Satellite search methods for improving time to first fix in a GPS receiver

• US 5,402,347 A
• Filed: 07/22/1993
• Issued: 03/28/1995
• Est. Priority Date: 07/22/1993
• Status: Expired due to Term

• Current Assignee: Trimble Navigation Limited (Trimble Inc.)
• Original Assignee: Trimble Navigation Limited (Trimble Inc.)
• Inventors: Weber, Lynn, McBurney, Paul W.
• Primary Examiner(s): Teska, Kevin J.
• Assistant Examiner(s): Walker, Tyrone V.
• Application Number:
  US08/095,953
• Time in Patent Office: 614 Days
• Field of Search: 364/443, 364/449, 364/447, 342/417, 342/418, 342/357, 342/385, 342/386
• US Class Current: 701/213
• CPC Class Codes: G01S 19/235 Calibration of receiver components
• CPC Class Codes: G01S 19/28 Satellite selection
• CPC Class Codes: G01S 19/29 carrier including Doppler, related G01S19/246 takes precedence

Abstract

An embodiment of the present invention includes a GPS receiver capable of concurrently tracking up to eight GPS satellites. The GPS receiver comprises computer-implemented methods for parallel search, split search and precomputation. The parallel search method causes the apparent Doppler frequency spectrum to be sectioned into several segments and a first GPS satellite signal is searched for in parallel in each of the segments. When the first GPS fix is not found after a predetermined time has expired, a split search is begun.

Claims

1. A method for quickly obtaining a first positional fix after a short period of non-operation with a multiple channel GPS receiver having a local oscillator with a frequency uncertainty that exceeds an actual Doppler carrier frequency shift and adds to the total apparent Doppler frequency shift, the method comprising the steps of:

pre-computing a table of codes associated with a plurality of constellations of orbiting GPS satellites predicted to be visible at a plurality of times in the future based on a current time and a GPS satellite almanac and a predetermined geographic position of said GPS receiver with said table indexed according to said future times;

fetching one part of said table of codes according to a time said GPS receiver is re-powered up after a period of power-off that followed the step of pre-computing said table;

searching for a signal from a first GPS satellite associated with said fetched part of said table of codes in parallel using said multiple channels of said GPS receiver such that a first frequency search range is positioned with a nominal frequency estimated for said GPS satellite signal at its mid-point and a first GPS receiver channel is appointed to search said first frequency search range and a second GPS receiver channel is appointed to search in parallel a second frequency search range adjacent to said first frequency search range;

splitting said multiple channels between searching for said first GPS satellite and a second GPS satellite if said first GPS satellite signal was not found; and

using an offset frequency value for controlling said local oscillator obtained from locking onto a first-found GPS satellite signal in subsequent searches for other GPS satellite signals, wherein said local oscillator frequency uncertainty contribution to said total apparent Doppler frequency shift is substantially reduced in magnitude, and the time required to acquire a subsequent satellite signal is thus substantially reduced.

2. The method of claim 1, wherein:

the pre-computing of said table includes ephemeris information obtained previous to said period of power-off and said table extends forward in time for period limited by the validity of said ephemeris information.

3. A method for quickly obtaining a first positional fix after a short period of non-operation with a multiple channel GPS receiver having a local oscillator with a frequency uncertainty that exceeds an actual Doppler carrier frequency shift and adds to the total apparent Doppler frequency shift, the method comprising the steps of:

pre-computing a table of pseudo random number (PRN) codes associated with GPS satellites predicted to be visible at a plurality of times in the future based on a current time and GPS satellite almanac and a predetermined geographic position;

fetching a PRN code from said table according to a time said GPS receiver is re-powered up after a period of non-operation that followed the step of pre-computing said table; and

using an offset frequency value for controlling said local oscillator obtained from locking onto a first-found GPS satellite signal in subsequent searches for other GPS satellite signals, wherein said local oscillator frequency uncertainty contribution to said total apparent Doppler frequency shift is substantially reduced in magnitude, and the time required to acquire a subsequent satellite signal is thus substantially reduced.

4. A method for searching in parallel for acquisition of a signal from a GPS satellite with a multiple channel GPS receiver having a local. oscillator with a frequency uncertainty that exceeds an actual Doppler carrier frequency shift and adds to the total apparent Doppler frequency shift, the method comprising the steps of:

pre-selecting a particular carrier frequency as a nominal frequency within an apparent Doppler frequency spectrum for GPS satellite signals that includes a relatively larger frequency uncertainty contributed by inaccuracies in said local oscillator;

partitioning said apparent Doppler frequency spectrum into a plurality of adjacent frequency search ranges such that a particular first frequency search range near the center of said apparent Doppler frequency spectrum includes said nominal frequency;

appointing a first of said GPS receiver channels to search said first frequency search range;

appointing a second of said GPS receiver channels to search a second frequency search range above and adjacent to said first frequency search range;

appointing any but said first of said GPS receiver channels to search a third frequency search range below and adjacent to said first frequency search range;

searching within said first and at least one of said adjacent frequency search ranges for said signal from a GPS satellite and terminating the search if said signal is found;

appointing any but said first of said GPS receiver channels to search a fourth frequency search range above and adjacent to said second frequency search range;

appointing any but said first of said GPS receiver channels to search a fifth frequency search range below and adjacent to said third frequency search range;

searching within said first and at least one of said adjacent frequency search ranges for said signal from a GPS satellite; and

using an offset frequency value for controlling said local oscillator obtained from locking onto a first-found GPS satellite signal in subsequent searches for other GPS satellite signals, wherein said local oscillator frequency uncertainty contribution to said total apparent Doppler frequency shift is substantially reduced in magnitude, and the time required to acquire a subsequent satellite signal is thus substantially reduced.

5. A multiple channel GPS receiver, comprising:

GPS navigational computer means for pre-computing a table of pseudo random number (PRN) codes associated with GPS satellites predicted to be visible at a plurality of times in the future based on a current time and GPS satellite almanac and a predetermined geographic position;

table indexing means for fetching a PRN code from said table according to a time said GPS receiver is re-powered up after a period of non-operation that followed the step of pre-computing said table;

radio receiver means having a local oscillator with a frequency uncertainty that exceeds an actual Doppler carrier frequency shift Which adds to the total apparent Doppler frequency shift and for searching for a signal from a first GPS satellite associated with said fetched PRN code in parallel using said multiple channels of said GPS receiver such that a first frequency search range is positioned with a nominal frequency for said GPS satellite signal at its mid-point and a first channel is appointed to search said first frequency search range and a second channel is appointed to search in parallel a second frequency search range adjacent to said first frequency search range;

multi-channel GPS receiver means for splitting said multiple channels between searching for said first GPS satellite and a second GPS satellite if said first GPS satellite signal was not found; and

local oscillator offset-control means for finding an offset frequency value to controlling said local oscillator, determined from locking onto a first-found GPS satellite signal, in subsequent searches for other GPS satellite signals, wherein said local oscillator frequency uncertainty contribution to said total apparent Doppler frequency shift is substantially reduced in magnitude, and the time required to acquire a subsequent satellite signal is thus substantially reduced.