Interpreting Radar Returns

There are many variables that come into play when trying to interpret weather radar returns. Aviation weather radar information comes in a number of different forms, including NEXRAD imagery transmitted into the flight deck and airborne weather radar return imagery generated by onboard systems. Return imagery will vary significantly based on the type of radar system employed and color settings selected to represent intensity levels. It is critically important to understand how varying levels of precipitation will be depicted by different sources. This article is intended to illuminate some things about radar that will help you to interpret weather threats to flight safety.

NEXRAD imagery is generated by ground-based systems that have very powerful beams and large antennas, making them capable of sensing very low levels of precipitation. Powerful computers running sophisticated software automatically change scan patterns and adjust other settings to produce the optimal return imagery for a given type of precipitation. Being ground-based, these systems produce little ground clutter, and often the little ground clutter that is present can be filtered out by software.

Individual site NEXRAD imagery is transmitted to a central computer system that combines all the individual site returns into a large, cohesive picture. This combined imagery is made available to vendors, who then make it available to pilots via either ground-based transmitters (ADS-B) or satellite-based service (SiriusXM and Garmin Connext). While this has significantly contributed to flight safety, there are limitations that must be understood. Most importantly, radar images take time to process and then be sent to vendors, who then transmit the data for your use. The radar return imagery you view in the flight deck could be several minutes old when received, while thunderstorm cells can move quickly, so it is important to use this tool only for strategic weather avoidance and keep plenty of distance between your aircraft and strong returns.

Airborne weather radar systems differ significantly from ground-based systems. While NEXRAD antennas are 28 feet in diameter, the typical general aviation airborne weather radar antenna is 10-18 inches in diameter. NEXRAD systems have peak power output of 700 kilowatts (700,000 watts), while airborne systems transmit at 40 watts. Fortunately, some good engineering practices and software management of the airborne systems allow for creation of very useful, real-time imagery. For understandable reasons, the imagery between ground-based systems and airborne systems will differ.

In addition to antenna size and system power output differences, return intensity (measured in dBZ) is depicted on different color scales by different systems. This can be seen in a comparison of the color scales shown below. Compare the dBZ values as depicted in the three scales.

* dBZ values for GWX 70 are approximate. See your Pilot’s Guide for actual values.

As you can see, the return intensity depictions will vary by the system that is generating the depiction. Be sure that you understand this important point when comparing returns from different sources. Some more recently developed airborne weather radar systems, such as the Garmin GWX™ 75/80 weather radar, now have a 16-color scale, which provides for finer differentiation between intensity depictions.

There is a great deal more to know about aviation weather radar products. We encourage pilots to look into the Garmin Aviation Weather Radar 2.0 eLearning course available on Garmin.com.

For pilots flying with airborne weather radar systems, your Garmin Aviation Training Team has created a comprehensive Quick Reference Guide that goes along with the online course.