This display requires that JavaScripts be enabled in your browser. For instructions, view http://www.nasa.gov/home/How_to_enable_Javascript.html

Instrument: LVIS : Laser Vegetation Imaging Sensor
View entire text

Associated Platforms
AIRCRAFT

Spectral/Frequency Information
Wavelength Keyword: Near Infrared
Spectral/Frequency Coverage/Range: 1064 nm

Related Data Sets
View all records related to this instrument


Description
SYSTEM OVERVIEW

LVIS is a pulsed laser altimeter and measures range by timing a short
(<10 ns duration) pulse of laser light between the instrument and the
target surface. The entire time history of the outgoing and return laser
pulses is digitized using a single detector, digitizer and timing clock.
This configuration unambiguously describes the range to the surface as
well as the vertical distribution of surfaces within each laser
footprint.

The LVIS system operates at altitudes up to 10 km AGL and has a 12
degree potential field-of-view (PFOV) within which footprints can be
randomly spaced across track. Scanning is performed using
galvanometer-driven scan mirrors that control the pointing of both the
laser and the telescope instantaneous field-of-view (FOV). Scan mirrors
are positioned in a stepped pattern, stopping to fire the laser and
integrate the return signal at each beam location. This raster scan
pattern efficiently covers 100% of the area within the data swath.
Footprint sizes from 1 to 80 m are possible, determined by the AGL
altitude of the airplane and the focal length of a diverging lens in the
output path. The dual axis transmit scanner allows the swath pattern to
remove forward motion of the aircraft from the collection pattern. Roll
compensation has also been incorporated into the software for keeping
collection directly below the aircraft.


OPTICAL SYSTEM

The receiver system consists of a 200-mm diameter, 5-power telescope
with a 25-mm exit pupil. The telescope has a 200-mm aperture, f/2
Petzval1(1 Petzval lens: a high speed, narrow FOV lens composed of two
achromatic lenses positioned about an aperture stop; named after the
Austrian optician Josef Petzvald) objective with a 400-mm focal length
directing light through a 50-mm focal length f/1.8 eyepiece, which
produces a 25-mm collimated beam. A scan mirror, located at the exit
pupil of the telescope, directs the beam through a 10-nm bandpass filter
and onto a 25-mm molded aspheric condenser lens which focuses onto the
0.8-mm Si:APD detector. The scan mirror is a 25x40 mm beryllium mirror
that was custom designed to be lightweight for fast scanning, yet stiff
enough to remain flat during and just after the intense acceleration of
scanning. The receiver box can accommodate two more detectors, enabling
the simultaneous collection of dual-wavelength, dual-polarization data.

Table 1: System characteristics of the LVIS altimeter
Telescope aperture 20 Telescope total FOV 200 mrad
Detector FOV 8 mrad
Detector band width 90 MHz
Bandpass filter band width 10 nm
Digitizer sampling rate 500 Msamp/s
Digitizer effective bits 7
Laser output energy 5 mJ
Laser pulse width 10 ns (FWHM*)
Laser spatial energy pattern TEM00 single mode
Pulse repetition rate rep-rate 100 to 500 Hz
Laser output wavelength 1064 nm
Data rate at 500 Hz rep-rate 300 Kbytes/s
Swath width at 10 km altitude 2.0 km
Footprint diameter 1 to 80 m
Maximum operating altitude >10 km
*Full width at half maximum.


LASER

The transmitter is a water-cooled, solid-state, diode-pumped, Nd:YAG
oscillator-only laser, designed and manufactured by Cutting Edge
Optronics (St. Louis, MO). The laser cavity is housed in a hermetically
sealed aluminum enclosure that measures 45x13x13 cm. Operating at rates
of up to 500 Hz, the laser emits 5 mJ, 10 ns, Gaussian-shaped
(temporally and spatially) optical pulses at a wavelength of 1064 nm.
Accurate ranging to a mean elevation in a wide laser footprint can be
confounded by a complex spatial energy distribution across the laser
spot, thus, the laser transmitter was required to have a single-spatial
mode (TEM00) energy pattern. A fiber-coupling lens is placed behind the
final turning mirror inside the laser enclosure to capture a small
amount (<1%) of the laser output and direct it through two optical
fibers (start pulse and calibration pulse with 100 m (300 ns) delay).
The laser output beam is directed through the output scanner box
containing filter wheels to control the output power to optimize return
signal strength, a diverging lens to control the size of the footprint
on the surface, a lockable pitch control for boresighting, and the
galvanometer-driven output scan mirror.

Online Resources
https://lvis.gsfc.nasa.gov/

Instrument Logistics
Data Rate: 500 Hz rep-rate 300 Kbytes/s
Instrument Owner: Laser Remote Sensing Laboratory, NASA Goddard Space Flight Center