Table of Contents

1 · What is Meteor Scatter?

Meteor Scatter (MS) is a radio propagation phenomenon where VHF signals (such as FM radio) reflect off the ionized trails left by meteors burning up in the Earth's upper atmosphere, typically at altitudes between 85 and 105 km.

Millions of meteoroids enter the atmosphere every day. While the visible "shooting star" may only last a fraction of a second, the ionized trail it leaves behind can reflect radio waves for several seconds (a "burst") or just a fraction of a second (a "ping"). Because these reflections occur high in the E-layer of the ionosphere, they allow communication and DX reception over distances far beyond the normal radio horizon — typically between 700 km and 2200 km.

Key factors that determine the success of meteor scatter reception:

• Distance: Due to the geometry of the Earth and the ~95 km reflection altitude, the optimal distance is usually between 1000 km and 1600 km. Distances below 700 km require very steep reflection angles, and distances over 2200 km are physically blocked by Earth's curvature.
• Transmitter Power (ERP): Higher effective radiated power dramatically increases the chance of receiving a decodable burst.
• Diurnal Variation: The Earth "sweeps" up more background sporadic meteors on its leading edge. This means meteor scatter is statistically best in the early morning (around 6 AM local time) and worst in the early evening (around 6 PM).
• Meteor Showers: Annual meteor showers (like the Perseids or Geminids) provide a massive spike in activity. During a shower, meteors originate from a specific point in the sky called the radiant, making forward scatter highly directional and predictable.

2 · Why a Plugin?

While sporadic meteors happen randomly all over the sky, active meteor showers present a distinct geometric advantage. The ionized trails formed by shower meteors align with the radiant. To bounce a signal effectively from a specific transmitter to your receiver, the geometry must satisfy strict reflection conditions relative to the radiant's position in the sky.

This allows the DX operator to aim their antenna and tune their receiver to the exact frequencies and directions that the current astronomical geometry heavily favors, rather than just guessing.

3 · Architecture – Data Sources and Components

4 · Simple Explanation – How the Plugin Works

Unlike airplane scatter, which tracks physical moving objects in real-time, meteor scatter prediction is statistical and geometrical. The plugin calculates the best potential paths for a meteor ping to occur, right now, based on astronomical data and transmitter properties.

4.1 · The Reflection Geometry

The optimal location for a meteor to reflect a signal from a transmitter (TX) to your receiver (RX) is near the geometric midpoint between the two, at an altitude of approximately 95 km. The plugin computes this midpoint and evaluates the geometric viability.

The distance is the most critical fixed factor. Because the reflection happens at ~95 km, paths shorter than 700 km require the signal to hit the meteor trail at a very steep angle, which is inefficient. Paths longer than 2200 km are blocked by the curvature of the Earth itself.

4.2 · Score – What does the Percentage Mean?

The hotspot score is a composite of astronomical and physical factors. It determines how likely you are to receive pings from that specific transmitter *right now*.

Score colour scale in the UI

4.3 · Meteor Showers & Sporadics

The plugin tracks major annual meteor showers automatically based on the current date:

• Quadrantids (Peak ~Jan 3)
• Lyrids (Peak ~Apr 22)
• Eta Aquariids (Peak ~May 6)
• Perseids (Peak ~Aug 12)
• Orionids (Peak ~Oct 21)
• Leonids (Peak ~Nov 17)
• Geminids (Peak ~Dec 14)

When a shower is active, the map will display the Ideal Lines (Forward Scatter) radiating from your receiver. Tuning to transmitters along these lines drastically improves the odds of catching bursts.

If no shower is active, or if you select "Sporadic (Background)" from the dropdown, the plugin relies purely on diurnal timing, distance, and ERP to score the best candidate transmitters.

4.4 · Transmitter Database (fmdx.org)

The TX database originates from maps.fmdx.org/api/ and is managed entirely on the server side by meteorscatter_server.js. By default, it queries transmitters between 700 km and 2200 km from your receiver.

Each transmitter entry provides:

• Frequency (87.5–108.0 MHz)
• ERP – Effective Radiated Power in kW
• Location – latitude, longitude, city name, ITU country code
• Station name / PS name – programme name

4.5 · The Map Panel – Display Elements

5 · Technical Deep Dive

5.1 · Scatter Score Calculation

The core scoring function calcMeteorScatter(rxLat, rxLon, tx, ctx) is evaluated for every transmitter passing the distance filter (700-2200 km).

Step 1 – Base Score (Distance & ERP)

Distance shapes the baseline score. The ideal reflection zone is 1200 - 1600 km. Penalties apply outside this window. ERP is added logarithmically.

let score = 100;
if (dist < 1200) score -= (1200 - dist) * 0.05;
if (dist > 1600) score -= (dist - 1600) * 0.05;

score += Math.min(15, Math.log10(Math.max(1e-6, tx.erp)) * 5);

Step 2 – Beamwidth & Terrain Blocking Penalty

In version 1.1, transmitters outside the configured rotor beamwidth suffer massive penalties. Also, a physical horizon check reduces scores if the signal path is obstructed.

