03-Kirchner 10 kHz - NASA From < 60 kHz to > 5 MHz (10 Hz to 50 kHz) 0 1000 2000 3000 4000 5000 6000

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  • PushingPushingPushingPushing Graz SLR Graz SLR Graz SLR Graz SLR fromfromfromfrom 2 kHz to 10 kHz 2 kHz to 10 kHz 2 kHz to 10 kHz 2 kHz to 10 kHz repetitionrepetitionrepetitionrepetition raterateraterate

    Georg Kirchner, Franz Koidl, Farhat Iqbal

    Institute for Space Research

    Austrian Academy of Sciences

    Kötzting, May 2011

  • 2IWF/ÖAW GRAZ

    FromFromFromFrom 2 kHz to 10 kHz2 kHz to 10 kHz2 kHz to 10 kHz2 kHz to 10 kHz

    Overview

    � Basic problems & limitations of higher repetition rates

    � #1: Overlap of transmit / receive pulses

    � #2: Signal / Noise ratio for LEO & HEO satellites

    � Question: What is the best (or highest ?) repetition rate ?

    Kötzting, May 2011

    � Technical problems, some solutions in Graz:

    � Boosting the High-Q-Laser

    � Adding the Riga ET for 10 kHz

    � Keeping the Graz ET for 2 kHz (out of these 10 kHz)

    � Routing of 2 kHz & 10 kHz start- and stop-pulses

    � Setup and test results

  • 3IWF/ÖAW GRAZ

    At 2 kHz: Overlap occurs at every 75 km range difference

    Noise Points / Second: LAGEOS 1 / night time

    Overlap Avoidance OFF

    0

    100

    200

    300

    400

    500

    600

    700

    20 30 40 50 60 70 80

    Elevation [°]

    P ts

    /S e c

    Basic problem #1: Overlaps

  • 4IWF/ÖAW GRAZ

    Graz / 400 µJ: During Overlaps > 50% noise

    Ajisai (day 80) increased noise inside one overlap

    (2634;160)

    (2626;1060)

    0

    200

    400

    600

    800

    1000

    1200

    2600 2610 2620 2630 2640 2650 2660

    One way Range[Km]

    N o is e

    P o in

    ts /s ec

    o n d

    During OVERLAP: Noise increases from 100 Hz to 1 kHz �

    (Overlap Avoidance switched OFF)

  • 5IWF/ÖAW GRAZ

    If Overlap Avoidance is switched OFF…

    Noise points/s vs. distance in [km] from detector into the atmosphere

    0

    250

    500

    750

    1000

    1250

    0 5 10 15 20

    Distance from detector[km]

    N o is

    e p o in

    ts /s

    Day 79

    Day 80

    Day 81

    Day 83

    Day 96

    Overlap backscatter: From distances up to 5 km; sometimes more …

  • 6IWF/ÖAW GRAZ

    The Overlap Problem

    Kötzting, May 2011

    � Overlap between transmit / receive pulses:

    � Strong backscatter from first 5 km of atmosphere;

    � Sometimes still backscatter from up to about 8 km of atmosphere

    � Depends on elevation, actual atmospheric conditions etc.

    � Should be avoided with Single Photon Systems (which are needed for high rep rate systems with their low energy/shot)

    � This limits the maximum repetition rate to about < 20 kHz

    � Above that: Intrinsic / continuous backscatter

    � Might become more tolerable with very low energy/shot

    � Avoidable if transmit / receive telescope are separated (some 10 m)

  • 7IWF/ÖAW GRAZ

    Kötzting, May 2011

    Basic problem #2: Signal / Noise Ratio

    Graz Laser: Constant ENERGY Laser (0.4 mJ)

    Wanted: Constant POWER Laser: Not (yet) available (?)

    Constant Power Laser:

    Energy per Pulse decreases with Rep Rate

    0.01

    0.1

    1

    10

    100

    1000

    10 100 1000 10000 100000

    Repetition Rate [Hz]

    E n e rg

    y / P

    u ls

    e [ m

    J ]

    1 W Laser

    5 W Laser

    0.8 W Graz Laser

    Graz Laser

    ‘Useful kHz‘ SLR Range’10 Hz‘ Systems

  • 8IWF/ÖAW GRAZ

    Kötzting, May/2011

    Another limitation: Signal / Noise Ratio

    - Increasing the laser rep rate: Energy per shot decreases: =>Decreases signal ����

    - Increasing the laser rep rate: Noise increases: =>Increases noise ����

    - Each Range Gate can produce a noise point (20-50% in Graz daylight)

    - SPAD dark noise increases: from 60 kHz@10 Hz to >5000 kHz@50 kHz

    - Decreasing signal ���� => S/N Ratio => S/N Ratio => S/N Ratio => S/N Ratio ↓↓↓↓↓↓↓↓ Increasing noise ����

    - Return identification gets difficult at low S/N ratios

    C-SPAD dark noise: Increases with Rep Rate:

    From < 60 kHz to > 5 MHz (10 Hz to 50 kHz)

    0

    1000

    2000

    3000

    4000

    5000

    6000

    0 10000 20000 30000 40000 50000

    Repetition Rate [Hz]

    C -S

    P A

    D D

    a rk

    N o

    is e

    [k H

    z ]

    Dark Noise

    C-SPAD: Dark Noise

    increases with repetition rate:

    e.g. 3 MHz noise at 20 kHz

  • 9IWF/ÖAW GRAZ

    Assumed: 5 W Constant Power Laser / C-SPAD / Graz Atmosphere:

    LEO: Significant increase of returns / s at least until 10 kHz

    Kötzting, May/2011

    LEO: How many returns per second can we expect ???

