U.S. patent application number 13/403671 was filed with the patent office on 2013-08-29 for rear looking snow helmet.
The applicant listed for this patent is Dennis D. McCrady, Jeffrey S. Tonigan. Invention is credited to Dennis D. McCrady, Jeffrey S. Tonigan.
Application Number | 20130219592 13/403671 |
Document ID | / |
Family ID | 48952026 |
Filed Date | 2013-08-29 |
United States Patent
Application |
20130219592 |
Kind Code |
A1 |
McCrady; Dennis D. ; et
al. |
August 29, 2013 |
Rear Looking Snow Helmet
Abstract
A technique for alerting a user of approaching moving objects
from the side or rear includes a rear looking radar with audio
alerts to the user. Feasibility of the Rear Looking Snow Helmet is
shown using the skiing and snowboarding applications where variable
frequency (distance related) audio alerts are provided to left and
right earphones depending on the location of the approaching
skier/snowboarder. The electronics driving the system are mounted
in the skier's/snowboarder's helmet while two antenna elements are
mounted on the rear of the helmet. A large ON/OFF switch is mounted
on the helmet for easy access. We show feasibility of the concept
using performance analysis and by proposing an implementation
architecture for the skiing and snowboarding applications. We claim
the system will meet an acceptable level of performance when
parameters are varied and traded off and that the system is
technology independent.
Inventors: |
McCrady; Dennis D.;
(Albuquerque, NM) ; Tonigan; Jeffrey S.;
(Albuquerque, NM) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
McCrady; Dennis D.
Tonigan; Jeffrey S. |
Albuquerque
Albuquerque |
NM
NM |
US
US |
|
|
Family ID: |
48952026 |
Appl. No.: |
13/403671 |
Filed: |
February 23, 2012 |
Current U.S.
Class: |
2/410 |
Current CPC
Class: |
A42B 3/046 20130101;
A42B 3/0426 20130101 |
Class at
Publication: |
2/410 |
International
Class: |
A42B 3/30 20060101
A42B003/30 |
Claims
1. A method used for warning a skier/snowboarder of an approaching
skier/snowboarder from the side or behind, comprising: a. A
transmitter portion of a rear looking radar system module mounted
within the skier's/snowboarder's helmet using: 2 MHz bandwidth, 5.8
MHz center frequency, 2 mW transmit power, direct sequence spread
spectrum with binary phase shift key modulation and 1024 chip
Hadamard code symbols and a radar range gate timer set to 4.5 s at
transmission; b. A two element antenna array mounted within or on a
rear surface of the skier's/snowboarder's helmet used in
conjunction with transmit and receive beamformers to form
.+-.30.degree. beams; c. A receiver portion of said rear looking
radar system module (mounted within the skiers/snowboarder's
helmet) using: said radar range gate with 10 m maximum range,
clutter processing to remove returns from stationary targets, a
correlator matched to the 1024 chip Hadamard code, a threshold
monitoring mechanism to declare a valid radar detection and turn
off the radar range gate timer, and a range determination
algorithm; d. A timing and control portion of said rear looking
radar system module that includes a sequencing and alerting
algorithm to control radar ping sequencing and user alerts; e.
Right and left earphones mounted inside the skier's/snowboarder's
helmet to which range proportional, variable frequency audio alerts
are directed indicating range and direction to a detected target;
f. A rechargeable battery mounted within said rear looking radar
system module with a recharging interface mounted on a surface of
the helmet; and g. A large ski glove friendly ON/OFF switch mounted
on a surface of the skier's/snowboarder's helmet for easy access
when transitioning to high false alarm rate areas.
2. The method of claim 1 wherein: a. The rear looking radar system
technology, parameters, mechanical configuration and physical
mounting configuration can be varied; b. The audio alert function
technology, parameters, mechanical configuration and physical
mounting configuration can be varied; c. The ON/OFF switch
technology, parameters, mechanical configuration and physical
mounting configuration can be varied; and d. The battery
technology, parameters, mechanical configuration and physical
mounting configuration can be varied.
3. The method of claim 1 except used for warning a pedestrian of an
approaching pedestrian (intending or not intending harm) or vehicle
from the side or behind wherein: a. The rear looking radar system
technology, parameters, mechanical configuration and physical
mounting configuration can be varied; b. The audio alert function
technology, parameters, mechanical configuration and physical
mounting configuration can be varied; c. The ON/OFF switch
technology, parameters, mechanical configuration and physical
mounting configuration can be varied; and d. The battery
technology, parameters, mechanical configuration and physical
mounting configuration can be varied. e. The system could be
mounted within a hat or object of clothing or standalone.
