The Mars 94/96 Mission will carry, among others things, the balloon probe experiment. The balloon with the scientific cargo in the gondola underneath will drift in the Mars atmosphere, its altitude will range from zero, in the night, up to 5 km at noon. The accurate gondola altitude will be determined by an altimeter. As the Balloon gondola mass is strictly limited, the altimeter total mass and power consumption are critical; maximum allowed is a few hundred grams a few tens of mWatts of average power consumption. We did propose, design, and construct the laser altimeter based on the single photon ranging technique. Topics covered include the following: principle of operation, altimeter construction, and ground tests

Hamal, Karel

Pershin, S.

Prochazka, Ivan

Sopko, B.

English

NASA Technical Reports Server

N94-15-3i9
MARS LASER ALTIMETER BASED ON
A SINGLE PHOTON RANGING TECHNIQUE
I.Prochazka, K.Hamal, B.Sopko
Faculty of Nuclear Science and Physical Engineering, Czech Technical University
Brehova 7, 115 19 Prague 1, Czechoslovakia
ph/fax +42 2 848840, fix 121254 fjfi c, bitnet TJEAN@CSEARN.BITMET
S.Pershin
Space Research Institute,Moscow,Russia
GENERAL
The MARS 94/96 mission [1] will carry, among others, the balloon probe experiment.
The balloon with the scientific cargo in the gondola underneath will drift in the Mars
atmosphere, its altitude will range from zero, in the night time, up to 5 km at noon. The
accurate gondola altitude will be determined by an altimeter. As the balloon gondola mass
is strictly limited, the altimeter total mass and power consumption are critical, maximum
allowed is a few hundreds grams and a few tens of mWatts of average power consumption,
only. We did propose, design and construct the laser altimeter based on the single photon
ranging technique.
PRINCIPLE OF OPERATION
The system operation is based on the single photon laser ranging technique : the short
and low power laser pulse is transmitted in the nadir direction, the signal reflected from the
planet surface is collected by the receiver lens and detected by the photon counting device.
The light time of flight is measured. Completing the series of measurements, the altitude, the
range to the surface, may be evaluated. The number of individual laser fires needed for one
altitude determination is limited by the altitude itself, the background illumination intensity
and hence the signal to noise ratio and the balloon gondola vertical velocity. Thanks to the
relatively high repetition rate of the laser transmitter (currently 7 kHz, the 25 kHz version
is expected to be available in the near future), one altitude measurement may be completed
within fractions of a second.
The single photon ranging technique has several key advantages in this application :
* the low power, high repetition rate diode laser may be used as a transmitter_
* the wavelength of such a transmitter well coincides with the maximum sensitivity of the
semiconductor photon counting detector,
* no analog signals are treated within the device, all the information is obtained from the
statistical treatment of the set of range measurements. This fact is resulting in overall
simplification of the device and its performance stability within a wide temperature range.
All the electronics may be based on digital, low power CMOS circuits only, what is resulting
in low mass and low power consumption,
* the laser ranging system may be used, for the atmospheric clouds, haze and aerosols
monitoring, as well.
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ALTIMETER CONSTRUCTION
As a transmitter, the integrated laser diode pulser LD191 is used. It delivers uniform,
fast dsetime pulses at 880 nanometers wavelength 100 nanoseconds long with the energy of
2 micro/oules. The receiver consists of a Single Photon Avalanche Diode detector package
[2], its operating voltage is controlled by a newly designed circuit in a wide temperature
range -60 to +20 Centigrade. The diode's quantum efficiency reaches 20 % at this
wavelength range. The transmitter and receiver share the same optics of 40mm aperture, the
optical path is separated by a polariser. To keep the optics as simple as possible, the diode
aperture and hence the receiver field of view is not perfectly matched to the laser transmitter
one. By this compromise the optics mass and alignment precision requirements have been
significantly reduced on the expense of the energy budget link. The bandpass filter 5
nanometers wide having the transparency of 60% is used to reduce the background
illumination. The altimeter electronics consists of a flying time counter having 33
nanoseconds resolution, the programmable range gate generator, the control logic and
interface to the board computer.
To facilitate the single photon ranging even under the Mars daylight conditions, the
combination of the detector limited field of view, optical bandpass filtering, the adjustable
