Nano-SAR Case study of synthetic aperture radar

63 rd International Astronautical Congress, Naples, Italy. Copyright ©2012 S. Engelen et al. All rights reserved. 

IAC-12-D1.2.3 

NanoSAR – CASE STUDY OF SYNTHETIC APERTURE RADAR FOR NANO-SATELLITES 

Steven Engelen 

Delft University of Technology, The Netherlands, s.engelen@tudelft.nl 

Maarten van den Oever 

Delft University of Technology, The Netherlands, maartenvdoever@gmail.com 

Pooja Mahapatra 

Delft University of Technology, The Netherlands, p.s.mahapatra@tudelft.nl 

Prem Sundaramoorthy 

Delft University of Technology, The Netherlands, p.p.sundaramoorthy@tudelft.nl 

Eberhard Gill 

Delft University of Technology, The Netherlands, e.k.a.gill@tudelft.nl 

Robert Meijer 

TNO Delft, University of Amsterdam, The Netherlands, robert.meijer@tno.nl 

Chris Verhoeven 

Delft University of Technology, The Netherlands, c.j.m.verhoeven@tudelft.nl 

Nano-satellites have a cost advantage due to their low mass and usage of commercial-off-the-shelf technologies. 

However, the low mass also restricts the functionality of a nano-satellite’s payload. Typically, this would imply 

instruments with very low to low resolution and accuracy, essentially ruling out applications such as remote sensing. 

However, multiple nano-satellites can cooperate to improve the overall system performance, for example by 

increasing the frequency of the observations. The objective of this study is to design a radar system that can be 

accommodated in a nano-satellite, and investigate the feasibility of using multiples of these nano-satellites to 

perform high temporal resolution remote sensing. 

In this paper therefore, the concept of a nano-satellite sized Synthetic Aperture Radar (Nano-SAR) is 

investigated. Nano-satellites have very constrained power and volume budgets, and there are limits to how much 

surface area they can unfold for use in radar. Given these constraints, a SAR system for use in a nano-satellite in a 

350 km orbit was sized, and approaches to tackle the deficits in the radar link budget are proposed. When applying 

state-of-the-art technologies, both on the component level, as well as on an architectural level, one arrives at a closed 

link budget. 

The proposed radar system consists of a patch antenna array with a span of 1.14 m by 0.18 m, operating at a 

frequency of 5.8 GHz. Power amplification and phase shifting is performed on the panel, using digital radio 

frequency (RF) integrated Complementary Metal Oxide Semiconductor (CMOS) circuits. This results in a swath 

width of 60 km, with pixel sizes of 10 m in elevation direction. Given these performance values, coupled with the 

increased revisit times, it was obvious this radar, when flown in a larger swarm of nano-satellites, would allow faster 

now-casting for weather prediction. With significant investment in technology development, it could be possible to 

use this system for SAR interferometry, for near-real-time monitoring of fast ground deformation phenomena such as 

earthquakes and volcanoes. Other applications could lie in the field of near-real-time ship motion detection and oil 

spill spread detection. 

Many technical challenges need to be solved still and platforms need to be designed, capable of supporting this 

system, before this payload would be ready for deployment. Preliminary design suggests the cost of such an 

instrument is substantially higher than what is common for nano-satellite components. However, the potential of 

such a system is extremely promising, and merits further investigation. 

I. INTRODUCTION 

With the prospect of nano-satellite constellations 

and swarms [1], a novel application area can be 

envisaged, in which multiple cooperating satellites 

orbiting Earth reduce revisit times of arbitrary areas to 

several hours, or even less. Interesting as this may seem, 

nano-satellites traditionally haven’t had the best of 

instruments, as they are limited in size, mass and 

perhaps most importantly, power. They do have a cost 

advantage [2] over traditional satellites, as they 

primarily use off-the-shelf components, which allow 

using them in larger numbers. 