Description
CROSS REFERENCE TO RELATED APPLICATIONS (IF ANY)
[0001] None.
STATEMENT OF FEDERALLY SPONSORED RESEARCH/DEVELOPMENT (IF ANY)
[0002] None.
BACKGROUND OF THE INVENTION
[0003] There are many situations during a typical skiing or
snowboarding day in which skiers and snowboarders would appreciate
having eyes in the back of their heads. Whether cruising down a
wide open slope, merging with an adjacent trail, or transiting a
slope or trail on a catwalk, being able to "see" sideways or
backwards provides a new level of safety to skiers and snowboarders
of all abilities. Not all skiers and snowboarders are cautious or
make a concerted effort to obey the rules of skiing or snowboarding
etiquette. It is not uncommon to observe skiers and snowboarders of
limited ability barreling down a slope out of control colliding
with or nearly missing unsuspecting, controlled skiers and
snowboarders on the same slope. Other skiers and snowboarders can
be seen projecting themselves out of the trees at the side or
bottom of a trail without concern for passing skiers or
snowboarders. The same situation occurs at trail merges where the
out of control skier or snowboarder on one trail could be just
above an unsuspecting skier or snowboarder on the other trail as
they merge. Catwalks across slopes provide multiple situations for
concern. Skiers or snowboarders traversing the slope on the catwalk
are vulnerable to uphill skiers or snowboarders and the uphill
skiers' or snowboarders' ability to avoid them. Also in play are
the skiers and snowboarders below the catwalk should the uphill
skier or snowboarder decide to jump off the edge of the catwalk
without seeing the skier or snowboarder below the catwalk. The
uphill skier or snowboarder will be airborne when seeing the skier
or snowboarder below the catwalk making it very difficult to avoid
a collision.
[0004] The danger in all of the above examples can be reduced or
avoided if the vulnerable skiers and snowboarders had the ability
to "see" to the side or behind themselves as they proceed downhill.
The ability to "see" can take many forms. Skiers or snowboarders
could constantly turn their heads from side to side in search of
encroaching skiers or snowboarders turning themselves into
partially blind, dangerous projectiles. Skiers and snowboarders
could wear rear view mirrors on their helmets to reduce the amount
of time required to scan behind them. But they still need to focus
on the rear view mirror; time spent not looking for other skiers or
snowboarders in their own downhill path. Rear view mirrors are also
susceptible to frost, fog, and snow, all of which reduce visibility
and lead to periodic cleaning, an irritant taking away from the
free skiing or snowboarding experience. Skiers and snowboarders
could mount video cameras to the rear of their helmets and use a
heads up display in their goggles to view the rearward video scene.
This approach suffers the same disadvantages as the rear view
mirror in addition to being very expensive.
[0005] The approach proposed in this patent allowing skiers and
snowboarders to "see" beside or behind themselves avoids the
problems described above. In general the proposed approach uses a
rear looking radar with audio alerts. The small size, power, and
weight of the radar allow it to be mounted in the helmet. The audio
alert warning of an encroaching skier or snowboarder allows the
skier or snowboarder to continue to look downhill while performing
an evading maneuver. Evading maneuvers could be quick turns to the
right or left or stops to the right or left depending on the audio
alert and the situation.
[0006] There are radar warning systems for automobiles that look
forward.sup.1, sideways.sup.2, and to the rear.sup.3,4,5 for
example, but none fit the skiing and snowboarding applications. The
skiing and snowboarding applications require small, lightweight,
low dissipated power, low transmit power (to maximize user safety),
limited field of view, low delay, quick reaction time, and rear
looking radar detecting a forward moving target. While the
inventors have not found a suitable, off-the-shelf device in the
open literature, there are capable systems' that could be modified
to meet the specific skiing and snowboarding requirements. In
addition, the inventors claim that a system similar to that
described herein could be built with different system parameters
while meeting similar requirements and objectives for the
skiing/snowboarding application and for a pedestrian application.
