Active sensing with satellites

What the Activity is for

Some satellites send out pulses, and then time how long these pulses take to return, in order to see what lies beneath them. Here you'll model that process.

What to Prepare

• Plasticine, or other modelling clay
• straws
• graph paper and pencil
• an ultrasonic ruler if possible (available from many DIY stores)
• prepared electrical ducting

What Happens During this Activity

Introduce the concept of working out how far away a reflector is by timing an echo. Either use the ultrasonic ruler or clap and listen. Draw out the importance of the round trip time, from which you can calculate the distance.

Now show the prepared channels, explaining how the top surface represents the path of the satellite, and hidden below this is the landscape. Make some landscape and place it inside the ducting. Ask how to find out what distance below the satellite the landscape is using the straw.

Point out that from repeated readings you can build up a map.

This is how radar maps of planets are built up, as are maps of the sea floor.

Now put each pair to work making a landscape, then sealing it into the ducting. Each group passes their landscape on to the next group, who use the straw to gather data, before plotting a map on the graph paper.

One subtlety is that the straw gives a map of depth below the satellite, but often people want a map of heights above some agreed datum. Subtracting the depth from a constant gives the required map. Faster classes might be left to puzzle this out. For classes needing more support you might ask for a map of depth, or tell them how to process the data to give a map of height.

After the maps are made a lively discussion can ensue. Only those cheating can find the finer detail of a landscape; this type of probing can only reconstruct the landscape on a scale determined by the spatial resolution of the sampling.