Note: Descriptions are shown in the official language in which they were submitted.
P 42 24 645.8
Method of Correcting Measurement Errors
Caused by Clock Deviations in a Secondary
Radar System
The present invention relates to a method as set forth
in the preamble of claim 1.
European Patent Application 92144119.0 discloses a
secondary radar system for Mode S operation in which
the position of an aircraft is determined from stored
arrival times of a reply signal from a station aboard
the aircraft, transmitted in response to an interroga-
tion signal from a ground station, at the receivers of
at Least three ground stations situated at different
geographical locations by using hyperbolic position
finding techniques.
In such a system, interrogators in the ground stations
must operate in strict synchronism so that signal tran-
sit times and eventually distances can be calculated
from the stored interrogation and arrival times associat-
ed with one another.
To maintain synchronism, the above patent application
(see claim 6, for example) proposes to synchronize the
interrogators in the ground stations with the aid of
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21.06.93
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geostationary satelt.ites. Such synchronization therefore
requires in all ground stations receivers for the trans-
mitted time signals and antennas for satellite reception.
It is the object of the invention to provide a method
of the above kind whereby clock deviations and/or time
measurement errors caused thereby can be very accurately
corrected without the use of an additional, external de-
vice.
According to the present invention, there is provided a
method of correcting measurement errors caused by clock
deviations in ground stations (S1-S4) of a secondary radar
system which determines the positions of aircraft by
measuring transit times of interrogation signals addressed
to airborne stations (BS) and of reply signals transmitted
by said airborne stations and received in at least three
ground stations, characterized in that it comprises steps
of transmitting, by means of all the ground stations (S1-
S4) involved in determining the position on an aircraft,
interrogation signal to the airborne station (BS) of the
aircraft in a cyclic sequence and at fixed points of time
within a time frame predetermined by the clock of a ground
station (S1); receiving reply signals from the airborne
station by means of all the ground stations, which
determine the times of reception, which, by themselves or
together with the address of the interrogating ground
station, taken from the reply signal of the airborne
station, are exchanged among the ground stations or
communicated to a master station (S1); calculating in the
master station or in one or more of the other ground
stations, for changing pairs of ground stations, the
elapsed time between the transmission of an interrogation
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signal in one of the ground stations of each pair and the
reception in the other ground station of the reply signal
transmitted by the airborne station in response to the
interrogation for both directions; and interpreting half
the absolute value of any difference between the two times
as a deviation of the clocks of the two ground stations
from each other and taken into account in the position
determination to correct the signal transit times.
The solution uses a principle as is known for performing
time comparisons between satellite ground stations used
for communication Csee, for example, articles by
D. Kirchner et al and D.A. Howe in "IEEE Transactions
on Instrumentation and Measurement", Vol. 37, No. 3,
September 1988, pages 141 et seq. and 418 et seq.).
A particular advantage of the method according to the
invention is that, apart from the devices required for
secondary radar Mode S operation, no further devices
are needed for the correction. The necessary calculations
and data transmissionsare performed by existing pro-
cessors and transmission equipment, respectively.
Further advantageous aspects of the method according tc
the invention are defined below.
Preferably, signal transit times from a master station to
one other ground station and back are calculated. This
makes the synchronization of the clocks of all ground
stations with the clock of the master station particularly
simple.
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According to preferred embodiments, the method according to
present invention prevents the clocks of the ground
stations from deviating from each other too much during
times in which no airborne station is available for
interrogation and form transmitting a reply signal, i.e.,
in which the method described above cannot be carried out.
The method according to the invention will now be de-
scribed in detail with reference to the accompanying
drawing, in which:
F.ig. 1 shows the principle of the invention in
a system with two ground stations, and
Fig. 2 shows a secondary radar system with
several ground stations with synchronized
clocks.
Fig. 1 shows two ground stations S1, S2 with interroga-
tors which are interconnected by a data link 1!.'Located
within the coverages of.both interrogators i.s an aircraft
with an airborne station BS which, after a fixed delay
following the reception of an interrogation signal trans-
mitted by one of the interrogators, transmits a reply
signal which is received by both interrogators. The
distance from the airborne station BS to the interrogator
of the ground station S1 is d1, and that to the inter-
rogator of the ground station S2 is d2.
