Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-11-04
2002-08-27
Hunter, Daniel (Department: 2684)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S428000, C455S430000, C455S431000, C455S003010, C455S003020, C455S003060, C455S012100, C455S013100, C455S013200, C375S260000, C375S316000, C244S158700, C244S159200
Reexamination Certificate
active
06442385
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method and apparatus for selectively operating tundra orbit satellites to reduce receiver memory requirements in a time diversity satellite broadcast system.
BACKGROUND OF THE INVENTION
Radio frequency transmissions are often subjected to multipath fading. Signal blockages at receivers can occur due to physical obstructions between a transmitter and the receiver or service outages. For example, mobile receivers encounter physical obstructions when they pass through tunnels or travel near buildings or trees that impede line of sight (LOS) signal reception. Service outages can occur, on the other hand, when noise or cancellations of multipath signal reflections are sufficiently high with respect to the desired signal.
Communication systems can incorporate two or more transmission channels for transmitting the same program or data to mitigate the undesirable effects of fading or multipath. For example, a time diversity communication system delays the transmission of program material on one transmission channel by a selected time interval with respect to the transmission of the same program material on a second transmission channel. The duration of the time interval is determined by the duration of the service outage to be avoided. The non-delayed channel is delayed at the receiver so that the two channels can be combined, or the program material in the two channels selected, via suitable receiver circuitry. One such time diversity system is a digital broadcast system (DBS) employing two satellite transmission channels.
With reference to
FIG. 1
, a DBS
10
with time diversity is shown. An uplink facility comprises a splitter
12
for providing multiple channel time division multiplexed (TDM) content
11
to each of two transmission channels
14
and
16
. The first transmission channel
14
is transmitted to a first satellite
20
at a first frequency f
1
via uplink components indicated at
18
. The second transmission channel
16
is delayed by a selected time interval, as indicated at
22
, prior to being transmitted to a second satellite
24
at a second frequency f
2
via uplink components indicated at
26
. A dual arm receiver receives the early and late signals from the satellites
20
and
24
, respectively, at a downconverter
28
. A delay unit
30
delays the early signal from the satellite
20
via a time interval corresponding to the time interval used to delay the second transmission channel at the transmitter. The delay is applied to all of the channels in the multiple channel TDM content
11
. The delayed output from the delay unit
30
can then be synchronized with the late signal and combined, as indicated at
32
. A channel selector
34
extracts content corresponding to a particular one of the channels in the multiple channel TDM content in response to a user input, for example.
In a particular implementation of a DBS with time diversity, three satellites
20
,
24
and
36
operate in respective ones of tundra orbits
50
,
52
and
54
, as illustrated in FIG.
2
. In other words, the satellites
20
,
24
and
36
are in respective ones of three inclined, elliptical orbits which are each separated by approximately 120 degrees. The combination of the 120 degree separation and the rotation of the earth yields a common ground track
60
for all three orbits which is illustrated in FIG.
3
. In addition to an approximately 120 degree spatial separation, the orbits
50
,
52
and
54
are temporally separated by T/
3
or one-third of an orbit period T (e.g., one-third or eight hours of a 24 hour geosynchronous orbit).
With continued reference to
FIG. 3
, the satellite ground track
60
is a figure-eight, having a northern loop
62
that is smaller than the southern loop
64
. The northern and southern loops
62
and
64
share a common ground track point hereinafter referred to as the crossover point
66
, as shown in FIG.
4
. At the crossover point, satellites descending from the northern loop
62
to the southern loop
64
have the same orbital position as satellites ascending from the southern loop
64
to the northern loop
62
. Each satellite
20
,
24
and
36
spends approximately one-third (e.g., eight hours) of its orbit time south of the equator
68
and, correspondingly, two-thirds (e.g., sixteen hours) of its orbit time north of the equator. Thus, when one satellite
20
is at perigee, as shown in
FIG. 5
, the subsatellite points of the other two satellites
24
and
36
cross paths and are therefore in the same sky position at the crossover point
66
.
As indicated in
FIG. 6
, when one satellite
36
is at apogee, the other two satellites
20
and
24
are at essentially equal latitude near the equator
68
. Of these two satellites, (e.g., satellites
20
and
24
in FIG.
6
), one satellite
20
appears to be rising in the southeast, while the other satellite
24
appears to be setting in the southwest. The rising satellite commences transmitting, while the setting satellite ceases transmitting to comply with international coordination and interference concerns with respect to the allocation of bandwidth for satellite operations. By symmetry of the elliptical orbit, this situation of two satellites at nearly the same latitude occurs halfway through an orbit following the time of perigee, that is, at time T/
2
(e.g., 24/2 or 12 hours) past perigee.
In a time diversity system as described above in connection with
FIG. 1
, the satellites
20
,
24
and
36
operate as either the “early” satellite (i.e., the satellite transmitting the nondelayed channel
14
) or the “late” satellite (i.e., the satellite transmitting the delayed channel
16
), depending on the position of the satellite along the satellite ground track
60
.
For example, when the satellites
20
,
24
and
36
are located along the ground track
60
as depicted in
FIG. 6
, the satellite
20
is the late satellite for illustrative purposes and is switched on shortly after it ascends past the equator along the southern loop
64
.
Correspondingly, the satellite
24
is switched off for its travel along the portion of the southern loop
64
that is below the equator
68
. The satellite
36
is the early satellite.
When each satellite commences its ascent north of the equator along the southern loop
64
, the satellite is switched from “early” to “late”, or “late” to “early”, depending on its “early” or “late” status during its traverse of the previous northern loop
62
. Thus, the “early” or “late” status of a satellite changes in an alternate manner after the completion of the period during which the satellite is switched off, that is, while traversing the southern loop
64
when the orbital position of the satellite is at a latitude below the equator
68
. Accordingly, in the previous example, when the late satellite
36
reaches a latitude near the equator while descending in the southern loop
64
, the early satellite
20
is at apogee, and the satellite
24
is switched on and is commencing its ascent above the equator, approximately eight hours later. The satellite
36
is therefore switched off and the satellite
24
is the late satellite. The uplink components
18
and
26
are each controlled using data from a telemetry, tracking and command (TTC) system
27
which monitors and controls the flight operations of the satellites
20
,
24
and
36
, as shown in FIG.
1
. In accordance with this TTC system data, the uplink components
18
and
26
are commanded to transmit the content on the transmission channels
14
and
16
, respectively, to the selected ones of the satellites, depending on their orbital positions and corresponding positions along the ground track
60
. Each satellite is capable of receiving either of the frequencies corresponding to the late or early satellite signals as commanded by the TTC system.
In view of the above-described system for operating early and late satellites in tundra orbits, a compromise exists between the elevation angle and the availability of spatial and/or time diversity. When elevation
Harry Andrew T
Hunter Daniel
Roylance Abrams Berdo & Goodman LLP
XM Satellite Radio Inc.
LandOfFree
Method and apparatus for selectively operating satellites in... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for selectively operating satellites in..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for selectively operating satellites in... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2895811