U.S. patent application number 09/147300 was filed with the patent office on 2002-02-28 for method and a system for monitoring plurality of movable objects.
Invention is credited to OLESEN, LYKKE.
Application Number | 20020024448 09/147300 |
Document ID | / |
Family ID | 20402673 |
Filed Date | 2002-02-28 |
United States Patent
Application |
20020024448 |
Kind Code |
A1 |
OLESEN, LYKKE |
February 28, 2002 |
METHOD AND A SYSTEM FOR MONITORING PLURALITY OF MOVABLE OBJECTS
Abstract
A method and a system for monitoring a plurality of movable
objects, wherein each of the objects is equipped with a transponder
with which a stationary node is arranged to communicate. Each
transponder is caused to reply to a unique signal from the node and
each transponder that sends a response signal gives to the response
signal a transponder identification addition. A node is caused to
detect the speed and direction at which each transponder moves
towards and away from the node. Movement of the transponder
relative to the node is evaluated by node associated means on the
basis of the speed and direction of the transponder. A register
over those transponders with which the node shall communicate is
setup for each node. A plurality of separate nodes are provided and
each node is provided with such a register. The nodes are arranged
to communicate with one another, so as to enable a node to record
in its monitoring register a transponder from another node.
Inventors: |
OLESEN, LYKKE; (DJURHAMN,
SE) |
Correspondence
Address: |
JACOBSON HOLMAN PLLC
400 SEVENTH STREET N.W.
SUITE 600
WASHINGTON
DC
20004
US
|
Family ID: |
20402673 |
Appl. No.: |
09/147300 |
Filed: |
January 20, 1999 |
PCT Filed: |
May 23, 1997 |
PCT NO: |
PCT/SE97/00848 |
Current U.S.
Class: |
340/8.1 ;
340/10.1; 342/44 |
Current CPC
Class: |
G01S 13/878 20130101;
G01S 19/16 20130101; G08B 21/0263 20130101; G01S 19/51 20130101;
G08B 21/0227 20130101; G08B 21/023 20130101; G01S 2205/007
20130101; G01S 13/765 20130101; G01S 13/78 20130101; G01S 5/0027
20130101 |
Class at
Publication: |
340/825.49 ;
340/10.1; 342/44 |
International
Class: |
G01S 013/78; G08B
005/22 |
Foreign Application Data
Date |
Code |
Application Number |
May 23, 1996 |
SE |
9601971.6 |
Claims
1. A method of monitoring a plurality of movable objects, wherein
each object is equipped with a transponder and a stationary node is
arranged to communicate with said transponders, wherein each
transponder is caused to respond to a unique signal from the node,
wherein each transponder that sends a response signal gives the
response signal a transponder identification addition, wherein the
node is caused to detect the speed of movement and direction of
each transponder towards and away from the node and with the aid of
node associated means is caused to evaluate movement of the
transponder in relation to the node on the basis of transponder
speed and direction, characterized by setting-up for each node a
register of the transponders with which the node shall communicate;
providing a plurality of separate nodes, each having such a
register; and enabling the nodes to communicate with one another
and therewith enable a node to take over in its monitoring register
a transponder from another node.
2. A method according to claim 1, characterized by causing said
node associated means to deliver an alarm signal when the distance
of the transponder from the node deviates from a permitted
value.
3. A method according to claim 1 or 2, characterized by delivering
an alarm signal when the speed of the transponder exceeds a
predetermined value.
4. A method according to any one of claims 1-3, characterized by
causing the node to effect detection by the Doppler effect.
5. A system for monitoring a plurality of movable objects, wherein
each object is provided with a transponder, wherein a stationary
node is arranged to communicate with the transponders, wherein each
transponder is adapted to respond to a unique signal from the node,
wherein each transponder that sends a response signal is adapted to
give said signal a transponder identification addition, wherein the
node includes means for detecting the speed at which each
transponder moves towards or away from the node and is adapted to
deliver, with the aid of node associated means, an alarm signal
when movement of the transponder towards or away from the node
exceeds a predetermined value, characterized in that the node has a
register of those transponders with which the node shall
communicate; in that the system includes a plurality of nodes; and
in that the nodes are adapted to communicate such as to enable a
node to record in its monitoring register a transponder from
another node.
