Last lesson, you learned a little bit about satellite busses and bus-driving. Almost all satellite busses are similar to each other. If you looked at a RapidEye satellite bus specifications and a Digitalglobe satellite bus, you’d likely think they might be one and the same.
But the payload is different. The satellite payload (and orbit) is very dependent on the mission the satellite is to perform. In the case of RapidEye and Digitalglobe, the mission is taking pictures of all the goings-on all over the Earth’s surface. But maybe the satellite belongs to NOAA, something like the Polar-orbiting Operational Satellite (POES).
POES doesn’t have just one payload, but three: the Advanced Very High Resolution Radiometer (AVHRR), the Advanced TIROS Operational Vertical Sounder (ATOVS) suite, and the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) Microwave Humidity Sounder (MHS) instrument. None of these payloads are close to an imagery payload, but then taking pictures is not POES’ mission. Because POES is government-run, they have not just one, but several important sounding missions (from their website): Data from the POES series supports a broad range of environmental monitoring applications including weather analysis and forecasting, climate research and prediction, global sea surface temperature measurements, atmospheric soundings of temperature and humidity, ocean dynamics research, volcanic eruption monitoring, forest fire detection, global vegetation analysis, search and rescue, and many other applications.
And this is typical of government satellites and satellite programs. On almost every government satellite, there is likely to be more than one, probably more than two payloads. If you look at the Geostationary Operational Environmental Satellite (GOES), its satellite bus hosts many different payloads: imagery (visible and infrared), a Sounder, a magnetometer, an X-ray sensor, high energy proton and alpha detectors, and energetic particles sensor. Some also have a solar x-ray imager (SXI), an extreme ultraviolet sensor, Emergency Position-Indicating Radio Beacon (EPIRB), and Emergency Locator Transmitter receivers.
That’s quite a few, right? The primary reason for all of that on one single satellite bus is: money. The government’s programs want to advertise they are getting the most bang for the taxpayer buck, but, insidiously, this can also cause something called requirements creep. Requirements creep (or scope creep), unfortunately, is a natural outgrowth of satellite and space acquisitions programs (actually, any program). I won’t go much into government acquisitions programs, but will say they tend to cost a lot of taxpayer money, which is ironic, because they are always put in place with the mission of being custodians and disbursers of taxpayer money.
Reasons for requirements creep in acquisitions programs vary, but ultimately, new payloads and missions are added on, changes to the satellite bus (the design of which was finalized months or years ago) are implemented, etc.
So government satellites tend to have a lot of payloads, are very costly (acquisitions programs tend to run for years—sometimes decades), but the resulting satellites tend to be very capable (in spite of using technology that probably was designed 10 years ago—again, very long acquisitions programs). But there’s also a lot of risk, because now a lot of money is tied up in one satellite—and what happens if that goes boom?
For more on how US taxpayer money is helping fund space systems, for NOAA, go here (it’s a start); for hosted payloads, go here; for general Department of Defense, go here and here. Lots of issues for all of them, what the military call “a target rich environment.”
On the other hand, it appears, at least on the imagery side, that Digitalglobe and RapidEye are not only keeping things simple, with one payload per bird, but they are also making a profit. We will start talking more about their imagery operations in the next lesson.