// Rotor beamwidth check
const halfBeam = S.beamwidth / 2;
if (adiff > halfBeam) {
    score -= ((adiff - halfBeam) * 2.5);
}

// Terrain Blocking (if enabled)
const maxScatterDist = 2000 + radioHorizonRx + radioHorizonTx;
if (dist > maxScatterDist) {
    score -= 40;
}

Step 3 – Shower Radiant Alignment (If Active)

If a meteor shower is active, the algorithm determines the two ideal forward-scatter azimuths (±90° from the radiant azimuth). A Gaussian curve evaluates how close the transmitter's bearing is to these ideal lines.

const diff1 = Math.abs(txBrg - ctx.ideal1), adiff1 = Math.min(diff1, 360 - diff1);
const diff2 = Math.abs(txBrg - ctx.ideal2), adiff2 = Math.min(diff2, 360 - diff2);
const minDiff = Math.min(adiff1, adiff2);

// Apply Gaussian alignment reward
const alignment = gaussianAlignment(minDiff, 45);
score *= (0.25 + 0.75 * alignment);

Step 4 – Diurnal Variation

Sporadic background meteors peak roughly at 6:00 AM local solar time and hit a trough at 18:00. This interacts with seasonal variations as well.

5.2 · Astronomy & Radiant Computation

The plugin internally tracks time via Julian Dates to compute celestial mechanics without relying on external APIs.

Radiant Azimuth & Altitude

Using the shower's Right Ascension (RA) and Declination (Dec), the plugin calculates the Local Sidereal Time (LST) and transforms the equatorial coordinates into local horizontal coordinates (Azimuth, Altitude) to determine exactly where in your sky the meteors are coming from.

function getRadiantAzAlt(raDeg, decDeg, lat, lon, date) {
    // Converts RA/Dec to Az/Alt via Greenwich Mean Sidereal Time (GMST)
    ...
    return { az: azRad / PI_180, alt: altRad / PI_180 };
}

Solar Altitude & Diurnal Curve

Sun altitude is used for optional D-layer weighting. The local solar hour is calculated based on the receiver's longitude to model the Earth's leading edge sweeping up meteors.

5.3 · Elevation Data and Topographic Profile

Like Airplane Scatter, terrain elevation data is fetched from APIs and cached on the server (elevation_cache.json) to prevent rate limits.

The profile is built from 75 equidistant great-circle sample points. However, instead of tracking a 10 km cruising aircraft, the profile highlights the 95 km meteor layer at the midpoint.

The profile canvas accounts for Earth's curvature drop mathematically, ensuring that paths over 2000 km correctly show the horizon blocking effect.

5.4 · TX Spatial Grid Index & Database Optimization

To keep the browser responsive, the server script (meteorscatter_server.js) performs a pre-filter. When the client requests the database, the server already culls transmitters outside the requested radius (e.g., 2200 km) and below the minimum ERP (e.g., 100 kW).

5.5 · fmdx.org TX Database – Server-Side Cache

The full 50 MB fmdx.org dataset is managed on the server with a priority cascade identical to Airplane Scatter:

5.6 · GPS & PST Rotator WebSocket Listener

The plugin listens to /data_plugins. If GPS data is broadcasted, it automatically updates the receiver's QTH. If a PST Rotator is active, the red rotor line is rendered on the live map.

5.7 · Local Node Proxy Server

The meteorscatter_server.js backend attaches to the FM-DX-Webserver HTTP server and exposes proxy endpoints for audio streams and elevation APIs, securely dodging CORS restrictions.

6 · Configuration Parameters

The plugin contains several hardcoded parameters at the top of meteorscatter.js related to the astronomical databases and UI updates.

// Major meteor showers (ZHR = Zenithal Hourly Rate)
const METEOR_SHOWERS = [
    { id: "quadrantids",   name: "Quadrantids",   start: [1, 1],  end: [1, 5],  ... },
    { id: "perseids",      name: "Perseids",      start: [7, 17], end: [8, 24], ... },
    ...
];

// Score colour thresholds
const SCORE_EXCELLENT     = 80;
const SCORE_HIGH          = 60;
const SCORE_MEDIUM        = 40;

7 · User Settings

The plugin offers a variety of customizable parameters accessible via the gear icon in the map header. These settings allow you to tailor the scatter prediction engine to your specific location, equipment, and preferences.

General & Filters

• Min/Max Distance (km): Sets the minimum and maximum distance for the great-circle path between the transmitter and receiver. Airplane scatter typically occurs between 400 km and 1500 km.
• Min TX ERP (kW): Filters out weak transmitters. Higher ERP values generally produce more reliable scatter reflections.
• Min Score (%): Hides candidates falling below this percentage score.
• Strict Min Score: When enabled, the plugin strictly enforces the minimum score, potentially showing fewer results if none meet the threshold. When disabled (auto relax), the plugin may lower the threshold to ensure some candidates are always displayed.
• Frequency Filter: Allows you to apply a whitelist or blacklist to ignore local, strong, or unwanted frequencies. See Section 7.1.

