GPS near-far resistant receiver

Details GPS near-far resistant receiver GPS near-far resistant receiver

2002-09-12

2004-08-03

Blum, Theodore M. (Department: 3662)

Communications: directive radio wave systems and devices (e.g.,

Directive

Including a satellite

C342S357490, C701S213000

Reexamination Certificate

active

06771214

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to the field of radio-navigation receivers and more particularly to a method for mitigating constructive interference in a received radio-navigation signal by modeling the interference, and then subsequently removing the interference from the signal.
BACKGROUND
An example of a radio-navigation satellite system (RNSS) is the United States Global Positioning System (GPS). The GPS was established by the United States government, and employs a constellation of 24 or more satellites in well-defined orbits at an altitude of approximately 26,500 km. These satellites continually transmit microwave L-band radio signals in two frequency bands, centered at 1575.42 MHz and 1227.6 MHz., denoted at L
1
and L
2
respectively. These signals include timing patterns relative to the satellites onboard precision clock (which is kept synchronized by a ground station) as well as a navigation message giving the precise orbital positions of the satellites, an ionosphere model, and other useful information. GPS receivers process the radio signals, computing ranges to the GPS satellites, and by triangulating these ranges, the GPS receiver determines its position and its internal clock error.
GPS's designers assumed that all transmitters would be aboard satellites at a large and relatively constant distance from all user receivers, consequently generating signal levels at the receivers that would be weak and relatively constant. This assumption drove a number of trade-offs in system and satellite transmitter design and continues to influence receiver development even today.
Despite this assumption ground-based transmitters (known as PLs, pseudo-satellites, or simply pseudolites) have been used to complement the GPS satellites from the very beginning. In the foreseeable future, PLs may be incorporated in Unmanned Aerial Vehicles (UAVs). A PL transmits a signal with code-phase, carrier-phase, and data components with the same timing as the satellite signals and the same format. A GPS receiver acquires such a PL signal and derives code-phase pseudo-ranges or carrier-phase measurements to be used in a navigation algorithm in substantially the same manner as for a GPS satellite. The major differences are that a PL typically does not contain a high-accuracy atomic clock and that the PL position must be described in geographical terms rather than in orbital elements.
Precision navigation and landing systems require reliable and highly accurate position, velocity and time information (these aggregately denoted herein as PVT information) not achievable by standalone GPS. Precision-guided weapons require reliable PVT information to achieve acceptable Circularly Error Probable (CEP) targeting errors. To meet these requirements additional radio-navigation transmitters are needed. These transmitters can be additional satellites as specified in the Wide Area Augmentation System (WAAS) or PLs based on the ground as specified in the Local Area Augmentation System (LAAS), or on board ships, or even UAVs loitering in the air above an area of interest. WAAS and LAAS can transmit either correction data (i.e., differential data) or provide additional ranging information. When these transmitters use the GPS spectrum, as is the case for UAVs, PLs, and satellites providing ranging information, additional interference is added. This constructive interference is seen as noise to the receiver, which can degrade and in some cases prevent a receiver from acquiring and tracking the satellites.
Moreover, introduction of PLs violates one of the key assumptions of the designers of GPS. Thus, the distance between a user receiver and a PL can be large or quite small, so PL signal levels at a receiver can vary significantly. Relatively strong PL signals have the potential to overwhelm satellite signals and jam a receiver, whereas weak PL signals may be too feeble to allow receiver tracking. This is the basis for a wireless communication difficulty known in the art as the “near-far” problem.
Equally problematic is the sharing and encroachment of the GPS radio frequency spectrum from other users. Mobile Satellite Systems (MSS) downlinks, wind profiler radar, space based radar, ultra-wideband systems, GPS expansion and the European Radio Navigation Satellite System known as Galileo, have or have filed for frequencies in and around the GPS spectrum. These additional systems are potential sources and targets of interference from and to existing RNSS systems.
Another type of interference is self interference, which is the result of signals from a radio-navigation transmitter interfering with the reception of radio-navigation signals at the receiver. This type of interference often occurs when a RNSS receiver and transmitter are located physically near (or identical to) each other. Self interference is an extreme case of the “near-far” problem.
Another RNSS interference concern is spoofing and meaconing. Spoofing is a technique for causing an active radio-navigation receiver to lock onto legitimate-appearing false signals and then be slowly drawn off the desired path causing significant PVT errors. In addition, spoofers can effectively jam large geographical areas. Meaconing is a technique for the reception, delay, and rebroadcast of radio navigation signals that can confuse a navigation system or user.
In general, spoofing is more difficult to achieve than generic jamming, and is often targeted to an individual user. Spoofing, however, can achieve the same effect (widespread disruption) as jamming. This is because a spoofer can inject misleading data within a localized area and its pseudo-random number (PRN) signal will act as a highly effective jammer over large distances. A spoofer can defeat nearly all anti-jamming equipment.
In conclusion, many types of radio navigation interference exist within the RNSS RF spectrum, and it is desirable to have a method and apparatus to identify and remove such wireless interference that compromises the usefulness of legitimate radio navigation signals. In particular, it would be advantageous to reduce or mitigate the near-far problem in radio navigation.
Abbreviations
The following abbreviations are used herein.
ADC: Analog to Digital Converter
AFRL: Air Force Research Lab
C/A code—Coarse/Acquisition or Clear/Acquisition Code
CDMA—Code Division Multiple Access
CEP—Circular Error Probable
DARPA—Defense Advanced Research Projects Agency
DGPS—Differential GPS
DLL—Delay Locked Loop
DOP—Dilution of Precision
E code—European code
DSP—Digital Signal Processing
FLL—Frequency Locked Loop
GNSS—Global Navigation Satellite System (ICAO definition)
GPS—Global Positioning System
IF—Intermediate Frequency
IMU—Inertial Measurement Unit
INS—Inertial Navigation System
LAAS—Local Area Augmentation System
MAS—Multiple Access System
MFD—Matched Filter Detector (Technique used in most GPS receivers)
MSD—Matched Subspace Detector (Technique used in DFC next-generation receiver)
MSS—Mobile Satellite System
NF—Near Far
NFR—Near Far Resistant
P(Y) code—Precision (Encrypted) code
PL—Pseudolite, pseudo-satellite
PLL—Phase Locked Loop
PRN—Pseudo Random Noise code, e.g., C/A Gold codes and the P(Y) codes.
PVT—Position, Velocity, and Time
RAIM—Receiver Autonomous Integrity Monitoring
RF—Radio Frequency
RNSS—Radio Navigation Satellite System
ROC—Receiver Operating Characteristic
SA—Selective Availability
SNR—Signal to Noise Ratio
SV—Space Vehicle (e.g., an RNSS satellite)
VCO—Voltage Controlled Oscillator
UAV—Unmanned Aerial Vehicles
UMP—Uniformly Most Powerful
USAF—United States Air Force
WAAS—Wide Area Augmentation System
Terms and Definitions
It is advantageous to define several terms before describing the invention. It should be appreciated that the following definitions are used throughout this application. Where the definition of terms departs from the commonly used meaning of the term, applicant intends to utilize the definitions provided below, unless specifically indicated.
GPS Codes: Each GPS satellite or PL at least transmits two different codes such codes typically

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