Avionics News April 2012 - 22

dAtAliNk weAther

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Latency Compounded: On Data Update Rates, Download Cycles Times and Processing Priorities How may I slow you? Let me count the ways. The first source of latency in datalink weather occurs on the ground. For example, it generally takes six antenna rotations to build a complete weather-radar picture from a Doppler system. From the collection, the data must be relayed to the broadcast system – satellites for most datalink services and via a ground-station network for broadcast to ADS-B In clients – that adds a bit more time. Then, the data works within the limits of its speed and bandwidth to be received by compatible receivers and queued for display. When the full download is complete, the waiting display system loads the graphics. Remember, the graphics downloaded (Doppler weather radar, lightning data, graphics for TAF and METAR information) are not all updated at the same time, every time. The radar and lightning updates are the most frequent. Ideally, a system runs fast enough to offer updated graphics every six minutes, or 10 times per hour. The receiving/displaying systems of software, hardware and screen, are ready and able to load and use the data the instant it’s available. Dedicated Versus Multifaceted Once the download is complete and the image is ready to play, it’s automatically displayed after the receiver and processor do their jobs on the data. Most systems also will show a gauge or meter that indicates how old the image data is – both to help you weigh what you see against what’s outside, but also to keep you alert to the next update. Panel-mounted systems and dedicated
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EFBs tend to handle workloads quickly and reliably. Again, the narrower demands on computing power and the availability of processing memory helps. You can look at this from the other side: the machine and its software, panel-mounted or dedicated EFB, are not also tasked with maintaining a third or fourth generation Internet connection or a Bluetooth link; the processors of the panel-mounted systems and dedicated EFBs also don’t have to prioritize their image displays to always make the graphics the first priority. With tablet computers, the variables can add up and compound the weather data’s inherent latency with other latency issues, such as how quickly the datalink receiver can load the data into the tablet, whether other programs are sharing the processing power and available memory, the screen refresh rate and how the tablet processor prioritizes the updated images. One tablet user told me that if he uses another program, his device wants to play all the accumulated graphics updates before getting to the most-current image. “It’s not terrible, but I could go without the animated GIF every time I toggle back to the datalink,” the user shared. Conversely, he found using a wired datalink receiver a better option than using a Bluetooth alternative. “It runs faster with the Bluetooth turned off,” he said. Great for Plates, Charts, for Off-Duty… Revisiting this with datalink-weather subscribers using tablet computers, a few helpful hints emerged. First, consider limiting what the tablet runs – to nothing but the weather, or the weather and charts. Even when out of range of a Bluetooth connection, the tablet will continue to look for a past one if left on. Ditto for Wi-Fi – turn off the transceiver; this will save battery power and eliminate a potential slow-performance

inducer. Surfing in-flight isn’t really a good use of a tablet in a flying cockpit. If possible, try different services or receivers; a different receiver may help speed the processing time, which should improve position accuracy. Finally, before you think you’ll need it, try to work out the tablet and its datalink receiver around real weather and note any errors in the display of weather. A number of tablet-based datalink users conceded they seldom see the displacement of the level experienced in July 2011 at AirVenture – seldom, as opposed to never. “Sometimes, it seems as if two different programs (GPS navigation and weather graphics) are at work independently – and disagree on location,” said one tablet user. He noted that he needs an external GPS sensor to feed position information, so he can use the tablet for VFR navigation. “Sometimes, no weather playing, it’s off a bit in the image generated to go with the GPS position. Throw in weather datalink and it gets a bit worse.” But, he added that playing datalink weather on his portable GPS navigator with an integral datalink receiver seldom errs enough to notice. Information: Enemy of Uncertainty Avionics makers, software engineers and hardware technicians offered some suggestions for avoiding the surprise of a disparity like those seen between tablets and dedicated aviation units. They generally apply universally and are worth considering as suggestions for shop customers. First, familiarization is the most-frequent suggestion. Load the unit as you would typically use it. Fire up the device and everything you would use – external GPS sensor and datalink receiver, charts, plates and flightplan programs – and recruit a friend to take you for a drive. Ideally, real-weather features easily eyeballed would be the best reference for gauging the latency issues of your system. If not previously familiar, take this

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Avionics News April 2012

Table of Contents for the Digital Edition of Avionics News April 2012