Radio frequency broadcasting systems and methods using two low-cost geosynchronous satellites

High quality audio broadcasts at radio frequencies to mobile receivers at or near the earth's surface are provided by substantially simultaneous transmission of the same signal from two geosynchronous, spatially-separated satellites on the geosynchronous orbit which virtually eliminates multipath fading and foliage attenuation and thereby permits the use of a low-cost space segment.

BACKGROUND OF THE INVENTION 
Over the past several years, proposals have been made in the United States 
at the Federal Communications Commission (FCC) and, internationally, at 
the International Telecommunications Union (ITU) to broadcast radio 
programs from geosynchronous satellites to receivers in mobile platforms 
(e.g., automobiles) and in other transportable and fixed environments. 
Since geosynchronous satellites are located in near-equatorial orbits 
approximately 42,300 kilometers from the earth's surface, such satellites 
appear stationary to an observer on the ground. The satellite views 
roughly one-third of the earth's surface below it, which allows radio 
broadcast coverage of such a large area or, by using directional antennas 
on the satellite, a sub-area such as a particular country. This potential 
national coverage area of many tens of millions of square kilometers for 
providing radio service throughout the continental United States (or other 
country/region) is the main feature of satellite radio broadcasting, since 
normal terrestrial AM/FM radio stations typically cover a much smaller 
area. 
Radio broadcasting from satellites involves use of special receivers in 
mobile or fixed platforms because of technical implementation and 
frequency allocation/interference requirements. Consequently, proposals 
for building such systems have generally used UHF frequencies in the range 
of about 300 to about 3,000 MHz. FIG. 1 shows a typical satellite radio 
broadcasting system. Additional satellites can be used with the satellite 
system shown in FIG. 1 for providing redundancy, additional channels or 
both. FIG. 1 shows the most important transmission path, the path from the 
satellite to the mobile or fixed platforms. Since a mobile platform 
requires an antenna which can receive satellite signals from all azimuths 
and most elevation angles, the mobile platform antenna gain must be low 
(e.g. 2-4 dBi gain is typical). For this reason, the satellite must 
radiate large amounts of radio frequency transmitter power so that the 
mobile platform receiver can receive an adequate signal level. 
In addition to the need for a high power transmitter in the satellite is 
the need for extra transmitter power, called "transmission margin", to 
overcome multipath fading and attenuation from foliage. Multipath fading 
occurs where a signal from a satellite is received over two or more paths 
by a mobile platform receiver. One path is the direct line-of-sight or 
desired path. On other paths, the signal from the satellite is first 
reflected from the ground, buildings, or trucks, and then received by a 
mobile platform receiver, as FIG. 2 shows. These other paths are 
interfering in amounts that depend on factors such as losses incurred 
during reflection. Among the methods for reducing multipath fading in 
radio systems, are the following: 
1. Providing a second path for a desired signal between a transmitter and a 
receiver that is physically different from the first path for the signal. 
This is called space diversity, and is effective where only one of the two 
paths is strongly affected by multipath fading at any instant; 
2. Providing a second transmission frequency for a desired signal between a 
transmitter and a receiver. This is called frequency diversity, and is 
effective where only one of the two frequencies is strongly affected by 
multipath fading at any instant; and 
3. Providing signal modulation resistant to multipath fading such as spread 
spectrum. This method is effective where some resistance results from the 
large modulated frequency bandwidth used, and some resistance results from 
the receiver's rejection of an undesired signal's spreading code. 
The transmission margin necessary to overcome multipath fading or 
attenuation from foliage has been both measured and estimated by experts 
to be in the range of about 9 to about 12 dB for satellite radio broadcast 
systems operating at UHF frequencies. Fortunately, multipath and 
attenuation from foliage seldom occur simultaneously. However, the need 
for 9-12 dB transmission margin means that satellite transmitter power 
must be increased by a factor of 8 to 12 over its initially high level. 
Radio broadcasting satellites operating at such power levels would be 
extremely large, complex and costly. To date, no commercial system of this 
kind is in use because of this high cost. 
The systems and methods of this invention overcome these problems, by 
sending the same radio broadcast signals substantially simultaneously 
through two or more geosynchronous satellite sources separated by a 
sufficient number of degrees of orbital arc to minimize the effects of 
multipath fading and foliage attenuation, as FIG. 3 shows. 
A receiver on a mobile or fixed platform receives the two signals through 
two physically distinct paths in space diversity methods, and selects the 
stronger signal, or combines the two signals. The signals can be at the 
same radio frequency using a modulation resistant to multipath 
interference, or at a different radio frequency, with or without a 
modulation resistant to multipath. Foliage attenuation is minimized 
because trees and other foliage are seldom in the line-of-sight to both 
satellites at the same time. 
