Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2001-09-10
2002-10-15
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S357490, C701S213000
Reexamination Certificate
active
06466164
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to ranging receivers, e.g. receivers used for receiving navigation information from the well-known global positioning system, and more particularly to the calculation of pseudoranges by ranging receivers.
BACKGROUND OF THE INVENTION
In the well known Global Positioning System (GPS), a time-stamped signal is transmitted by a GPS satellite as the basis for a pseudorange calculation by a GPS receiver, as part of the determination by the GPS receiver of the position of the GPS receiver or other navigation information useful to the GPS receiver. The GPS receiver measures (records) the time of arrival (TOA) of a target signal fragment (a particular point on the signal, the point that happens to be received at the instant the receiver schedules for a position measurement) based on the GPS receiver clock. If the GPS receiver clock were synchronized with the GPS satellite clock, since the signal includes a time stamp so that the time of transmission (TOT) of the target signal fragment can be determined, the GPS receiver would know the time it took for the target signal fragment to propagate from the GPS satellite to the GPS receiver, which, when multiplied by the speed of light, would yield the range (actual distance) between the GPS satellite and the GPS receiver. (The receiver would simply note the time it received the target signal fragment received at the moment scheduled for a position determination, and would determine when the target signal fragment was transmitted by finding the last time-stamped signal fragment received before the target signal fragment, and based on a knowledge of the structure of the signal, determining how much later the target signal fragment was transmitted, compared to the time-stamped fragment of the signal.)
Each GPS satellite includes a clock that is approximately synchronized with GPS time, and each GPS satellite broadcasts information needed by a GPS receiver to correct the GPS satellite clock to essentially exact GPS time. Thus, each GPS receiver knows how to correct the time stamp of a signal fragment by a GPS satellite so that the time stamp is translated to precise GPS time. However, GPS receivers usually include relatively inexpensive clocks, which must be synchronized with GPS time each time a receiver is turned on, and periodically thereafter. Therefore, when a GPS receiver determines its range from a GPS satellite based on the time-stamp of a signal fragment and the time (according to the GPS receiver clock) it received a target signal fragment, the result is off from the true range, and is conventionally called the pseudorange to indicate the error. The pseudorange is the true range plus the error due to the GPS receiver clock being offset from GPS time. A GPS receiver typically determines its position by acquiring pseudoranges from usually four satellites, which allows the GPS receiver to simultaneously determine its position as well as the offset of its clock compared to GPS time.
GPS satellites broadcast navigation data (including their ephemerides and health information) using a direct sequence spread spectrum signal. Doing so allows all of the satellites to share the same frequency spectrum. Each satellite modulates the same carrier frequency with a PRN code (via binary phase shift key modulation) as well as with the navigation data for the satellite. A receiver must acquire and track the signal from a GPS satellite in order to read the navigation data from the satellite. The acquisition and tracking of a GPS signal for a particular one of the GPS satellites amounts to synchronizing the received PRN code for the GPS satellite (obtained from the received signal after removing the carrier frequency) with a replica of the PRN code generated by the GPS receiver. A correlator determines at what relative position the replica PRN code is in phase with the received PRN code.
In actual operation of a GPS receiver in good signal conditions (so that the GPS receiver can decode the time-stamp of the signal fragment), the GPS receiver determines precisely when a target signal fragment was transmitted by a satellite using a procedure that depends on aligning a replica of the PN code for the satellite. A GPS signal from a satellite includes a navigation data component at 50 Hz, and one or another pseudorandom number (PRN) sequences. One such PRN sequence, called the C/A (coarse acquisition) sequence, is a 1023 bit-long code broadcast as a 1.023 MHz signal; the 1.023 C/A signal thus has a period (epoch) of 1023 bits (called chips, to distinguish these from the bits of the navigation data message). A bit of the navigation data therefore has a duration of 20 ms, and a chip of the C/A code therefore has a duration of 1 ms. A receiver schedules a position measurement at a certain moment, and at the scheduled moment target signal fragments arrive from the set of satellites for which pseudoranges will be determined. The navigation data for a satellite is provided in five subframes of 300 bits each, making a frame of 1500 bits, broadcast over a period of 6 seconds. Each subframe provides a so-called TOW time stamp, which indicates the GPS time at which a corresponding particular bit of the navigation message was transmitted. (The GPS time so indicated is off from true GPS time, but the navigation data includes corrections to it, so that for all practical purposes, the TOW is according to true GPS time.) For each such satellite, the time of transmission of the target signal fragment is determined using:
T
TOT
k
=TOW
k
+N
bit
k
+N
ms
k
+N
chip
k
+&Dgr;
chip
k
, (1)
where TOW
k
is the time of week at which the bit corresponding to the TOW time stamp was transmitted, and the other terms add to that time of week the time increments needed to arrive at the time the target signal fragment was transmitted. Of the other terms, N
bit
k
is the time duration corresponding to the whole number of (20 ms) data bits transmitted after the TOW time stamp data bit (i.e. the data bit for which the TOW is given) and before the target signal fragment, N
ms
k
is the time duration corresponding to the whole number of (1 ms) epochs of PRN codes transmitted after the last whole data bit and before the target signal fragment, N
chip
k
is time duration corresponding to the whole number of (1 microsecond) chips transmitted after the last whole PRN epoch and before the target signal fragment, and &Dgr;
chip
k
is the time duration corresponding to the fraction of a chip transmitted after the last whole chip and before the target signal fragment. The quantity N
chip
k
+&Dgr;
chip
k
is called the sub-millisecond component of the time of transmission.
Once the TOT for a target signal is determined for enough satellites, the receiver (or a computing facility in communication with the receiver) determines the receiver position as well as the difference in the receiver clock and GPS time at the time of arrival of the target signal fragment using for example the method of least squares to solve simultaneously for the receiver position and the offset of the receiver clock from the true GPS time at the TOA of the target signal fragment. In other words, once the TOT for a satellite is determined, say the k
th
satellite, the pseudorange &rgr;
k
for that satellite is calculated using simply the TOA for the signal fragment based on the reading of the receiver clock at the moment the target signal arrives. The pseudorange &rgr;
k
is calculated according to:
&rgr;
k
=(
T
GPS
−T
TOT
k
)·
c,
(2)
where T
TOT
k
is the TOT for the k
th
satellite, T
GPS
is the TOA (in GPS time) of the target signal fragment (which is the same for all satellites), and c is the speed of light. Pseudoranges from usually at least four satellites are then used to solve a set of equations relating the pseudoranges for the satellites to the geometrical (true) ranges to the satellites and to the offset in the receiver clock; the set of equations are:
&rgr;
k
=∥{overscore (x)}
sv
k
(
T
TOT
k
)−
Akopian David
Syrjārinne Jari
Blum Theodore M.
Nokia Mobile Phones Ltd.
Ware Fressola Van Der Sluys & Adolphson LLP
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