Abstract:
A microaltimeter measures altitude or range highly accurately from an orbiting vehicle. The microaltimeter has a low power solid state laser that is pulsed at a rate above 1 kilohertz. The pulses are delivered to a small telescope which sends them to a planetary surface and receives return reflections. A high efficiency photon detector measures received photons and supplies received photon signals to a process or which makes a time-based bin-wise comparison to find the time of flight and hence the range.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     Priority is claimed from copending U.S. provisional application No. 60/129,232, filed Apr.14, 1999. 
    
    
     ORIGIN OF INVENTION 
     The invention described herein was made by an employee of the United States Government, and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefore. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to laser altimeters and in particular to space-borne laser altimeters for mapping planetary surfaces and the like. 
     BACKGROUND OF THE INVENTION 
     Spaceborne laser altimeters typically use modest energy 50 to 100 milliJoules solid state lasers, large telescopes having apertures of 50 to 100 centimeters in diameter, and high detection thresholds to achieve unambiguous surface returns with few or no false alarm resulting from solar background noise. As a result of this conventional design philosophy, spacecraft prime power and weight constrains typically restrict operations to modest repetition rates on the order of a few tens of Hz which, at a typical earth orbit round velocity of seven kilometers per second limits along-track spatial sampling to one sample every few hundred meters. There is great motivation in obtaining higher along-track resolution and/or better cross-track coverage, but achieving this capability through a simple scaling of the laser fire rate or power is not practical from spacecraft. This is especially true of altimeters for use in orbit about other planets where instrument mass and prime power usage is severely restricted. In ground-based systems the conventional high signals-to-noise ratio approach to laser altimetry does not make efficient use of the available laser photons. 
     The surface return rate of an Earth orbiting altimeter can be increased by the two orders of magnitude for a given laser output power by emitting the available photons in a high frequency (few kilohertz) train of low energy, approximately one milliJoules pulses as opposed to a low frequency train of high energy pulses by employing single photon detection. This mode of operations reduces the risk of internal optical damage to the laser, thereby improving long-term reliability and makes the beam inherently more eyesafe to a ground-based observer. In addition these high return rates can be accomplished with much smaller telescope apertures on the order of 10 centimeters diameter. Indeed the contrast of the terrain signal against the solar-induced noise background is actually enhanced through the use of a small receive telescope. 
     However a problem with such a ground-based system is that it relies on the accuracy smoothness or reliability of a satellite track. Such a system is unsuitable for use in an orbiting altimeter for providing high resolution of rapid terrain changes across steep slopes. The ground-based system in part obtains its relatively large signal-to-noise ratio by means of the predictability of the orbital motion or velocity of displacement from the satellite to the receiving head along the path of the laser beam. 
     What is needed then is an improved microaltimeter system for use in spacecraft and in particular in deep space probes. The system should use relatively low power but which provide very high resolution of terrain, atmospheric, oceanic features and the like. 
     SUMMARY OF THE INVENTION 
     Apparatus embodying the present invention includes a microlaser associated with a small diameter telescope. The microlaser emits pulses of coherent light at a high repetition rate. The emitted laser pulses are collimated by the telescope and transmitted to a ground track. Photons are reflected off the ground. The return photons are filtered through a spectral filter and through a spacial filter to reduce solar background noise. The telescope then receives the return photons and supplies them to a multi-pixel or a multi-faceted single photon detection system. 
     The use of relatively simple software algorithons based on post-detection Poisson filtering techniques enables the identification and extraction of surface sampling data from the more abundant optical background noise prior to on-board storage or transmission of a data to the ground station. 
     The roughly order of magnitude reduction in required telescope diameter greatly simplifies the mechanics of scanned system and allows the use of relatively inexpensive, modest diameter optical wedges or holographic optical elements to simultaneously scan boet the transit and receive beams for-cross-track interrogation of the terrain while maintaining narrow fields of view for background noise suppression. 
