Pulsed, pseudo random position fixing radio navigation method and system and the like

This disclosure is concerned with a technique and apparatus for reducing CW and other inband interference in a broad bandwidth RF pulse navigation system to inobtrusive noise through the use of pulse-position random modulation, including, where desired, superimposed random phase modulation, with signal-random modulation code cross-correlation in reception.

The present invention relates to navigation or position fixing systems and 
methods relying upon particular propagation characteristics of 
electromagnetic waves, as in the radio frequency (RF) spectrum; being more 
particularly though not entirely concerned with such systems as Loran, 
Omega, Decca, GPS, etc., wherein the positional accuracy attained depends 
upon the stability of the velocity of propagation of the electromagnetic 
waves. 
Such systems use two or more transmitting stations, either transmitting 
continuous wave (CW) RF signals at different frequencies for purposes of 
identifying the different stations, or pulsed RF signals with different 
code formats for purposes of identifying the different stations. If only 
two stations are employed, as an illustration, position fixing may be 
obtained by determining the range to each station from the user location. 
The intersection of two range circles, for example, around respective 
stations, will define the location of the user; such systems often being 
referred to as range-range position fixing systems. To determine these 
ranges, the user must know the relative location of the stations, the 
exact time of day, and the time of transmission from the two stations; and 
with such knowledge, and measuring the times of arrival of the transmitted 
waves, the user can calculate the ranges to the transmitting stations. A 
cesium clock or other standard is currently usually employed by the user 
to keep track of the exact time of the day (to better than 100 nsec). When 
three and more stations are used, these stations are synchronized in time, 
and the user selects one station as a reference, often referred to as the 
master station; and measures the time difference of arrival of signals 
from the other stations with respect to the master station. Lines of 
constant time difference between the master station and another, form 
hyperbolae; and these types of position fixing systems are often referred 
to as hyperbolic navigation systems of which Loran is an important 
example. 
As previously mentioned, the position accuracy of such systems depends upon 
the constancy of the velocity of propagation of electromagnetic waves, 
with the carrier frequencies used in these systems covering the range from 
very low frequencies (VLF) to ultra high frequencies (UHF). The 
propagation characteristics of electromagnetic waves, however, vary widely 
over this tremendous frequency range. At the low frequency range, such as 
employed in the Omega system, the carrier frequency of which is 
approximately 10,000 Hz, the surface of the earth and the ionosphere form 
a wave guide that determines the propagation characteristics of these 
radio waves. The velocity of propagation of the waves guided by the earth 
and the ionosphere is a function of the effective height of the ionosphere 
above the earth. The effective height of the ionosphere varies 
considerably from night to day and from season to season since it is 
affected by solar radiation; and periods of sun flares also greatly 
influence the effective height of the ionosphere. Large unpredictable 
errors in position fixing (as much as 6 nautical miles) can thus result 
from the solar perturbations of the ionosphere. 
Similarly, the navigation system Loran-C is affected by the ionosphere. 
This system operates at a carrier frequency of 100 kHz, at which frequency 
the ray theory is usually applicable to explain the propagation 
characteristics of the radio waves. Loran-C transmissions consist of RF 
pulses of a tearshaped form, as described, for example, in my earlier U.S. 
Pat. Nos. 3,889,263; 3,786,334 and 3,711,725, with appropriate user 
receivers for enabling reception thereof being described, for example, in 
U.S. Pat. Nos. 3,882,504 and 3,921,076. The RF pulse emitted from the 
Loran antenna travels to and is received by the user by means of a ground 
wave, a first-hop skywave singly bounced from the ionosphere, a second-hop 
skywave reflected from two locations of the ionosphere, and so on, as 
illustrated in connection with hereinafter described FIGS. 1A and 1B 
hereof. Since the height of the ionosphere depends upon solar radiation, 
the times-of-arrival of the skywaves are unpredictable. The ground wave, 
however, is stable and this wave may therefore be used for position 
determination by the user. Only the first few RF cycles in the ground wave 
are used for position determination, as explained, for example, in the 
above-mentioned receiver patents, in order to insure that the first-hop 
skywave does not interfere. The higher-order-hop skywaves, however, may 
overlap succeeding transmitted pulses and thereby none-the-less cause 
interference. 
This interference is avoided by a special signal format, as shown in 
later-described FIGS. 2(A) and (B), the code of which is selected to avoid 
correlation of the delayed sky-wave signals with the ground wave signal. 