~~~~ 2
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The interrogators of both ground stations are equipped
with highly accurate clocks which operate synchronously
in a fixed system time frame. These clocks serve as a
time standard for measurements of the transit times of
interrogation and reply signals. With them, the time of
transmission of a predetermined pulse edge of an in-
terrogation signal and the time of reception of a
likewise predetermined pulse edge of a reply signal
transmitted by the airborne station after a fixed delay
can be precisely measured in relation to a synchroniza-
tion mark of the system time frame. The time elapsed
between the transmission of an interrogation signal and
the arrival time of an associated reply signal can then
be calculated at the interrogating ground station. If the re-
ceiving station is not the interrogating station, the arrival
time; must be transmitted to the interrogating station.
The calculated time is composed of the signal transit
times between the interrogating ground station and the
airborne station, t1, and between the airborne station
and the receiving ground station, t2, and the fixed de-
lay t~ in the airborne station. Depending on which
ground station is interrogating and in which ground
station the time of reception of the reply signal is
being determined, the signal transit time is composed
of the fixed delay to and twice the distance d1 (in-
terrogation and reply evaluation by ground station S1),
the distance d1 * d2 (interrogation by ground station
S1, reply evaluation by ground station S2) or twice
the distance d2 (interrogation and reply evaluation
by ground station S2). If the reply signal is evaluat-
ed by the interrogators of both ground stations, and
the measured signal transit times are exchanged via the
data link V, the distances d1 and d2 can be calculated,
since the wave velocity (speed of light c) is known.
To determine the position of the aircraft using a
hyperbolic system, at least one additional ground
station is necessary whose clock must also be synchronized
with the system time frame.
Mutual synchronization of the interrogators necessitates
measuring the existing clock deviation and, to this end,
two successive interrogationlreply cycles with signal
transmissions taking place over the same distances in
opposite directions. For example, the reception at the
station S2 of a reply signal from the airborne station
BS which was elicited by the interrogator S1 is
followed, after a predetermined waiting time, by an
interrogation from the station S2 and the reception
of the reply signal from the airborne station BS by
the ground station S1. The time measured between the
transmission of the interrogation signal and the re-
ception of the associated reply signal is the same in
both directions provided the interrogators operate in
exact synchronism and the interrogations follow in
such rapid succession that any distance meanwhile
traveled by the airborne station will be of no conse-
quence. If the synchronization is not correct, a time
error will be measured for both directions of trans-
mission. The measured error is the same for both direc-
tions of transmission but has a different sign for each
direction.
If t1 is the signal transit time over the distance d1
in Fig. 1, t2 the signal transit time over the distance t2,
~:~~~2~.
and t~ the delay in the airborne station, the total
time for an interrogation/reply cycle directed from
ground station 1 to ground station 2 or oppositely is
Tl~z -_ T2~1 -_ t1 + t~ + t2
In the presence of a synchronization error tf (e. g.,
lag of the clock of the ground station S2), a total time
of
T1 ~2 -_ t1 + t~ + t2 - tf
will be measured for the interrogation/reply cycle
initiated by the station S1, while a total time of
T2~1 _ t~ + t~ + t1 + t f
will be measured for the interrogation/reply cycle in
the opposite direction, The two total times thus differ
by 2 tf. Accordingly, the deviation of the clock in
the ground station S2 from the clock in the ground sta-
tion S1 is tf, half the measured difference.
Tf it is possible to readjust the clock in the ground
station S2, this can be done after transmission of the
magnitude and sign of the detected time error tf to the
station S2. Exact synchronization of the two clocks is
thus restored. The deviation can also be stored and
taken into account as a correction value in calculat-
ing distance for the purpose of locating the position
of the aircraft. Thus, the erroneous measured value
need not be discarded.
_,_
The delay in the airborne station does not enter into
the measured total time. It is therefore inconsequen-
tial if the delays in the airborne station. of different
aircraft are not exactly equal. The delay in a single
airborne station, however, should be so constant that
it does not measurably change during two successive
interrogation/reply cycles.
In practice, this requirement cannot always be met. De-
lay fitter occurs which, according to the regulations
of the ICAO for secondary radar systems, must not exceed
50 nanoseconds. In addition, randomly distributed
measurement inaccuracies are also likely when measur-
ing the times of reception in the interrogators of the
ground stations. These randomly distributed transit-time
and measurement errors can be reduced by making multiple
measurements and averaging the measured values.
