Meteor M2 LRPT Decoder

1 System Overview

The Meteor M2 decoder is a complete digital ground station implemented primarily in FPGA fabric, capable of receiving and decoding Low Rate Picture Transmission (LRPT) signals from Russian Meteor-M series weather satellites.

System Architecture

┌─────────────┐    ┌─────────┐    ┌─────────┐    ┌──────────────────────────────────┐
│ QFH Antenna │───▶│   LNA   │───▶│   SAW   │───▶│           RTL-SDR                │
│  (137 MHz)  │    │ +20 dB  │    │ Filter  │    │    (2.4 MSPS I/Q samples)        │
└─────────────┘    └─────────┘    └─────────┘    └──────────────┬───────────────────┘
                                                               │ USB
                                                               ▼
┌──────────────────────────────────────────────────────────────────────────────────┐
│                              ZYNQ FPGA                                           │
│  ┌────────────────────────────────────────────────────────────────────────────┐  │
│  │                         Programmable Logic (PL)                            │  │
│  │                                                                            │  │
│  │   ┌─────┐   ┌─────┐   ┌───────┐   ┌─────────┐   ┌─────────┐   ┌────────┐  │  │
│  │   │ DDC │──▶│ RRC │──▶│Costas │──▶│ Gardner │──▶│ Viterbi │──▶│  Reed  │  │  │
│  │   │     │   │Filter│   │ Loop  │   │   TED   │   │ Decoder │   │Solomon │  │  │
│  │   └─────┘   └─────┘   └───────┘   └─────────┘   └─────────┘   └────┬───┘  │  │
│  │                                                                    │      │  │
│  └────────────────────────────────────────────────────────────────────┼──────┘  │
│                                                                       │         │
│  ┌────────────────────────────────────────────────────────────────────┼──────┐  │
│  │                      Processing System (PS)                        ▼      │  │
│  │                                                              ┌──────────┐ │  │
│  │    ┌─────────────┐    ┌──────────────┐    ┌──────────────┐   │  Frame   │ │  │
│  │    │  RTL-SDR    │    │    Pass      │    │    Image     │◀──│  Sync &  │ │  │
│  │    │   Driver    │    │  Prediction  │    │   Assembly   │   │  Parse   │ │  │
│  │    └─────────────┘    └──────────────┘    └──────┬───────┘   └──────────┘ │  │
│  │                                                  │                        │  │
│  └──────────────────────────────────────────────────┼────────────────────────┘  │
│                                                     │                           │
└─────────────────────────────────────────────────────┼───────────────────────────┘
                                                      │
                                                      ▼
                                               ┌─────────────┐
                                               │   Output    │
                                               │ HDMI / Web  │
                                               └─────────────┘

2 Orbital Mechanics

Understanding the satellite's orbit is crucial for pass prediction, antenna tracking, and interpreting the received imagery.

Sun-Synchronous Orbit

Meteor M2 operates in a sun-synchronous orbit at 98.7° inclination. This special orbit precesses at exactly the rate Earth orbits the Sun, meaning the satellite crosses the equator at the same local solar time each day (~09:30 ascending node).

Orbital Parameters

The Polar Gap

Due to the 98.7° inclination (not exactly 90°), there's a circular polar cap of approximately 8.7° radius at each pole that the satellite never passes directly over. The maximum latitude reached is 180° - 98.7° = 81.3°.

Pass Geometry

From Eugene, Oregon (44.05°N, 123.09°W), typical pass characteristics:

3 Signal Characteristics

RF Parameters

Why 137 MHz?

Link Budget

A link budget accounts for all gains and losses in the signal path from satellite to receiver. Understanding where each value comes from is essential for validating the design.

Physics-Derived Values (Calculated from Formulas)

These values are computed from well-established physics equations and are highly reliable.

Specification-Based Values (From Standards & Datasheets)

These values come from equipment specifications or industry standards.

Estimated Values (Require Verification)

These values are approximations from community observations or incomplete documentation. They should be validated during development through actual signal measurements.

Complete Link Budget Summary

// Link budget calculation (zenith pass)
// ═══════════════════════════════════════════════════════════════
// TRANSMIT SIDE
Satellite EIRP:              +7 dBW        // ⚠ Estimated - verify

// PATH LOSSES
Free space loss (820 km):  -134 dB        // Physics: ITU-R P.525
Atmospheric loss:            -1 dB        // Standard: ITU-R P.676

// RECEIVE SIDE
Antenna gain (QFH):          +3 dBi       // Spec: antenna design
Cable loss (short run):      -1 dB        // Spec: coax datasheet
// ═══════════════════════════════════════════════════════════════
Received signal power:    -126 dBW

// NOISE
Noise floor (120 kHz BW):  -148 dBW       // Physics + ~5 dB system NF
// ═══════════════════════════════════════════════════════════════
Signal-to-Noise Ratio:    ~22 dB        // At zenith, good conditions
SNR at horizon:           ~12 dB        // Additional 10 dB path loss
SNR (worst case):          ~5 dB        // Low elevation + interference

References

• ITU-R P.525 - Free-space propagation
• ITU-R P.676 - Atmospheric attenuation
• ITU-R P.618 - Earth-space propagation
• SatNOGS Database - Community satellite observations
• Antenna-Theory.com - Antenna fundamentals

4 Doppler Processing

At 7.5 km/s orbital velocity, the satellite's signal experiences significant Doppler shift that must be tracked by the receiver.

