An Indian startup, is trying to build a multi- payload satellite having both optical and SAR sensor on the same satellite. The look angles of the these payload are quite different and it is very difficult. Although the images/ data captured from these sensors would have the same temporal resolution, the optical spatial resolution would be heavily downgraded and it would not be possible to fuse the date sets and obtain analytics.

Many potential users think, that it is a great idea, but why there must be a reason, why other nations have not put this idea to work.


3 Answers 3


This may have more to due to finance/politics than space/technology.

Asking investors for money to build a first satellite is much harder than for second etc where you can invite them to see your control center, real data and (maybe) sales.

There are a number of existing small sat companies doing earth surveillance and pitching investors with 'we want to enter an existing and saturated market' is hard, much better to be pitching 'our innovation allows two systems for the price of one'. Even if it later turns out your business is better with dedicated platforms you will have got that first bird into orbit proving the business and technology is (hopefully) worthy of further investment.

There may also be domestic Indian politics in play, having an organic operator with at least a basic level of capability may be seen as useful insurance against outside pressure, be it business (monopoly pricing) or political (sanctions).

It may not be clear what the real answer is unless they actually fly the proposed system.


The look angles would be different, but very well-known. So, in practice, we would still be able to use all datasets simultaneously after some pre-processing and geometry corrections. Scientists combine information from SAR images and Sentinel-2 (optical) to look at and monitor areas of interest. For example, one can obtain information on the location of a newly formed sinkhole (or other hazard) through the optical channel (something you can't see directly from SAR in all cases) and then analyse the deformation in the surrounding environment through the coherence of the inSAR product.

To obtain the inSAR images, one has to apply orbit corrections (which are very important to the correctness of the analysis). I imagine that having the 2 instruments in one satellite would mean that you can do this process once to generate your final product.

So, in general you may use the optical information for land classification and combine it with deformation rates. Optical sensors also don't produce information during the night, so depending on the application, radar can not only be complementary, but the main source of information.

Something else to consider: Maybe this start-up is planning to generate the corrected products already, before sending the info back on Earth, or pre-process it with their models and release them to their clients ready-for-use.

It could also be more cost-efficient to design a single satellite with multiple payloads instead of multiple satellites with one instrument each.

  • $\begingroup$ Do you have references for your assertions such as "To obtain the SAR images, one has to apply orbit corrections" ? As written, that does not seem to make sense. Not sure what this means either "The look angles would be different, but very well-known" Was this AI generated? $\endgroup$ Dec 23, 2022 at 13:56
  • $\begingroup$ yes, sorry I meant inSAR. Here is a tutorial on how to do interferometry: eo4society.esa.int/wp-content/uploads/2021/02/D4P1a_LTC2015.pdf . On page 10 you see that an orbit correction is taking place. So, before you generate the interferogram you need to orbit correct. Why you assume that I generated this with an AI? You are very mean organic marble.. $\endgroup$ Dec 23, 2022 at 14:37
  • $\begingroup$ As for the look angles, those are the angles at which the sensor is pointed toward the Earth's surface. The post indicates that the look angles of the antennas will be different. However, this doesn't matter since they are still well-known and corrections can be applied easily. $\endgroup$ Dec 23, 2022 at 14:39
  • $\begingroup$ additional sources and tutorials. I am surprised you don't know about orbit correction, because it's an essential part of pre-processing of inSAR. HERE asf.alaska.edu/how-to/data-recipes/… and HERE dges.carleton.ca/courses/IntroSAR/… $\endgroup$ Dec 23, 2022 at 14:43
  • $\begingroup$ Consider editing some of those sources into your answer, comments can get deleted. $\endgroup$ Dec 23, 2022 at 14:46

What would be the advantages and disadvantages of a LEO Earth observation satellite, carrying both Electro optical and SAR payload be?

If it's SAR + image analysis/decision making computer + optical, then it might be a big advantage.

SAR can spot ships at sea because it can scan large swaths and pick out bright radar reflections, and if it detects one that can't be correlated with the satellite's most recently uploaded Automatic identification system or AIS listings it can point an optical telescope directly at it to try to see what it's up to (if the Sun is up)

See also

From Star-shaped artifacts in SAR images of the "Suez Canal traffic jam seen from space"

cropped detail from ESA multimedia via Wikimedia: Suez Canal traffic jam seen from space

Source and original ESA source

[...] The two identical Copernicus Sentinel-1 satellites carry radar instruments to provide an all-weather, day-and-night supply of imagery of Earth’s surface, making it ideal to monitor ship traffic.

The sea surface reflects the radar signal away from the satellite, and makes water appear dark in the image. This contrasts with metal objects, in this case the ships in the bay, which appear as bright dots in the dark waters.


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