Antenna & Rotor Setup

• Antenna Beamwidth: Defines the horizontal opening angle (in degrees) of your receiving antenna (e.g., 50°). Aircraft and transmitter paths falling outside this cone will receive a severe off-beam gain penalty.
• Static Azimuth (No Rotor): If you do not have an active motorized rotor connected to the webserver, set your antenna's fixed pointing direction here. The map will project your beam cone based on this value, and the off-beam penalty will be calculated against it.

Display & Scoring

• Target candidates: The target number of high-scoring candidates the prediction engine should try to find and display in the side panel.
• Map draw limit: The maximum number of hotspots/aircraft pairs drawn on the Leaflet map simultaneously (to prevent browser lag).
• Sun weighting: When enabled, applies a daytime absorption penalty to the score based on the current solar elevation angle at the receiver's location.

Terrain Configuration

• Real Terrain Blocking: When enabled, the plugin performs a topographic line-of-sight check near your receiver. If local hills or mountains physically block the required take-off angle towards the scatter event, the candidate's score is drastically reduced (x0.15) to reflect the blocked signal path.
• RX/TX antenna height AGL (m): The height above ground level for your receiving antenna and the assumed height for transmitting antennas. Used to calculate radio horizons more accurately.
• Collapse groups by default: Determines whether the candidate list on the left side is expanded or collapsed upon loading.

Live Radar Integration

• Live Radar Data: The plugin periodically fetches live radar telemetry to determine current atmospheric scatter activity. Depending on the current live status, global score multipliers may be applied dynamically to reflect real-world propagation conditions.

System UI

• Use Metric System: Toggles distance and altitude displays between kilometers/meters and miles/feet.
• Web server automatically right-aligned: If checked, the main FM-DX-Webserver dashboard UI will shift to the right when the plugin is active, preventing the side panel from overlapping your tuner controls.

7.1 · Blacklist and Whitelist Configuration

Place plain text files in the plugins/MeteorScatter/ directory on the server:

• blacklist.txt – frequencies to exclude (one per line, e.g. 98.00)
• whitelist.txt – frequencies to include exclusively (all others hidden)

Lines starting with # are treated as comments and ignored. Only values in the FM broadcast range (87.5–108.0 MHz) are accepted.

8 · Recommended Companion Plugins

9 · Tips, Tricks & Best Practices

• Patience is Key: Meteor scatter requires waiting. Set your receiver to an empty frequency, point your antenna according to the hotspot map, and wait for "pings" (short whistles) or "bursts" (sudden audio fragments).
• Trust the Distance: Unlike tropo, 300 km is a *terrible* distance for MS. Aim for hotspots in the 1200 - 1500 km range.
• Watch the Radiants: During major showers like the Perseids (August) or Geminids (December), ignore the background and strictly follow the cyan "Ideal Lines" radiating from your QTH on the map.
• Early Morning is Best: If no shower is active, wake up early. 6 AM local time is statistically the peak of daily background sporadic meteor activity.

10 · Changelog

Version 1.1 – April 2026 Current

• Live Radar Model: Added server-side simulated radar data (via /api/meteorscatter/live_radar) that feeds dynamic scoring multipliers based on background bursts.
• Antenna Beamwidth & Rotor Penalties: Hotspots outside the defined antenna beamwidth (when connected to a PST Rotator) receive massive scoring penalties. The beamwidth is visualized as a red cone overlay on the map.
• Terrain Blocking Checks: Optional checking algorithm (3km and 10km path horizon checks) penalizes candidates that are physically obstructed by terrain.
• Refined Shower Model: Gaussian modeling of the proximity to meteor shower peak dates and radiant elevation modeling (rewards mid-elevation alignments).

Version 1.0 – April 2026

• Meteor Scatter Prediction Engine: Core geometry and astronomy algorithms implemented for calculating optimal 95km reflection midpoints.
• Shower Tracking: Automatic tracking of major annual meteor showers (Quadrantids, Lyrids, Eta Aquariids, Perseids, Orionids, Leonids, Geminids) with radiant-based forward scatter lines.
• Diurnal & Sun Modeling: Scoring system accounts for Earth's leading edge and solar altitude.
• Server-Side Caching: High-performance TX and Elevation database caching via meteorscatter_server.js.
• Interactive UI: Live Leaflet map with hotspot markers, grouped list panel, and integrated audio streaming.
• Frequency Filtering: Support for whitelist/blacklist to hide local splatter frequencies.
• Auto Update Checker: The plugin now alerts users if a new version is available on GitHub.
• UI Additions: Web server interface can now automatically align to the right.