6. A system according to claim 5, characterized in that said means
are adapted to deliver an alarm signal when the transponder moves
at a speed greater than a predetermined value.
7. A system according to claim 5 or 6, characterized in that said
means are adapted to determine changes in the position of the
transponder on the basis of said detected speeds of movement.
8. A system according to any one of claims 5-7, characterized in
that the detection means of said node are adapted to operate with
Doppler effect.
9. A system according to any one of claims 5-8, characterized in
that a monitoring node is adapted to contain criteria for which an
alarm shall be triggered when a transponder leaves a node in which
the transponder is registered.
Description
[0001] The present invention relates to a method and to a system of
the kind defined in the preambles of the following method and
system Claims.
[0002] The invention is thus based on a technique in which each of
the objects is provided with a transponder and in which a
stationary node is arranged to communicate with the transponders,
wherein the node sends to each transponder a signal which is
identified solely by the transponder in question, wherein each
transponder sends to the node a response signal in response to the
signal from the node, wherein the response signal identifies the
transponder concerned. The node can thus be forced to detect the
speed and direction of each transponder towards and away from the
node. Node associated means can then be caused to evaluate movement
of the transponder in relation to the node, on the basis of the
speed and direction of the transponder. This technique is known
from U.S. Pat. No. 5,506,584, for instance.
[0003] The present invention finds use, for instance, in monitoring
boats in harbours, and it is, of course, of interest in this regard
to be able to limit the monitored area on the one hand, so that
communication between node and transponder can always be maintained
despite varying weather conditions. It is also of interest to be
able to measure whether or not the boat/transponder moves relative
to the node within the monitored area.
[0004] An object of the present invention is to provide a method
and a system with which transponder movement and distance of the
transponder from the node can be monitored, and with which the
transponder can be tracked between different nodes.
[0005] This object is achieved with a method according to claim 1
and with a system according to claim 5 respectively. Further
developments of the invention are set forth in the dependent
Claims.
[0006] The fact that the transponder delivers a unique signal
enables the unique transponder signal to be associated with a
telephone network subscriber. For instance, by coupling the node to
a telephony system, the subscriber is able to send a message to the
transponder via the telephony system when the node discovers that
the transponder concerned is located outside its permitted area or
moves at a speed which exceeds a predetermined value. For instance,
the node can be caused to detect a Doppler-shift response from each
transponder, this Doppler shift constituting a measurement of the
speed at which the transponder moves towards or away from the
node.
[0007] The transponders normally respond to a coded microwave
signal from the node with a coded Doppler-shift response. The node
sends coded signals to the transponders in accordance with a given
sequence at pre-programmed time intervals, preferably constant time
intervals. The signal from the node is coded so that only the
intended transponder is able to react to the node signal. The
transponder includes a computer which is programmed to activate the
transponder for transmission when the code of the node signal
agrees with the identity of the transponder. When the transponder
returns a coded signal to the node, this reply signal is also given
a code that identifies this transponder. Naturally, the computer in
the transponder may also be programmed to add additional
information to the transponder output signal. For instance, a theft
alarm or burglar alarm may be coupled to the computer of the
transponder, so as to cause the computer to add corresponding
information to the transponder reply signal, thereby enabling an
alarm signal to be sent by the node to an alarm receiver, via a
communications system (telephony system). For instance, the
transponder may have an identity corresponding to a telephone
subscriber number, so as to enable an alarm to be signalled to the
subscriber with a message corresponding to the current alarm state
associated with the transponder in question, this signal being sent
directly from the node via a telephony system, for instance the GSM
system.
