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Since satellites are always in different locations, it can be challenging to repeat a test environment for CRPA testing.

Historically, testing Controlled Reception Pattern Antenna (CRPA) systems in a laboratory has been a very difficult process — dynamics were limited and test duration was usually short.

Often, dynamic testing was conducted in outdoor ranges using aircraft. But using aircraft for testing purposes is very tough to control and almost impossible to repeat.

Additionally, due to the constraints, limitations and difficulties that can be encountered while conducting radiated testing on a range, it is very difficult to test the performance of CRPA-based systems.

CRPA Function QuoteHow CRPA works

CRPAs are operationally used in a jammed environment and function as a way to reduce the effects of jamming. A CRPA antenna has multiple RF receptors (antenna elements).

The RF receptors receive signals (both GPS and jamming) at slightly different phases because of the spacing between antenna elements.

The CRPA is attached to an Antenna Electronics (AE) module, which then combines the incoming signal and noise in such a way as to maximize the signal (GPS) and minimize the noise (jammers). An easy way to understand the concept is that a CRPA works similar to noise-cancelling headphones.

Photo: CAST Navigation.

Challenges to CRPA testing

One of the fundamental problems of testing CRPA antennas in the past has been the ability to generate a set of coherent signals that can be controlled well enough to produce an environment that, to the individual antenna elements or antenna electronics, has the proper timing (phase) relationship that is representative of both the geometry of the individual antenna element locations relative to the GPS RF signals as well as the jammers.

The approach that results from using a Butler matrix to attempt to reproduce the geometry, uses a single signal simulator and a tapped delay line to adjust the relative time of arrival of the signals. Thus, at best, this can provide the instantaneously correct relationship at only one time.

Remember that the satellites are moving in orbit at about 500 meters per second. This means that even for a stationary user, the geometry is constantly changing. Thus, this approach, realistically, only supports the stationary use case.

Video: CAST Navigation.

Approaches to testing CRPA in a laboratory environment

Two other basic approaches can be taken to test CRPA antennas and their signal-processing capabilities in a laboratory environment. Either one of these approaches can generate a series of time- or phase-coordinated signals that represent the signals seen at each element of the CRPA.

The first approach is accomplished by spatially separating radiators in an anechoic chamber, and the second approach is to use multiple satellite signal generators to create a wavefront generator.

Approach 1:

CAST-5000 GPS Wavefront Generator. Photo: CAST Navigation.

CAST-5000 GPS Wavefront Generator. Photo: CAST Navigation.

The first approach can be accomplished using an individual radiator for each satellite composite signal. In this approach you can locate the radiators around the periphery of a chamber, with the spatial orientation of each satellite roughly that of the depicted constellation.

Additional radiators would be used to radiate the jammer signals from their appropriate spatial orientation. The key problem here is the extreme difficulty in controlling and synchronizing the individual signals to the required precision.

This approach requires generating satellite motion by physically moving the radiators in the anechoic chamber. Both the GPS satellites and the jammers would have to be installed on tracks which would be able to change their position in a realistic manner.

Approach 2:

The second approach would be to use a wavefront generator. A wavefront generator generates a separate constellation of GPS satellites representative for each element of a CRPA antenna.

Due to the difficulties described earlier, the signals that are derived for each element of the CRPA antenna are input to the antenna electronics.

Each element of the CRPA antenna electronics assembly will see a complete GPS constellation which is in full phase agreement with all the other CRPA elements. By introducing offsets to each element of the antenna, dynamic testing of the CRPA should be possible under conditions that closely match those of the real world.

This level of control allows the possibility of a far more effective approach to testing the CRPA system and should allow for testing of the CRPA system performance, even under high dynamic, long duration testing situations.

Photo: CAST Navigation.

Testing CRPA with the CAST-5000

John Clark, CAST quoteIn general, the approach to do this with a conventional GPS satellite signal simulator would be to modify the existing satellite signals in such a way as to present a single satellite to each of the radiator elements in the chamber.

The CAST-5000 not only adjusts the GPS RF based on the spacing of the antenna elements, CAST also has implemented a coherent jammer capability. These jammers are received by the antenna elements at slightly different phases. The user may specify up to four different jammer types with the basic coherent jammer.

Typical types of jamming waveforms could be pulsed, swept, or CW. These jammers may be turned on and off and appear at different locations during the test scenario. By combining both GPS and jamming signals, a user can evaluate the effectiveness of the CRPA.

The CAST-5000 can be configured as an RF-only simulator, or it can be combined with the CAST EGI test simulator to allow a test set-up that evaluates an EGI with CRPA antenna electronics attached. This allows the navigation system to be evaluated as a whole.

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To learn more about CAST Navigation products, visit www.castnav.com. Interested in understanding more about how the CAST-5000 works? View the product spec sheet below.

Download the CAST-5000 Product Brochure

This page was produced by North Coast Media’s content marketing staff in collaboration with CAST Navigation. NCM Content Marketing connects marketers to audiences and delivers industry trends, business tips and product information. The GPS World editorial staff did not create this content.