Receivers on mobile and fixed platforms receive the two signals through the 
two physically distinct transmission paths and select the stronger signal 
or combine the two signals to same radio frequency and avoid interfering 
with each other by use of spread spectrum modulation with code division 
multiple access, or by transmitting the radio signals from each satellite 
with opposite polarizations (e.g. cross or orthogonal polarizations such 
as horizontal linear/vertical linear or left circular/right circular). 
Where isolation of the two signals is achieved by opposite polarizations, 
any analog or digital signal modulation may be used. Alternatively, the 
two signals can be transmitted from the two satellites at different radio 
frequencies, which has the advantage of achieving frequency diversity 
capability in addition to the space diversity capability. Where different 
satellite frequencies are used, the signals may be transmitted using any 
analog or digital modulation. 
In preferred embodiments, these systems and methods provide radio 
broadcasts from geosynchronous satellites with one-eighth or less the 
power needed with a single satellite. Since satellite cost is directly 
proportional to satellite transmitting power, the radio broadcast 
satellite system of this invention uses satellites about one-eighth or 
less as costly and as heavy as single satellite systems. The reduced 
satellite mass also permits the use of a lower capability, lower cost 
launch vehicle. Even if two launch vehicles are needed, the satellite 
portions of the subject system are still only about 25% as costly as a 
single satellite transmission system. 
The subject system substantially improves reception quality by eliminating 
many blockage outages. Blockage outages occur when physical objects such 
as buildings or hills lie in the line-of-sight between the satellite and 
the receiver. As FIG. 4 shows, such blockage seldom occurs simultaneously 
on both satellite paths. FIG. 4 also shows that signal attenuation from 
foliage is minimized, because such attenuation results from partial signal 
blockage. 
SUMMARY OF THE INVENTION 
This invention relates to a system of two or more satellites moving in 
spatially separated positions on substantially the same geosynchronous 
orbit, each sending or relaying, substantially simultaneously, preferably 
at UHF frequencies in the range of about 300 to about 3,000 MHz, the same 
radio broadcast signal to receivers at or near the earth's surface. The 
spatial separation of the satellites is sufficient to minimize multipath 
fading, foliage attenuation, or both. Preferably, the separation between 
any two satellites is in the range of about 25.degree. to about 
50.degree.. These signals are preferably digitally modulated for high 
fidelity, but may also be analog.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the co-frequency embodiments of FIGS. 5 and 6, two satellites in 
substantially the same geosynchronous orbit send or relay substantially 
the same signal at substantially the same radio frequency. As a result, 
the receiver for the radio signals preferably resists multipath 
interference and prevents mutual self-interference that would result in 
signal jamming. Methods such as spread spectrum modulation (e.g., direct 
sequence or frequency hopping) are preferably used to achieve Code 
Division Multiple Access (CDMA). 
A preferred receiver for use in a mobile platform such as a vehicle is a 
standard, one-channel direct sequence spread spectrum detection device. 
This device is adapted to acquire the code of the signal from any of the 
satellites in the system. Preferably, this code is the same for the 
signals from both satellites, which is accomplished by having the 
satellites receive the radio signals to be transmitted to the mobile 
platform receiver from an up-link station on the earth's surface. Such an 
up-link station could delay one of the two codes in time to permit faster 
acquisition. In the mobile receiver, when the signal level drops a fixed, 
predetermined amount below a threshold value, such as an amount greater 
than 2 dB, a code loop s opened, and re-acquisition is performed on any 
signal stronger than the threshold value, as FIG. 5 shows in block diagram 
form. 
In FIG. 5, the antenna receives the radio frequency signals from each of 
the two satellites. The signals are amplified by the radio frequency 
amplifier. The signals are changed from radio frequency to an intermediate 
frequency (IF) by the down converter. The specific intermediate frequency 
is chosen by the frequency of the local oscillator. One of the two signals 
is acquired and detected by the spread spectrum demodulator on a random 
basis and the other signal ignored. The signal level of the detected 
signal is sent to the Signal Level Memory and Threshold Comparator. The 
detected signal is then sent to an audio amplifier and loudspeakers for 
listening. The Signal Level Memory continuously receives the signal level 
of the detected signal and compares it with the previously sent values of 
signal level. When the current value of signal level falls a certain 
amount (i.e., to a preset threshold), the spread spectrum demodulator is 
forced to re-acquire a signal, and attempts to do so until a signal is 
re-acquired whose level is greater than the threshold level. 
Alternatively, the receiver in the mobile platform can have common antenna, 
radio and intermediate frequency (IF) equipment. The IF feeds two 
correlators, each namely an independent spread spectrum code acquisition 
circuit and a detection circuit, as shown in FIG. 6. 
