(TT&C) and Data Transmission Integrated Signal in TDD Mode

Article Design of Tracking, Telemetry, Command (TT&C) and Data Transmission Integrated Signal in TDD Mode

Linshan Xue 1, Xue Li 1,2,*, Weiren Wu 1,3 and Yikang Yang 4 1 School of Mechanical and Electrical Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China; [email protected] (L.X.); [email protected] (W.W.) 2 School of Microelectronics and Communication Engineering, Chongqing University, Chongqing 400044, China 3 Lunar Exploration & Space Engineering Center, Beijing 100190, China 4 School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, China; [email protected] * Correspondence: [email protected]

Received: 7 September 2020; Accepted: 9 October 2020; Published: 13 October 2020

Abstract: For satellite or aircraft networks, tracking, telemetry, and control (TT&C) and data transmission between diﬀerent nodes are necessary. Traditional measurement mostly adopts the frequency division duplex (FDD) mode and uses a continuous measurement system to achieve high-precision measurement. However, as the number of network nodes increases, the mode suﬀers from complex frequency domain allocation, and high-cost measurement and data transmission equipment is required. This paper proposes the integrated signal in time division duplex (TDD) mode to improve frequency utilization to address these circumstances. The proposed signal can transmit the TT&C and data at the same frequency. In addition, the high-precision time-frequency synchronization and relative measurement technology in the TDD mode for distributed spacecraft or aircraft networks are studied. The simulation results show that the signal can work normally when the Doppler extrapolation error is less than a quarter of the integration frequency. The distance extrapolation error should be less than a quarter of the length of a chip. The integrated signal reduces the frequency band occupation and realizes the integration of TT&C and data transmission. In addition, the measurement performance is reduced by only 2~3 dB compared with that of the traditional pure TT&C signal.

Keywords: binary oﬀset carrier (BOC); integrated signal; TT&C; data transmission

1. Introduction In traditional satellite network systems, the responsibilities of the intersatellite link (ISL) are divided into two independent functions, TT&C and data transmission; that is, the data transmission link and TT&C link are undertaken by two independent sets of equipment [1,2]. Refs. [3–5] introduce TT&C’s typical methods and related technologies for the intersatellite, including ranging, Doppler frequency measurement, and angle measurement. The two completely independent functions not only have complex structures but also occupy diﬀerent frequency bands. However, as the number of low earth orbit (LEO) satellites increases, such as OneWeb, Starlink, Telesat, etc., the lack of frequency resources is one of the most important restrictions in the spacecraft and aircraft networks [6,7]. In addition, the TT&C services adopt the FDD mode to realize high-precision measurement. Under standard operating conditions, the FDD mode will produce problems such as a complex frequency distribution and increased requirements for TT&C equipment. Due to the lack of spectrum resources, it is diﬃcult to obtain the paired spectrum needed in the FDD mode [8,9].

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Ref. [10] adopts space-based TT&C technology to solve the problem of resources dissatisfaction of ground-based TT&C system. However, this method is limited by the number of ground stations. Refs. [11–14] place the TT&C and data transmission services in the same frequency band to alleviate the problem. An unbalanced signal system with a low information rate was adopted in [11]. In the paper, the in-phase (I) channel transmitted data, and the quadrature (Q) channel transmitted the TT&C signal. Due to the emergence of the unified spread spectrum TT&C system, a TT&C and data transmission system based on code division multiple access (CDMA) was proposed [12,13]. Ref. [14] inserts the ranging pseudo-noise (PN) code into the high-speed data transmission channel to achieve an integrated signal. However, the signal combination of these methods is complex, which increases the complexity of the modulation and demodulation process. In sum, the existing methods seeking to improve the frequency band utilization in satellites are code division multiple access and improved modulation methods. In the mobile communication system, the extreme shortage of spectrum resources can be alleviated by the TDD mode. By transmitting and receiving at different times, it can utilize all available bandwidth efficiently. TDD technology plays an important part in the long-term evolution (LTE) standard, saves spectrum resources, and addresses users’ needs [15,16]. For satellite and aircraft systems, compared with those in FDD, the frequency resources in TDD are easier to obtain, and the costs are lower. However, TDD technology is not suitable for high-precision ranging and cannot measure velocity. Because the transponder cannot receive the signal during the transmission period, the tracking loop is interrupted periodically. With these observations in mind, in this paper, we increase the utilization of the frequency band by using an integrated signal and the TDD mode. The BOC and amplitude-phase shift keying (APSK) integrated signal is proposed. The TT&C signal is modulated by the BOC, and the data transmission is modulated by APSK. The BOC signal is expected to be used in future navigation systems because of its advantages. The BOC signal offers better performance for code tracking and multipath rejection performance. In addition, compared with the traditional spread spectrum modulation, the existence of subcarriers provides designers with more freedom to design modulation mode and reduce the interference of other signals at the same frequency [17,18]. It can be regarded as a direct spread spectrum sequence (DSSS) that multiplies the square wave subcarrier sequence [19]. The modulation subcarrier causes the frequency spectrum to shift to both sides of the carrier frequency. The BOC signal can free the bandwidth of the center frequency according to the characteristics, and it can be used as the TT&C signal when the data transmission signal is transmitted at the same frequency. To inherit the advantages of the TDD mode, we must guarantee TT&C performance. In [20], the nodes are measured by the ground station, and the results are sent to each node. However, this method increases the ground station’s burden and can synchronize with the ground station only when there is a link between the node and station. The measurement and control system in [21] records telemetry data and plays back the data to realize non-real-time TT&C functions after the communication link is interrupted. Since the tracking loop is interrupted and retracked continuously, the measurement accuracy cannot be high enough. In the paper, we propose a method that can achieve high measurement accuracy by exploring the tracking loop. Although the tracking error is increased as the extrapolation time increases, the method can ensure the TDD’s stable tracking. For data transmission services, phase-shift keying (PSK) modulation system has a characteristic such as constant envelope, easy implementation and the robustness against channel effects, but its spectrum utilization is not suitable for high data transmission services [22]. Quadrature Amplitude Modulation (QAM) modulation system has high spectrum utilization, but the strong effect of nonlinearity and complex implementation limits its application to satellite [23]. Combining the advantages of both, APSK modulation mode is adopted in the paper. The APSK has advantages as follows.