.sup.1 R. Stevenson, "A Driver's Sixth Sense," IEEE Spectrum,
October 2011, pp 50-55..sup.2 Delphi Automotive Systems,
http://www.prnewswire.com/news-releases/delphis-collosion-avoidance-syste-
ms-take accident-prevention-to-the-next-level, Feb. 23, 1998..sup.3
M. Rao, "Accident Avoidance During Vehicle Backup," U.S. Pat. No.
7,772,991 B2, Aug. 10, 2010..sup.4 B. Osborne,
http://www.geek.com/articles/gadgets/audiovox-offers-easy-wireless-collis-
ion-avoidance-solution, Jun. 17, 2008..sup.5 P. Seiler, et al,
"Development of a Collision Avoidance System," Society of
Automotive Engineers, 98PC-417, 1998.
BRIEF SUMMARY OF THE INVENTION
[0007] A technique for alerting a skier/snowboarder of approaching
skiers/snowboarders from the side and rear includes a rear looking
radar with audio alerts to the user. Feasibility of the Rear
Looking Snow Helmet (RLSH) will be shown. The electronics driving
the system are mounted in the skier's/snowboarder's helmet while
two antenna elements are mounted within or on the rear of the
helmet. A large, ski glove friendly ON/OFF switch is mounted on the
outer shell of the helmet for easy access when the
skier/snowboarder reaches the bottom of the run, for example, where
false alarms could be generated by the motion of
skiers/snowboarders approaching ski lifts or walking to the
cafeteria. Stationary objects like trees, ski lift towers, or
resting skiers/snowboarders are eliminated using standard radar
clutter rejection techniques. The system uses .+-.30.degree. rear
looking beams and provides an audio alert to the left earphone if a
skier/snowboarder is detected in the +30.degree. beam and an audio
alert to the right earphone if a skier/snowboarder is detected in
the -30.degree. beam. A higher audio frequency indicates a closer
range and a lower audio frequency indicates a longer range to the
approaching skier/snowboarder. The RLSH is transparent to the user
until an approaching skier/snowboarder triggers an alert that may
require the user to make a protective maneuver.
[0008] Sophisticated radar systems are available today that are
more than capable of fulfilling the needs of the RLSH. We suggest
that current technology could be modified and simplified to
implement our proposed system. Future technology could be used to
further simplify and reduce size, power, weight, and cost. We
maintain that the RLSH is technology independent. Our main goal
here is to show that the system is feasible today and deserving of
patent coverage. We provide Key Performance Parameters for the
skiing and snowboarding applications that yield an operational
system design coupled with performance analysis to demonstrate
functionality and precision. Conclusions from our performance
analysis show that the RLSH will detect approaching
skiers/snowboarders in the 10m, .+-.30.degree. beam observation
window with a probability of detection >99.9%, a probability of
false alarm <10.sup.-6, and with a range accuracy of 1.5 m. In
addition, a Sequencing and Alerting Algorithm will have access to
data from multiple pings and provide range proportional audible
alerts so the user can evade or prepare for the encroaching
skier/snowboarder.
[0009] The above and further features and advantages of the
invention will become apparent after considering the following
descriptions and figures. While these descriptions and figures go
into specific details of the invention for a specific skiing and
snowboarding application, it should be understood that variations
may and do exist and would be apparent to those skilled in the art.
For example, many of the system parameters could be varied and
another system could be designed to meet similar performance
requirements and objectives for skiers/snowboarders or
pedestrians.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING (IF ANY)
[0010] FIG. 1 is a sketch of the Rear Looking Snow Helmet concept
for the skiing and snowboarding applications depicting the
operational use of the system as well as showing how the system is
mounted within and on the snow helmet.
[0011] FIG. 2 is a table that lists key performance parameters for
the Rear Looking Snow Helmet for both skiing and snowboarding
applications.
[0012] FIG. 3 is a system block diagram of the key functions and
signal flow of the Rear Looking Snow Helmet.
[0013] FIG. 4 is a snapshot of a Rear Looking Snow Helmet user with
encroaching snowboarder to illustrate RF link budget and timing
calculations.
[0014] FIG. 5 shows the antenna array layout, the beamformer, the
beamformer equations, and the beampattern for the .+-.30.degree.
beams of the Rear Looking Snow Helmet.
[0015] FIG. 6 is a plot showing the number of achievable pings
between the 10 m initial detection and 2 m threshold versus three
realistic differential velocities for the Rear Looking Snow
Helmet.
