PHAST™ is the Prismatic mission availability tool for High Altitude, Long Endurance (HALE) vehicles. HALE systems utilising solar power for “eternal” flight need to fly using the minimum of power and whether the vehicle is an aircraft or an airship this means flying at the lowest viable speed to maintain the mission where it is needed.

Against this need to fly at the lowest viable speed is the wind present at the flight altitude. To maintain the mission the vehicle needs to fly faster than this wind.

The average winds as a function of altitude and time of year are widely  published with a typical form as shown below

This example of Southern Europe shows the typical jest steam activity at altitudes between 8km and 15km through the winter and spring which moves away during the summer months – typical of a “Mediterranean” type climate.

This average form shows the rationale expressed by nearly all HALE entrants now to try and maintain flight between 15 and 20km where the winds are lower.

Why not higher?

For an aircraft the power required to fly higher increases steeply due to the ever reducing atmospheric density and necessarily faster airspeed required to maintain lift. For an airship the reduced atmospheric density means a larger volume of balloon is required to provide the required buoyancy. So higher is always harder, bigger and more expensive.

So the drive to minimise power pushes the design to want to fly lower to reduce size and power without considering the wind whilst the increasing wind speed at lower altitudes pushes the design to fly higher to avoid having to fly faster (and hence use more power) to maintain station.

Understanding the wind as a function of location, altitude and time is therefore absolutely fundamental to the design and operation of any HALE vehicle – and just looking at the averages is not sufficient.

PHAST addresses this issue by using real wind data over many years not only to form a statistically accurate data set but also to run flight scenarios against real, historic wind data to determine the actual impact on the wind on the system. A visual indication of the importance of such a scenario based analysis is shown in the video below. This video shows the actual wind data on a 12 hour bassi from one site in Southern Europe over a whole year. The general form of the jet stream and Mediterranean climate can be seen, but take a close look around days 5 and 165. This show that for short periods the wind speeds can be significantly higher than the average and this might mean that the vehicle is blown so far off station that it may need to be returned to base – with all the inherent loss of service and risk of loss during the recovery and re-launch period.

On the other hand you can see that December in this year (2016) was very benign and well below the long term averages for wind speed at all altitudes. This is why PHAST uses multiple years to produce reliable statistics for design and application planning.


PHAST integrates a performance model of the aircraft or airship to enable the difference between wind speed and flight speed to be assessed for all the wind data points collected over multiple years. PHAST then analyses the difference in wind and flight speeds in two ways.

Power over wind. 

This assessment is a relatively simple assessment of what percentage of time at the cruise altitude (which changes through the year due to the variation in availability of solar energy) can the craft fly faster than the actual wind observed at that time at that altitude. It gives a good general overview of the viability of providing reliable services.

Station Keeping

This tool runs a scenario using the correct sequence of winds in the chosen month and plot where the craft will actually be having flown in the measured winds for the 12 hour period between measurements. It then considers what fraction of time is the craft within the user defined area of interest (AOI), giving a direct measurement of “availability” from a station keeping perspective. This therefore accounts for periods of any particularly high wind speeds which could move the craft many kilometres from the AOI and which might then take a few days to recover.

Location, Location, Location. 

The ability to fly and persist in one area is strongly dependent on the location both for a perspective of available solar power and wind characteristics. PHAST enables wind data to be imported from over 350 locations around the world to give an accurate understanding of the availability of services at any location and for any time of year.


PHAST is a service planning tool. The actual availability of PHASA-35 to provide high value services over the vast majority of the World’s population all year round can be further enhanced by active navigation of the vehicle knowing the expected winds.  Varying altitude and location to minimise negative wind events, using the higher altitudes achievable during the day and utilising excess power to increase flight speed for the short periods all enable PHASA-35 to provide reliable, high availability services even when the raw wind statistics might indicate such a mission is not viable.