(1) High spectrum utilization; (2) Small amplitude ﬂuctuation which can eﬀectively resist nonlinear distortion; Remote Sens. 2020, 12, 3340 3 of 21

(3) The constellation is circular, which requires less power; (4) It is easy to realize variable rate modulation and meet the hierarchical transmission requirements of satellite communication.

At present, 16APSK and 32APSK have been selected into DVB-S2 standard, and are widely used in European remote sensing satellite and other military and civil satellite communication systems [24]. Considering those advantages, we adopt 16APSK as the modulation mode of the data transmission signal. The major contributions of this article are as follows:

(1) Considering the lack of spectrum resources, this paper proposes the BOCcos and APSK integrated signal to achieve TT&C and data transmission services. (2) Considering the shortage of frequency resources in the case of large numbers of nodes, we propose the high precision measurement method for the TDD mode.

The outline of the remainder of this paper is as follows. First, the BOCcos and APSK integrated signals are designed in Section2. The design of the acquisition and tracking algorithm in the TDD mode is described in Section3. The application scenarios and performance analysis of the measurement system are given in Section4. Section5 provides the discussion on the measurement and data transmission performance of integrated signals. Besides, it also gives the method to improve performance. In Section6, we provide a conclusion and propose future work.

2. The Modulation System for the BOCcos and APSK Integrated Signal In this section, we will give a signal model of an integrated signal and brieﬂy introduce the concept of modulation and demodulation.

2.1. Signal Mode Based on the slit-spectrum characteristics, BOC and APSK signals are proposed to be used in the design of the TT&C and data transmission integrated signal. Mathematically, the BOC signal can be expressed as (1): stt c(t) = d(t) g(t) c(t) (1) & · · where d(t) is navigation data, g(t) represents the PN sequence and c(t) is the subcarrier sequence. For the BOC signal subcarrier, which can be chosen as sine phase, cosine phase, or non-zero dual polarity codes. Diﬀerent subcarrier will inﬂuence the characteristics of the BOC signal. The BOCcos signal has the deepest null-forming in the center frequency than the other BOC signals. This property makes it easier to extract the 16APSK signal at the receiving end. Thus, we adopt BOCcos signal to transmit TT&C services. In a BOCcos signal, g(t) and c(t) can be expressed as (2) and (3):

X∞ g(t) = α pT (t kTc) (2) k c − k= −∞

c(t) = sign[cos(2π fst)] 0 t Tc (3) ≤ ≤ Here, α = 1. Tc is the width of the PN code. pT is the PN sequence. k ± c To express the split spectrum characteristics of the BOC signal more intuitively [25], the power spectrum of the BOCcos signal is shown in (6):

 2 π f 2 π f cos sin   fc 4 fs  PBOC cos( f ) = fc  (4)  π f   π f cos  2 fs Remote Sens. 2020, 12, x FOR PEER REVIEW 4 of 21

RemoteThis Sens. paper2020, 12 uses, 3340 APSK to achieve data transmission services [24]. The baseband signal of A4PSK of 21 is expressed as (4):

This paper uses APSK to achieve data transmission2 services [24]. The baseband signal of APSK is s t D t R texp j  i  i  0,1, , n  1 expressed as (4): dt       d (5) "  nd !# 2π s (t) = D(t)R(t) exp j i + θ i = 0, 1, , n 1 (5) dt n d where Dt  is the data and Rt  is the radius∗ ofd the constellation diagram.··· − nd is the signal points ofwhere a concentricD(t) is the circle. data  and representsR(t) is the the radius initial of phase. the constellation diagram. nd is the signal points of a concentricThe signal circle. modelθ represents of the integrated the initial signal phase. at baseband is shown as (5): The signal model of the integrated signal at baseband is shown as (5): 푠푡(푡) = 푠푡푡&푐(푡) + 푠푑푡(푡) = 푑(푡)푔(푡)푠푖푔푛[푐표푠(2휋푓푠푡)] + 푠푑푡(푡) (6)

