Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2003-06-20
2004-07-13
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S357490, C701S216000
Reexamination Certificate
active
06762715
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method, which is adopted in a positioning system based on satellites and is used for determining the position of a positioning receiver, as well as relates to the positioning receiver itself.
As a positioning system based on satellites, a GPS (Global Positioning System) developed by the US is widely used.
In the GPS, a GPS receiver receives diffusion codes and navigation messages including time information and locus information, from GPS satellites moving around the earth. The GPS receiver then calculates the position of the receiver itself. Finally, the GPS receiver displays the calculated position to the user.
To put it concretely, a diffusion code received by a receiver designed for the public is a spectrum diffusion signal referred to as a C/A (Clear and Acquisition) code in an L1 band of 1575.42 MHz.
A C/A code is typically obtained as a result of a BPSK (Binary Phase Shift Keying) modulation process carried out on a carrier with a frequency of 1,575.42 MHz by using a signal diffusing 50-bps (bits per second) data through the use of a PN (Pseudorandom Noise) code having a chip rate of 1.023 MHz and a code length of 1,023. An example of the PN code is a gold code. As shown at the top of
FIG. 20
, the C/A code comprises a repetition of diffusion codes (PN codes) at a period of 1,023 chips. The period of 1,023 chips is 1 msec in length.
The diffusion code varies from satellite to satellite. However, the receiver is capable of determining which satellite generates a received diffusion code. A satellite generates a diffusion code, which is assigned to the satellite, synchronously with an atomic clock employed in the satellite, and transmits the diffusion code.
As shown in
FIG. 20
, the data of a navigation message is transmitted at 50 bps, that is, 1 bit per 20 msec, which accommodate 20 periods of the diffusion code. For a bit with a value of 1, the 1,023 chips in one period of the diffusion code, that is, the 1,023 chips in 1 msec, are opposite to those for a bit with a value of 0.
A navigation message is transmitted in main-frame units. Each main frame having a duration of 30 seconds comprises five sub-frames, namely, first to fifth subframes. Each sub-frame having a duration of 6 seconds comprises 10 words (or 300 bits). Thus, each word has a duration of 600 msec and comprises 30 bits.
As shown in
FIG. 21
, the first and second words of each of the first to fifth sub-frames are a TLM (telemeter) word and a HOW (Handover Word) respectively.
The first word used as a TLM word includes a preamble at the first to eighth bits and a TLM message at the ninth to twenty-second bits. The preamble has a fixed prescribed pattern independent of the data included in the TLM message. The ninth bit and the twenty-second bit are referred to as MSB (Most Significant Bit) and LSB (Least Significant Bit) of the TLM message respectively.
The second word used as a HOW includes a TOW (Time of Week) count message at the first to seventeenth bits and a sub-frame ID, which is an identification code, at the twentieth to twenty-second bits. The first bit and the seventeenth bit are referred to as MSB and LSB of the TOW count message respectively.
In each satellite, the TOW count message, which is time information, is counted up every 6 seconds, beginning at a predetermined start time, synchronously with the atomic clock employed in the satellite. The period of 6 seconds is the duration of a sub-frame.
From the TOW count message, the receiver is capable of detecting the time, at which a diffusion code is transmitted from the satellite, every 6 seconds. During the period of 6 seconds, digits of the diffusion-code transmission time expressing a number equal to or greater than 1 msec are generated on the basis of an epoch or a data acquisition time interval of 1 msec obtained in a process of demodulating information on a locus.
In addition, since the period of a diffusion code is 1 msec in length, the phase of the diffusion code corresponds to the value of the digits of the diffusion-code transmission time expressing a number smaller than 1 msec. Thus, by detecting the phase of the diffusion code, it is possible to confirm the pattern of the digits of the diffusion-code transmission time expressing a number smaller than 1 msec.
The TOW count message is received for every sub-frame, which has a duration of 6 seconds. In actuality, however, in order to confirm the information on time, it is necessary to recognize the head of the sub-frame and acquire data A corresponding to the TOW count message. In addition, it is necessary to further acquire data B corresponding to the TOW count message at a location, which is considered to be the next sub-frame, and to verify that the equation B=A+1 holds true. By carrying out these operations, the boundary between the sub-frames and the time information are confirmed at the same time.
For this reason, after the receiver is activated, it takes at least about 6 seconds to confirm the transmission time of a diffusion code. When the receiver receives data, starting with that in the middle of a sub-frame, it takes up to about 12 seconds to confirm the transmission time of a diffusion code.
Locus information unique to each satellite is inserted into the third and subsequent words of 3 sub-frames, namely, the first to third sub-frames, transmitted by the satellite. The unique locus information is referred to as ephemeris information. Locus information common to all satellites is inserted into the third and subsequent words of 2 sub-frames, namely, the fourth to fifth sub-frames, transmitted by the satellites. The common locus information is referred to as almanac information.
The ephemeris information is a parameter used for finding the locus of a satellite transmitting the ephemeris information. The ephemeris information is locus information having high precision. The ephemeris information is transmitted by the satellite repeatedly for each main frame and updated in a relatively frequent manner in accordance with control executed by a control station located on the ground.
The receiver stores the ephemeris information in a memory to be used for calculation of the position of the receiver. From the precision point of view, however, the life of the ephemeris information or the period in which the ephemeris information can be used is only about 2 hours. For this reason, it is necessary to monitor the time lapsing since a point of time at which the ephemeris information is stored in the memory. As the length of the monitored time exceeds the life of the ephemeris information, the information stored in the memory is updated by storing new ephemeris information.
In order to acquire new ephemeris information to be used for updating that stored in the memory, however, at least 3 sub-frames, that is, the first to third sub-frames with a total duration of 18 seconds are required. When the receiver receives data, starting with that in the middle of the main frame, it takes up to about 30 seconds corresponding to the duration of a main frame to update the ephemeris information stored in the memory.
The almanac information includes information showing approximate positions of all satellites and information indicating which satellites can be used. In order to obtain the entire almanac information, data of 25 main frames or a master frame with a duration of 750 seconds is required. The almanac information is locus information updated at intervals of several days in accordance with control executed by a control station located on the ground.
The life of the almanac information is several months. If the almanac information is stored in a memory employed in the receiver, normally, the information is updated at intervals of several months. By storing the almanac information in a memory employed in the receiver, it is possible to determine which satellite is to be assigned to any channel after the power supply is turned on.
In the calculation of a position, as shown in
FIGS. 22A and 22B
, symbol ti denotes the diffus
Awata Hideki
Takada Masayuki
Blum Theodore M.
Frommer William S.
Frommer & Lawrence & Haug LLP
Sony Corporation
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