As later more fully explained, tight tolerances are thus imposed upon the 
front-end filtering and processing circuits of the receiver channels to 
insure that each received signal is identically processed and that the 
time delay for each is precisely the same--the filter tolerance 
requirements being thus extremely strict and difficult to meet with 
relatively inexpensive components. 
An object of the present invention, accordingly, is to provide a new and 
improved method of and apparatus for eliminating this long-standing 
problem of tight component tolerances in the RF front end of such 
navigation receivers. 
Another serious problem with present systems is their susceptibility to 
interference from in-band transmissions from other stations, such as 
low-frequency military transmissions and medium-frequency broadcast 
transmissions. Mutual intereference between navigation systems also 
exists, moreover, such as between DECCA Navigator and Loran-C. The reverse 
problem in which the navigation system interferes with broadcasting 
systems, both civil and military, also exists. A further objective of this 
invention, therefore, is to minimize such mutual interference, thereby 
permitting navigation and communication systems to co-exist in the same 
frequency band without degradation of performance. 
An additional object is to provide a novel technique and apparatus 
involving random pulse position coding and also random phase modulation, 
where desired, to obviate such interference and related problems, as in 
Loran-type and other signal transmission systems, as well. The use of such 
coding in the short pulse system is different from GPC where biphase 
coding alone is used to increase ranging accuracy and provide code 
division (as opposed to time division) signal multiplexing. 
Other and further objects will be explained hereinafter and are more fully 
delineated in the appended claims. In summary, however, from one of its 
important aspects, the invention embraces a method of rendering 
multi-station radio frequency pulse navigation relatively insensitive to 
inband interference, that comprises, transmitting groups of 
radio-frequency pulses from each station with a group repetition rate 
selected to enable transmission of a pulse group from each station with 
sufficient time between each pulse group that signals from two or more of 
the stations cannot overlap in time anywhere in the navigation coverage 
area; randomly pulse-position modulating pulses within groups in 
accordance with a predetermined code and over a sufficiently large number 
of groups to insure that the random sequence contains a sufficiently large 
number of pulses that adequate skywave-groundwave rejection of the 
transmitted pulse groups occurs everywhere the same are received in said 
navigation coverage area. Preferred details and best mode applications are 
later presented.

Turning, first, to an example of the seriousness of the previously 
described strict filtering tolerances in navigation receivers, consider 
the Loran-C technology. As before mentioned, the special signal format 
selected to avoid overlap of higher-order-hop skywave signals with the 
ground wave signals is currently a format involving the transmission by 
each Loran-C station of groups of eight pulses spaced one millisecond 
apart; FIG. 2(A). These groups of pulses are transmitted at a fixed 
repetition rate and they are phase-coded in accordance to the sequence of 
plus and minus signs shown, for example, in the XMTR 1 box of FIG. 2(B). A 
"plus" sign indicates a pulse of zero degree carrier phase, and a "minus" 
sign indicates a pulse of 180.degree. carrier phase. The code is selected, 
as previously stated, such that delayed skywaves are uncorrelated with the 
ground wave. 
The other transmitters in the chain (XMTR2 and XMTR3, FIG. 2(B)) transmit 
their signals in the allocated spaces shown, and the group repetition rate 
is selected such that it contains time for transmission of the pulse group 
from each station and sufficient time between each pulse group so that 
signals from two or more stations cannot overlap in time anywhere in their 
coverage area. Thus, the transmitted signals are time shared so that all 
signals pass through the same front end of the receivers. Since the 
position determination is based on the time difference measurement 
(difference in time-of-arrival of the electromagnetic waves from the 
transmitting stations), it is important, as before mentioned, that each 
received signal is processed in an identical manner so that the time delay 
in the front end of the receiver is the same for all signals. 
In CW hyperbolic navigation systems such as DECCA Navigator and Omega, on 
the other hand, parallel channels are used in the receiver, each channel 
receiving a particular transmitted signal. The time delay in each of these 
channels must be almost identical over a wide dynamic signal range. 
Receivers of this type are quite expensive. As an example of the degree of 
stability required, consider the DECCA Lambda system wherein a receiver 
front-end bandwidth of 15 Hz is used. Assuming that the filter therein 
employed is first order, then the signal delay is 0.01 seconds, which 
corresponds to a distance of 3180 km. A receiver front end error of 10 
nsec. is introduced if the bandwidths of the receiving channel differ by 
only 0.000390%. Thus, the tolerances of the filter components must be of 
the same order of magnitude; and only with temperature-controlled crystal 
filters is it possible even to approach this kind of stability. Even with 
a receiver front-end bandwidth of 15 kHz, as in Loran-C receivers, the 
tolerance requirements on filter components are difficult to meet. 
