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NASA Transponder Experiments. Jan McGarry & Tom Zagwodzki with inputs from D. Smith, X. Sun, G. Neumann, J. Abshire, J. Degnan and many others Transponder Session ILRS Workshop at Eastbourne October 2005. NASA Transponder Experiments. MOLA: Mars Orbiter Laser Altimeter

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Nasa transponder experiments

NASA Transponder Experiments

Jan McGarry & Tom Zagwodzki

with inputs from D. Smith, X. Sun, G. Neumann, J. Abshire, J. Degnan

and many others

Transponder Session

ILRS Workshop at Eastbourne

October 2005

Nasa transponder experiments

NASA Transponder Experiments

  • MOLA: Mars Orbiter Laser Altimeter

    - 2 unsuccessful 2-way ranging attempts while in cruise in 1996/97:

    1996 clouded out. 1997 spacecraft developed problems.

    - September 2005 & January 2006 1-way attempts while orbiting

    Mars (~ 80 Mkm):

    Sept05: Day 1 (laser problems), Day 2 (clouds), Day 3 (success!)

  • MLA: Mercury Laser Altimeter

    - Successful 2-way transponder experiment in daylight during Earth fly-by

    with satellite 24 Mkm away (May 2005).

  • LOLA: Lunar Orbiter Laser Altimeter (instrument on LRO)

    - Future 2-way transponder experiment to calibrate instrument in-flight.

    - Similar to MLA experiment.

    - During “cruise” – Fall 2008.

  • LRO: Lunar Reconnaissance Orbiter

    - Future 1-way ranging to improve orbital knowledge.

    - Launch Fall 2008. Laser ranging required through 2009.

    - Requesting ranging support from ILRS stations.

Mars orbiter laser altimeter mola
Mars Orbiter Laser Altimeter (MOLA)

One of the science payload instruments on the Mars Global Surveyor (MGS) by NASA JPL;

Launched Nov. 7, 1996 and still operating around Mars;

Dave E. Smith,

Maria T. Zuber, Deputy

Currently in circular orbits around Mars at 400km altitude and 2 hour orbit period.

Nasa transponder experiments

Mars Orbiter Laser Altimeter (MOLA)

26 kg;

34 watts;


Receiver field of view: 0.85 mrad

Minimum detectable signal at telescope: ~ 0.1fJ/pulse at >90% detection probability.

Mars orbiter laser altimeter mola1
Mars Orbiter Laser Altimeter (MOLA)

  • Laser altimetry measurements of Mars from 3/3/1999 to 6/30/2001, with 650 million laser shots and achieving all the science goals;

  • Anomaly developed in the clock oscillator or the divider circuits with the laser and the rest of the receiver still functioning;

  • Operation continued in the enhanced passive radiometry mode, 10/10/2001 to present.

Nasa transponder experiments

MOLA-Earthlink Experimentat NASA’s 1.2 meter telescope

J. Abshire, X. Sun, G. Neumann, J. McGarry, T. Zagwodzki, P. Jester, + many others

Experiment Objectives - Demonstrate:

• Laser pulse detection at Mars distances

Assess Earth laser to Mars orbit detection probabilities

• Laser communications at ~ 100 Mkm distance

Detection of an on-off modulated (ie chopped) laser pulse train

MOLA can count # of pulses received per 125 msec window

• Laser pulse timing

Transmit a pulse sequence which moves relative to MOLA counter read rates.

Estimate sequence arrival time relative to MOLA noise counter read time.

Nasa transponder experiments

MOLA-Earthlink Ground Station at NASA’s 1.2 meter telescope

Laser: Continuum Inlite

Detector: spare MOLA

Laser fire rate: 49 Hz (chopped 6 ON, 6 OFF). Energy per pulse: ~ 90 mJ.

Divergence: ~ 60 microrad. Pulsewidth: ~10ns. Wavelength: 1064 nm.

Fire time recording: 50 psec shot-to-shot, 100 nsec tie to UTC.