DETAILED DESCRIPTION OF THE INVENTION
[0016] Introduction
[0017] The invention overcomes the aforementioned dangers of
downhill skiing and snowboarding. Using a rear looking radar with
audio alerts mounted in the skier's/snowboarder's helmet allows the
skier/snowboarder to focus attention on the fall line below knowing
an audio alert will warn of encroaching skiers/snowboarders from
the side or behind. This enables the skier/snowboarder to ski more
confidently, enjoying a safer free skiing and snowboarding
experience while improving the safety of fellow skiers/snowboarders
as well.
[0018] As shown in the non-scale concept sketch in FIG. 1, the RLSH
contains a small rear looking radar system 101 mounted on and
inside the helmet. This system detects encroaching
skiers/snowboarders then warns the user. The electronics driving
the system are mounted in the helmet liner while a two element
antenna array 102 is mounted within or on the outer shell of the
helmet. A large, ski glove friendly ON/OFF switch 103 is mounted on
the outer shell of the helmet for easy access when the
skier/snowboarder reaches the bottom of the ski trail, for example,
where false alarms from slow moving, benign targets could be
generated. The benign targets could be skiers or snowboarders on or
off their skis or snowboards moving toward a ski lift or the
cafeteria. The system uses .+-.30.degree. rear looking beams that
provide rear and ample side coverage. If an approaching
skier/snowboarder is detected in the +30.degree. beam 104, the
system will generate a variable frequency audio alert to the left
earphone 105, where a higher audio frequency indicates a closer
range and a lower audio frequency indicates a longer range to the
approaching skier/snowboarder. Likewise, if an approaching
skier/snowboarder is detected in the -30.degree. beam, the system
will generate an audio alert to the right earphone. A rechargeable
battery is included in the electronics package with a recharging
interface 106 on the surface of the helmet. The RLSH is transparent
to the user until an approaching skier/snowboarder triggers an
alert that may require the user to make a protective maneuver.
[0019] A key component in the RLSH is the radar system.
Sophisticated radar systems are available today.sup.1 that are more
than capable of fulfilling the needs of the RLSH. We suggest that
current technology could be modified (simplified) to implement the
RLSH. Future technology could be used to further simplify and
reduce size, weight, power, and cost, i.e. the RLSH is not
technology dependant. Key performance parameters for the generic
rear looking radar for the skiing and snowboarding applications are
shown in FIG. 2. Note that size, weight, and power (SWAP) are
technology dependent. The initial prototypes could be implemented
with Altera or Xilinx Field Programmable Gate Arrays (FPGAs) which
may not initially meet the desired SWAP goals. When a final design
is crafted in state-of-the-art Application Specific Integrated
Circuit (ASIC) technology such as that used by the Infineon/Bosch
team.sup.1 or Freescale Semiconductor.sup.1, the SWAP goals should
be easily met. Reducing the 500 MHz bandwidth used by
Infineon/Bosch to 2 MHz for the RLSH provides an approximate
clocking reduction scale factor of 250 leading to our optimism for
meeting the SWAP goals in a production system. Further detail will
be provided below for the Key Performance Parameters as they relate
to the System Block Diagram in FIG. 3 and as they are used to show
feasibility of the RLSH. FIG. 3 shows a feasible architecture for
implementing the RLSH. This architecture is based on digital signal
generation in the transmitter and digital signal processing in the
receiver. There are many architectures that could be applied to the
RLSH involving various mixes of digital and analog hardware. There
will be additional architecture choices as technology advances in
the future. The purpose here is to show that the RLSH concept is
feasible today while not limiting the current and future
implementation possibilities.
[0020] System Description
[0021] Referring to the transmitter side of FIG. 3, the Transmit
Signal Generator 301 and the Intermediate Frequency (IF) 302 are
typically prototyped in an FPGA, as mentioned above, in a digital
baseband design. Current FPGA densities and speeds available from
Altera or Xilinx are more than adequate given the sampling and chip
rates being used. Once prototyped and tested, the final design can
be implemented in ASIC technology, as mentioned above. The Transmit
Signal Generator is where the 2 Mcps (Mega-chips per second), 1024
chip BPSK (Binary Phase Shift Keyed) signal is implemented and
transmitted to start the precision timing for the radar range gate
used to measure the distance to the encroaching skier/snowboarder.
