How is Jason Doing It?
Jason-1 is equipped with an altimeter, a radiometer, a laser retroreflector array, DORIS and TRSR.
The Poseidon-2 Altimeter is a a radar that uses two different radio frequencies to measure the height of the satellite above the sea surface.It does this by sending out radar pulses at two frequencies and listening for their echoes from the sea surface. It carefully measures the time for the radar pulse to go down and back. When we divide the time by two and multiply by the speed of light we get the distance from the satellite to the ocean surface. This is the altimeter range in the picture.
The Jason Microwave Radiometer listens to the radio waves produced by the ocean surface, by clouds, and by water vapor in the atmosphere. It uses these signals to calculate the amount of water vapor in the atmosphere. Water vapor slows the signal from the altimeter. This causes the altimeter to calculate that the ocean surface is further away than it really is by about 10 cm. This is a small error but we need to remove it.
The laser retroreflector array has mirrors that reflect laser beams aimed at the satellite from lasers on the ground. The reflected laser light is used to measure the position of the satellite so that scientists on Earth can know where Jason is above the Earth and how high it is.
DORIS is used to determine the velocity of the satellite. DORIS stands for Doppler Orbitography and Radiopositioning Integrated by Satellite. DORIS measures the Doppler shifts of two radio frequencies transmitted to DORIS from beacons on Earth.
The Turbo Rogue Space Receiver TRSR is a very fancy global positioning system (GPS) receiver. TRSR measures the position of the satellite by tracking the signals from GPS satellites and ground systems simultaneously. It can listen to 16 GPS signals at once. Using the signals it can measure its position with an accuracy of about 10 cm.
As you may have noticed, these last three are all instruments that help Jason to know where it is. There needs to be three because Jason's position must be known to within a few centimeters at all times in order for the data it is collecting to be meaningful.