For a BOCcos signal,st(t) = thestt & parametersc(t) + sdt(t ) (=m,dn() t) ofg (t a) sign BOC[coscos((2mπ,nfs)t )] signal+ sdt ( aret) determined by the(6) spectrum shape, the signal bandwidth and the bandwidth of the APSK signal. m is the multiple of For a BOCcos signal, the parameters (m,n) of a BOCcos(m,n) signal are determined by the spectrum the subcarrier frequency fs relative to the reference frequency fbase , which means fs m f base . n shape, the signal bandwidth and the bandwidth of the APSK signal. m is the multiple of the subcarrier is the multiple of the spread spectrum code frequency fc relative to the reference frequency , frequency fs relative to the reference frequency fbase, which means fs = m fbase. n is the multiple of and thus fc n f base . Here, fbase =1.023 MHz [26]. × the spread spectrum code frequency fc relative to the reference frequency fbase, and thus fc = n fbase. To transmit voice, image and video, high-speed data transmission service often needs several× Here, fbase = 1.023 MHz [26]. MbpsTo or transmit even hundreds voice, image of Mbps. and Therefore, video, high-speed we set the data bandwidth transmission of data service transmission often needs as 10MHz. several UsingMbps 16APSK or even hundredscan achieve of the Mbps. transmission Therefore, rate we of set hundreds the bandwidth of Mbps. of data transmission as 10 MHz. UsingThe 16APSK BOCcos(15,2.5) can achieve signal the transmissionis used to modulate rate of hundredsthe TT&C of data Mbps. in Galileo systems [25]. Besides, the bandwidthThe BOCcos(15,2.5) of the 16APSK signal signal is used should to modulate be less thethan TT&C the bandwidth data in Galileo of BOCcos’s systems null [25].-forming. Besides, Thus,the bandwidth we use BOCcos(30,5) of the 16APSK signal signal with should the same be less properties than the bandwidth as BOCcos(15 of, BOCcos’s2.5) to transmit null-forming. TT&C signal.Thus,we The use spreadBOCcos(30,5) spectrum signal code with freque thency same properties is 5.115 MHz, as BOCcos(15,2.5) and the subcarrier to transmit frequency TT&C signal. is 30.69The spread MHz. spectrum The 16APSK code signal frequency is 10fc MHz,is 5.115 which MHz, is and used the subcarrierfor data transmission frequency f s services.is 30.69 MHz. The constellationThe 16APSK signaldiagram is 10of MHz,16APSK which is shown is used in for Figure data 1a transmission. It has two services.concentric The circles. constellation The inner diagram circle hasof 16APSK four points, is shown and the in Figure outer1 circlea. It has has two twelve concentric points circles.[27,28].The inner circle has four points, and the outerThe circle spectrum has twelve of the points integrated [27,28 ].signal at the intermediate frequency (IF) end is shown in Figure 1c. The spectrum of the integrated signal at the intermediate frequency (IF) end is shown in Figure1c.

(a)

Figure 1. Cont.

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(b)

(c)

FigureFigure 1.1. TheThe characteristicscharacteristics ofof thethe BOCcos(30,5)BOCcos(30,5) andand 16APSK16APSK integratedintegrated signal.signal. ((aa)) TheThe constellationconstellation ofof 16APSK16APSK ((bb)) TheThe spectrumspectrum ofof BOCcos(30,5)BOCcos(30,5) atat thethe basebandbaseband ((cc)) TheThe spectrumspectrum ofof BOC(30,5)cosBOC(30,5)cos andand 16APSK16APSK atat thethe IF.IF.

2.2.2.2. TheThe ConceptConcept ofof ModulationModulation andand DemodulationDemodulation TheThe designdesign and implementationimplementation scheme scheme of of the the BOCcos BOCcos and and 16APSK 16APSK integrated integrated signal signal system system is isshown shown in in Figure Figure 2.2 . In In the the transmission transmission link, the TT&C and and data data transmission transmission service service signals signals are are generatedgenerated byby thethe BOCcosBOCcos basebandbaseband signalsignal andand 16APSK16APSK basebandbaseband signal,signal, respectively.respectively. Then,Then, thethe twotwo basebandbaseband signalssignals areare modulatedmodulated toto 44.9644.96 MHzMHz asas aa digitaldigital IFIF signal.signal. AfterAfter that,that, thethe systemsystem generatesgenerates aa 44.9644.96 MHzMHz analoganalog IFIF signalsignal throughthrough thethe digital-to-analogdigital-to-analog converterconverter (DAC).(DAC). InIn thethe radioradio frequencyfrequency (RF)(RF) transmissiontransmission channel, the signal signal is is up up to to the the R RF,F, which which is is filtered ﬁltered and and power power-ampliﬁed.-amplified. Then, Then, it ittransmits transmits the the signal signal to tothe the antenna. antenna. In addition, In addition, the data the data transmission transmission signal signal power power can be can kept be 10 kept~30 10~30dB higher dB higher than the than TT&C the TT&C signal. signal. A more A detailed more detailed analysis analysis will be will carried be carried out in outSection in Section 5.2. 5.2. InIn thethe receivingreceiving link, link, when when the the signal signal is is received received by by the the antenna, antenna, the the RF RF signal signal is downconvertedis downconverted to 44.96to 44.96 MHz MHz in the inRF the channel. RF channel. Then, Then, the digital the digital IF signal IF is signal generated is generated by the analog-to-digital by the analog- converterto-digital (ADC).converter In (ADC). addition, In addition, the BOCcos the signalBOCcos in signal the IF in is the acquired IF is acquired and tracked. and tracked. The tracking The tracking results results can eliminatecan eliminate the Doppler the Doppler signal signal to ensure to ensure the demodulation the demodulation of 16APSK. of 16APSK.