[0008] The technique of detecting movement of the transponder
towards or away from the node with the aid of a Doppler effect for
instance, also enables the absolute position of the transponder to
be calculated by numerical integration of the speed fixes, although
absolute determination of the transponder position will preferably
be carried out at close intervals. However, the invention is not
limited to the use of a Doppler effect for detecting transponder
movement and speed.
[0009] The invention enables movement of the transponder to be
detected, or sensed, within the permitted transponder area that
lies well within the range of the node/transponder, therewith
giving a clear indication that the transponder is moving away from
its expected position while the transponder is located within the
communication range of its node.
[0010] The fundamental concept of the invention enables several
nodes to be used, each of which monitors its respective group of
transponders, said nodes communicating with one another. When a
transponder moves away from its node, out of the permitted area,
the node is thus able to communicate with nearby nodes and initiate
the nodes to search for the transponder concerned, the code of
which is then passed to these remaining nodes. When one of these
remaining nodes receives a response from the transponder concerned,
the transponder can be dismissed from the groups of objects
monitored by the other nodes. For example, the nodes may lie at a
distance of 3 km apart, to enable such a handover to take place.
The nodes may in turn, communicate with a main node which monitors
the permitted movement area of a given transponder. The area can
then be represented by a specific group of nodes. It is thus fully
possible to connect the main node to a computer that monitors the
permitted area of a given transponder. For instance, the main node
may allow a given transponder to move within the permitted areas
corresponding to overlapping of the permitted transponder areas of
a predetermined number of nodes, the main node otherwise triggering
an alarm to the person responsible for the transponder concerned.
For example, a yacht may have been hired with under the provision
that it must not leave an area defined by pre-determined permitted,
mutually overlapping communication areas of the nodes with the
transponder, wherewith the main node is able to initiate an alarm
of some kind if the transponder concerned disappears from the
permitted area. The main node may, in turn, be connected to other
main nodes via known telecommunications equipment.
[0011] The nodes may be comprised of mobile units that have an own
GPS equipment that determines the location of the node. The node
may, in principle, include a transmitter, a receiver and logic that
enables the node to monitor the transponders that are in the area
monitored by the node and registered therein.
[0012] The invention will now be described in more detail with
reference to an exemplifying embodiment thereof and also with
reference to the accompanying drawings.
[0013] FIG. 1 illustrates schematically an inventive monitoring
system.
[0014] FIG. 2 is a schematic illustration of several interlinked
systems.
[0015] FIG. 3 is a schematic illustration of communication between
a node and a transponder.
[0016] Referring first to FIG. 3, there is shown a transponder 10
that includes a receiver 11, a computer 12 and a transmitter 13.
Also shown is a node 20 which includes a digital transmitter 21,
logic 22, position determining equipment 23 and a receiver 24.
[0017] The transponder 10 is mounted on an object to be monitored.
The transponder 10 may be passive or may have its own power source
so as to provide better signal transmission back to the node.
[0018] The node 20 includes a register of a number of transponders
10 to be monitored. Each transponder has a unique identity/address.
This address may consist of a signal code. The node 20 sends a
signal to respective transponders 10 at predetermined time
intervals. The node 20 sends the signal with the transponder code,
so that only the intended transponder will recognize the signal.
The transponder then sends a response signal to the node,
preferably directly. The computer 12 provides the response signal
with information concerning the identity of the transponder 10, so
that the node 20 is able to decide whether or not the transponder
concerned has actually replied. The computer 12 provided in the
transponder 10 may be coupled to a burglar alarm or other alarm
means, so that corresponding information can be transmitted to the
node 20.
[0019] Although the node 20 is normally stationary, it may be
mobile. The node therefore includes GPS equipment 23 which
automatically monitors the position or location of the node 20. The
node 20 has communications equipment for communication with other
nodes.