Doppler Shift Calculation

Doppler Rate

The rate of Doppler change is highest at closest approach (when the satellite is moving most perpendicular to the line of sight). This can be used to determine elevation angle.

AFC Implementation

// AFC tracking loop (pseudocode)
module afc_tracker (
    input  clk,
    input  [15:0] phase_error,    // From Costas loop
    output [31:0] freq_correction  // To NCO
);
    // Second-order loop filter for frequency tracking
    // Bandwidth ~100 Hz to track Doppler rate
endmodule

5 QPSK Demodulation

QPSK Constellation

QPSK encodes 2 bits per symbol using four phase states separated by 90°. The Meteor M2 signal uses standard Gray-coded QPSK.

          Q (Imaginary)
              │
        01    │    00
          ╲   │   ╱
           ╲  │  ╱
            ╲ │ ╱
    ─────────┼─────────── I (Real)
            ╱ │ ╲
           ╱  │  ╲
          ╱   │   ╲
        11    │    10
              │

    Symbol mapping (Gray coded):
    00 →  45° = (+1, +1)
    01 → 135° = (-1, +1)  
    11 → 225° = (-1, -1)
    10 → 315° = (+1, -1)

Carrier Recovery: Costas Loop

The Costas loop simultaneously tracks carrier frequency and phase for QPSK signals. It uses a characteristic S-curve error detector that is insensitive to data transitions.

Symbol Timing Recovery: Gardner TED

The Gardner Timing Error Detector (TED) works on the received samples to align the sampling instant with the optimal point (center of symbol).

6 Forward Error Correction

Meteor M2 uses concatenated coding: an inner convolutional code (decoded with Viterbi) and an outer Reed-Solomon code. This combination provides robust error correction for the noisy satellite channel.

Convolutional Code (Inner)

Viterbi Decoder

The Viterbi algorithm finds the maximum likelihood path through the code trellis. Key implementation considerations:

• Soft decisions: Using 3-4 bit soft symbols provides ~2 dB gain over hard decisions
• Path metrics: Need sufficient bit width to prevent overflow
• Traceback depth: Typically 5× constraint length = 35 symbols minimum
• Survivor path storage: Major FPGA resource consumer

Convolutional Deinterleaver

Between the inner and outer codes, a convolutional interleaver spreads burst errors across multiple Reed-Solomon codewords. The deinterleaver reverses this process.

Reed-Solomon Code (Outer)

7 LRPT Protocol

Frame Structure

After FEC decoding, the data is organized into CCSDS-compatible frames:

┌──────────────┬─────────────┬───────────────────────────┬─────────┐
│  Sync Word   │   Header    │         Payload           │  Parity │
│   (32 bits)  │  (48 bits)  │       (8144 bits)         │(128 bits)│
└──────────────┴─────────────┴───────────────────────────┴─────────┘
                     │
                     ▼
        ┌────────────────────────────────┐
        │ VCID (6 bits) - Virtual Chan ID │
        │ SCID (8 bits) - Spacecraft ID   │
        │ Frame Count (24 bits)           │
        │ etc.                            │
        └────────────────────────────────┘

CCSDS Sync Word

// 32-bit sync word (and bit-inverted version for phase ambiguity)
localparam SYNC_WORD     = 32'h1ACFFC1D;
localparam SYNC_WORD_INV = 32'hE53003E2;  // 180° phase

Image Data Organization

8 FPGA Implementation

Target Platform

Resource Estimates

Clock Domains

// Clock structure
sample_clk    : 2.4 MHz   // RTL-SDR sample rate
symbol_clk    : 72 kHz    // Symbol rate
system_clk    : 100 MHz   // Main processing clock
ps_clk        : 667 MHz   // ARM processor clock

Implementation Notes

9 Hardware Setup

Bill of Materials

QFH Antenna Construction

The Quadrifilar Helix (QFH) antenna provides right-hand circular polarization with a hemispherical pattern - perfect for tracking satellites from horizon to horizon.

RF Chain

QFH Antenna ──▶ Short Coax ──▶ LNA ──▶ SAW Filter ──▶ Coax ──▶ RTL-SDR
     │              │            │          │           │
     │         < 1m to          +20 dB    Bandpass    Route to
     │         minimize        gain      137±5 MHz    indoor
     │          loss                                  location
     │
  Mount with clear sky view
  (rooftop, pole, etc.)

10 Development Log

Chronological record of development progress, challenges, and insights.

11 References

Standards & Specifications

• CCSDS 131.0-B-3: TM Synchronization and Channel Coding
• CCSDS 132.0-B-2: TM Space Data Link Protocol
• ECSS-E-ST-50-05C: Radio Frequency and Modulation

Software References

• LeanHRPT - Open source LRPT decoder
• meteor_demod - Software demodulator reference
• SatSignal - David Taylor's weather satellite resources

Technical Resources

• PySDR - SDR fundamentals
• The Scientist and Engineer's Guide to DSP - Free DSP textbook
• Proakis & Salehi - Digital Communications (textbook)

Community

• r/amateursatellites
• RTL-SDR.com - SDR tutorials and news