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16APSK(10) for data transmission Upconverter to 44.96MHz servives Antenna IF front-end RF front-end Up conversion to processing RF processing module module

BOC(30,5)cos Upconverter to for TT&C 44.96MHz services

(a)

16APSK(10) Time delay for data transmission Antenna Low-pass filter Doppler frequency services

RF front-end IF front-end processing Down converter to processing 44.96MHz module module

BOC(30,5)cos High-pass filter Acquisition process for TT&C for BOC(30,5)cos services

Rough results

Tracking process for BOC(30,5)cos

(b)

Figure 2. TheThe BOC(30,5)cos BOC(30,5)cos and and 16APSK 16APSK integrated integrated signal signal system. system. (a) (Transmissiona) Transmission link link structure structure (b) (Receivingb) Receiving link link structure structure..

3. The Receiving Method of the Integrated Signal in the TDD Mode In this section, we will elaborate on on the the acquisition method method used used to to obtain obtain the the TT&C TT&C results. results. However, considering that the tracking process of TT&C and the demodulation of 16APSK have no didifferencefference from from the the traditional traditional method, method, and willand not will be discussed not be discussed here. Finally, here. the motion Finally, compensation the motion methodcompensation in the TDDmethod mode in the is shown. TDD mode is shown. 3.1. The Acquisition Process of Integrated Signal 3.1. The Acquisition Process of Integrated Signal The acquisition process should give the rough code phase and Doppler frequency to the tracking The acquisition process should give the rough code phase and Doppler frequency to the tracking process. Serial acquisition, parallel acquisition and parallel code phase searching are the three major process. Serial acquisition, parallel acquisition and parallel code phase searching are the three major methods for acquisition [29]. For BOC signals, due to the existence of subcarriers, the acquisition methods for acquisition [29]. For BOC signals, due to the existence of subcarriers, the acquisition process needs to preprocess the signal. The acquisition methods of a pure BOCcos signal are shown process needs to preprocess the signal. The acquisition methods of a pure BOCcos signal are shown in [30,31]. Ref. [32] gives a BPSK-like technique to acquire the BOC signal as two BPSK signals. in [30,31]. Ref. [32] gives a BPSK-like technique to acquire the BOC signal as two BPSK signals. However, the method will cause a 3dB signal loss. This paper restores the upper and lower sidebands However, the method will cause a 3dB signal loss. This paper restores the upper and lower sidebands to center frequency for processing, which will effectively avoid 3dB signal loss. The technique extracts to center frequency for processing, which will effectively avoid 3dB signal loss. The technique extracts the signals of the upper and lower sidebands of the BOC signal to the baseband and regards them as the signals of the upper and lower sidebands of the BOC signal to the baseband and regards them as a C/A code for acquisition. And the acquisition method for C/A code is discussed in [33,34]. In this a C/A code for acquisition. And the acquisition method for C/A code is discussed in [33,34]. In this paper, the TT&C signals are extracted by using this technique, and there is no need for an independent paper, the TT&C signals are extracted by using this technique, and there is no need for an filtering process of APSK signals independent filtering process of APSK signals The preprocessing in the acquisition process is as follows. The preprocessing in the acquisition process is as follows. Step 1. The control unit shifts the frequency of the carrier numerically controlled oscillator (NCO) Step 1. The control unit shifts the frequency of the carrier numerically controlled oscillator (NCO) by f . In addition, the carrier NCO generates the in-phase and orthogonal carrier signals based on the by If . In addition, the carrier NCO generates the in-phase and orthogonal carrier signals based on frequencyI tuning word and mixes with the input signals. the frequency tuning word and mixes with the input signals. Step 2. The subcarrier NCO generates the in-phase or orthogonal subcarrier signals and mixes Step 2. The subcarrier NCO generates the in-phase or orthogonal subcarrier signals and mixes them with the output signal of step 1. them with the output signal of step 1. Step 3. A low-pass filter is used to filter the signal and retain the baseband signal with a Step 3. A low-pass filter is used to filter the signal and retain the baseband signal with a 10 MHz 10 MHz bandwidth. bandwidth. The signal acquisition diagram is shown in Figure 3a.