[0020] The node 20 monitors a plurality of transponders 10 in its
local area. By detecting whether or not a transponder moves towards
or away from the node by means of the Doppler effect, the node
logic 22 is able to decide whether or not the transponder concerned
is located within a permitted area that lies well within the
communications area. If a transponder 10 moves significantly, the
node is able to deliver a signal to this effect either to an alarm
centre or to the owner of the object on which the transponder 10 is
mounted.
[0021] The node logic is able to determine the distance of the
transponders, by integrating the speeds at which the transponders
move established by the Doppler effect, although parallel absolute
measurements of the position of the transponders 10 in relation to
the node are preferably carried out.
[0022] If a transponder is detected to move in a non-permitted
manner or to be located outside a permitted area in relation to the
node 20 in which the transponder is registered, the node is able to
communicate with neighbouring nodes, as illustrated in FIG. 2, such
that the node 1 (FIG. 3) sends information P relating to the
transponder concerned to a neighbouring node 2, so that said node
can search for the transponder concerned in its monitoring area. If
a neighbouring node, thus the node 2, detects the transponder
concerned, the transponder can be inserted in the monitoring
register of node 2 while deleting said transponder from the
register of the previous node at the same time.
[0023] As illustrated in FIG. 2, the nodes 1, 2, etc., are also
able to communicate with a main node which defines the conditions
that are permissible with respect to a given transponder, and which
also defines the conditions with respect to the transponder that
shall initiate an alarm.
[0024] It can be assumed that a GSM telephone subscriber subscribes
to a transponder 20 having a unique identity, for instance the
telephone number of the subscriber. The transponder is mounted on a
vessel in a harbour and monitored by a node. If the boat leaves its
position or leaves the monitored area without permission, there is
initiated via the node or via the main node 30 which can have a
link to GSM or IMMARSAT, an alarm which passes to the subscriber
telephone via the telephony system, so that the subscriber obtains
this information. Alternatively, the information can, of course, be
sent to a monitoring station 31 of some kind or other, e.g. an
alarm centre. When the transponder is mounted on a rental or hired
boat or a hired car, the rental company is able to readily
ascertain whether or not the hired object has left the area in
which it is allowed to move in accordance with the rental contract.
Furthermore, it is beneficial for the rental or hire company to be
able to trace the hired objected/transponder if/when it leaves the
permitted area.
[0025] With reference to FIG. 1, it will be seen that the node
communicates with a plurality of transponders 20 (A, B, C, D)
within its monitoring area. In this regard, the node may monitor a
transponder 20 (A) with respect to a condition whereby it may not
experience any movement towards or away from the node 1.
Secondarily, the node may detect whether or not a transponder is
located within a permitted area in relation to the node 20, i.e.
that a transponder 10 is located within a permitted distance from
the node 20.
[0026] The monitoring system is based on a node/base station and a
plurality of units whose positions shall be monitored and which are
scattered in the space within a distance R from the node. These
units are equipped with transponders that respond to a coded
microwave signal from the node with a coded response signal.
[0027] The code is described below, wherein solely the problems
with and possible position determining solutions with the aid of
the Doppler effect are considered. Because of the nature of the
system, it is assumed that the node has at most a 3-DB antenna
amplification (semi-directional), whereas the transponder has an
omni-directional antenna (ER). The antennas will preferably be
circular polarized, so as to be orientation insensitive.
