Dual antenna null elimination

RF signal nulling and loss of RF lock (i.e., loss of a communications or command link) is resolved by changing the phase of signals derived from first and second (+Z and -Z) antennas so that they add in phase instead of canceling. This is accomplished automatically by using a computer or processor to measure the RF signal level output by a receiver and change the phase of the incoming signal using a phase shifter to achieve the highest level. A squelch circuit in the receiver is used to determine the maximum obtainable RF signal level. The squelch level is compared in a computer to a threshold when the system looks for a higher squelch level. The phase of the phase shifter is changed using a computer to maximize the received signal level. A feedback loop from the receiver through the computer to the phase shifter provides a path to shift the phase of the incoming RF signal in one of the paths so the respective signals add instead of cancel, which eliminates the signal nulling problem.

BACKGROUND 
The present invention relates generally to spacecraft, and more 
particularly, to a system that eliminates nulls created by summing signals 
derived from two broadbeam antennas on a spacecraft that point in 
different directions. 
The telemetry, command and ranging (TC&R) antenna on many spacecraft is on 
the earth side of the spacecraft and a similar antenna is disposed on the 
opposite (anti-earth) side of the spacecraft, along the +Z and -Z axis. 
Typical spacecraft configurations apply the signal from both antennas to a 
single receiver through a coupler that normalizes signal levels from both 
antennas. The normal configuration involves the +Z antenna facing the 
earth so that the -Z antenna is completely masked by the spacecraft. 
During orbit raising, and in a loss of lock scenario, the spacecraft can 
face in any direction. The incoming signal transitions from the +Z to the 
-Z antenna as the spacecraft rotates with respect to the earth. A problem 
occurs near the center (X/Y plane) where the spacecraft receives 
approximately equal signals from both antennas. The resulting signal has a 
"null" region. At the frequency of operation of the antennas, the 
wavelength is typically shorter than an inch. This means that the summing 
point for the +Z and -Z signals can add or subtract the signals depending 
on the exact orientation of the spacecraft. When two RF signals almost 180 
degrees apart are added a strong signal is produced. However, when the 
signals subtract, the RF level is almost canceled (nulled out ) and the 
link to Earth is lost. 
Previous approaches incorporate additional receivers with antennas having 
narrow beamwidths so that the signals can be combined at baseband. The 
baseband is sufficiently lower in frequency so that the nulling effect is 
not noticed. The prior approaches have also been less reliable as a result 
of assigning a different receiver to a signal shifted by 180 degrees. 
Various approaches have been proposed to eliminate or ignore the antenna 
null. One approach involves constantly moving the spacecraft to assure 
that it will not be in a null situation for a significant period of time. 
This approach was determined to be undesirable since the same motion will 
put a good signal into a null. Another approach involves orienting the 
spacecraft so that the null will rarely face the earth, but this cannot be 
guaranteed during a loss of lock scenario. 
Another solution involves switching the incoming signals between the two 
antennas. This solution sounds good until the reliability and failure 
modes are taken into consideration. A failure could lock the system on the 
wrong antenna and prohibit receiving a signal. This could be resolved by 
an additional receiver for each antenna which is quite expensive. 
Therefore, it is an objective of the present invention to provide for a 
system that eliminates nulls created by two oppositely pointing antennas 
disposed on a spacecraft and thus eliminates loss of communication with 
and control of the spacecraft. 
SUMMARY OF THE INVENTION 
To meet the above and other objectives, the RF signal nulling and 
associated loss of RF lock (i.e., loss of the communications or command 
link) is resolved in accordance with the present invention by changing the 
phase of signals derived from first and second (+Z and -Z) antennas so 
that they add in phase instead of canceling. This is accomplished 
automatically by using a computer or processor to measure the RF signal 
level output by a receiver and change the phase of the incoming signal by 
means of a phase shifter to achieve the highest level. A squelch circuit 
in the receiver is used to determine the maximum obtainable RF signal 
level. The squelch level is compared in a computer onboard the spacecraft 
to a threshold level. When the system is below the threshold, it looks for 
a higher squelch level. The phase of the phase shifter is changed using a 
computer to maximize the received signal level. A feedback loop from the 
receiver through the computer to the phase shifter provides a path to 
shift the phase of the incoming RF signal in one of the paths so the 
respective signals add instead of cancel, which eliminates the signal 
nulling problem. 
