR analysis.

Q10. How are satellite positions propagated?

Astra propagates live satellites from their TLEs with SGP4. It then converts the positions into ground-relative geometry for elevation, slant range and off-axis angle. Astra computes positions and angles in Earth-Centered Earth-Fixed (ECEF), East-North-Up (ENU) and Local-Vertical-Local-Horizontal (LVLH) antenna frames. Synthetic scenarios step through a configurable simulation window. Live scenarios poll in real time. Each step gives a complete link budget at every ground point.

Q11. What exactly is in the link budget?

The chain runs from the satellite EIRP down to received power at the ground terminal:

• EIRP, receive gain, and antenna pattern gain evaluated at the true off-axis angle
• Free space path loss from the slant range
• ITU-R P.618 atmospheric losses: rain fade, gaseous absorption, cloud attenuation, and tropospheric scintillation, with link availability by exceedance conversion
• Clutter loss from ITU-R P.2108-1 or P.833-10, selected by the ESA WorldCover class at the point
• Polarization loss, additional losses, and a system margin

Each component is a separate column in the output CSV. When you click a heatmap point in the browser, Astra shows the same breakdown for that point.

Q12. Which antenna pattern models are available?

• Gaussian: a simplified analytical model
• Bessel (3GPP TR 38.811): circular aperture
• ITU-R S.672-4: the FSS reference pattern
• Phased array (URA): beam shaping for 5G NTN, to 64 × 64 elements
• Custom: CSV gain tables over off-boresight and azimuth angles, or Ansys HFSS Far-Field Data (FFD) files, with 2D interpolation for asymmetric beams and measured sidelobes

The View Pattern tool renders any of these patterns before a run. It shows the beam shape and the sidelobe structure for the analytical models and for imported patterns.

Q13. Can a satellite beam stay on a fixed ground target?

Yes. Set Antenna pointing to Earth-fixed (targeted), and enter the targets in a table, each as a label, a latitude and a longitude. At each time step, each satellite steers its beam to one target that it can reach:

• The target must be inside the maximum steering angle, which is 60° by default, and above the minimum elevation angle.
• If several targets qualify, Astra selects the target with the highest elevation or the highest C/N.
• If no target qualifies, the beam is idle.

Earth-fixed pointing works in point and coverage analyses. It does not apply to multibeam runs or to the phased-array antenna model. The coverage log adds columns for the serving target, the steering angle and the target elevation.

A steered beam covers a larger ground area at a lower received power than a nadir beam. Earth-fixed pointing does not add beams. It moves beams from other areas to the targets. The NetSim Astra page compares the two modes for 12 satellites over India.

Q14. How is clutter loss modelled?

Astra looks up the ESA WorldCover land-cover class at each ground point, and the class selects the clutter model. Built-up land uses the Earth-space and aeronautical statistical clutter loss model of ITU-R P.2108-1, Section 3.3. Tree cover and mangroves use the site general model for slant paths of ITU-R P.833-10, Section 2.2.2. All other classes get 0 dB.

Three inputs set the loss for each link: the carrier frequency, the elevation angle of the satellite, and a location percentile (default 50%). Low-elevation links get the largest loss, so the loss changes during a satellite pass. At 20 GHz and a 50% percentile, built-up land gives 14.7 dB at 10° elevation and 4.6 dB at 30°. Clutter is off by default. To use it, set Clutter Mode to ESA WorldCover in the RF Parameters tab, where you can also set the percentile.

P.2108-1 Section 3.3 is valid from 10 GHz to 100 GHz. From 0.5 GHz to 10 GHz, Astra extrapolates the same equation, so treat built-up values in that range as interim. Outside 0.5 GHz to 100 GHz, Astra applies 0 dB.

Q15. Beyond received power, which link-quality metrics does Astra produce?

• C/N₀ and C/N, in dB-Hz and dB, from the receiver system noise temperature and the channel bandwidth. They show whether the link closes against a required threshold.
• G/T, the receiver figure of merit in dB/K. If you give no system noise temperature, Astra uses 290 K.
• E_b/N₀, from the C/N and the modulation and coding scheme (MCS)
• MCS and peak data rate, from the C/N at each point and time step
• Doppler shift, from the radial velocity between satellite and ground station. A 550 km LEO satellite reaches about ±280 kHz at Ku-band and about ±37 kHz at L-band.

Q16. Is bit error rate computed, and for which modulations?

Yes, in the point, coverage and multibeam analyses. Astra computes BER from E_b/N₀ for BPSK, QPSK, OQPSK, 8PSK, 16QAM, 64QAM and 256QAM. It also gives a coded-BER estimate from the code rate, and the symbol rate. Astra evaluates BER at every grid point, so the coverage grid holds a BER value for each point, in addition to received power.

Q17. Does Astra calculate the MCS and the data rate?