Designations
[0028] index r=received
[0029] index t=transmitted
[0030] 'designates transponder membership (without ' designates
base units)
[0031] P=power
[0032] R=distance base unit-transponder
[0033] G=antenna amplification
[0034] A=effective antenna area
[0035] g'=transponder amplification (may be smaller or greater than
1)
[0036] f=frequency
[0037] f.sub.o=base unit transmitting frequency
[0038] B=base unit reception bandwidth
[0039] kT=Boltzmann's constant.times.temperature (Kelvin)
[0040] v=transponder velocity relative base unit
[0041] c=speed of light
[0042] .DELTA.f=Doppler shift
[0043] .lambda.32 c/f.sub.0=transmitted wave length
Power considerations
[0044] The starting equations are
P.sub.r'=P.sub.t*G.sub.t* A.sub.r'/4/.pi.R.sup.2
P.sub.r=P.sub.t'*G.sub.t'*A.sub.r/4.pi./R.sup.2
[0045] With respect to the antennas it can be assumed that
G=2*.pi.*A/.lambda..sup.2 (50% antenna efficiency). With
P.sub.t'=*g'*P.sub.r' there is then obtained
P.sub.r=P.sub.t*g'*(G*G'/2).sup.2*(.lambda./2/.pi./R).sup.4
[0046] provided that the response frequency is essentially equal to
f.sub.o. Assume that we have a passive transponder (without its own
power source). Since the response from the transponder must--in
addition to the Doppler shift--also be frequency shifted in
relation to the base frequency so as not to be drenched in other
reflected signals, g' may be at highest in the order of 0.01. G=2
and G'=1. With f.sub.o=2.45 Ghz, R=500 m and P.sub.t=1 W there is
then obtained P.sub.r<2.3*10.sup.-20 W.
[0047] Assume that v=+/-10 km/h is the maximum speed that can be
expected. This corresponds to the Doppler shift
.DELTA.f=2*v*fo/c=+/-45 Hz.
[0048] Thus, the receiver bandwidth must be chosen as B=100 Hz,
which gives P.sub.noise=k*T*B=4*10.sup.-19 W. Because the signal is
coded with more than 1 kb/s, the noise may be increased by a
further power of 10. Thus, the base unit must have a power output
of at least 200 W, in order for the response not to be drenched in
noise. The conclusion is that an active transponder should be
chosen even for the relative short distance of 500 m.
[0049] With an active transponder, Pr'=1.5*10.sup.-9 W under the
same conditions. Thus, it is possible to reduce the power output to
-30 dBm without being troubled by noise, and still obtain a range
somewhere in the region of 3 km (B=1 kHz, signal/noise ratio 10).
Naturally, this presumes that the transponder will also deliver an
output signal of comparable strength.
Stability Considerations
[0050] As before mentioned, it is necessary to add to the
transponder a frequency shift in the order of at least 1 kHz in
order to filter out undesirable echoes in the base station
receiver. The frequency stability must be in the order of 1 Hz (v=6
cm/s). It is possible to include single-sideband or double-sideband
modulation (Premid). Alternatively, frequency can be doubled or
halved. This eliminates the stability requirement of the
transponder, but may, instead, require separate receiving and
transmitting antennas in both base station and transponder in order
to fulfil the angular covering requirement. Furthermore, collision
with permitted maximum radiation in these other bands may
occur.
Measuring Time and Detectable Movement
[0051] A. Incoherent detection
[0052] At f.sub.o=2.45 Ghz, a resolution of 1 Hz is required to
indicate 6 cm/s, which gives a measuring time of at least 1 second.
When 50 objects are to be monitored, it will take at least 50
seconds between the observations of an object. The object can have
moved 3 m at 6 cm/s or 140 m at 10 km/h in this space of time. If
lower requirements are placed on the speed resolution, both
measuring time and time intervals can be reduced. The smallest
detectable movement will still be 3 m, although the value are 10
km/h will decrease proportionally.
[0053] In order to lower the bottom limit of detectable movement,
it is necessary either to increase f.sub.o or to reduce the number
of monitored objects.
[0054] The position of the monitored object must be calculated by
numerical integration of the speed determinations. This quickly
gives rise to large positional errors, particularly when monitoring
vessels that are moored in a harbour, due to the reciprocatory
motion that occurs. Furthermore, only movement towards or away from
the base station is indicated, not sideways movement. In order to
avoid false alarms or non-occurrent alarms, it will probably be
necessary to effect absolute determination of the position at close
intervals.