The transmitted signal is then passed to the IF. The baseband
signal is translated to a digital IF within the bandwidth of the
Digital-to-Analog (D/A) Converter then converted to an analog
signal by the D/A converter 303. A typical IF is 70 MHz leading to
a D/A converter sampling rate on the order of 100 MHz, a reasonable
rate for current 10-12 bit D/A's, providing ample resolution for
the RLSH application. The Radio Frequency (RF) function 304
translates the signal to 5.8 GHz, the third Industrial, Scientific,
and Medical (ISM) band where few restrictions apply, interference
is unlikely, and off the shelf components are available. The
Beamformer 305 consists of Transmit (Tx) and Receive (Rx)
functions. The Tx Beamformer generates the two beams at
.+-.30.degree. used to discriminate whether a skier/snowboarder is
approaching from behind on the right or left side. A simple
.+-.90.degree. phase shift yields .+-.30.degree. beams,
respectively. The Power Amplifiers 306 amplifies the phase shifted
RF signals from the beamformer to 2 mW and applies them through the
Transmit/Receive (T/R) Switch 307 to the Antenna Array 102. Timing
and Control 308 is implemented in a General Purpose Processor (GPP)
and controls timing of the transmit signal from generation through
setting of the T/R Switch in the transmit position at the
appropriate time. The GPP can be purchased from Intel or
implemented as a standard building block in an FPGA depending on
desired functionality and complexity.
[0022] Referring to the receive side of FIG. 3, the Timing and
Control GPP 308 sets the T/R switch in the receive position at the
appropriate time and the signals from the antenna array are routed
through Preamplifiers 309 to the Rx Beamformer 305. Like the Tx
Beamformer, a simple .+-.90.degree. phase shift yields
.+-.30.degree. beams, respectively. The 30.degree. beams are
translated by the RF to the same IF 310 used by the transmitter
where they can be bandpass sampled by the Analog-to-Digital (A/D)
Converter 311 at a sampling rate of 40 Msps, a reasonable rate for
current 10-12 bit A/D converters, providing ample resolution for
the RLSH application. The Detection Processing 312 is typically
performed in the same FPGA, for the prototype, or ASIC, for the
production system, used for transmit signal generation. Bandpass
sampling at 40 Msps moves the IF frequency to 10 MHz. Detection
Processing includes translating the spectrum to baseband from the
digital IF at 10 MHz and filtering. Once at baseband, the sampling
rate can be decimated to the minimum sampling rate of twice the
bandwidth, 4 Msps, to minimize the required processing load.
Digital clutter processing, using a two pulse canceller.sup.6, is
applied to remove returns from stationary targets such as trees,
ski lift towers, or resting skiers and snowboarders. A digital
correlator matched to the 1024 chip Hadamard code is then performed
while the correlator output is monitored for threshold crossings
indicative of detecting a valid radar return. The threshold will be
set to guarantee a probability of detection (P.sub.d)>99.99% and
a Probability of False Alarm (P.sub.fa)<10.sup.-6. A valid radar
return stops the radar range gate timer started by the transmitter
and provides the accumulated round trip time (to and from the
target) to the range determination algorithm. Note that proper care
must be taken to calibrate (precisely measure) the transmitter and
receiver delays in order to refer the true, over the air signal
timing measurement to the antenna, while excluding all system
delays. The Signal-to-Noise Ratio (SNR) at the correlator output,
used to determine P.sub.d and P.sub.fa, will be estimated below in
the Performance section. Once an appropriate radar return is
detected in one of the 30.degree. beams, an appropriate Audio Alert
313 will be generated and channeled to the corresponding Left 314
or Right 315 Earphone. The receiver Timing and Control function in
the GPP 308 controls the flow of the signal from the T/R switch
through the receiver to the earphones where the user is alerted to
the encroaching skier/snowboarder. .sup.6 A. Oppenheim,
Applications of Digital Signal Processing, Prentice Hall, Englewood
Cliffs, N.J., 1978, Performance page 310.
[0023] Performance
[0024] FIG. 4 will be used to illustrate the calculation of SNR and
time based parameters in order to determine feasibility of the
RLSH. SNR is used to determine P.sub.d versus P.sub.fa and location
accuracy. Time related variables include coherence time and total
reaction time.