Large numbers of nano-satellites do present a 

potential space-debris issue [3], which is why in this 

IAC-12-D1.2.3 Page 1 of 6


paper, we assumed them to be orbiting in very low earth 

orbits. These orbits would be “self-cleaning” in case of 

a loss of orbit maintenance operations of the satellite in 

question, as the average orbital lifetime of a 350 km 

orbit is less than 100 days. This strategy, though 

expensive in terms of propellant and the requirement of 

an active propulsion system effectively eliminates faulty 

satellites from the system. It has another bonus, in that 

the satellites are much closer to their target, allowing for 

higher resolution sampling, with more modest 

instruments. 

Our paper attempts to make a case for a nanosatellite 

synthetic aperture radar (Nano-SAR), and 

investigates the plausibility of such a system in 

combination with a nano-satellite platform. 

II. NANO-SATELLITE BUDGETS 

Nano-satellites are defined as “satellites with a wet 

mass of less than 10 kg” [4]. The most common 

platform however is a so-called CubeSat [4], which 

measures the satellites in units of 100x100x100 mm, 

generally with a wet mass of around 1 kg per “Cube”. 

Given these mass constraints, and also the ensuing 

volume constraints, the incoming power, and the 

thermal envelope of such satellites, it should be clear 

these satellites are severely restricted in their operations. 

To put his into perspective: the orbit average 

incoming power for a nano-satellite in low earth orbit 

varies between 5 to 15 watts [5], when sufficient surface 

area is present, and is ever increasing, especially when 

applying deployable solar panels [6]. Where higher 

power levels are concerned, thermal issues in removing 

the excess heat becomes a major obstacle, as dissipating 

over 20W in a small nano-satellite is a challenge indeed. 

Attitude control accuracies are generally low, with 

common values being reported as 5° pointing 

knowledge, and 10° pointing accuracy. However, higher 

performance solutions are surfacing on the market, with 

advanced attitude determination systems using Earthhorizon 

scanners [7] and even miniaturised star trackers 

combined with reaction wheels for more precise control, 

allowing pointing knowledge at levels of hundreds to 

tens of arcseconds, and pointing accuracies well below a 

degree [8]. 

Given these constraints, a high power, high 

resolution, high accuracy system will not be feasible. 

In order to circumvent some of these constraints, our 

system assumes the presence of an energy storage 

device, such as a battery, allowing for higher peakpower 

availability. This reduces the demand on the solar 

panel size, at the expense of requiring a pulsed and/or 

duty-cycled system. Furthermore, we assume a total 

available payload power of 15 Watts over the course of 

its operations, effectively leaving around 5 W for the 

spacecraft bus operations. 

In return, the data downlink is assumed to be 

handled by the radar system, relieving the spacecraft 

somewhat of its tight power constraints. 

Also, the large surface area of the radar panel can be 

used in combination with solar cells, to provide more 

power to the satellite. 

III. THE RADAR SYSTEM 

When the radar is used for SAR, the swath width is 

limited by multiple factors. The first factor that is 

important is the Pulse Repetition Frequency (PRF). The 

PRF is determined by the speed of the platform and the 

antenna length. To create a synthetic aperture without 

ambiguities in the final image, the radar needs to 

transmit two pulses per antenna length travelled. Thus a 

smaller antenna and a higher speed, i.e. a lower orbit, 

require a higher PRF. The drawback of a higher PRF is 

that the unambiguous range of the radar decreases. 

For an orbit of 350 km and an antenna of 1.14 m in 

azimuth the minimum PRF is in the order of 20 kHz. 

Such a high PRF leads to an unambiguous range of 

about 6 km. A swath of 6 km is common in small 

satellite systems [9]. However an ambiguous range of 

6 km will cause ambiguous projections that are still in 

the main beam to show up in the final image. This is a 

reason that such a high PRF is not feasible for our 

system. 

A solution to overcome these problems is to lower 

the PRF, and use measurements from the other satellites 

to fill the synthetic aperture. However such a solution 

causes problems in the demodulation of the signal, 

requires tight synchronisation, and requires accurate 

orbit control and pointing. 

In [10] a technique is presented that enables the 

utilisation of small antennas with Digital Beam Forming 

capabilities, which enable lowering of the PRF. With 

the utilisation of these techniques the PRF times the 

number of sub apertures needs to be higher than the 

Doppler bandwidth. 