[0055] In order to decide whether the speed is directed towards or
away from the base station, down-mixing is effected with an
intermediate frequency in the order of at least 1 kHz, and not down
to the baseband. If this is not generated in the transponder
(sideband modulation), e.g. when doubling or halving the frequency,
it must be generated in the base station.
[0056] B. Coherent detection
[0057] A large number of the aforesaid problems can be
circumvented, by effecting coherent detection to the baseband and
by sampling the various monitored objects much more frequently. A
choice can be made between homodyne detection or heterodyne
detection. As before mentioned, homodyne detection produces
difficulties in respect of determining direction. It is possible
that such difficulties can be overcome with a high degree of
accuracy in A/D conversion and the choice of an intelligent
interpretation algorithm, although there will always be a danger of
wrong interpretation of the velocity direction at some time point,
resulting in a sudden change. A better solution is to insert a low
intermediate frequency when down-mixing in the base unit. This
greatly reduces the accuracy requirement of the A/D converter (4-5
bit accuracy suffices) and jitter in the sampling time-points is of
but small importance. However, high requirements are placed instead
on the accuracy at which the product of local oscillator frequency
multiplied by sample time-point is calculated when interpreting the
position of the object, or target. Because it is the absolute value
of the accuracy in this product calculation that is of importance,
the relative error in position determination will increase linearly
with time, and a 0-position will probably be required at regular
intervals if this linear error growth cannot be eliminated (see the
following solution to the problem).
[0058] We double or half the frequency in the transponder and
detect the signal heterodyne-wise in the base station, with a
frequency double/frequency halved local oscillator signal converted
up with a further intermediate frequency (.+-..sub.LO. The voltage
from the detector obtains the following form:
V(t)=V.sub.c*cos[B 2*.pi.*(f.sub.LO*t+N*2*R(t)/.lambda.)+.phi.],
N=2 when doubling or N=0.5 when halving freq.
[0059] where V.sub.e=the voltage envelope, which is slightly
time-dependent due to aspect variations of the antennas. .phi. is a
constant phase angle. For an unequivocal result, it is necessary
that the time derivative of the phase in the cosine-function is
always positive. Thus, if V=dR(t)/dt is maximum +/-10 km/h, it is
necessary that f.sub.LO is at least N*50 Hz, where f.sub.o=2.45
GHz.
[0060] This voltage is sampled at a rate at which the phase change
between samples is kept beneath .pi./2 in the cosine function. With
maximum v=10 km/h, the sampling frequency is at least N*400 sa/s,
where f.sub.o=2.45 GHz and f.sub.LO is chosen as N*50 Hz. The
choice of a sampling frequency that is a multiple of f.sub.LO
should be avoided, since synchronism can make determination of
V.sub.e difficult (the peak values in the voltage response that are
assumed to vary with a time constant much greater than 1/f.sub.LO).
Arc cos [V(t)/V.sub.e] is then calculated. Since arc cos is not
unequivocal, it is necessary when making the interpretation to rely
on the condition that the phase change between mutually sequential
samples is greater than 0 but less than .pi./2, and on an
intelligent algorithm. Generally speaking, it is preferably ensured
that Ve is underestimated rather than overestimated; the normalized
values V(t)/Ve that exceed 1 are simply made equal to 1 in the
interpretation. The characteristic of the detector (linear,
quadratic) plays a relatively small part. As before mentioned,
f.sub.LO*t is a critical magnitude in the evaluation. With the
sampling frequency f.sub.s and a counter S of the number of samples
from start t=0 results in
f.sub.LO*/S f.sub.LO/f.sub.s
[0061] For accurate determination of the value of the product,
f.sub.s will then be chosen as a multiple of f.sub.LO, or
preferably (in accordance with the earlier discussion) that both
frequencies are multiples of a third frequency, e.g.
f.sub.LO=2*N*25 Hz and f.sub.s=17*N*25 Hz.