[0025] Performance--SNR Related Parameters
[0026] SNR determination begins with transmit power and a link
budget. A low value of transmit power (2 mW) was selected to
eliminate harmful RF radiation to the user. The link budget is used
to predict the received SNR back at the RLSH after a radar signal
is transmitted, propagates to the approaching skier/snowboarder,
reflects off the approaching skier/snowboarder then propagates back
to the RLSH.
[0027] Referring to FIG. 4, Effective Isotropically Radiated Power
(EIRP) is:
EIRP=P.sub.T-L.sub.C+G.sub.A (1)
where:
[0028] P.sub.T=10 LOG(2 mW)=3 dBm,
[0029] L.sub.C=cable losses=0.5 dB (assumed), and
[0030] G.sub.A=antenna gain (or loss)=-5.5 dB (assumed),
yielding
[0031] EIRP=3-0.5-5.5=-3 dBm.
[0032] As shown in FIG. 5, the Antenna Array 102 consists of two
0.5''.times.0.5'' dielectric antenna elements (by TOKO, for
example) that are combined through a simple beamformer 305 to
provide .+-.30.degree. beams at -3 dB 501. Equations for the
beamformer 502 are also shown in FIG. 5 where a phase shift of
(-)90.degree. is used to generate the (-)30.degree. beam,
respectively 501. Antenna gain (or loss) above is determined at the
worst case magnitude, -3 dB, along with an assumed additional -2.5
dB to account for other antenna related losses resulting in a total
loss of -5.5 dB.
[0033] Referring again to FIG. 4, the radar signal is transmitted
from the antenna at EIRP=-3 dBm and propagates to the prospective
target (approaching skier/snowboarder) over a range R, reflects
according to the Radar Cross Section (RCS) of the
skier/snowboarder, propagates back over the same range, R, and the
signal, S, is received by the user. The Two-Ray Propagation
model.sup.7 is used to determine the Propagation Loss (PL) of the
radar signal as it propagates to and from the target: .sup.7 T.
Rappaport, Wireless Communications Principles and Practice, IEEE
Press, NY, N.Y., and Prentice Hall PTR, Upper Saddle River, N.J.,
1996, page 89.
PL=20 LOG(H1)-40 LOG(R)-40 LOG(R)+20 LOG(H2) (2)
where,
[0034] H1=H2=helmet mounted antenna height from ground=5'9''=1.75 m
(assumed)
[0035] R=10 m (maximum range)
therefore,
[0036] PL=20 LOG(1.75)-40 LOG(10)+20 LOG(1.75)-40 LOG(10)
[0037] PL=5-40+5-40=-70 dB.
[0038] Given the propagation loss, a link budget can be derived for
the received signal strength, S, at the input to the user
receiver:
S=EIRP+PL+RCS+G.sub.A-L.sub.C (3)
where,
[0039] RCS of the approaching skier/snowboarder=-3 dB.sup.8 .sup.8
T. Doraru and C. Le, "Validation of Xpatch Computer Models for
Human Body Radar Signatures," Army Research Laboratory,
ARL-TR-4403, March 2008.
[0040] S=-3-70-3-5.5-0.5=-82 dBm.
[0041] Knowing that a typical off-the-shelf, 3.sup.rd ISM band
receiver.sup.9 has a noise floor, N=-92 dBm, the SNR at the
receiver input, SNR.sub.i, can be determined: 9 Altan ALT5801, 5.8
GHz Transceiver Module, Altan Technologies, June 2009.
SNR.sub.i=S-N (dB) (4)
[0042] SNR.sub.i=-82-(-92)=10 dB.
[0043] The SNR at the output of the correlator, SNR.sub.o, is then
determined as:
SNR.sub.o=SNR.sub.i+PG (5)
where,
[0044] PG=Processing Gain=10 LOG(correlator length)=10 LOG(1024)=30
dB.
[0045] Substituting into (4) yields,
[0046] SNR.sub.o=10+30=40 dB.
[0047] From Whalen.sup.10, an output SNR.sub.o=16 dB supports
P.sub.d=99.9% and P.sub.fa=10.sup.-6 leaving a 24 dB margin. The
entire SNR.sub.o=40 dB is needed to achieve reasonable location
accuracy for the example system being proposed. Other and future
systems could tradeoff increased bandwidth (BW) versus SNR.sub.o to
reduce transmit power or antenna efficiency, for example, to reduce
the margin and associated costs. As shown below, bandwidth is
directly proportional to location accuracy in time, .sigma..sub.t,
which is estimated using the Cramer-Rao Bound (CRB).sup.11: .sup.10
A. Whalen, Detection of Signals in Noise, Academic Press, New York,
1971, p 248..sup.11 H. Poor and G. Wornell, Wireless Communications
Signal Processing Prospectives, Prentice Hall, Upper Saddle River,
N.J., 1998, p 383.