In addition to the important interference problem, the technique of the 
invention relaxes these receiver tolerance requirements and simplifies the 
required equipment. 
As above stated, the preferred navigation type system of said U.S. Pat. No. 
4,151,528, has been selected for the exemplary illustration of the present 
invention; but it is to be understood that other system pulse shapes may 
also be used. Similar to the Loran-C system of navigation, each station of 
the system of FIG. 4 (XMTR 1, XMTR 2 and XMTR 3) transmits groups of 
pulses in the sequence shown, with each RF pulse of the groups of 
transmitted pulses having the preferred shape of FIG. 3 for the 
unambigious cycle selection reasons described in said U.S. Pat. No. 
4,151,528, with several half-cycle navigation pulses having a single RF 
cycle as an intermediate part thereof with either or both of a unique sum 
amplitude and zero crossing, as shown at the second and third half cycles 
of the pulses of FIG. 5. The group repetition rate is selected such that 
it contains time for transmission of the pulse group from each station and 
sufficient time between each pulse group so that signals from two or more 
stations cannot overlap in time anywhere in the coverage area, as 
previously discussed. 
In accordance with the present invention, the pulses within a group are 
pseudo-randomly position and/or phase modulated as shown in FIGS. 5 and 6. 
In fact, the modulation may be continued over many groups. 
Pseudo-random pulse position modulation is the apparently random, but known 
variation in the time spacing between successive pulses (See FIG. 5). The 
pseudo-random position modulation insures that only a small percentage of 
the received pulses suffer skywave interference (see FIG. 7). It also 
guarantees that the error introduced by the pulses which are interfered 
with is small. In other words, even though the variance of a location 
estimate based on the distorted pulses alone is larger, the bias of this 
estimate is small. Pulse position modulation has the same effect on errors 
introduced by other interfering signals. It also facilitates signal 
acquisition, station identification and secure position fixing. 
Pulse phase modulation is the apparently random, but known variation of the 
polarity of the pulses (see FIG. 6). It also controls the bias of position 
estimates based on pulses suffering skywave or other signal interference. 
Phase modulation may be used to contribute to system security. It is also 
useful in signal acquisition and station identification. 
A pseudo-random trigger generator initiates the transmission of the pulse, 
controlling the phase and/or time of the transmission in a pseudo-random 
manner. Transmitter control circuits 7, FIG. 8, respond to the 
pseudo-random trigger by providing the signals that the high power section 
2 needs to transmit a pulse. They may also analyze the antenna current by 
feedback 4 to provide control over the temperature-sensitive high power 
section. 
A suitable type of phase-adjusting driving circuit 3 is described, for 
example, on pages 2-57 through 2-62 of Accufix Instruction Manual, 
Megapulse Inc., Bedford, Mass., 1973-4. A useful form of trigger generator 
5 may be of the type described on pages 60-64 and 85-91 of Spread Spectrum 
Systems, by R. C. Dixon, John Wiley and Sons, 1976; and typical control 
circuits 7 of the type described on pages 2-82 through 2-101 of said 
Accufix Instruction Manual may be employed, as may other well-known 
variants for performing such functions. 
The random pulse position code being transmitted in accordance with the 
invention is also stored in each receiver, with the receiver performing a 
cross correlation between the received signal and the stored pulse 
position code as later described in connection with the illustrative 
receiver of FIG. 9. CW interference present in the received signal is thus 
sampled in a random way and is therefore transformed to random noise. 
The effect of inband CW signals, accordingly, is merely to increase the 
receiver noise level--with no other malfunctions such as loss of lock or 
uncorrectable bias errors occurring. In the light of the broad bandwidth 
of the transmitted signal, furthermore, narrow band notch filters can be 
used significantly to reduce the effect of CW interference. Conversely, 
since the energy in the pseudo-random navigation signal of the invention 
is spread over a wide band and is almost continuous with frequency, the 
energy density is very low, thereby causing negligible interference with 
CW or other periodic signals. 