In FIG. 6, the antenna receives the radio frequency signal from each of the 
two satellites. The signals are amplified by the radio frequency 
amplifier. The signals are changed from radio frequency to an intermediate 
frequency (IF) by the down converter. The specific intermediate frequency 
is chosen by the frequency of the local oscillator. The down converter 
output is split in half by the splitter, and presented to each spread 
spectrum demodulator. Each spread spectrum demodulator acquires and 
detects one of the two signals. The two signals can be recognized by 
either using a different code sequence for each signal, or by having an a 
priori time offset between the two signals' identical code sequence. Each 
spread spectrum demodulator sends the detected signal to either the 
Amplitude Sensor Switch, which outputs the stronger (higher level) one to 
an audio amplifier and loudspeakers for listening, or to the Phase 
Corrector and Adder, which shifts the signals so they are in phase with 
each other and then sums them. The sum is outputted to an audio amplifier 
and loudspeakers for listening. Alternatively, the phase correction can be 
accomplished in the Spread Spectrum Demodulators. The codes of the signals 
from the satellites can be substantially identical, but offset in time or 
orthogonal to one another, as are Gold codes. Each of the detected signals 
is derived from the correlators. The signals can then be selected 
individually, or combined with one another to produce a single, summed 
output signal. 
The receiver preferably outputs a signal by one of two methods. The simpler 
method compares the amplitudes of the signals from the two satellite 
sources, and chooses the stronger signal for output. Alternatively, the 
phases of the two signals are adjusted until they are identical to one 
another. The two signals are then summed to produce an output signal. This 
method avoids switching the receiver from one signal to another, and 
provides better quality signals when the transmission paths of the two 
signals are unaffected, or are only partially attenuated by multipath 
fading or foliage. The previously mentioned phase adjustments are 
necessary because, although both satellite sources send substantially the 
same signal at substantially the same time, these signals reach the mobile 
platform receiver with different phases since the platforms are generally 
at a different distance from each satellite. 
In the dual-frequency embodiments, both satellites send or relay 
substantially the same broadcast signal, but at two substantially 
different frequencies. These embodiments achieve less multipath fading 
because both space and frequency diversity are attained simultaneously. 
These embodiments further permit the use of multipath resistant 
modulation. However, the receiver is more complex. As FIG. 7 shows, such a 
receiver includes two down converters, intermediate frequency amplifiers 
and demodulator circuits. In FIG. 7, the antenna receives the radio 
frequency signal from each of the two satellites. The signals are 
amplified by the radio frequency amplifier. The radio frequency amplifier 
output is split in half by the Splitter and presented to each down 
converter. The signals are changed from radio frequency to an intermediate 
frequency (IF) by the down converters. The local oscillators are set to 
the proper frequencies so that the signal frequencies F.sub.1 and F.sub.2 
are converted to the same IF. The IF from the down converters feeds the 
demodulators. The demodulators remove the signal modulation, and send the 
detected signals to either the Amplitude Sensor Switch, which outputs the 
stronger (higher level) one to an audio amplifier and loudspeakers for 
listening, or to the Phase Corrector and Adder, which shifts the signals 
so they are in phase with each other and then sums them. The sum is 
outputted to an audio amplifier and loudspeakers for listening. 
Alternatively, the phase correction can be accomplished in the 
demodulators. 
Dual-frequency embodiments can be as shown in FIG. 7, or can be of a type 
which switches rapidly between the frequencies of the two signals, or can 
utilize digital signal processing. The output signals from the receiver 
can be selected by comparing the amplitudes of the two input signals, and 
using the stronger signal, or the input signals can be adjusted to the 
same phase and summed to produce an output signal. 
Alternatively, the receiver in the mobile platform can have an antenna 
which accepts two orthogonal (or cross) polarized radio frequency 
transmissions (or two antennas, each accepting one of the two 
polarizations); radio frequency amplification, down conversion, 
intermediate frequency (IF) and demodulation equipment; and equipment for 
either selecting the stronger signal or phasing the two signals and then 
combining them as shown in FIG. 8. 
In FIG. 8, one or more antennas receive the same radio frequency 
transmission from each of the two satellites at orthogonal polarizations. 
One satellite sends its radio frequency transmission at one polarization 
(e.g., right hand circular or vertical), and the second satellite sends 
the same radio frequency transmission at the orthogonal polarization 
(e.g., left hand circular or horizontal). The signals are electrically 
separated from each other by the cross polarization amplified by radio 
frequency amplifiers, and then converted from radio frequency to 
intermediate frequency (IF), as by the down converters. The specific 
intermediate frequency is chosen by the frequency of the local oscillator. 
The embodiment in FIG. 8 assumes that both satellites transmit their 
signals at the same radio frequency. However, different radio frequencies 
could be used by adding a second local oscillator. The down converters 
feed demodulators which are chosen to match the modulation used, since any 
type of analog or digital modulation can be employed. The demodulators 
remove the signal modulation, and send the detected signals to the 
amplitude sensor switch, which outputs the stronger, or higher level 
signal to an audio amplifier for listening, or to the phase corrector and 
adder, which shifts the signals so they are in phase with each other and 
then sums them. The summed signal is outputted to an audio amplifier and 
loudspeakers for listening. Alternatively, phase correction can be done in 
the demodulators.