range gate electronics _md the data processing software have to be applied. One out of three
main strategies may be selected for setting up the range gate :
1. 'Start from scratch' all the possible range gate settings are consequently applied, the echo
signal is recognized. This is an emergency solution not requiring any a priori knowledge of
the altitude to be measured.
2. ' Morning take off' starts from the range gate interval 0 - 150 meters. It is based on the
presumption, that the gondola will take off after the sunrise.
3. ' Routine operation ' the range gate setting is predicted on the basis of the previously
measured altitude and vertical velocity. The signal search in the +/- 1 range gate interval
may be carded out, if necessary.
GROUND TESTS
To proof the feasibility of the device, the experimental system has been constructed
at the Space Research Institute, Moscow
and tested in a series of ground test at the
Czech Technical University. The ultimate
single shot range resolution and the mea-
sured range dependence on the echo signal
strength (the range bias) have been mea-
sured. The LDI91 laser diode pulser, the
single photon detection package and a high
resolution time interval unit has been used.
The results are plotted on Figure 1 together
with the return rate. From the single shot
range resolution of 4-5 meters one can
conclude, that the ranging precision of one
meter may be obtained averaging more than
25 returns. The ground target ranging with-
in the ranges 0-3.5 km has been obtained
bias,jilter [meters I return rate
0,1
0 _ _H,. .................. , ...... 0.01
o._ , _o _oo _ooo
signal slrenglh [pholonslecho]
I.P_echlb/_lt.S.Perskil. P,lgue. NotOgll9
Figure 1 Indoor calibration test
using the optics aperture of 17 mm only. The example of the ranging result is on Figure 2,
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Ground target ranging, 3.5 km distance
Fig. 2
g.
_SER pul_ses 60000
elevation angl e:5g_le9
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Vertical humidity haze monitoring, daylight
multiple haze layer may be seen
13-76
Fig. 3
this is a horizontal ranging to the 3.5 km distant target in a light haze visibility conditions.
Employing the full 40 mm aperture optics of the f'mal version, the full range will be guaran-
teed.
The single photon ranging system may be used for the atmospheric transparen-
cy/backscatter monitoring [3]. Currently, the proposal, that such a system may be used for
the Mars atmosphere monitoring, is under investigations. Either using the independent unit
installed in the 'Small Mars Station' [4] or directly from the balloon gondole in the night
time, when is it located on the surface and it is not used for altimetry. In another words, the
system may be used for the clouds distance and density profile monitoring, as well. On
Figure 3 there is an example of the results of the clouds monitoring. The laser rangefinder
was pointed to the zenith, the lower cloud boundary and its density profile may be
recognized. The clounds may be monitored day and night up to the height of 1 km.
CONCLUSION
The altimeter based on the single photon ranging technique outranged the microwave
and Nd YAG laser systems by low weight, low power consumption and the compact design.
The mass, dimensions and price of such a system make it attractive not only for Mars
mission, but also for mobile, air and space born remote sensing applications. The excellent
stability of both the laser transmitter, and the single photon detector permits tO calibrate the
receiver sensitivity to high accuracy and thus to use the system as a precise visibility and
surface reflectivity sensor. The applications of a visibility sensor in automated meteo stations,
traffic safety, air and ship safety systems seem to be quite promising. In all these
applications, the eye safety of the laser transmitter is a significant advantage of the device
decsign. The air and space born remote sensing application in agriculture, geology, large area
forest monitoring, soil humidity and radioactive Cesium dust monitoring is under
consideration.
References
[1] R. Sagdeev, Physics Today, 5, 30 (1988)
[2] I. Prochazka, K.Hamal, B.Sopko, G.Kirchner, Technical Digest of the Conference on
Lasers and Electro Optics, CLEO 91, Baltimore, May 1990
[3] I.Prochazka, K.Hamal, S.Pershin, Capabilities of Single Photon Detection in LIDAR for
Atmospheric Studies, International Symposium on Radars and Lidars in Planetology, Cannes,
France, Sept. 1991
[4] I.Prochazka, K.Hamal, et al, MARS 94 Mission Backscatter LIDAR for Small Mars
Stations, Gound Tests Results, European Geophysical Society XVII General Assembly,
Edinburg, April 1992
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Mars laser altimeter based on a single photon ranging technique

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