The wide bandwidth of the navigation signal of the invention, of course, 
requires wide bandwidth front-end receivers. At the before-mentioned 
carrier frequency of 100 kHz, for example, the typical RF front-end 
bandwidth is 100 kHz, which causes a delay of 1.6 usec. To obtain 
front-end delay errors of less than 10 nsec, the variation in front end 
filter component values must only be of the order of 2%, which is quite 
reasonable and a far cry from the strict tolerances of the prior art, 
before discussed. 
A block diagram of a suitable linear pseudo-random navigation system 
receiver is shown in FIG. 9. This receiver identifies the transmitted 
signals, tracks the phase of these signals and provides time difference 
information. As before explained, it uses the pseudo-random codes to 
distinguish the different transmitters to avoid skywave interference and 
to render in-band interference incoherent. 
The received signal flows from the antenna, so-labelled in FIG. 9, to the 
front end 6. The front end includes a low noise antenna coupler, bandpass 
filters and appropriate gain stages. It also includes notch filters, which 
eliminate strong narrow band interference, wherein some of these notches 
require manual tuning, but others automatically identify and notch 
interference. The output of the front end 6 is multiplied at 8 be a local 
version of the desired signal. This product is integrated in the following 
stage 10, with integration beginning when a local pulse begins, and ending 
when a local pulse ends. When the local pulse ends, moreover, the output 
of the integrator is converted to digital form at 12; and finally, the 
integrator 10 is cleared, as is well-known. 
The digitized data is used by a microprocessor 14 to acquire and track all 
transmitters. While acquiring, the microprocessor 14 searches for a 
correlation between the local signal and the received signal. It tracks by 
maintaining a peak output from the integrator 10, using a second order 
tracking loop to eliminate errors due to vehicle velocity, and statistical 
algorithms to eliminate data polluted by strong noise. The microprocessor 
14 calculates approximate time differences (T1, T2, T3). It also provides 
outputs to be displayed (especially the time differences) at 16. 
Triggers for initiating the internal pulse trains are provided by the 
slewable trigger circuit 18, with these triggers being slewed in 
accordance with the microprocessor time difference estimates. This circuit 
employs a stable clock (say "10 MHZ") as it's basic timing base. 
An imitation of the desired signal is provided by a pseudo-random pulse 
train generator 20, the imitation being analog and modulated by the same 
pseudo-random code as the transmitted signals of FIG. 8. 
Suitable circuits for performing the function of the pseudo-random pulse 
train generator 20 are described in Spread Spectrum Systems by R. C. Dixon 
(John Wiley and Sons, 1976). A useful microprocessor 14 is, for example, 
the Motorola 6809. The slewable trigger circuit 18 may be realized using 
standard TTL-MSI integrated circuits or could be incorporated in the 
microprocessor. Other well known circuits may also be used to achieve 
these results. 
The receiver of FIG. 9, as before mentioned, is a linear-type receiver; but 
a receiver may also be of the non-linear type, as shown, for example, in 
FIG. 10. This receiver also uses knowledge of the pseudo-random code to 
identify and track the transmitted signals. It tracks the phase of these 
signals and displays time difference and hence position information. 
The front end 6 of this receiver is identical to the linear receiver front 
end of FIG. 9. In this case, however, the front end output is hard-limited 
at 9. This process retains signal phase information while limiting the 
signal amplitude. This is done to limit the effect of impulsive noise and 
to avoid the dynamic range problems inherent in linear designs. 
The microprocessor 14 uses knowledge of the pseudo-random codes to acquire 
the desired signal and track its phase. It acquires by sampling at 11 the 
hard-limiter output periodically, searching for coherent energy (signal) 
and performing a modified cross-correlation. It tracks the signal phase by 
using a plurality of time-of-zero crossing detectors 13 in addition to the 
sample and holds. The microprocessor phase lock loop is of second order so 
that errors due to vehicle velocity are eliminated. It also contains 
statistical algorithms to identify and eliminate data corrupted by strong 
interference or noise. The microprocessor has algorithms to correct for 
the drift of its oscillator relative to the transmitter clock. It outputs 
the estimated time differences to a display so that the operator can 
estimate position, all as is well-known in this art. 
While the invention has been described in connection with the preferred 
Loran-C pulses of previously referenced U.S. Pat. No. 4,151,528, the 
pseudo-random pulsing concept thereof, properly also described as "random" 
as before discussed, can also be applied to more conventional Loran or 
other navigation pulses, and to other radio transmitting systems, as well, 
where similar operational problems may exist; such and other modifications 
occuring to those skilled in this art being considered to fall within the 
spirit and scope of the invention as defined in the appended claims.