The Doppler bandwidth is given by twice the 

satellite speed, divided by antenna length. In our case 

the Doppler bandwidth is about 14 kHz, and the 

resulting PFR is 2 kHz with 7 sub apertures. The 

unambiguous range is 60 km in such a solution. 

Since the footprint of the radar is about 600 km in 

elevation ambiguities remain. A possible way to solve 

this is through utilisation of an encoded pulse. Coding 

for conventional radar is presented in [11] and [12] . 

This technique can also be used to reduce ambiguities 

through cycling through different codes. In doing so, 

reflections of each pulse can be distinguished, as they 

are encoded with a different code. 

In SAR the phase of the signal is important for the 

mapping of the received power. If coding is used the 

phase still needs obtained for the mapping of the receive 

power. In our paper we assume the phase to be known. 



An advantage of using coding is that the code can be 

used to increase the signal to noise ratio. In our system 

the coding is used for this purpose, however the 

processing to perform SAR needs to be adjusted, which 

is considered outside of the scope of this work. Also the 

gain of coherent addition of the signals is lost since 

demodulation is performed before addition of the 

measurements. 

The resolution in azimuth in a radar system is given 

by half the antenna length; and, for non-coded systems, 

in elevation by 

Δ𝑅 = 𝑐 

2𝐵 

[1] 

where B is the bandwidth. The effective ground 

range resolution is given by, 

Δ𝑅 𝑔 = Δ𝑅 

sin 𝜗 𝑖𝑛𝑐 

[2] 

with ϑ the incidence angle. For a bandwidth of 

40 MHz the ground range resolution would be 10 m for 

an incidence angle of 24 degrees. 

The power required to operate a radar system can be 

calculated using the radar formula. 

𝑃 𝑟 = 𝑃 𝑡𝐺 𝑡𝐺 𝑡𝜆 2 𝜎 

(4𝜋) 3 𝑅 4 𝐿 𝑠 

[3] 

where Pt represents the transmitted power, Pr the 

received power, G the antenna gain, 𝜆 the wavelength, σ 

the radar cross section of the target, L the system losses 

and R the range form the platform to the target. 

Due to the relatively large distances involved in 

satellite communications the transmitted power needs to 

be increased. The power needs to be increased with the 

range to the power four to receive the same power 

compared to a conventional system. If the radar is used 

for Synthetic Aperture Radar (SAR) the evaluation of 

the system needs to be adapted. We introduce the Signal 

to Noise Ratio (SNR), which is given by 𝑃 𝑟/(𝑘𝑇𝐵𝑓). 

The Noise Equivalent Sigma Zero (NESZ) is a 

measure for the sensitivity of the SAR radar instrument. 

Introducing the integration of the SAR system the radar 

equation can be rewritten to: 

𝑁𝐸𝑆𝑍 = 2(4𝜋𝑅)3 𝑘𝑇𝐹𝐿 𝑠𝑉 

𝑃 𝑡𝐺 2 𝜆 3 Δ𝑅 𝑔 

[4] 

where V is the orbital velocity and Pt the average 

power transmitted. With this formula the sensitivity of 

the SAR system can be determined. 

III. CODING GAIN 

In the radar system the coding is going to be used to 

reject ambiguities. By carefully choosing the codes the 

multiple signals can be distinguished. Thus ambiguous 

projections that normally would show up in the swath 

can be distinguished and removed in the processing. 

Another advantage of coding is that the SNR increases, 

thus the signals with low power levels can still be 

recovered. A disadvantage is that the actual power level 

of the signal is not easily obtained. This introduces 

severe problems in the processing. The entire processing 

of the signals to obtain the final image needs to be 

changed in order to work with coding. In this first study 

the effect of the coding on the processing is not 

considered, coding is only used to increase the SNR. 

The actual gain by coding in dB is given by 

10 𝑙𝑜𝑔(2𝐿), in which L represents the code length. For 

a code length of 40000 this gain amounts to 49dB. The 

received power, excluding the coding gain, is shown in 

Table 1. 

