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Presentation Summary. How Beacons are Located Using Low Earth Orbit (LEO) Satellites. Requires Highly accurate Orbit information and Very Stable Beacon Oscillator. Description Of Doppler Shift. Ambiguity Resolution (side “A” or “B” Determination). Satellite Types. Two types of satellites:

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  1. Presentation Summary • How Beacons are Located Using Low Earth Orbit (LEO) Satellites • Requires Highly accurate Orbit information and Very Stable Beacon Oscillator • Description Of Doppler Shift • Ambiguity Resolution (side “A” or “B” Determination)

  2. Satellite Types • Two types of satellites: w LEO satellites in near-polar orbit w Geosynchronous earth orbiting (GEO) satellites (cannot provide Doppler info)

  3. Determining Beacon Locations From LEO Doppler Data GEOMETRY FOR OVERHEAD PASS T2 850 km 3400 km T1 T3 Distress Beacon DOPPLER SHIFT CAUSES FREQUENCY TO APPEAR HIGHER THAN ACTUAL Frequency TIME OF CLOSEST APPROACH (AND ACTUAL FREQUENCY) High DOPPLER SHIFT CAUSES FREQUENCY TO APPEAR LOWER THAN ACTUAL Actual Low Time T3 T1 T2

  4. COSPAS-SARSAT Processor and On-Board Memory • SAR Processor • Extracts the beacon ID • Measures the signal’s frequency • Time tags the frequency measurement • On-Board Memory • Stores the processed data • Continually transmits the stored data (typical hold time is several hours) 406 MHz Beacon SAR Processor LUT Ground Station Local User Terminal

  5. Doppler Curve (Processed by LUT) Satellite S4, orbit 12587 1991 MAR 05 18:16:31.80 1 2 3 4 5 6 7 Frequency (kHz) 8 9 10 Max Elevation 60o 11 12 13 14 15 Time (secs)

  6. Time of Closest Approach (TCA) Satellite S4, orbit 12587 1991 MAR 05 18:16:31.80 1 2 3 4 5 6 7 Frequency (kHz) 8 9 10 TCA Occurs at Zero Doppler Shift 11 12 13 14 15 Time (secs)

  7. Typical Doppler Curves Doppler Frequency vs. Time 10 8 Pass 1 (High Elevation) 6 4 2 Frequency (KHz) 0 -2 Pass 2 (Low Elevation) -4 -6 TCA -8 -10 0 5 10 15 20 Time (minutes)

  8. First Location (A / B Ambiguity) Latitude 1B 1A Gnd Trk Pass 1 (time is measured along gnd trk) Longitude TCA

  9. A / B Ambiguity Resolved Latitude 1B 1A Gnd Trk Pass 2 2A Gnd Trk Pass 1 2B Longitude

  10. Two Pass, Merged Location Latitude 1B Gnd Trk Pass 2 Gnd Trk Pass 1 2B Longitude Merged Location

  11. Ground Track and Field of View T1 T2 T2 -T1 = 101 minutes D longitude – 25 Deg.

  12. Resolving Location Ambiguity 2B 2A 1A 1B 2 1 Two Pass Solution for a Beacon Located in Brazil LEGEND: ground tracks of successive spacecraft orbits 1A, 1B Real and Image solutions from pass 1 2A, 2B Real and Image solutions from pass 2 1 2

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