     By using state of the art photon counting detectors, which are capable of providing centimeter level ranging resolution, as well as angularly resolving the source of a single photon event within the receiver field-of-view performance is further enhanced. With high angular resolution of the single photon source the measured range becomes nearly a point-to-point measurement. That is from an internal altimeter reference point to a small area of uncertainty on the surface. This is determined by the angular resolving power of the photodetector and can be very small compared to the total beam area on the ground. The ranging precision then is limited by the laser pulsewidth, the timing capabilities of the range receiver, and the much-reduced residual spreading caused by the roughness of the surface and slope within the very small zone of range uncertainty. This ability to measure the near point-to-point-time-of-light of an individual photon, together with the high effective signal to noise the ratio, avoids much of the range ambiguity inherent in current high energy altimeters. Such systems will require multiple photons reflected from anywhere within illuminated spot to be recorded by waveform digitizers which consume relatively large amounts of power. The signals would then have to be deconvoluted using sophisticated and not completely reliable algorithms in order to decipher the results and obtain a single range measurement. 
     The apparatus and method embodying the present invention will provide significantly greater spatial resolution in either the along-track or cross-track directions or both, as well as greatly reduced demands on spacecraft resources such as prime power, volume and weight allocations. Potential altimeter targets are land, ice, and water surfaces as well as distributed or soft targets such as clouds, planetary boundary layers, tree canopies and other vegetation. The much-reduced signal levels of the microaltimeter embodying the present invention relative to conventional altimeters are largely offset by a corresponding reduction in the detection thresholds to one photoelectron or less so that instrument sensitivity is enhanced. As a result geoscience applications including development of high resolution, high accuracy topographic databases of land surfaces useful for studying hydrogical runoff, the effects of clouds on radiation balance, changes in sea, lake, or reservoir levels, changes in ice sheet thickness, tree canopy heights and biomass assessment are all possible through use of the apparatus and method embodying the present invention. Applications to extraterrestrial science missions including low power high resolution topographic mapping of other planets, moon, asteroids and comets within the Solar Systems may also be carried out through he apparatus and method of this invention. The apparatus and method of this invention may also be used for aerial surveying of cities and towns and/or generating of local topographic maps from high aircraft cruise altitudes which do not require special Federal Aviation Administration waiver. 
     It is a principal aspect of the present invention to provide an apparatus and method for low power, highly accurate ranging detection from a high altitude aircraft or an orbiting spacecraft or the like. 
     Other aspects of the invention will become obvious to one of ordinary skill in the art upon a perusal of the following specification and claims in light of the accompanying and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a apparatus embodying the present invention; and 
     FIG. 2 is a graph showing details of a signal processing technique embodying the present invention 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings and especially to FIG. 1, an apparatus embodying the present invention is shown therein and generally referred to by reference numeral  10 . The apparatus  10  comprises in this embodiment, a microaltimeter  10 . The microaltimeter  10  includes a source of coherent, substantially monochromatic light  12 . Coherent light source  12  is a solid state microlaser  12 . The microlaser  12  is a pulsed Q-switch solid state laser which emits light at a wavelength of 532 nanometers in pulses having a duration of less than one nanosecond to provide high spatial resolution. The laser is pumped from a light source through a fiber optic link and an appropriate collimating lens which supplies light to the gain material comprising the laser. The laser emits the pulses at a rate of about 10 kilohertz, each pulse being 50 to 100 milliJoules in energy. The pulses are sent to a telescope  14  which then collimates the laser light, sends it the ground and receives reflected laser photons back. The reflected photons are then fed to a photon time-of-flight detector  16  which is connected to a processor  18  which draws conclusions as to the range data. 
     In further detail, the microaltimeter laser  12  is a Nd:YAG laser. The laser  12  emits a beam  20  which is received by a mirror  22 , forwarding beam to a second mirror  24  which ultimately sends the light to the concave off-axis paraboloidal mirror  26  of the telescope  14 . The paraboloidal mirror then launches a collimated pulse beam  30  which may be sent through a optical scanning wedge  32  to a surface. The beam then returns along substantially the same beam path, is received back at the primary mirror  26 , fed to the mirror  24  and a portion of the beam is fed to a spectral and spatial filter  40 . That filter comprises a 140 microradian spatial filter for removing extranians solar background photons as well as a 3 nanometer bands pass spectral filter centered around the frequency of the laser light for removing further solar background photons which will further reduce the signal-to-noise ratio of the detector. The resulting filter and light is sent in a stream  50  to a quad photomultiplier tube that is a photomultiplier tube assembly having four areas which are able to provide some ground resolution. 
