Investigating Altimetry
Background:
Laser and radar altimetry reflect electromagnetic waves off the snow and ice in order to measure the distance from the instruments to the ice or snow surface (see Understanding Altimetry for more background information). The speed of the waves (be they radio or light) is known and the time it takes for the waves to leave the source, bounce off the ice and reflect back to a detector is measured. Using this information scientists are able to find the distance the waves travelled, hence how far away the snow/ice surface was. The basic equipment setup is shown below:
Combining information from EM measurements, the radar and the laser allows the thickness of the snow and ice layers to be calculated. As in the illustration below:
Activity:
The following activity allows students to simulate the principles behind finding a distance when a speed and a time are known. They will also learn about repeating measurements and taking averages to improve the accuracy of their results.
Materials:
Stop watch (per group of 2 students)
Tape measure
Chalk (or something to mark distances on the ground with)
Procedure:
Take students outside if the weather is good or to the gym if it’s not, ask them to get into pairs with one student being the ‘laser’ and one being the ‘radar’. Mark out 10m and get one student to time how long it takes the other to walk 10m and then they can swap over. Tell them to walk normally as they will need to walk at the same speed every time if they want good results. They should time themselves walking the 10m three times and take an average time. From this they can calculate their walking speed in metres per second.
e.g. 10m takes X seconds
students walk 10 ÷ X metres in 1 second
Which gives walking speed (v) in m/s
Students now have a speed for the 'radio waves' emitted by the radar and a speed for the 'light waves' emitted from the laser.
Once students know their walking speed they can time themselves walking different unknown distances, then try calculating the distance from their walking speed.
Mark out a known distance (the larger the better depending on space available), this will indicate the distance from the laser/radar to the ice-water boundary. Along this line mark 2 points representing the snow-air and snow-ice boundaries.
Students then take the role of a laser or radar pulse and time themselves walking to and from the different boundaries depending on whether they are the laser or the radar. Students should repeat this at least 3 times so they can calculate an average time for them to travel that distance.
Students now have their average walking speed and the time it took them to walk an unknown distance, from this they can calculate the distance from the radar/laser to the appropriate boundary using the equation below:
Distance = Walking Speed × (Time ÷ 2)
NB the time is divided by 2 as the time measured was there and back and therefore double the distance that you wanted to measure.
Once students have found their distance you can give them the measurement to the ‘bottom of the ice’ and students can calculate ice and snow thickness using the equations below:
To calculate the thickness of the snow and ice layers:
Snow Thickness = Radar Distance – Laser Distance
Ice Thickness = EM distance – Radar Distance
When students have done their calculations they can measure the actual distances to see how accurate they were.
Discussion Questions:
- What are the sources of error in your simulation? How could they be reduced?
- What problems might the scientists have in Antarctica when using their equipment? (e.g. snow and ice are irregular and don’t reflect perfectly, the cold affects electronic instruments etc)
- Have a look at some actual results from the sled-based radar team taken from ice station 6 on the SIPEX voyage. Can students interpret the graph?