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The signal acquisition diagram is shown in Figure3a. Remote Sens. 2020, 12, x FOR PEER REVIEW 7 of 21 Step 1 converts the IF signal to the baseband without ﬁltering the high-frequency signals. The powerStep spectrum 1 converts of thethe processedIF signal to signal the baseband is shown without in Figure filtering3b. Stepthe high 2 restores-frequency the signals. BOCcos The signal to thepower spread spectrum spectrum of signal.the processed In addition, signal is it shown will move in Figure the baseband 3b. Step 2 APSKrestores signal the BOCcos to the signal 30 MHz to + fI frequency.the spread In this spectrum stage, thesignal. power In addition, spectrum it will of the move signal the baseband changes toAPSK that signal shown to inthe Figure 30 MHz3c.  Finally,fI step 3frequency. can ﬁlter In the this spread stage,spectrum the power signalspectrum in theof the baseband signal changes as Figure to that3d. shown in Figure 3c. Finally, Afterstep 3 the can preprocessing filter the spread of spectrum the IF signal, signal thein the integrated baseband signalas Figure can 3d be. compatible with the existing BOC signalAfter processing the preprocessing system. of the In theIF signal, paper, the weintegrated adopt signal the fast can Fourierbe compatible transform with the (FFT) existing for the BOC signal processing system. In the paper, we adopt the fast Fourier transform (FFT) for the correlation to obtain rough estimates of the PN code and Doppler frequency. correlation to obtain rough estimates of the PN code and Doppler frequency.

Local prn Results FFT code cache Doppler frequency

Preprocessing for IF signal IFFT Max Dectection IF Signal BOC(30,5)cos+16 Code phase offset APSK(10) Down Data Subcarrier Low pass filter Resample FFT converter cache

Carrier NCO Doppler Subcarrier frequency NCO switching

(a)

(b)

(c)

Figure 3. Cont.

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(d)

Figure 3. Design of TT&C signal processing. (a) Signal acquisition (b) Normalized power spectrum of Figure 3. Design of TT&C signal processing. (a) Signal acquisition (b) Normalized power spectrum of the IF signal (c) Normalized power spectrum of the signal after the desubcarrier (d) Normalized power the IF signal (c) Normalized power spectrum of the signal after the desubcarrier (d) Normalized spectrum of the signal after the low-pass filter. power spectrum of the signal after the low-pass filter. 3.2. Motion Compensation Method in TDD Mode 3.2. Motion Compensation Method in TDD Mode For multi-node spacecraft/aircraft networks, it is necessary to synchronize the time and eliminate the clockFor multi difference-node betweenspacecraft/aircraft different nodes.networks, After it is that, necess theary unified to synchronize time standard the time can and be eliminate adopted inthe the clock whole difference network between [35]. different In addition, nodes. the After network that, doesthe unified not need time all standard nodes tocan use be adopted the global in navigationthe whole network satellitesystem [35]. In (GNSS) addition, or the a ground network station does for not timing need all and nodes positioning, to use the which global greatly navigation saves resources.satellite system In the (GNSS) ground or system, a ground clock station synchronization for timing and technology positioning, mainly which includes greatly average saves resources. time and double-diIn the groundfference system, or triple clock di synchronizationfference observation technology [36–38 ].mainly For intersatellite includes average/aircraft time links, and the double time- synchronizationdifference or triple and true difference distance canobservation be measured [36– via38].dual For one-way intersat rangingellite/aircraft (DOWR) links, [36 ,39 the,40 ]. time synchronizationHowever, the and DOWR true distance method can needs be measured to measure via the dual pseudo-range one-way ranging and velocity (DOWR) at the[36, same39,40]. time. In theHowever, TDD mode, the the DOWR pseudo-range method needs and velocity to measure results the obtained pseudo-range by the and local velocity measurements at the same between time. diInff theerent TDD nodes mode, are the not pseudo obtained-range simultaneously. and velocity Here,results we obtained take nodes by the A local and B measurements as an example. between different nodes are not obtained simultaneously. Here, we take nodes A and B as an example. The pseudo-ranging and velocity results of node A can be expressed as ρA(t1) and vA(t1), and the  t vt resultsThe of pseudo node B-ranging can be expressed and velocity as ρresultsB(t2) and of nodevB(t 2A). cant1 and be expressedt2 are the as times A  when1  and the resultsA  1  , and are obtainedthe results by of local node satellites B can orbe aircraft, expressed respectively. as B t2 Because and vt theB  TDD2  . t1 mode and cannott2 are receivethe times and when transmit the t t signalsresults atare the obtained same time, by local the relationsatellites between or aircraft1 ,and respectively.2 can be expressedBecause the as TDD (7): mode cannot receive and transmit signals at the same time, the relation between and can be expressed as (7): t1 = t2 + Tsp + ∆t (7)

t12 t  Tsp   t (7) where Tsp is the switching period between the receiving and transmitting processes. ∆t denotes the errorwhere in theTsp measurementis the switching process. period between the receiving and transmitting processes. t denotes the errorVia the in the following measurement steps, theprocess. time of A and B measuring the pseudo-range and velocity can be correctedVia the to thefollowing same time. steps, the time of A and B measuring the pseudo-range and velocity can be correctedStep 1.to Calculatethe same time. the radial acceleration value aA(t) between the spacecraft using the spacecraft orbitStep elements 1. Calculate or the inertial the radial navigation acceleration system value (INS). atA () between the spacecraft using the spacecraft Step 2. According to (15), we can calculate the correct velocity v B(t ) using the radial acceleration orbit elements or the inertial navigation system (INS). 0 2 value a(t): Step 2. According to (15), we can calculate the correct velocity vt'()B 2 using the radial Z t2 acceleration value at(): v B(t2) = vA(t1) + aA(t)dt (8) 0 t1 t2 v'()()()BAA t21 v t a t dt (8) t1

Step 3. The Doppler frequency fd can be calculated by (9).