Telescope pointing: 1 arcsec open-loop pointing accuracy, 1 arcsec jitter (night)

Nasa transponder experiments

1.2 m telescope satellite tracking in preparation for MOLA telescope

  • Reasons for satellite ranging:

    - To verify system pointing & ranging performance

    - To determine beam divergence

    - Independent determination of system delay (esp from

    passes collocated with MOBLAS-7)

  • System configuration for transponders:

    - No range gate

    - Set detector threshold high to greatly reduce noise

    - Use bandpass filter during daylight

    - Operator display shows return events minus laser fire

    modulo laser PRF

    - Current detector has good QE at both 532nm and 1064nm

Nasa transponder experiments

1.2 m telescope telescope


Nasa transponder experiments

MOLA-Earthlink Experiment Results telescope

  • Performed on 3 scheduled dates with spacecraft (9/21, 9/24, 9/28): at ~ 08:00 UTC.

  • Each tested lasted ~ 45 minutes and involved 2 spacecraft scans of earth.

  • Maximum time earth laser in MOLA FOV per scan line: ~8 seconds.

  • MOLA saw earthshine in channel 2 detector on all 3 dates – very repeatable.

  • First date (9/21) we had laser problems, 2nd date (9/24) was cloudy.

  • On Wednesday 9/28, MOLA recorded events in Channel 1 (high threshold) corresponding to laser fires from 1.2 meter telescope. Analysis of this data is just beginning.

Nasa transponder experiments

MGS scans about earth. Earthshine is seen in telescope

MOLA receiver ch#2 as

red-orange-yellow in plot from 9/21/2005.

Each day’s experiment consisted of two back-to-back scans.

Scans were very repeatable.

~ 3 mrad

Nasa transponder experiments

MLA telescope

Nasa transponder experiments

Mercury Laser Altimeter (MLA) telescope

One of the science payload instruments on the MEcury Surface, Space ENvironment, Geochemistry, and Ranging (MESSENGER) spacecraft by Johns Hopkins APL, which launched August 3, 2004.

Dave E. Smith,

Maria T. Zuber, Deputy

MESSENGER is en-route to a 2011 rendezvous with Mercury.

Nasa transponder experiments

MLA instrument characteristics: telescope

Laser PRF: 8Hz

Wavelength: 1064 nm

Energy per pulse: 20 mJ

Laser divergence: 80 urad

Receiver FOV: 400 urad

Telescope area: 417 cm^2

3 receiver channels with

four dynamically controlled


Abstract from paper “The Mercury Laser Altimeter” by D.E.Smith, J.F.Cavanaugh, et al:

Nasa transponder experiments

MLA-Earthlink Experiment telescopeat NASA’s 1.2 meter telescope

X. Sun, G. Neumann, J. Cavanaugh, J. McGarry, T. Zagwodzki, J. Degnan, + many others

Experiment Objectives:

In-flight calibration of instrument – determine instrument pointing relative to spacecraft and laser boresight, verify laser characteristics, verify ranging system performance.

Laser ground system characteristics:

Laser PRF: 240 Hz Wavelength: 1064 nm

Energy per pulse: 15 mJ Laser divergence: 55 urad

Receiver FOV: ~260 urad

Event time recording: 50 psec shot-to-shot, accurate to UTC to within 100 nsec

Telescope pointing: 1 arcsec open-loop accuracy, several arcsec jitter during daylight.

Detector: spare MOLA detector

Laser: HOMER (B. Coyle Laser Risk Reduction Program developmental laser)

Nasa transponder experiments

MLA-Earthlink Experiment Results telescope

  • Performed on 3 scheduled dates with spacecraft (5/26, 5/26, 5/31) at ~ 17:00 UTC

  • Each test lasted ~ 5 hours and involved spacecraft scan of earth over 7 x 7 mrad area.

  • Maximum time earth laser in MLA FOV: ~ 5 seconds.

  • Passive radiometry scan of earth by MESSENGER was performed earlier in the month and verified spacecraft pointing.

  • MLA laser pulses were detected at the ground. MLA also detected laser pulses from ground laser.

    First successful two-way optical communication at interplanetary distances.

Nasa transponder experiments

LOLA telescope

Nasa transponder experiments

LOLA Instrument telescope

LOLA is one of the science payload instruments on the Lunar Reconnaissance Orbiter (LRO), being developed by GSFC, and scheduled to be launched Fall 2008.

