Instrument Checks GPS Noise Immunity

July 1, 2003
This single-unit system can create arbitrary waveforms as wide as 40 MHz and CW signals to 2 GHz for checking the immunity of GPS receivers to noise and interference at L1, L2, and L5 frequencies.

Global Positioning System (GPS) receivers (Rxs) now have widespread use in aircraft landing systems, shipborne navigation systems, and E-911 civil emergency location systems. Due to political threats and increasingly occupied bandwidth, dual-frequency GPS Rxs have been developed to overcome the effects of interference. Since such Rxs must perform accurately even under conditions of interference, Noise Com (Parsippany, NJ) has developed the GPS7500 Noise & Interference Generator, a single self-contained instrument that supports all GPS interference testing requirements, including for L1, L2, and L5 frequencies.

The effects of noise and interference on a GPS Rx range from decreased accuracy to loss of lock. Documents DO-229, DO-235 and DO-253 from the Radio Technical Commission for Aeronautics describe 13 test cases for GPS interference, including broadband noise interference, continuous-wave (CW) interference, and pulse interference. In addition, potential emerging interference sources, such as ultrawideband (UWB) (pulsed) communications signals, have been studied and added to this documentation.

GPS interference testing is not trivial. Stable, low-level GPS signals must be combined with known levels of noise and interference, for example, generating pulsed interference at levels above +20 dBm with on/off ratios exceeding 164.5 dB while also preventing phase noise from skewing the test results.

The GPS7500 (Fig. 1) was developed as a solution for such testing. The modular instrument can be supplied with one or two arbitrary waveform generators for simultaneous testing of L1 (1575.42 MHz) and L2 (1227.60 MHz) or L5 (1176.45 MHz) GPS signals. It also includes frequency upconverters and an elaborate signal switching and combining network. The waveform generators incorporate digital filtering and over sampling with interpolation to create nearly spurious-free intermediate-frequency (IF) signals which are then upconverted to the desired RF range. The GPS7500 also features two 1-to-2-GHz CW frequency generators for out-of-band interference signals at levels beyond +20 dBm. A built-in power meter ensures accurate calibration of signal and interference levels, while a calibrated noise source serves as a low power reference in support of a 100-dB dynamic range (Fig. 2).

The interference and noise generator has three operating modes: synthesized CW modem arbitrary interference mode, and pulse mode. In the CW mode, the instrument offers an operating range of 1000 to 2000 MHz with 20-kHz frequency resolution and ±0.5 PPM frequency stability. In the CW mode, the phase noise is −122 dBc/Hz offset 100 kHz from the carrier and spurious levels are −80 dBc offset 21.5 MHz from the carrier.

In the arbitrary interference mode, the generator can create arbitrary noise and interference signals, such as amplitude modulation (AM), with 3-dB bandwidths from 0 to 40 MHz. The phase noise is −46 dBc/Hz offset 20 kHz from the carrier while spurious levels are better than −52 dBc offset 21.5 MHz from the carrier. In the pulse mode, the generator can create pulses as narrow as 6.67 ns with duty cycles from 0 to 100 percent at pulse repetition rates of 5 Hz to 75 MHz. The generator achieves a minimum pulse on/off ratio of 164.5 dB. It can control signal levels from −126.5 to +10 dBm from 1000 to 2000 MHz, and has a low-noise mode that drops the signal/noise floor to −170.5 dBm measured in a 1-Hz bandwidth.

The GPS7500 operates under the control of a Pentium processor running the Windows XP operating system. Since GPS Rxs capture and decode signals at levels close to the natural noise floor, the GPS7500 employs special calibration routines to measure and calibrate in-band interference. The instrument's internal hard drive includes time-saving preset templates, which follow the bandwidth, power, and frequency combinations for DO-253A testing (Fig. 3). Additional arbitrary waveforms with as much as 42 MHz of bandwidth can be created by means of the GPS7500's embedded personal computer (PC) or imported using the Ethernet interface. Noise Com, Inc., 25 Eastmans Rd., Parsippany, NJ 07054; (201) 261-8797, FAX: (201) 261-8339, e-mail: [email protected], Internet:

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