Introduction
CubeSats are small spacecrafts with a modular structure based on one CubeSat unit (1U), which is a cube with the side equal to 10 cm [1], [2]. This modular structure enables versatile spacecraft designs with regular shapes and various sizes among which the most common are the 1U, the two-unit (2U) with dimensions of 10 cm
Among the various electronic components of a CubeSat, the radio communication system is a critical one, since it enables the CubeSat to exchange information and interact with ground terminals as well as with other CubeSats. The first step in designing the communication system of a CubeSat involves a link budget analysis to determine power requirements, choose appropriate hardware, and establish modulation parameters for signal transmission and reception. However, unlike traditional large satellites for which radio link budgets have been studied extensively and full details on designing the satellite communication system are available in the literature [3], [4], only limited studies of link budgets for CubeSat radios are available in the literature, related to specific CubeSat missions [5], [6]. These, along with the high-level presentation of the communication systems of various CubeSat missions given in the survey papers [7], [8], provide only a narrow perspective on designing communication systems for CubeSats. However, more general studies of link budgets for CubeSats that are decoupled from the specific details of the CubeSat missions are desirable and will be helpful in assessing the software-defined radio (SDR) implementations proposed recently for CubeSat communication systems [9]–[11]. These have emerged in the wider context of software defined electronics [12], which offers flexible implementations for modern telecommunication and measurement systems by using programmable hardware components that can be reconfigured through software. For communication systems, SDRs have been successfully used since the late 1990s and early 2000 years to improve interoperability of the various commercial radio systems and to reduce development and deployment costs [13], [14], and they have the potential to produce a radical change in the way space communication systems are designed and implemented.
Prompted by the limited number of link budget studies for CubeSat radio systems, this paper aims at augmenting existing literature with a study that considers salient parameters influencing the communication system design such as the CubeSat trajectory altitude, inclination, or inter-satellite slant range, along with constraints implied by SDR platforms.
The paper is organized as follows: Section II reviews characteristics of CubeSat missions in low Earth orbit (LEO) followed by an outline of radio link design in Section III. Sections IV and V present the analysis of the CubeSat power budget for the ground station and inter-satellite links, respectively. Final remarks and conclusions are given in Section VI.
Cubesat Missions in Low Earth Orbit
The CubeSat standard for small satellites was developed in the late 1990s and early 2000 years, being formally introduced in [16] and [17], with over 100 manifested CubeSat missions documented in 2013 [18] and more than 250 CubeSats currently included in the NORAD two-line element (TLE) data sets [15]. Many CubeSat missions consist of a single satellite launched and operated individually to perform specific science experiments, but they can also include multiple CubeSats that are deployed in clusters and establish inter-satellite links to form distributed satellite sensor networks in space [19]–[21].
In our study of radio link power budgets for CubeSat missions we will assume that CubeSats are placed in low Earth orbit at altitudes ranging between 200 km and 800 km, with circular trajectories and inclinations of either 52° or 98°. This assumption is supported by the CubeSat orbital parameters recorded in the TLE data maintained by NORAD and shown in Fig. 1. As can be seen from Fig. 1(a), of the 258 CubeSats whose information is available in [15], the majority have trajectories with eccentricities very close to zero. Furthermore, the apogee and perigee altitudes of most CubeSat missions are very similar, which also indicates almost circular orbits. In addition, from Fig. 1(b)–(c) we note that, with few exceptions, most CubeSat missions launched at altitudes below 500 km have trajectory inclinations of 52°, while those launched above 500 km have trajectory inclinations of 98°.
A CubeSat flying in LEO is equipped with multiple subsystems, which are needed to provide support and power to the science instruments and to transmit the collected data to a ground station for further processing, analysis, and archiving. While physical configuration of CubeSats depends on the actual science experiment to be performed, the main components of a CubeSat are independent of its science mission and are outlined in the block diagram shown in Fig. 2. As seen from Fig. 2, beside the science instruments which are supposed to capture the data related to the observed parameters, a CubeSat spacecraft includes a power subsystem, which is required to power-up all the other subsystems, an onboard computer that performs the data acquisition and processing and controls all of the other CubeSat functions, and the communications subsystem that establishes links with the mission ground station and/or with other CubeSats that may be part of the mission. In missions involving a single CubeSat only the ground station transceiver shown in Fig. 2 is present, while in missions designed for cluster operation of CubeSats the inter-satellite transceiver is also included to establish inter-satellite communication links as required by the mission.