The present system incorporates an RF power monitor or a squelch circuit in 
the receiver that is coupled by way of a feedback loop through the 
computer to the RF phase shifter in one of the two paths (+Z or -Z) whose 
signals are combined and applied to the receiver. When the system detects 
low RF power or the squelch signal, the phase shifter is commanded by the 
computer to shift the phase to optimize the received power or maximize the 
squelch level. This function may be disabled when the spacecraft is on 
orbit and activated if there is a loss of lock. Other variations of this 
approach may be used to introduce a discrete phase shift of 180 degrees or 
90 degree increments. This variation does not require a RF power monitor 
and may be implemented using a squelch signal produced by the squelch 
circuit. 
The present invention eliminates telemetry, command and ranging antenna 
blackout null areas located in a 50 degree donut shaped pattern around a 
spacecraft using only +Z and -Z (first and second) antennas by combining 
RF signals derived from the antennas. The present invention provides for 
an increase in link margin by adding 6 dB (derived from coherently 
combining two signals) to currently available -4 dB antenna gain. The 
present invention provides improved RF performance and reliability (with 
respect to dropouts).

DETAILED DESCRIPTION 
Referring to the drawing figures, FIGS. 1a and 1b illustrate front and side 
views, respectively, of a spacecraft 10 comprising first and second (+Z 
and -Z) antennas 11, 12 that face the Earth and away from the Earth, 
respectively. More specifically, FIG. 1a shows the typical locations of 
the antennas 11, 12 relative to the XZ plane, while FIG. 1b shows the 
locations of the antennas 11, 12 relative to the YZ plane. The present 
invention may be implemented in such a spacecraft 10 in a manner described 
below. 
By way of introduction, telemetry, command and ranging provisions on the 
spacecraft 10 incorporate multiple antennas to provide complete coverage 
during orbit raising. System simplification has reduced the number of 
antennas to two antennas 11, 12 located on the +Z and -Z axis. For 
reliability reasons, signals from the two (+Z and -Z) antennas are added 
in an RF section located prior to a receiver 15. Unfortunately, this 
creates a donut shaped pattern around the center of the spacecraft 10 
between .+-.40 degrees from the Y axis in the YZ plane and a lopsided 30 
to 50 degree area from the -X to the +X respectively in the XZ plane where 
the RF signals cancel periodically so that no signal or a significantly 
reduced signal is generated, resulting in a loss of command capability. 
The peak to peak separation between nulls occurs every 0.13 degrees change 
in the spacecraft 10 for a Ku-Band system. 
FIGS. 1c-1f illustrate the antenna and null patterns produced by the 
antennas 11, 12 shown in FIGS. 1a and 1b and discussed in the preceding 
paragraph. In particular, FIG. 1c shows the antenna pattern for the +Z 
antenna 11, and FIG. 1f shows the antenna pattern for the -Z antenna 12. 
FIG. 1d shows the null area relative to the XZ plane and FIG. 1e shows the 
null area relative to the YZ plane. The +Z antenna 11 has the broadest 
beam and the best coverage. The null area relative to the XZ plane was 
measured to be between 30 and 50 degrees and is shown in FIG. 1d. The null 
area relative to the YZ plane of .+-.40 degrees is shown in FIG. 1e. FIG. 
3 is a graph of normalized magnitude versus angle in radians showing the 
null pattern that is eliminated by the present invention. 
FIG. 2 illustrates a system 20 in accordance with the principles of the 
present invention that eliminates nulls created by the antennas 11, 12 on 
the spacecraft 10. In the system 20, RF signals derived from the +Z and -Z 
antennas 11, 12 are processed by redundant processing paths, one of which 
will be described below. The +Z antenna 11 comprises a 14 GHz broadbeam 
receive antenna 11. The -Z antenna 12 comprises a receive horn antenna 12. 
