This past week has been smooth sailing as we tested both of our instruments, the radar and the radiometer. The radar’s high power transmitter was turned on as the spacecraft was flying over the North Atlantic and then over Greenland. The telemetry data looked great and the signals received clearly showed the transition from ocean to land (yay!). There were some biases with the data that still need to be worked out, however everything is working and we will have a better sense of the quality of the radar data once the antenna starts spinning. During this past week we also turned on the radiometer and the overall signal and signal changes as the satellite orbited the Earth were jaw-dropping GREAT. It is not uncommon to encounter calibration issues early on but when we compared the radiometer data to other calibrated datasets, there was a 99% match, which is pretty darn awesome!
For those of you not familiar with how these instruments work, the radar is similar to an ultrasound scanner and the radiometer is similar to a thermal camera. The radar emits a signal (1.215-1.3 GHz) towards the surface of the Earth and we measure the portion of that signal that reflects back to the satellite. The radiometer detects energy emitted from the Earth, similar to a thermal camera, but it operates in a different frequency range (1.4 GHz). Both instruments operate in the microwave range of the electromagnetic spectrum. This allows SMAP to see the surface of the Earth under almost any kind of weather condition or regardless of day or night conditions. Microwave frequencies are also very sensitive to water in the soil and to the frozen or thawed state of the surface (yes, SMAP will detect that too). The A in SMAP stands for active because the radar provides its own illumination source. The P stands for passive because the radiometer is not transmitting any signals but rather detecting radiation emitted from the Earth. The radar was built at JPL while the radiometer was built at Goddard. Here is a great video explaining how the radiometer works:
You are probably wondering how a radiometer that listens for very faint signals is not overwhelmed by the high-powered signal transmitted from the radar. Well, that is another one of the great engineering achievements of SMAP. The radiometer uses a different frequency than the radar and only listens for a signal when the radar is not transmitting. Also, there are many filters built into the system to keep the radiometer and radar signals apart. Prior to launch we tested the instruments together to make sure these measures functioned as expected. The instruments were also tested individually for conditions expected during launch and in space (e.g. vibration, extreme temperatures, radiation exposure, etc.). When we turned on both instruments this past week, we were ecstatic to learn that they worked as designed.
Looking forward, one of the major milestones will be to place the spacecraft in its final intended orbit, 685 kilometers above the Earth. This will be achieved through multiple orbital maneuvers over the next six weeks. A test was successfully performed yesterday morning to ensure that the thrusters used to maneuver the spacecraft were behaving as expected.
As you can imagine, this past week was very exciting… to say the least. As a scientist, receiving our first data and seeing its great quality has been particularly special. In a way this represents the first initial steps in achieving the scientific goals of SMAP and the start of a new journey.