In the case of single satellite missions, the CubeSat collects data related to the science experiment being carried out, performs some local processing of this data, and then transmits the data through a radio link to a ground station where it is further processed, interpreted, and stored. In such missions there is usually no need for an inter-satellite link transceiver, unless the CubeSat needs to establish a link also with other CubeSats or an existing satellite network, such as GlobalStar or Iridium for example, which may be used as alternatives to collect satellite experiment data.
In the case of multiple CubeSats operating in formation, the satellites establish also inter-satellite radio links [6] and set up a space wireless network over which they share observed science data along with ancillary information (position, timing, etc.) that enables them to perform joint/distributed processing of the data. In such a scenario, it is not necessary for all CubeSats to establish radio links to ground stations, as only one (or maybe a few of them) can act as gateways to transmit the science data to a ground station. We note that the CubeSats acting as gateways require to have both radio transceivers present, one to establish the radio link with the ground station, and another one to exchange information with the other CubeSats over the inter-satellite wireless network. The other CubeSats will only need to have the inter-satellite link transceiver to be able to establish the space wireless network.
Radio Link Design
The goal of radio link design is to ensure that a reliable communication link can be established between a radio transmitter and its associated receiver. In the context of digital communication systems, link reliability is evaluated through the bit error rate (BER) associated with the specific digital modulation scheme that is used to transmit information over the radio link, which depends on the signal-to-noise ratio (SNR) at the radio receiver. Thus, the main objective of radio link design is to establish if sufficient power is available at the radio receiver to close the link, that is to meet a specified SNR value.
For digital modulation schemes, the SNR at the receiver is given by the ratio of the received energy per bit \begin{equation} E_{b} = \frac {P_{r}}{R}, \end{equation}
\begin{equation} N_{0} = k T_{s} \end{equation}
\begin{equation} T_{s} = T_{ant} + T_{r}, \end{equation}
\begin{equation} T_{r} = \frac {T_{0}}{L_{r}} (F - L_{r}), \end{equation}
Combining eqs. (1) and (2) the received SNR is written as \begin{equation} \mbox {SNR} = \frac {E_{b}}{N_{0}} = \frac {P_{r}/R}{k T_{s}}. \end{equation}
Given the available power \begin{equation} P_{r} = \frac {P_{t} G_{t} G_{r}}{ L_{p}}, \end{equation}
\begin{equation} L_{p} = \left ({ \frac { 4 \pi d \hspace {0.025cm} f}{c} }\right )^{2}, \end{equation}
\begin{equation} \mbox {SNR} = \frac {E_{b}}{N_{0}} = \frac {P_{t} G_{t} G_{r}}{k T_{s} R L_{p}}. \end{equation}
Because the link budget equation (8) is essentially a succession of product operations of multiple terms, it can be written also in a more convenient form that involves the decibel (dB) representations of the individual terms that appear in it:\begin{align} \mbox {SNR}_{dB}=&10 \log _{10} \left ({ \frac {E_{b}}{N_{0}} }\right )\notag \\=&10 \log _{10} \left ({ \frac {P_{t} G_{t} G_{r}}{k T_{s} R L_{p}} }\right ) \notag \\=&P_{t, {dBm}} - 30 + G_{t, { dBi}} + G_{r,{dBi}} - L_{p,{ dB}} \notag \\&- 10 \log _{10} k - 10 \log _{10} T_{s} - 10 \log _{10} R. \end{align}
Using eq. (8) or (9) one can now evaluate the data rate
Power Budget for the Ground Station Link
In general, a CubeSat is connected to a ground station through a duplex radio link consisting of uplink, over which the CubeSat transmits data to the ground station, and downlink, over which the ground station transmits commands to the CubeSat. The value of the propagation path loss, which is shown in eq. (7), depends on the link distance
A. Determining Distance for the Ground Station Link
The distance \begin{equation} d_{\min } = R_{E} \frac {\sin (\lambda _{\min })}{\sin (\eta _{\min })}, \end{equation}
\begin{align} \sin (\lambda _{\min })=&\sin (lat_{pole}) \sin (lat_{gs}) \notag \\&+ \cos (lat_{pole}) \cos (lat_{gs}) \cos (\Delta long),\qquad \end{align}
\begin{align} lat_{pole}=&90^\circ - incl \notag \\ long_{pole}=&L_{node} - 90^\circ . \end{align}
The minimum nadir angle is implied by \begin{equation} \tan (\eta _{\min }) = \frac {\sin (\rho ) \sin (\lambda _{\min })}{1 + \sin (\rho ) \cos (\lambda _{\min })}, \end{equation}
\begin{equation} \sin (\rho ) = \frac {R_{E}}{R_{E} + H} \end{equation}
Similarly, for \begin{equation} d_{\max } = R_{E} \frac {\sin (\lambda _{\max })}{\sin (\eta _{\max })}, \end{equation}
\begin{equation} \sin (\eta _{\max }) = \sin (\rho ) \cos (\varepsilon _{\min }) \end{equation}
\begin{equation} \lambda _{\max } = 90^\circ - \varepsilon _{\min } - \eta _{\max }, \end{equation}
Beside the distance between CubeSat and ground station, the orbital parameters of the CubeSat trajectory determine also the total time \begin{equation} T = \frac {P}{180^\circ } \cdot \arccos \left [{ \frac {\cos (\lambda _{\max })}{\cos (\lambda _{\min })} }\right ], \end{equation}
\begin{equation} P = 1.658669 \cdot 10^{-4} \cdot \sqrt {(R_{E} + H)^{3}} \end{equation}
\begin{equation} T_{\max } = P \frac {\lambda _{\max }}{180^\circ }. \end{equation}
In Tables 1 and 2 we illustrate the distance and view times for CubeSats with different trajectory inclinations and altitudes, and a ground station located in Norfolk, VA,2 while in Tables 3 and 4 we show the values for a ground station located in San Antonio, TX.3
Summarizing the values in Tables 1–4 we note that the distance between a CubesSat and a ground station located in the continental United States ranges from 500 km to about 2,600 km, with view times of 5 – 10 minutes during which the CubeSat and the ground station may exchange information.