The particular antenna type is not limited to those specified in this 
example. This patent covers all applications where antenna beams overlap 
and cause an RF cancellation or reduction in power resulting from the 
summing of 2 received. 
The +Z antenna 11 is coupled by way of a coupler (CP) 13 and a bandpass 
filter 14 to the receiver 15. A coupler 13 is used instead of a hybrid so 
that there will be lower loss through the +Z path because its signal is 
weaker as a result of the larger antenna pattern of the +Z antenna 11. The 
output of the receiver 15 is coupled to a bit detector and decryptor 16 
which is coupled to and controlled by a central processing unit (CPU) 31 
or processor 31. The CPU 31 is coupled by way of a MIL-STD 1553 bus 32 to 
a data concentration unit (DCU-A) 33. The output of the DCU-A 33 is 
coupled to a telemetry transmitter 21. The RF output of the telemetry 
transmitter 21 is amplified by a traveling wave tube amplifier (TWTA) 22 
and is sent through a circulator 23, a bandpass filter 24 and a coupler 
25. One output of the coupler 25 is coupled by way of a switch 26 to a 12 
GHz broadbeam transmit antenna 17. The telemetry transmitter 21 has a 
second output that is coupled by way of a second circulator 27 to an 
alternate position of the switch 26. A second output of the coupler 25 is 
coupled to a -Z transmit horn antenna 16. A second position of the switch 
26 is coupled by way of a multiplexer 18 to a communications antenna 19. 
In accordance with the present invention, the CPU 31 is coupled to the 
receiver 15 and receives a squelch level signal from a squelch circuit 15a 
therein. Alternatively, a RF power monitor 15a may be used in place of the 
squelch circuit 15a. The CPU 31 measures the squelch level signal. The CPU 
31 is coupled to a phase shifter 34 that is connected between the -Z 
receive horn antenna 12 and the coupler 13. The CPU 31 outputs a control 
signal to the phase shifter 34 that changes the phase generated by the 
phase shifter 34 in accordance with a predetermined algorithm, and which 
is added to or subtracted from the phase of the RF signal derived from the 
-Z receive horn antenna 12. Thus, a feedback loop 35 is formed from the 
receiver 15 through the CPU 31 to the phase shifter 34 which is used to 
shift the phase of the signals received by the -Z receive horn antenna 12 
so that the occurrence of antenna nulls is eliminated. 
The phase shifter 34 is preferably a computer controlled ferrite phase 
shifter 34 that is disposed in the path between the -Z receive horn 
antenna 12 and the coupler 13. A ferrite phase shifter 34 is used because 
of its low loss and fast response. However, it is to be understood that 
any phase shifter 34 may be employed and the present invention is not 
limited to only ferrite phase shifters 34. Also, the present invention may 
be implemented using a two step (180 degree) phase shifter 34 or a 
continuously variable phase shifter 34. 
The algorithm that operates the phase shifter 34 is programmed into the CPU 
31 and is activated when the squelch level of the receiver 15 is 
approached (such as when a predetermined threshold level is reached), 
indicating that the RF signal processed by the receiver 15 is about ready 
to drop out. This initiates an optimization algorithm in the CPU 31 that 
controls the phase of the RF signal derived from the -Z receiver horn 
antenna 12 that is input to the coupler 13. 
Irrespective of the type of phase shifter 34 that is employed, the squelch 
level of the receiver 15 is measured before and after the phase is 
changed. The RF signal output from the receiver 15 is selected that 
generates the largest squelch level signal. If the new squelch level is 
larger than the initial squelch level, the new squelch level is maintained 
until it drops to the threshold level. If the new squelch level is lower 
than the initial squelch level, the phase shift supplied by the phase 
shifter 34 is changed in the opposite direction. 
Thus, a system that eliminates nulls created by oppositely facing antennas 
on a spacecraft and thus eliminates loss of control of the spacecraft has 
been disclosed. It is to be understood that the described embodiment is 
merely illustrative of some of the many specific embodiments which 
represent applications of the principles of the present invention. 
Clearly, numerous and other arrangements can be readily devised by those 
skilled in the art without departing from the scope of the invention.