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Step 3. The Doppler frequency fd can be calculated by (9).

fRF f (t ) = v B(t ) (9) d 2 0 2 · c

Here, c is the speed of light. fRF is the radio frequency. Step 4. v B(t ) is used to modify the pseudo-range value ρ B(t ) of the spacecraft via (10): 0 2 0 2

Z t2 " Z t2 # ρ B(t2) = ρA(t1) + vA(t1) + aA(t)dt dt (10) 0 t1 t1

The second term on the right side of (11) is the pseudo-distance modifier. Step 5. v B(t ) and ρ B(t ) are used to conduct DOWR. f and ρ B(t ) are sent to the carrier 0 2 0 2 d 0 2 tracking loop and code tracking loop to guarantee the stable tracking of the loop without receiving a signal. The acceleration value a(t) of spacecraft can be obtained by spacecraft orbit elements or the INS. The former should consider oblate earth model, atmospheric resistance, lunar gravitation and sun gravitation perturbations, etc. And it uses J4 orbit propagator or high precision orbit perturbations 3 2 (HPOP) to calculate the acceleration value. In [41], the error of orbit propagator is less than 10− m/s . These elements can be uploaded in each satellite through the ground station at regular intervals. The latter method uses INS to obtain the acceleration value of the spacecraft. The messages are obtained autonomously by INS. The [42] shows the measurement value of INS is less than 104 m/s2. However, the acceleration direction measured by the two methods is different from the direction of the two spacecraft, and we need to project the acceleration vector to the direction between the spacecraft. With the above extrapolation method, first, the times of the distance and velocity measurements by nodes A and B are synchronized. Thus, DOWR can be used to eliminate the distance error caused by the clock difference, and the integrated signal can provide high-precision measurements [43]. Second, the extrapolation ensures that the tracking loop remains locked during the TDD mode signal transmission period, which enables the loop to converge faster when receiving the signal input.

4. Performance Analysis of the TT&C Signal and Data Transmission Signal In this section, we will investigate the performance of the integrated signal in terms of the acquisition time, tracking performance, bit error and measurement error. In addition, we will discuss the methods of reducing measurement error. Considering that data transmission services require more power than TT&C services, we set the power ratio of APSK and BOCcos to 10:1.

4.1. The Design of the Simulation Scenario The design of the simulation scenario comes from the satellite toolkit (STK). For intersatellite links, we set the maximum distance and relative velocity between diﬀerent nodes as 783 km and 602 km/h, respectively. From [31], The relationship between satellite’s transmitting signal power and receiving power is shown in (11). λ PR = PT + GT + GR + 20lg L (11) 4πd − A

Here, PR and PT is the received signal power and transmitted signal power. GR and GT is the antenna gain at receiver and transmitter. λ is the wavelength. d represents the distance between the receiver and transmitter. LA is the atmospheric loss, which can be ignored for intersatellite links. We set the satellite’s transmitting power PT is approximately 10 W, and the satellite’s antenna gain GT is less than 37.1 dB. The receiver antenna gain GR is 0. The Ka band is used for the intersatellite links [6,7,44]. The carrier-to-noise ratio (CNR) of the receiving end is higher than 55 dB-Hz. The trajectory in this process is obtained by the STK simulation, as shown in Figure4a. The orbit parameters of satellites are shown in Table1. Figure4b) shows the distance between the visible Remote Sens. 2020, 12, x FOR PEER REVIEW 10 of 21 satellites in adjacent orbital planes. The value changes following a cosine curve in the range of [482.293 km, 783.774 km]. The inter-aircraft link scenario is shown in Figure 4c. The range between different nodes are less than 40 km, and the trajectory includes uniform motion, acceleration motion and S-curve motion. However, the carrier-to-noise ratio between UAVs is higher than the satellites, and the result is better than the satellite scene. Therefore, we only give the simulation results of the satellite scene. And the simulation results of aircraft scenes in some environments are given in Appendix A.

Table 1. The orbit parameters of satellites.

Satellite number 3364

Remote Sens. 2020, 12, 3340 Orbit inclination 51 10 of 21 Orbit altitude 508 km Orbital plane 58 satellites in adjacent orbital planes.The satellites The value in each changes plane following58 a cosine curve in the range of [482.293 km, 783.774 km]. Number 3364

(a)

Remote Sens. 2020, 12, x FOR PEER REVIEW 11 of 21 (b)

(c) FigureFigure 4. 4.SimulationSimulation scenario scenario design. design. ( (aa)) SatelliteSatellite orbit.orbit. ( (b)) The The distance distance between between visible visible satellites satellites (24 (24h) h)..(c ()c )Aircraft Aircraft scenario. scenario.