Item Value 

[linear] 

Value 

[dB] 

𝑷 𝒕 750 W 58,75 dBm 

𝑮 𝒕 

24 

𝑮 𝒓 

40 

𝝈 

0 

𝜆 2 0,06 m 2 -24,437 

𝑳 𝒔 

0 

𝑹 𝟒 1.5 ∙ 10 22 m 4 222 

𝑷 𝒓 

-156,42 dBm 

Table 1: Received power parameters, and result, 

excluding coding gain 

Due to the utilisation of the patch antenna the gain is 

not the same for transmit and receive. The received 

power of -156 dBm is low and might be a problem. 

Without coding the SNR in such a system would be -28 

dB which does not lead to feasible applications. 

However with the coding used the SNR is over 20 dB. 

Now if the radar is going to be used for SAR the 

impact of the coding is large, since coherent addition of 

the samples is not trivial, and the detecting of the phase 

and the power levels is more difficult. However with 

sufficient computing power it should be possible to 

perform SAR-like operations. In that case indication of 

the performance can be obtained from the formula for 

the NESZ. The duty cycle during operation is set to 

0.02. Without the coding considered a NESZ of 25 dB is 

obtained which is not sufficient. However if the coding 

could be utilized a NESZ of -24 dB would be obtained. 



IV. DIGITAL RF POWER AMPLIFIERS AND 

SIGNAL GENERATORS 

Another novelty in this system is in the use of socalled 

“digital RF [13]” power amplifiers and signal 

generators. In such a system, the signal is generated 

fully digitally. 

This results in extremely high efficiencies for the 

power amplifier, which resembles a switch-mode 

amplifier. The signal generation, since it is fully digital, 

is also entirely flexible, simplifying the phase matching 

in the phased array significantly. Also, any type of 

signal can easily be generated, allowing for accurate 

beam-forming and pattern generation on the antenna. 

As these components can be (mass-)produced in 

bulk silicon, each patch antenna could easily be 

transformed into an active patch antenna, with its own 

signal generation and TX/RX front-end; reducing cable 

and connector losses. Simultaneously, the pulse power 

is generated on the radar panel, allowing for much 

easier thermal management compared to a power 

amplifier confined to the satellite main body. 

V. RESULTS 

V.I. Radar properties 

The specifications of the resulting radar system are 

summarised in Table 2. It can be used in an altimeter 

mode (“radar mode”) and in SAR mode. When used as 

a SAR, the resolution is dramatically increased; at the 

expense of a 10 times reduction in swath width. 

Value Unit 

Input power 15 W 

Power Amplification Efficiency 60 % 

Frequency 5,8 GHz 

Phased array length 118 cm 

Phased array width 18 cm 

Patch dimensions 0,5 𝜆 2 

Efficiency 0,7 - 

Pulse duty cycle 2 % 

Phased Array Element, max 

beam width 60 ° 

Phased Array Element gain 6 dB 

Code length 40000 chips 

Number of antennas 322 - 

Module peak output power 2,33 W 

Phased Array Receiver antenna 

gain 31 dB 

Transmit (pulse) power 750 W 

Bandwidth 40000 kHz 

Pulse duration 40 µs 

Energy per pulse 0,042 J 

Table 2: Specifications of the proposed radar system 

V.II. Revisit times 

Satellites in a low 350 km orbit have a fast ground 

swath velocity, and a small field of view, compared to 

higher orbiting satellites. In order to assure continuous 

global coverage, a constellation of satellites is required. 

The number of satellites, and also the number of distinct 

inclinations of their orbital planes is defined 

predominantly by their instrument field of view, and the 

desired revisit time. Also, the overlap between swaths 

and the potential gaps in observations are defining the 

number of required satellites. Earth’s rotation at these 

low orbits does play a role, but it is minor, compared to 

satellites in higher orbits. 

Table 3 lists the revisit times for various orbital 

planes and numbers of satellites, for a stationary Earth 

at 350 km, with a given swath width of 600 km in radar 

mode, as well as the effective revisit time when the 

system is used in pure SAR mode. This effective revisit 

time results from the smaller effective swath width in 

case the radar is used purely in SAR mode. 

