NASA Transponder Experiments

Jan McGarry & Tom Zagwodzkiwith inputs from D. Smith, X. Sun, G. Neumann, J. Abshire, J. Degnan

and many others

Transponder SessionILRS Workshop at Eastbourne

October 2005

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.

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, PIDavid.E.Smith@nasa.gov

Maria T. Zuber, Deputy PImtz@mit.edu

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

Receiver field of view: 0.85 mrad

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

26 kg;34 watts;~0.5x0.5x0.5m

Mars Orbiter Laser Altimeter (MOLA)

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.

MOLA-Earthlink Experimentat NASA’s 1.2 meter telescope

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.

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

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)

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

Laser: Continuum InliteDetector: spare MOLA

1.2 m telescope satellite tracking in preparation for MOLA

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

1.2 m telescopelaser

MOLA-Earthlink Experiment Results

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.

MGS scans about earth. Earthshine is seen in MOLA receiver ch#2 asred-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

MLA

Mercury Laser Altimeter (MLA)

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, PIDavid.E.Smith@nasa.gov

Maria T. Zuber, Deputy PImtz@mit.edu

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

MLA instrument characteristics:

Laser PRF: 8HzWavelength: 1064 nmEnergy per pulse: 20 mJLaser divergence: 80 uradReceiver FOV: 400 uradTelescope area: 417 cm^2

3 receiver channels withfour dynamically controlledthresholds.

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

MLA-Earthlink Experimentat 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 nmEnergy per pulse: 15 mJ Laser divergence: 55 uradReceiver FOV: ~260 uradEvent time recording: 50 psec shot-to-shot, accurate to UTC to within 100 nsecTelescope pointing: 1 arcsec open-loop accuracy, several arcsec jitter during daylight.

Detector: spare MOLA detectorLaser: HOMER (B. Coyle Laser Risk Reduction Program developmental laser)

MLA-Earthlink Experiment Results

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.

LOLA

Telescope

Beam Expander

DOE

Laser

Aft optics and Detectors (5)

Instrument housing

Digital Electronics & Power Converter

Laser & Analog Electronics

LOLA Instrument

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, PIDavid.E.Smith@nasa.gov

Maria T. Zuber, Deputy PImtz@mit.edu

LOLA has prime two objectives

– 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.

LOLA-Earthlink Experiment

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.

Laser Ranging to LRO

Overview of LRO Tracking

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.

Laser Ranging Design

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.

1-way laser ranging

28 Hz synch or 1 to 150 Hz asynch

Range window opens at 28 Hz

One LOLA detector does both earth uplink and lunar surface

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.

EARTH RANGEWINDOW (8ms)

LUNAR RWIN(5ms)

35.7 msec (28 Hz)

Start of LOLA laserfire period (T0)

Start of next LOLA laser fire period

LOLA laser fires(~ 9ms after T0)

Ground station: SLR2000 at GGAO

We ask for support from ILRS community in ranging to LRO

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).