4.2. Acquisition Performance of Tablethe TT&C 1. The Signal orbit parameters of satellites. Because this paper adoptsSatellite an integrated number signal, the subpeak 3364 of the APSK signal will be interfered Orbit inclination 51 due to the existence of the BOC signal. In addition, the signal power◦ of the data transmission signal Orbit altitude 508 km is higher than that of the TT&COrbital signal plane to ensure the bit error rate. 58 This will further interfere with the acquisition of the BOCcosThe signal. satellites In in Figure each plane 5, the acquisition probability 58 of the CNR is given. Obviously, the acquisitionNumber probability of the integrated signal 3364 is lower than that of the single BOCcos signal. The degradation of performance is approximately 2~3 dB. However, for intersatellite links and aircraft links, the CNR is higher than that of the satellite ground link. Therefore, the performance degradation is acceptable.

Figure 5. The acquisition probabilities of an integrated signal and a BOC signal.

The clock cycle required for one acquisition process is shown in Table 2. Since the sampling

frequency fs is set to 245.52 MHz, 409,105 sample points will be cached within 0.2 ms. After that, downsampling is realized 12 times, and the sample point number is reduced to 4093. Then, the data are zero-padded to 4096 points and sent to the FFT.

Table 2. Required clock cycles for once acquisition process.

Process Clock Cycle Downsampling 4096 FFT of the downsampled signal 4096 12 Complex multiplication between the local PN code and downsampled signal 4096 IFFT of the multiplication result 4096 12 Maximum searches 4096 Threshold calculation 1280 Total 111872 Remote Sens. 2020, 12, x FOR PEER REVIEW 11 of 21

Remote Sens. 2020, 12, 3340 11 of 21

The inter-aircraft link scenario is shown in Figure4c. The range between di ﬀerent nodes are less than 40 km, and the trajectory includes uniform motion, acceleration motion and S-curve motion. However, the carrier-to-noise ratio between UAVs is higher than the satellites, and the result is better Figure 4. Simulation scenario design. (a) Satellite orbit. (b) The distance between visible satellites (24 than the satellite scene. Therefore, we only give the simulation results of the satellite scene. And the h). (c) Aircraft scenario. simulation results of aircraft scenes in some environments are given in AppendixA. 4.2. Acquisition Performance of the TT&C Signal 4.2. Acquisition Performance of the TT&C Signal Because this paper adopts an integrated signal, the subpeak of the APSK signal will be interfered Because this paper adopts an integrated signal, the subpeak of the APSK signal will be interfered due to the existence of the BOC signal. In addition, the signal power of the data transmission signal due to the existence of the BOC signal. In addition, the signal power of the data transmission signal is is higher than that of the TT&C signal to ensure the bit error rate. This will further interfere with the higher than that of the TT&C signal to ensure the bit error rate. This will further interfere with the acquisition of the BOCcos signal. In Figure 5, the acquisition probability of the CNR is given. acquisition of the BOCcos signal. In Figure5, the acquisition probability of the CNR is given. Obviously, the acquisition probability of the integrated signal is lower than that of the single Obviously, the acquisition probability of the integrated signal is lower than that of the single BOCcos signal. The degradation of performance is approximately 2~3 dB. However, for intersatellite BOCcos signal. The degradation of performance is approximately 2~3 dB. However, for intersatellite links and aircraft links, the CNR is higher than that of the satellite ground link. Therefore, the links and aircraft links, the CNR is higher than that of the satellite ground link. Therefore, the performance degradation is acceptable. performance degradation is acceptable.

Figure 5. The acquisition probabilities of an integrated signal and a BOC signal. Figure 5. The acquisition probabilities of an integrated signal and a BOC signal. The clock cycle required for one acquisition process is shown in Table2. Since the sampling The clock cycle required for one acquisition process is shown in Table 2. Since the sampling frequency f is set to 245.52 MHz, 409,105 sample points will be cached within 0.2 ms. After that, frequency sf is set to 245.52 MHz, 409,105 sample points will be cached within 0.2 ms. After that, downsamplings is realized 12 times, and the sample point number is reduced to 4093. Then, the data downsampling is realized 12 times, and the sample point number is reduced to 4093. Then, the data are zero-padded to 4096 points and sent to the FFT. are zero-padded to 4096 points and sent to the FFT. Table 2. Required clock cycles for once acquisition process. Table 2. Required clock cycles for once acquisition process. Process Clock Cycle Process Clock Cycle Downsampling 4096 Downsampling 4096 FFT of the downsampled signal 4096 12 × ComplexFFT multiplication of the downsampled between signal 4096 12 Complex multiplicationthe between local PN the code local and PN code and downsampled4096 signal 4096 downsampledIFFT of the multiplication signal result 4096 12 IFFT of the multiplication result 4096 12 Maximum searches × 4096 Maximum searches 4096 ThresholdThreshold calculation calculation 1280 1280 TotalTotal 111,872 111872

Considering other waiting times, the acquisition process in the FPGA set the margin to 200 clock (111872+200) cycles. The acquisition time Tacq = 245.52MHz = 0.456 ms. For intersatellite links, the Doppler frequency search range is generally set to 6 kHz. Therefore, if 500 Hz is used as the step, the time ± for searching all frequency points is 11.4 ms. However, due to the existence of missed detections, Remote Sens. 2020, 12, 3340 12 of 21 the acquisition time will be longer. Due to the tong detector is used to reduce the false probability, the resident time will increase at each frequency [33,43]. Each frequency will be searched eight times at low SNR and three times at high SNR. Take six times as acquisition time, which is equal to 66.57 ms. For the TDD mode, the hardware can search the code phase and Doppler frequency when in the TDD mode receiving stage. Thus, the duration of the TDD mode receiving and transmitting stage should not less than acquisition time.