B. Frequency Selection and Link Characteristics
CubeSats commonly use frequencies in the VHF and UHF bands for establishing communication links with ground stations, with focus on the amateur radio frequencies in the 2 m VHF bands (144 MHz to 148 MHz) and 70 cm UHF bands (420 MHz to 450 MHz) [7]. To minimize the value of the propagation path loss and to reduce the power requirements for the CubeSat transmitter, the 2 m bands are used for the uplink (CubeSat-to-ground station), while the 70 cm bands are used for the downlink (ground station-to-CubeSat). Assuming a maximum distance of 2,600 km between the CubeSat and the ground station, the uplink path loss value at 146 MHz is 144 dB, while the down link path loss value at 437 MHz is 153.56 dB, corresponding to a 9.56 dB lower loss in the uplink compared to the downlink.
In terms of antennas, CubeSats rely mostly on omnidirectional dipole antennas for transmission in VHF bands with no directional gains (0 dBi), while ground stations usually have directional antennas with satellite tracking abilities and antenna gains between 10 and 15 dBi. Such gain values are usual for Yagi antennas in the VHF and UHF radio bands, and our link budget analysis assumes that the ground station transmit and receive antenna gains are 12.34 dBi and 15.5 dBi, respectively.4
For the CubeSat transceiver minimal losses of approximately −0.2 dB will be included in the link budget calculations. This corresponds to an absolute loss term of
For the ground station losses of about −3 dB will be considered in the link budget, corresponding to an absolute loss term
For the ground station uplink the antenna noise temperature is taken to be equal to the reference temperature
C. Link Budget Analysis
For the power budget analysis we assume that the CubeSat transmitter operates at low power levels, which may be achieved by using a software defined radio (SDR) transmitter with no additional amplifiers. This assumption is motivated by the fact that CubeSats rely exclusively on their onboard power system, which has limited capabilities to support batteries and solar panels due to the small physical volume of the CubeSat spacecraft. Specifically, our link budget analysis assumes a transmit power level of 15 dBm (about 31.62 mW) for the CubeSat, corresponding to a Universal Software Radio Platform (USRP) B-200 with no additional amplifiers [22]. We note that similar power levels have been reported in the literature for CubeSat transmitters [23], and this value is conservative, as transmit powers as high as 30 dBm (1 W) have also been mentioned for CubeSat missions [24].
For the ground station transmitter higher transmit power levels may be assumed. This is motivated by the fact that for the ground station transmitter the only restriction on transmit power is implied by its operating license, and in our link budget analysis we assume a transmit power level of 50 dBm (100 W), which can be achieved by using amateur radio equipment. However, this assumption does not restrict the implementation of the ground station transmitter to amateur radio equipment, and if it is desired to implement the ground station transmitter using a SDR such as the USRP, a two-stage amplifier with 10 dB in the fist stage and 25 dB in the second stage may be used to increase the output power of the USRP transmitter to reach the desired level of 50 dBm.
With these power levels for the CubeSat and ground station transmitters, we note from the power budget summarized in Table 5 that, with digital modulation schemes commonly used in satellite communication systems that include forward error correction (FEC) coding [4, Sec. 13.3.3], low data rates of the order of 2.4 kbps can be achieved in the uplink, while data rates as high as 1 Mbps are available in the downlink, with link margins of 3.2 dB and 1.85 dB, respectively.