Dave E. Smith,

Maria T. Zuber, Deputy



Beam Expander


Instrument housing

Digital Electronics & Power Converter

Aft optics and Detectors (5)

Laser & Analog Electronics

Nasa transponder experiments

LOLA has prime two objectives telescope

  • Produce a high-resolution global topographic model and global geodetic framework that enables precise targeting, safe landing, and safe mobility on the Moon’s surface.

  • Characterize the polar illumination environment, and image permanently shadowed regions of the Moon to identify possible locations of surface ice crystals in shadowed polar craters.

Nasa transponder experiments

LOLA-Earthlink Experiment telescope

  • LOLA instrument characteristics:

    Laser PRF: 28 Hz

    Wavelength: 1064 nm

    Energy per pulse: 2.4 mJ (split 5 ways using a Diffractive Optical Element)

    Laser divergence: 100 urad

    Fiber bundle couples received light to 5 detectors.

    QE of APDs: ~ 40%

    FOV: 400 urad

    Telescope aperture: 14 cm

    Orbit of LRO: nominally 50 km above lunar surface.

  • Ground system: NASA’s 1.2 meter telescope.

  • Experiment objectives: in-flight instrument calibration (similar to MLA).

  • Timeframe: Fall 2008 - probably one to two week period.

Nasa transponder experiments

Overview of LRO Tracking telescope

  •  Laser ranging, together with the S-band and the LOLA altimeter (cross-over analysis), can provide the orbital precision needed to meet all LRO’s post-processing orbital requirements (1 meter radial and 25-50 meters horizontally).

  •  Make one-way range measurements to < 10 cm at rate of once/sec.

  •  Analyze laser and S-band data to make initial improvement in gravity field and LRO orbit. Introduce LOLA altimeter cross-over data to make substantial improvement in gravity model, particularly on the lunar far-side.

Nasa transponder experiments

Laser Ranging Design telescope

  •  One-way laser link with LRO time-stamping the received laser pulses with on-board oscillator.

  • Use small, passive optical receiver on the LRO High Gain Antenna (Earth-pointing) and send the optical signal to the timing receiver via an optical fiber bundle.

  • Use an ultrastable OCXO as the LRO time base (Symmetricom 9500).\

  • Make use of existing LOLA detectors and electronics for detection and timing.

  • Synchronize ground laser fires from primary ground station with LRO clock to maximize detection probability in the LOLA earth range window.

  • Precisely time stamp ground laser fires. Derive the range from ground station to LRO from laser fire times and LRO event times.

Nasa transponder experiments

1-way laser ranging telescope

28 Hz synch or 1 to 150 Hz asynch

Range window opens at 28 Hz

Nasa transponder experiments

One LOLA detector does both earth uplink and lunar surface telescope

  • Two range windows in one detector: 8 msec earth and 5 msec lunar.

  • Need to minimize the earth events in the lunar surface window to prevent on-board signal processing algorithm from following the wrong signal.

  • This can be achieved by synchronizing the ground laser fires to LOLA (LRO timing and range-rate of spacecraft), OR by low frequency laser fire rates.

Start of LOLA laser

fire period (T0)

LOLA laser fires

(~ 9ms after T0)

Start of next LOLA

laser fire period

35.7 msec (28 Hz)

Nasa transponder experiments

We ask for support from ILRS community in ranging to LRO telescope

  • Ground station requirements:

    Energy density at LRO (400,000 km): 2 fJ / cm^2

    Laser pulsewidth: < 10 ns FWHM

    Laser fire rate: 28 Hz synchronized to s/c time & LRO range-rate (primary system);

    1 to 150 Hz for an unsynchronized system

    Timing of laser fire: < 200 psec (relative), within 100 nsec of UTC (absolute)

    Laser wavelength: 532.2 +- 0.1 nm.

  • MLRS is expected to participate:

    discussions started with LOLA instrument Scientist (X.Sun).

  • MOBLAS systems cannot participate as currently configured:

    fire time tag accuracy issue and also laser energy at spacecraft.

  • Station scheduling will most likely be required to minimize ambiguity of events at LRO.

  • As always, data will be shared with community (archived to CDDIS).