2. 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)

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

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

5. Processor and On-Board Memory
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

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

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

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

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

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

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

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

13. Resolving Location Ambiguity1
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 2 2B 1B 2A 1A
Two Pass Solution for a Beacon Located in Brazil