Power Requirement
[0062] Assume that a 32-bit address code is sent from the central
unit and responded to with 32-bit data. Also assume that 50 objects
are to be monitored. The data rate will then be at least
50*64*N*400=N*1.28 Mb/s. Each monitoring object is activated N*400
times and responds below 1% of the total time. With N*2 MHz
bandwidth in the receiver, P.sub.noise=N*8*10.sup.-15 W is
obtained. Because the transponder transmits on N times the
frequency and with the distance 500 m, the pulse power
P.sub.t'=N.sup.3*21*10.sup.-5 W is obtained for the signal/noise
ratio 10 and the receiver noise factor 6 dB. If a 1% efficiency is
assumed for the frequency doubling/halving, P.sub.t will also be
equal to the mean power consumption of the transponder. Halving of
the frequency reduces the transponder power requirement 64 times in
comparison with doubling the frequency, although a base-unit
antenna surface area that is 16 times greater is required in
return.
[0063] Under the same conditions as those described above (B=N*2
MHz, signal/noise ratio=10, noise factor=6 dB, R=500 m), the
continuous output power of the base unit on the frequency f.sub.o
will be at least P.sub.t=N*21*10.sup.-5 W.
[0064] At most, 10 .mu. W e.i.r.p. according to Televerkets
Radiodivision, 77-06-01, is permitted outside the nominal frequency
2450 MHz+/-20 Mhz. These values may be irrelevant, but they give an
indication that the frequency halving option is the most practical,
quite irrespective of the fact that it gives a more acceptable
battery life span in the transponder.
Considerations in Relation to Response Pulse Character
[0065] In the case of 50 monitored objects, the response time for
each object is only 25/N .mu.s on each activation occasion in the
above example. Only one sample is taken during this period. The
next sample is taken on the same object 2500/N is later. The
sampling rate of the base unit is, in total, 50 times higher, i.e.
N*20 ksa/s. Frequency broadening due to the pulse character of the
activation is in the order of 1/pulse length=N*40 kHz. Discrete
frequencies of N*400 Hz spacing? are obtained in this band. This
should not present a problem with regard to signal processing,
since the receiver bandwidth for Doppler detection is N*100 Hz in
the case studied. The noise power in this band is N*4*10.sup.-19 W.
If the receiver noise factor is 6 dB and the requisite signal/noise
ratio for interpretation is assumed to be 14 dB, the requirement
P.sub.1>N*4*10.sup.-17 W that is required (mean power in the
detected frequency band). With these above values, there is then
obtained P.sub.t>N.sup.3*2.6*10.sup.-8 W and a mean output power
from the transponder which is 100 times greater, since the spectrum
contains, in round figures, 100 similar frequency bands that are
not made use of. The requisite pulse output power of the
transponder is thus N.sup.3*26*10.sup.-5 W; thus roughly the same
requirement as that placed on data transmission.
[0066] If the low intermediate frequency f.sub.LO presents a
problem in detection, this intermediate frequency and the sampling
frequency f.sub.s, may either both be raised, or f.sub.s may be
retained and solely f.sub.LO raised. The following general
relationship must be fulfilled for unequivocal interpretation:
M*f.sub.s+.delta.f<f.sub.LO<(M+1/4)*f.sub.s-.delta.f M=0, 1,
2, . . .
[0067] where .delta.f=the maximum value of
Abs{2*N/.lambda.*[dR(t)/dt]} and M*2*n must be added in the
calculation of arc cos [V(t)/V.sub.e] in accordance with the
aforegoing. .delta.f=N*50 Hz in our example. The above condition
results in the condition f.sub.s>8*.delta.f. Sampling frequency
and intermediate frequency will preferably be synthesized from a
common fundamental frequency, as earlier. The permitted time jitter
of the sampling is related to f.sub.LO and must thus decrease
proportionally when f.sub.LO is increased by choosing M>0. A
sampling jitter of 0.01/f.sub.LO s can be tolerated in our
example.
* * * * *