CRB=1/(BW.times.SNR.sub.o.sup.1/2)=.sigma..sub.t (6)
[0048]
.sigma..sub.t=1/[2.times.10.sup.6.times.(10,000).sup.1/2]=5.times.1-
0.sup.-9=5 ns,
where the range error in meters is,
[0049] .sigma..sub.r=speed of
light.times.time=3.times.10.sup.8(m/s).times.5.times.10.sup.-9(s)=1.5
m.
[0050] A range error of .sigma..sub.r=1.5 m is sufficient to allow
several warnings of an encroaching skier/snowboarder first detected
at a range of 10 m.
[0051] Performance--Time Related Parameters
[0052] Moving on to the time related parameters, we begin with
differential velocity, .DELTA.v, between the skier using the RLSH
(shown in FIG. 4), moving at velocity 1 (v1) meters/second (m/s),
and the approaching snowboarder, moving at velocity 2 (v2) m/s:
.DELTA.v=v2-v1 m/s. (7)
[0053] The speed of average downhill skiers/snowboarders is on the
order of 20 mph..sup.12 Assuming that the encroaching
skier/snowboarder is approaching at 25-35 mph, .sup.12
http://www.trails.com/facts 9654
how-fast-do-downhill-skiers.html.
[0054] .DELTA.v.sub.min=5 mph (=7.33 feet/sec),
[0055] .DELTA.v.sub.max=15 mph (=22 feet/sec,=6.67 meters/sec).
[0056] In order to reduce false alarms, the field of view is
limited by the range gate to 10 m. The maximum and minimum total
reaction times, T.sub.Rmax and T.sub.Rmin, for the RLSH assuming a
maximum range of 10 m (33 feet) and the above differential velocity
assumptions are:
T.sub.Rmax=33/.DELTA.v.sub.min=33/7.33=4.5 sec, (8)
T.sub.Rmin=33/.DELTA.v.sub.max=33/22=1.5 sec. (9)
[0057] A simple Sequencing and Alerting Algorithm is used to
support the short reaction and alerting times and evade approaching
skiers/snowboarders. [0058] 1. Transmit (ping) every 0.5 second in
alternate (.+-.30.degree.) beams. [0059] 2. If approaching
skier/snowboarder is detected: [0060] a. Provide a warning and ping
in the same beam until the range <2 m, [0061] b. Provide
warnings with increased audible frequency after each ping as the
range decreases to <2 m, [0062] c. Warn RLSH user within 0.5
second during each ping cycle including detection processing,
keeping up with real time. [0063] 3. Resume by pinging in the
alternate beam. [0064] 4. Continue alternate beam pinging until
another approaching skier/snowboarder is detected and repeat the
algorithm.
[0065] As shown in FIG. 6 for examples of 15, 10, and 5 mph
differential velocities, there is time for 2, 3, and 7 pings,
respectively, between the 10 m initial detection and the 2 m
threshold where successive detections are indicated with increasing
audible frequency.
[0066] Coherence time, T.sub.c, the final time related parameter,
is the time over which a signal can be coherently integrated.
T.sub.c is related to wavelength, .lamda., of the transmitted
signal and the maximum differential velocity, .DELTA.v.sub.max,
between the user and the encroaching skier/snowboarder.sup.13:
[0067] .sup.13 T. Rappaport, Wireless Communications Principles and
Practice, IEEE Press, NY, N.Y., and Prentice Hall PTR, Upper Saddle
River, N.J., 1996, page 166.
T.sub.c.apprxeq.0.423.lamda./.DELTA.v.sub.max, (10)
[0068] T.sub.c.apprxeq.0.423.times.0.052/6.67.apprxeq.3.3 ms.
[0069] From FIG. 2, the required coherent correlation time is 0.5
ms, a value much less than T.sub.c indicating that the system will
achieve its maximum processing gain of 30 dB which was used in the
link budget above to calculate other performance related
parameters.
* * * * *
References