Yes. Digital Link Analysis has an Auto mode and a Fixed mode of MCS selection. In Auto mode, Astra reads the C/N at each point and time step. It then selects the highest modulation and coding scheme (MCS) that the C/N supports, from Table 5.1.3.1-2 of 3GPP TS 38.214 V18.11.0 (MCS index table 2 for PDSCH). This table has MCS 0 to 27, with QPSK, 16QAM, 64QAM and 256QAM. In Fixed mode, you set the modulation and the code rate.

In the two modes, Astra gives the peak data rate. It calculates this rate from the spectral efficiency of the MCS, the bandwidth, and an overhead that you set. If the C/N is too low for MCS 0, the point is out of range and its data rate is 0. The peak data rate is a planning estimate for one carrier that uses the full bandwidth. It is not a user throughput, because Astra has no scheduler and no traffic model.

The files point_analysis.csv and coverage_grid.csv hold the results in the columns mcs_index and peak_data_rate_bps. In the coverage view, a click on a grid point shows its modulation, data rate and E_b/N₀.

Q18. What does Astra do about power flux density and regulatory limits?

It reports the incident power density at the ground and checks it against a limit mask:

• Free-space PFD (dBW/m²) from the effective EIRP and the slant range
• PFD after atmospheric attenuation, when you enable the ITU-R P.618 model
• Spectral PFD normalized to a selectable reference bandwidth
• Nineteen built-in limit masks from Article 21, Section V, Table 21-4 of the ITU Radio Regulations. They cover S-band at 2.5 GHz to Q/V-band at 42.5 GHz. Where the limits differ, there are separate masks for geostationary and non-geostationary systems. You can also set custom limits and reference bandwidths.
• Automatic mask selection from the downlink frequency and the orbit class. In the same band, Astra checks a GEO source and a Walker or live-TLE source against different masks.
• A PFD margin on every sample, positive when the point is below the selected limit

This is a screening tool for engineering use, and it reports margins against a limit mask you select. It is not a filing, and it is not a substitute for a coordination or regulatory submission.

Q19. Can Astra evaluate multibeam payloads and co-channel interference?

Yes, for GEO satellites that you enter manually. Astra evaluates multibeam payloads with frequency reuse from FR1 to FR4. It produces a CINR heatmap across the service area, which shows the co-channel interference between beams. Astra writes the results for each beam to multibeam_cinr.csv.

Q20. What is in the access report?

Every point analysis produces an access report. It uses the same per-satellite time series as the link budget:

• Coverage percentage, total accesses, maximum simultaneous satellites
• Access-duration and gap-duration statistics, Time Average Gap, and mean and maximum response time
• A per-satellite visibility Gantt chart and a simultaneous-satellites step chart, with system-level gaps highlighted across both
• Collapsible interval tables with CSV downloads: access_intervals.csv, one row per satellite pass, and gap_intervals.csv, one row per system gap

Q21. What comes out of a run?

On the globe, Astra shows heatmaps, footprint polygons, satellite markers and ground links. Synthetic modes have playback at 1x to 50x and timeline scrubbing. Live modes have recording controls.

On disk, CSV throughout: point_analysis.csv, coverage_grid.csv, satellite_footprint.csv, multibeam_cinr.csv, access_intervals.csv, and gap_intervals.csv. The rows include the link-quality fields (C/N₀, C/N, G/T, E_b/N₀, BER and PFD margin), so you do not need a separate report. pandas or Excel can read the files directly.

Q22. How reproducible is a study that used live TLEs?

The project saves the live TLEs, with their capture time and source. If you reload the project with the saved TLEs, you get the original satellite positions. If you fetch fresh TLEs, you get the current positions. You can also record a live session and replay it later as captured, with the same playback and scrubbing controls as a synthetic run. Without the stored ephemeris, you cannot reproduce a live-constellation result after the TLEs change.

Q23. Can Astra run without the browser, for sweeps and automation?

Yes. Astra saves the three configuration tabs as one config.json, and the included CLI runs that file headlessly:

cli.py run config.json --set rf.eirp_dbw=60

You can override any field with a dotted path, so a parameter sweep is a shell loop over that command. The CLI returns typed exit codes for use in a CI pipeline. The CLI and the web UI read the same file, so a study that you configure in the browser runs unchanged on the command line. The exception in this release is an Earth-fixed coverage study (Q13). The CLI does not run it, so run it in the web UI.

Q24. How is Astra deployed for a team?

Astra is web-based, and you deploy it on your server. It supports multiple users with authentication and per-user projects. Users work in a browser, so there is no desktop install for each analyst and no local license tied to a machine. One installation serves the team.

Q25. What is the boundary between NetSim Astra and NetSim NTN?

Astra covers the constellation, the link budget, and the RF coverage, and stops at the radio link. NetSim NTN simulates the 5G NTN protocol stack and the traffic end to end at packet level, reporting throughput, latency, and error performance per beam. Astra outputs define the coverage and link conditions. NTN evaluates protocol and application performance under those conditions.

Q26. How is an evaluation arranged?

Write to sales for an evaluation license. You can configure a scenario that you already ran in another tool, and compare the outputs directly. The NetSim Astra product page covers the feature set in more detail.