As is the fashion in modern marketing, XM WX Weather offers three tiers of service-the more you spend, the more you get. While each package adds more features, it also essentially doubles the cost. The Aviator package was the launch product and it hasnt changed much in features and price since it was first introduced. At $49.99 per month, it has nearly a dozen weather products and its introduction fundamentally changed the rules for inflight weather awareness.

Probing for the cost-sensitive bottom of the market, a couple of years later, XM WX Weather introduced Aviator LT, a reduced version of the Aviator package that, at $29.99 per month, stripped off nearly two-thirds of the Aviator services features, leaving only the NEXRAD mosaic, METARs, TAFs, precipitation type and TFRs.

Just last year at AirVenture 2008, XM Wx Weather introduced a premium package called Aviator Pro at $99.99. Aviator Pro includes all of the features of its predecessor, but adds two icing products, high-level turbulence, convective outlooks and mesoscale discussions. At the same time, XM WX expanded the Aviator package by adding severe thunderstorm, tornado watches and pilot reports.

What package you choose depends on your mission and the capabilities of your satellite weather receiver. It may make better sense to err on the side of economy rather than gilding the lily. If you fly mostly in Florida, bouncing around at 5500 feet in a Cessna 172, you can hardly justify the Aviator Pro package; Aviator LT may be all you need. If you fly in Canada, especially during the summer, you may want the Aviator package, since it includes Canadian radar data. Or, if you fly above FL210 on your trips, Aviator Pro has some products you might want, but that arent absolutely necessary. Distance and speed matter. Any airplane thats capable of crossing half a continent before lunch will face more challenging weather

than a Piper Archer pilot going out to lunch.nch.

Receiver capability is a factor. Some panel mount displays support everything while many portables may not. Check with the manufacturer prior to purchase because these capabilities change. On the plus side, all of the receivers and displays support the most critical products: real-time NEXRAD, plus METARS and TAFs, TFRs and warnings. Lets get the easy one out of the way first. Aviator Pro is the premium service and has some rarified products. These include the Current Icing Product (CIP) icing analysis, Supercooled Large Drop (SLD) icing analysis, Graphical Turbulence Guidance (GTG) forecast, Storm Prediction Center (SPC) Day 1 convective outlook, SPC mesoscale discussion, visibility and the hurricane track product. Do you need this stuff? In a word, no.

Theres absolutely nothing here that adds a significant degree of situational awareness for the kind of tactical decisions pilots must make.

The CIP and SLD products are found on the Aviation Digital Data Service (ADDS) Website. (See www.adds.aviationweather.noaa.gov.) At best, the CIP and SLD icing analysis are 15 to 20 minutes old when broadcast and theyre updated once an hour. All they offer is a nice glimpse of the recent past.

Moreover, the icing analysis is displayed in 3000-foot increments even though the native resolution of the product is 1000 feet. A lot can happen in 3000 feet of icy clouds, especially SLD. The turbulence information is generated from the Graphical Turbulence Guidance (GTG) product and is the same product as available on ADDS. Unlike CIP, its available at altitudes only at and above FL210, despite what it says in the WxWorx documentation. GTG isnt an analysis of the current turbulence conditions, but a two-hour forecast based on the Rapid Update Cycle (RUC) for clear air turbulence (CAT) only. Its also limited to 3000-foot increments. Unless you fly a turbocharged or pressurized aircraft, this product wont be useful.

The Storm Prediction Center Day 1 convective outlook is updated four times a day and will hardly substitute for your real-time NEXRAD image, which is on the order of five to eight minutes old. This outlook represents a planning tool before you depart and doesnt add any significant awareness en route. The same applies to the SPC mesoscale discussion. As for hurricane tracks, if a storm will impact your route,

youll know before you depart, including its forecast track.

The Aviator Lite package includes county warnings, city forecasts, NEXRAD radar coverage, precipitation type, TFRs, METARs, terminal TAFs and the NEXRAD mosaic. Is this enough information to justify a $30 per month subscription? We sure think so. But how does it measure up against the original Aviator package? Is that level of service overkill? Are you paying $20 more a month for these borderline products you rarely use?