    LEO: Returns per Second vs. Reptition Rate

    5 W Constant Power Laser, Average Graz Weather

    10

    100

    1000

    10000

    100000

    10 100 1000 10000 100000

    Repetition rate[Hz]

    R et

    u rn

    s /

    s

    Champ 30°

    Champ 70°

    Envisat 30° Envisat 70°

    Ajisai 30°

    Ajisai 70°

    2 kHz 10 kHz

  • 10IWF/ÖAW GRAZ

    Assumed: 5 W Constant Power Laser / C-SPAD / Graz Atmosphere:

    HEO: NO increase of Returns / s above a few 100 Hz;

    LAGEOS: NO increase of Returns / s above a few kHz

    HEO: How many returns per second can we get ???

    HEO: Returns per Second vs. Reptition Rate

    5 W Constant Power Laser, Average Graz Weather

    1

    10

    100

    1000

    10 100 1000 10000 100000

    Repetition rate[Hz]

    R et

    u rn

    s /

    s

    Lageos at 30° Lageos at 70° Etalon at 30° Etalon at 70° GPS at 30° GPS at 70°

    2 kHz 10 kHz

  • 11IWF/ÖAW GRAZ

    FromFromFromFrom 2 kHz to 10 kHz2 kHz to 10 kHz2 kHz to 10 kHz2 kHz to 10 kHz

    Preliminary Conclusions for higher repetition rates:

    � 10 kHz Laser Repetition Rate is a good compromise

    � Overlaps still can be avoided

    � Direct increase of returns per second for LEO

    � Still some increase for LAGEOS

    � Limited additional value for HEO satellites (but 2 kHz still okay here)

    Kötzting, May 2011

    � Can be achieved in Graz with minor changes, no big investments ☺

    � Boosting the High-Q-Laser: Needs only a new Pockels Cell Driver

    � Needs Riga ET for 10 kHz: Already available

    � Needs an additional Windows PC

    � Needs a new PDM (Pulse Distribution Module)

  • 12IWF/ÖAW GRAZ

    Graz 2 kHz Laser: Limits for higher Rep Rates

    Oscillator + Regenerative Amp: CW Pumped => Could deliver up to ≈ 50 kHz !!!

    But: Original Pockels Cell: 5 kHz max; replaced by new Pockels Cell: 10 kHz max.

    Post Amp: Pulsed Pumping: 2 kHz max. limited; no way to increase it ….

    SEED (SESAM):

    70 MHz

    Isolator

    Regenerative

    Amp.

    Lambda/2

    P.C.

    HR

    Lambda/4

    Post Amp.

    Start Det. SHG

    CW

    URDM

    URDM

    CW

    URDM

    Pulsed

  • 13IWF/ÖAW GRAZ

    Post Amp: Remains at 2 kHz; Osc.+ Regen: 10 kHz

    Osc. + Regen. Amp.: CW Pumped => generating pulses at 10 kHz

    Post Amp: Pulsed Pumping: Remains at 2 kHz max

    New Pockels Cell installed: Allows up to 10 kHz (Regen. Amp.)

    Therefore: Pulses with alternating energy:

    1 pulse amplified by post amp: 400 µJ

    Next 4 pulses NOT amplified by post amp: 80 µJ

    2 kHz: 400 µJ (each pulse)

    10 kHz: 1*400 µJ, 4*80 µJ …

    80 µJ

    400 µJ

    Standard HQ Laser: 2 kHz

    400 µJ / Pulse; 0.8 W

    400 µJ

    Upgraded HQ Laser: „10“ kHz

    400 µJ / 80 µJ Pulse; 1.44 W

  • 14IWF/ÖAW GRAZ

    Graz ET:

    2 kHz

    HQ Laser C-SPAD Pulse Distribution

    Module PDM

    StopsStarts

    S tarts +

    S to

    p s

    DOS PC ISA FPGA Win PC

    Riga ET:

    10 kHz

    max

    Graz: 10 kHz Setup …

    S ta

    rt s

    + S

    to p s

    Start/Stop pulse colors: Symbolic to indicate STRONG (green) and WEAK (red) pulse energy …

  • 15IWF/ÖAW GRAZ

    First test results with Graz 10 kHz SLR

    Return Rate Increase vs. Perigee:

    10 kHz / 2 kHz

    1.0

    1.5

    2.0

    2.5

    3.0

    100 1000 10000 100000

    Satellite Perigee [km]

    R e tu

    rn R

    a te

    I n

    c re

    a s e F

    a c to

    r

    Laret

    Jason-2

    Lageos-2

    Etalon-2 1.44 W / 0.8 W = 1.82

    Max. Increase: 10 kHz / 2 kHz = Factor ≤ 5.00

    Min. Increase: 1.44 W / 0.8 W = Factor of 1.82

    Envisat

    1.44 W @ 10 kHz / 0.8 W @ 2 kHz = 1.82; for Single Photons, we will arrive at this ratio;

    For Single Photon Satellites (HEOs and LAGEOS): Theoretical Factor 1.82 expected;

    For Multi –Photon Satellites (LEOs): The Factor will increase, according to received energy (