     The quad photomultiplier tube is sufficiently sensitive that it can produce output signals for each of the photons it receives thereby acting as a photon time of flight detector. The four elements of the quad photomultiplier tube feed separate signals on a four line photomultiplier bus  52  to an amplifier set  54 . Then amplifies set then provides and amplified light detector output on amplifier bus lines  56  to a serial-to-parallel converter  58  and to a range gate generator control on or gate  60 . 
     A range gate generator  62  has a detector  64  which starts a timing sequence running and supplies a gate signal over a line  66  to the quad photomultiplier tube indicating whenever a laser pulse has been launched. The range gate generator  62  also sends a signal to a multichannel scalar or coarse receiver  70  which receives an output from the generalized output from a range gate control  60  from the amplifier set  54 . The multichannel scalar  70  provides coarse range data which is binned in the manner shown in  72  and then fed to the control and data acquisition system  18 . 
     That system includes a Windows NT based computer  74 , a VXI or fine range data processing computer  76  and a shared memory  78  for processing the information. In addition the NT computer receives time and position data through a line  90  from other portions of the system including an inertial navigation system, and gyroscopic compass  92  which may include a laser gyro system or the like which sends attitude information to a navigation imaging CPU and memory or processor system  94 . That system in term is able to communicate the epoch data to the anti computer. It also receives signals from a global positioning system receiver  100  located on the satellite or aircraft and from a wide field-of-view digital imaging system  102 . The system is able to process the information and store data on the disk drive and draw conclusions as to the rough instantaneous position of the vehicle in order to further find the range data. 
     The output amplified signals from the photomultiplier tubes are fed through the serial to parallel converter over a plurality of four buses  110  to a  16  channel time-to-digital converter  112  which provides detailed or fine range data of the type shown in  114  via a bus  116  to the VXI computer  76 . The VXI computer  76  is then programmed to provide binning operations of the range data. 
     The start pulse and one or more stop pulses will be recorded for each laser fire by the course and fine receivers  70  and  112 . They operate as correlation range receivers but with quite different range gates and resolution post detection filtering will identify which cells or tinning bins in the range gates of the output of the course and fine receivers are most likely to contain signal counts. The photon time of flight signals are then subtracted from aircraft or spacecraft altitude and corrected for pitch, roll, and yaw all of which is received by the line  90  at the control and data acquisition computers  18 . The final data analysis product will be a three dimensional plot of the terrain for a scanned swath of approximately 500 meters or less beneath the flight path. With the use of laser pulse energy of 10 microJoules and a 14 centimeter telescope aperture a mean signal strength of about 1.5 photoelectrons per laser pulse from a surface with low reflectance, on the order of 0.1 and a mean slope of 45° results. In this type of system that would result in a 76 percent probability of detection for the reference slope that would approach 100 percent for level terrain. 
     The correlation range receiver identifies the signal cell by summing the counts in each cell and comparing the total to a frame threshold value which has been previously chosen using poisson statistics. When the count exceeds the threshold the cell is tentatively identified as containing a signal indicating that a laser photon has been received. Otherwise, it is tentatively identified as containing noise. In the event that adjacent cells are unable to be determined to have signals, an N of M test will be applied to the data requiring that in N of M succesive frames making up a superframe one of the cells passes the threshold test and is displaced from signal cells and adjacent frames by no more than one range bin in vertical range. 
     In addition more sophisticated tracking algorithms which make examinations forward and backward along the time axis of the bins are also possible and could further accommodate steep or rapidly changing terrain features. For example, looking forward using adjacent range bin criteria and then working backward can lead to the identification of a middle cell which contains most of the reflected signal. 
     While there has been illustrated and described a particular embodiment of the present invention, it will be appreciated that numerous changes and modifications will occur to those skilled in the art, and it is intended in the appended claims to cover all those changes and modifications which fall within the true spirit and scope of the present invention.