It depends on how you fly and what other equipment is in the airplane. Besides lightning and winds, included in the Aviator package are Canadian radar, AIRMETs/SIGMETs, echo tops, satellite mosaic, freezing level forecast, severe weather storm tracks, surface weather maps and the two newest products, pilot reports and severe weather watches. Do you consistently make critical in-flight weather decisions with products other than METARs, TAFs and NEXRAD? Its obvious that the common denominator is NEXRAD, which all three levels have. During the spring, summer and fall, theres no more critical information than the radar image. Some pilots say they cannot do without the ground-based lightning. Really? Try this: Log onto your favorite Web sites NEXRAD loop. Unless you subscribe to a service such as WeatherTap, you wont see lightning overlaid on the radar image and even if you do, its unlikely to add value to the go/no-go process. Its helpful to pinpoint areas of embedded thunderstorms. But absolutely necessary? No.

It would be sweet to have a high-resolution visible satellite image in the cockpit, too. But thats not currently offered and pilots have gotten along without it. The NEXRAD image, coupled with the view outside of the cockpit and your preflight analysis, offers most of what you need to avoid dangerous convective turbulence.

XM WXs winds aloft are coarse, displayed in 3000-foot intervals. They arent actual winds, but forecast winds based on a two-hour RUC model. Mostly, the winds just give the pilot something to play with while staving off boredom. Will they save you the $20 per month on fuel? Doubtful.

How about Canadian radar? It has some issues with anomalous propagation. It tends to show precipitation that resembles thunderstorms even when the air is free of clouds. Filters employed by WxWorx attempt to mask this, but when clouds and precip are possible, these gross filters are not applied and anomalous propagation can be a real nuisance. If you dont fly in Canada, theres no need for this data.

Perhaps the best recent addition to the Aviator package is pilot reports. When we met with the technical staff at WxWorx a year before they announced their new

products, we encouraged them to include PIREPS in the Aviator service and not shove them into their premium service to match WSI. Were happy they agreed.

In addition to pilot reports, they also added severe thunderstorm and tornado watches to the Aviator package. Both of these are fantastic products to have in the cockpit, especially pilot reports. Echo top heights can be useful to determine if the area of precipitation is safe to penetrate. Any tops over 30,000 feet often equate to deep, moist convection containing severe or extreme turbulence.

The infrared satellite mosaic is also a borderline product. It will provide a more consistent cloud top height than echo tops, but doesnt show tops below 5000 feet. At a 5000-foot resolution, this product might not be of great value if you or your airplane are limited to below 10,000 feet.

AIRMETs and SIGMETs are a time-smeared forecast and are more useful for planning prior to flight than inflight. Convective SIGMETs, on the other hand, are issued hourly and are definitely a borderline product on the same level as lightning. These are most useful for depicting areas of embedded storms that may not have a classic thunderstorm radar signature.

Severe storm tracks (SCITs) typically show up on cells that exhibit reflectivity values of at least 55-dBZ. They dont typically add any additional value beyond what you already know from studying the radar image and may contain conflicting information. They show the direction and speed of movement, but dont always agree with reality, especially when the cells are evolving. While SCITs are refreshed quite frequently (every 1.25 minutes), new development can be confused with movement, causing two adjacent SCITs to point in opposite directions.

Last, the freezing level data is not an analysis of current conditions, but the two-hour RUC model forecast updated once an hour. The resolution is 2000 feet and wont likely change much while enroute.

If youre willing to pay $20 more a month to have the satisfaction of ground-based lightning and pilot reports, go for the Aviator package. The same advice applies if you fly or live in Canada-youll want the Canadian radar. Otherwise, cost/value relationships usually arent this easy to make: For the $29.95 it costs, XM WXs Aviator Lite is the slam dunk best value.

It provides all the data youll need to make sound inflight weather decisions and you can focus on improving your preflight analysis so youll have to react less to whats happening around you while inflight. In the end, thats the entire point of aeronautical decision-making.

Scott Dennstaedt teaches weather to pilots. See his site at www.avwxworkshops.com.