PushingPushingPushingPushing Graz SLR Graz SLR Graz SLR Graz SLR fromfromfromfrom2 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 [°]

Pts

/Sec

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]

Noise

Poin

ts/sec

ond

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]

Nois

e poin

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 neededfor 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]

Energ

y / P

uls

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

PA

D D

ark

No

ise

[kH

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]

Ret

urn

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]

Ret

urn

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-SPADPulse Distribution

Module PDM

StopsStarts

Starts +

Sto

ps

DOS PC ISA FPGA Win PC

Riga ET:

10 kHz

max

Graz: 10 kHz Setup …

Sta

rts

+ S

tops

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]

Retu

rn R

ate

In

cre

ase F

acto

r

Laret

Jason-2

Lageos-2

Etalon-21.44 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 ( distance …)

16IWF/ÖAW GRAZ

What we expect from High-Q-Laser et al:

CONSTANT POWER LASER; e.g. 5 W:

100 Hz / 50.0 mJ for HEO satellites

10 kHz / 0.5 mJ for LEO satellites

- Graz 10 kHz system is still under test – no 10 kHz NPs yet distributed

- DOS programs still ‚see‘ a 2 kHz system – Win PC handles RIGA / „10“ kHz

First test results with Graz 10 kHz SLR

17IWF/ÖAW GRAZ

New Dome in Graz New Dome in Graz New Dome in Graz New Dome in Graz ---- butbutbutbut still still still still „„„„oldoldoldold““““ 2 kHz2 kHz2 kHz2 kHz…………

Thank you ☺☺☺☺