We conclude the ground station link analysis by noting that, the low data rate achievable over the CubeSat uplink in conjunction with the limited time the CubeSat is visible from the ground station, imposes strict limits on the amount of information that may be transferred from the CubeSat to the ground station during one pass over the ground station. For example, at the rate of 2, 400 bps considered in the power budget example in Table 5, a short pass of 4.75 min would allow the transfer of about 684 kbits of data, while for a longer pass of about 9.4 min about 1.3 Mbits of data would be transferred. To put these numbers in perspective, we note that a VGA image with a size of
Power Budget for Inter-Satellite Links
Unlike the ground station link case, where one has to consider a duplex radio link with distinct parameters in the uplink and downlink, in the case of inter-satellite links one can focus on a simplex radio link where one CubeSat is the transmitter and another CubeSat is the receiver. In this case, the link distance
A. Frequency Selection and Link Characteristics
Because of the strict constraints imposed by the CubeSat design specifications [2] the antennas used for the inter-satellite radio link are expected to have small size and weight, while also providing link gains with low power consumption. Recently, various types of planar antennas have emerged as meaningful alternatives in this direction [28]. As reported in [28], microstrip patch and slot antennas operating in the L and S satellite frequency bands have sizes compatible with the CubeSat dimensions, and gains ranging from 2.3 dBi to 6.9 dBi depending on their shape and other characteristics.
In the current study of inter-satellite link budgets we assume that the CubeSat antenna gains are 5 dBi for both the transmitter and the receiver, and we consider link frequencies both in the L band (1 – 2 GHz) and in the S band (2 – 4 GHz). The path loss values associated with free-space propagation at specific frequencies in these bands and different CubeSat slant ranges are summarized in Table 6.
Furthermore, transceiver losses similar to those corresponding to the CubeSat transceiver in the ground station link are assumed: hardware/connector loss of about 0.2 dB (
B. Link Budget Analysis
For the analysis of the inter-satellite link power budget we start by considering again that the CubeSat transmitter operates at low power levels that correspond to the use of SDR transmitters with no additional amplifiers, and we assume the same transmit power level of 15 dBm (or 31.62 mW) for the CubeSat transmitter along with the use of similar digital modulation schemes with FEC. Under these assumptions, we note from the power budget summarized in Table 7 that, for the lowest inter-satellite link distance of 10 km data rates of the order of 3 Mbps can be achieved in the L band, while data rates of only 1 Mbps are available in the S band, with link margins of 1.08 dB and 0.1 dB respectively.
Because in practice link margins around 2 dB are desirable to ensure that the inter-satellite link is operational even with unforeseen variations of its parameters, establishing reliable high data rate inter-satellite links with 15 dBm transmit power is challenging, even over short distances. While for the 10 km link distance minor power increases of
Conclusions
In this paper, we studied radio link budgets to support communications between CubeSats and ground stations as well as inter-satellite communications. We outlined how the orbital parameters of the CubeSat trajectory determine the length of the ground station link and studied how the CubeSat slant range affects the path loss for the inter-satellite link. Formal power budgets that include achievable data rates and link margins have also been presented for the ground station and inter-satellite links.
The power budgets for both the ground station link and the inter-satellite link assumed that the CubeSat transmits at low power levels of 15 dBm, which correspond to the use of a SDR platform such as the USRP B-200 with no additional amplifiers. This constraint on transmit power limits the achievable data rates for the ground station uplink and for the inter-satellite link. An additional limitation for the CubeSat uplink to the ground station is implied by the limited time the CubeSat trajectory is visible at the ground station geographic location, which affects the amount of information that can be uploaded by the CubeSat to the ground station during one trajectory pass.
In future work we plan to study methods to overcome the aforementioned limitations on CubeSate data rates and amount of information that can be uploaded to the ground station. Advances in these directions are needed in order to bring CubeSats to the forefront of space exploration. CubeSat data rates may be increased by transmitting at higher power levels that exceed the usual 15 dBm power limit afforded by SDR platforms. This can be accomplished by using larger solar panels and batteries in conjunction with power amplifiers, and may require larger satellite volumes such as 3U or 6U CubeSats to accommodate them. Uploading large amounts of data from CubeSats may be accomplished through a network of ground stations at which the CubeSat is visible at different points in its trajectory and require precise coordination among ground station receivers along with the use of “pause & resume” protocols for information transfer.
ACKNOWLEDGMENT
The author is grateful to the Associate Editor who coordinated the review of the paper, Prof. Junfeng Wang, and to the anonymous reviewers for their comments, which improved the presentation of this work.