Note these scenarios produce oversampling of 

certain areas (e.g. the North- and South-pole). The 

constellation or swarm is therefore over-dimensioned. 

The question of whether full coverage of all of Earth 

is required is something left to the mission designers. 

However, when proposing for a mission requiring a 

large number of objects in low earth orbit, the unit cost 

should be as low as possible. 

V.III. Scientific use 

With a swath width of 60 km and a pixel size of 10 x 

3 m, the Nano-SAR system promises to be of 

comparable resolution characteristics with existing SAR 

satellites such as Radarsat-2 or even TerraSAR-X. With 

the possibility of revisit times going down to 25 days, 

Nano-SAR could be utilised for many of the 

conventional SAR applications that utilize 

amplitude/intensity information, such as change 

detection, oil spill detection, surveillance and so on. 

Higher revisit times also mean that faster-changing 

phenomena can be observed, i.e., the temporal sampling 

frequency of the observed phenomenon can be 

improved compared to existing single-satellite systems. 

Spatial and temporal resolutions comparable to 

constellations such as Cosmo-Skymed maybe be 

achieved more economically. 

For Nano-SAR, we have assumed the phase of the 

radar signal to be known. However, with some further 

computation cost, this phase information may also be 

extracted from the signal itself, and this could lead to 

the applicability of Nano-SAR for SAR interferometry 

applications. An additional requirement for this would 

be that precise positioning information is available on 

each of the nano-satellites. 



Desired 

satellite 

revisit time 

Required number of 

satellites per orbital 

RADAR MODE SAR MODE 

Swath width = 600 km, 

pixel size = 14000x15 m 

Swath width = 60 km, 

Pixel size = 10 x 3 m 

plane Number of orbital planes Total number of satellites 

Effective 

revisit time 

[min] [-] [-] [-] [days] 

150 0.61 66 40 63.13 

135 0.68 66 45 56.81 

120 0.76 66 50 50.50 

105 0.87 66 57 44.19 

90 1.02 66 67 37.88 

75 1.22 66 80 31.56 

60 1.52 66 101 25.25 

45 2.03 66 134 18.94 

30 3.05 66 201 12.63 

15 6.09 66 402 6.31 

Table 3: Revisit times as function of orbital planes and number of satellites 

With effective revisit times down to 45 minutes over 

the area of interest, it would be possible to build up a 

useful time series of data within about 15 hours, and 

using this, various effects of different atmospheric 

conditions during the acquisitions may be cancelled out, 

and ground deformation history measured from the 

residual phase. For a time series of all of Earth, the 

effective revisit time amounts to about 25 days, for the 

case of 100 satellites, which renders a useful time series 

of data within about 500 days. 

In case the radar is used in “radar mode”, nowcasting 

can be performed with pixel sizes of 14 km x 15 

m, with revisit times of 45 minutes or more, for a 

reasonable swarm size. 

VI. CONCLUSIONS 

Advances in nano-satellite platforms and innovative 

mission design enable the realization of novel space 

applications. Such enabler could be a Nano-satellite 

SAR, offering the possibility of medium resolution 

Earth observation at affordable costs. Nano-SAR 

combines the limited capabilities of multiple individual 

nano-satellites to provide both high spatial resolution 

and high revisit times making it suitable for an array of 

earth observation missions. 

In this paper, the initial specifications for a radar 

system, tailored to nano-satellite platforms have been 

outlined. This radar would provide a resolution of 14 

km x 15 meter, with a swath width of some 600 km; 

when flown in an orbit at 350 km altitude. The radar can 

also be used in SAR mode, in which it generates 7 subapertures, 

rendering a total swath width of 60km , with 

pixel sizes of 10 x 3 meter. 

It is also apparent that quite some technology 

development is required in order to allow demonstration 

of this system, but the applications could well prove to 

be worth the effort. 

VI. REFERENCES 

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[2] C. I. Underwood, M. J. Crawford en J. W. Ward, 

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