METARS – Delivering Weather Reports for SATCOM

Satellite communications is an exacting discipline, demanding a focus on things never visible to our end customers. We plan link budgets to the tenths, and sometimes hundredths, of decibels.  Antenna azimuth and elevation angles are calculated to an equal degree of exactitude, and if the orbital position is inclined, ephemeris elements are carefully predicted and accounted for to insure correct antenna alignment.  This degree of precision allows us to efficiently allocate the constrained resources of power and bandwidth to deliver reliable communications – aka providing the best possible service to our customers.

Still, with all this precision, one of the biggest variables in communication planning is the weather. As the old saying goes, everyone talks about the weather, but no one does anything about it! Rain fade particularly affects Ku and Ka band communications.  The energy at these frequencies is more readily absorbed because of the size of their wavelengths relative to the size of a raindrop.

Hence, heavy rain attenuates transmission and reception power in these bands, resulting in communication outages during periods of heavy rainfall and thunderstorms. To compensate for such potential outages, we can use temporal and geographic prediction charts of rainfall to include a buffer in our power and bandwidth planning. This allows us to calculate (and meet) specific availability targets with great accuracy.  

For example, every antenna in a given area on the earth will have a seasonal, historical record of rainfall. A rain fade margin is built into the power budget, with a higher fade margin used to engineer a link for 99.7% availability than a link engineered for 99.5% availability, for example. Dynamic power allocation mechanisms also reduce the effects of rainfall, where the equipment allows for it.

Still, all these calculations and mechanisms are of little solace to customers when heavy rain knocks down communications.  Often, just knowing that the outage is caused by a thunderstorm, rather than some other factor, is useful to the customer. It also saves staff time when troubleshooting the cause of the communications outage.

Weather reports from the teleport are useful — and in fact vital — for giving us and our customers a notice of weather degradation, but they are limited in two respects. First, they are a manual process, and thus prone to the usual interruptions and omissions of any manual procedure—too busy to send the report, did not think the rain was heavy, etc. 

Second, the report is limited to the teleport area. Heavy rain at a remote customer site will also degrade communications, and many remote sites have no human operators to make weather reports, at least not all the time.

To address these two shortcomings, we have a new trick up our sleeve to automatically alert operators and customers to heavy rain and thunderstorms in an area – Aviation METARs (The acronym comes from Meteorological Terminal Aviation Routine Weather Report).  These are formatted, machine-readable messages sent out approximately every hour by the National Weather Service. In addition to reports of wind speed, direction and cloud cover, these messages contain formatted symbols that denote weather conditions that satellite users care about. For example, “TSRA” may look like one of your kids’ text message abbreviations, but in fact this METAR means “Thunderstorms and rain showers in the area.”

METARs have two advantages over human weather reports: First, they are machine-readable.  A script is easily written that periodically requests the METAR and parses it for symbols in the message that depict heavy rain. If these conditions exist, the script sends an automatic, immediate email alert to a distribution list of customers and operators. The script periodically polls and reads the current METAR.  When the condition at the reporting station clears, it sends a second, “ALL CLEAR” email.

The second advantage is ubiquity: METARS are created, every hour, for over 9,000 sites worldwide!  In all likelihood, there is a reporting station (usually an airport) close to a teleport or remote site.  Once the script is set up for a given reporting site, it keeps a constant vigil, monitoring and reporting, with no further operator effort required.

We have begun rolling out this automated reporting to select sites and customers. METARS will not prevent weather disruptions, of course, and they are subject to some limitations. There are some weather conditions which might degrade communications but don’t show up in a METAR, and vice versa.

Still, METARS are one more way we are staying in close contact with our customers, and giving them a more transparent view of service availability. That improves situational awareness for both sides and contributes to a higher level of overall service.

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