Errors in the measured total times may also result from
the motion of the airborne station and the resulting
signal-path change during the time between two, successive
interrogation/reply cycles. Such errors will remain
small if the waitirigw time between the successive
interrogation/reply cycles is very short, e.g., on the
order of 1 millisecond. Time-measurement errors will
then remain below 1 nanosecond even with an unfavourable
spatial constellation of the ground stations and the
airborne station (airborne station vertically above one
of the ground stations).
Fig. 2 shows a group of ground stations S1 ... S4 of
a cellular network. In such a secondary radar network,
_8-
the position of an aircraft (airborne station BS) is
determined by measuring the times required for a reply
signal transmitted by the aircraft in response to an
interrogation signal from one of the ground stations
to travel to at least three of the ground stations.
If, according to an embodiment of the system described
in the European patent application referred to above,
the flight altitude is to be additionally determined
by evaluating the transit times of the reply signal,
a fourth, synchronously operating ground station is
required. To be able to synchronize the interrogators
of all required ground stations as described above or
to correct measurement errors caused by deviations of
the clocks, it is necessary to organize the interroga-
tion/reply mode of these ground stations in such a
manner that all ground stations send interrogations to
the same airborne station in as even a distribution as
possible, and that this does not result in intersec-
tions of signals at the receivers or in signals
arriving at the airborne receiver when the latter is
off during the transmission of a reply signal.
In a group of ground stations as shown in Fig. 2,
it is therefore advantageous to operate one of the
ground stations as a master station (or to provide an
additional master station>, to regard the timing sig-
nal of the latter as the system timing, and to syn-
chronize the clocks of the other interrogators with
the system time of the master station. The calculation
of the various distances a,nd the hyperbolic position
finding method are then carried out in a computing de-
vice in the master station after all measured values
have been transmitted to this station.
>v~~~.~~_
_ g _
In Fig. 2, the ground stations S1 to S4 belong together.
The ground station S1 is the master station, in which
the signal transit times needed for position deter-
mination are determined from the measured values of
all stations, and in which position determination is
performed. The clocks of all stations are first
coarsely synchronized with a system time which is pro-
vided by the master station to the stations S2-S4,
e.g., via data links V existing between the stations.
Synchronization with the aid of a public time stan-
dard transmitter or with the aid of a GPS timing signal
is also possible. Starting from a point 0 of the system
time or any agreed point of the public time standard,
the ground stations transmit interrogation signals
at fixed instants which are 1 millisecond apart, for
example. If the master station S1, for example, trans-
mits at the instant 0, the ground station S2 will fol-
low at instant 0+1, the ground station S3 at the instant
0+2, and finally, after 3 ms, the ground station S4.
After a fixed waiting time (e. g., 1 s), the master sta-
tion begins a new interrogation cycle. In this case,
the coarse synchronization of the ground stations must
be at least so good that the sequence of the stations
in~the interrogation cycle will be. preserved and no overlapping
of the interrogations will occur.
If an aircraft whose airborne station .(transponder) '
is~on comes near to the stations S1,to S4, the
stations will receive squitter signals from the airborne
station. From these squitter signals, they take the
identification of the aircraft, which i.s used to
~4~~~~
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selectively address the airborne station.
Also, a first coarse determination of the position of
the aircraft can be performed with the aid of the
arrival times of the squitter signals, which are re-
gistered in the ground stations.
If the airborne station can be addressed on a selective
basis, it will, after a fixed delay t0, transmit in re-
sponse to each interrogation signal containing its
identification a reply signal whose respective arrival
times in the individual ground stations are registered
and transmitted to the master station., It i.s then
possible to carry out the method of determining time
errors and correcting the synchronization of the ground
stations, described above for two ground stations, for
the station pairs S1/S2, S1lS3, and S1/S4, but also for
the three other possible pairs S2/S3, S2/S4, and S~/S4.
The initially possibly large synchronization error can
thus be reduced step by step, so that after a few
seconds, all ground stations involved, even if the
ground stations S1 to S4 are used without connection
with an areawide network, e.g., for air space sur-
veillance, will be very accurately synchronized with the
system time provided by the master station S1.