4.3. Tracking Performance for the TT&C Signal In the tracking process, the pseudo-range and Doppler frequency are the main measurements [42]. The ranging error refers to the code loop tracking error, and its main sources are thermal noise error and dynamic stress error. The latter is eliminated by carrier-assisted PN code technology, and the thermal noise error can be calculated as (12):

s " # 2d2B 4d σ = T LD 2(1 d) + (12) Ranging chip CNR − T CNR coh ·

Here, Tchip represents the width of a chip, which is 58.6 m for BOCcos(30,5). BLD is the bandwidth of the code tracking loop. CNR is the carrier-to-noise ratio of the TT&C receiving signal. d represents the chip interval of the delay locked loop (DLL). Tcoh represents the integration time [45]. For the Doppler frequency, the thermal noise error of the three-order phase locked loop (PLL) is calculated as (13). s ! BLP 1 σPLL = 1 + (13) CNR 2TcohCNR The Doppler frequency can be calculated as (14).

√2σ σ = PLL (14) Doppler 2π ∆t · Here, ∆t is the refresh interval time of the Doppler measurement, which is equal to 0.1 s [46,47]. Considering the convergence speed and tracking error of the DLL and PLL, we set BL = 1 Hz and BLP = 10 Hz. In this situation, the ranging error is 0.0755 m when CNR = 55 dBHz, and the Doppler frequency measurement error is 0.0127 Hz. If it is necessary to improve the measurement accuracy, it is very eﬀective to average the measurement results, increase the Tcoh or improve the CNR. In Section 3.2, the extrapolation method is used in the transmission stage of the TDD mode, which will cause errors. The acceleration measurement error and velocity measurement error are assumed to be ∆a and ∆v, respectively. Therefore, the velocity measurement error caused by the acceleration error is ∆a Tsp, and the ranging error is as shown in (15). × σce = ∆v + ∆a Tsp/2 Tsp. (15) × ×

The correction error σce shows that the shorter the switching period is, the smaller the extrapolation 3 2 error in the TDD process. For example, if ∆v 1 m/s, ∆a 1 10 m/s and Tsp = 100 ms, ≤ ≤ × − the pseudo-range correction error by extrapolation is less than 0.10005 m. If Tsp = 1 s, the correction error is less than 1.005 m. Considering the bit transitions, the relationship between the correction errors and whether the tracking process requires recapturing is shown in Table3. The threshold is related to the parameter selection of the frequency-locked loop and the DLL. Remote Sens. 2020, 12, x FOR PEER REVIEW 13 of 21

Remote Sens. 2020, 12, 3340 13 of 21 Table 3. The relationship between the correction errors and whether the tracking process requires reacquisition.

Table 3. TheThreshold relationship betweenWhether the the correction Tracking errors Process and Requires whether Reacquisition the tracking process

requires reacquisition. ce 0.25Tc chip  And 1 No   ThresholdDoppler Whether the Tracking Process Requires Reacquisition 4Tcoh

σce ce0.250.25TchipTc chip c And Or ≤ ± ·1 No σDoppler 1 Yes 4Tcoh  Doppler ≤ ± σce > 0.25Tchip4Tcohc Or ·1 Yes σDoppler > ± 4Tcoh The Doppler and pseudo-range measurements at different switching periods are shown in Figure 6. The Doppler and pseudo-range measurements at diﬀerent switching periods are shown in Figure6.

(a) The Doppler measurement result and tracking loop result (b) The Doppler measurement error at , at T100 ms ,   0.1m and   7.8Hz . sp ce Doppler and .

Figure 6. Cont.

Remote Sens. 2020, 12, x FOR PEER REVIEW 14 of 21 Remote Sens. 2020, 12, 3340 14 of 21

(d) The Doppler measurement result and tracking loop (e) The Doppler measurement error at ,

result at Tssp 10 , ce 10m and  Doppler  780Hz . and .

(f) The pseudo-range measurement result at , and .

(g) The Doppler measurement result and tracking loop result (h) The Doppler measurement error at , at Ts15 ,  14.65m and  1250Hz . sp ce Doppler and .

Figure 6. Cont.

Remote Sens. 2020, 12, x FOR PEER REVIEW 15 of 21 Remote Sens. 2020, 12, 3340 15 of 21

(i) The pseudo-range measurement result at Tssp 15 , ce 14.65m and  Doppler 1250Hz .

(j) The Doppler measurement result and tracking loop result (k) The Doppler measurement error at ,

at Tssp  20 , ce  20m and  Doppler 1560Hz . and .

(l) The pseudo-range measurement result at , and .

FigureFigure 6.6. TheThe measurementmeasurement resultsresults whenwhen CNRCNR= = 6060 dBHz.dBHz.

Excessive extrapolation errors of the Doppler frequency and ranging during the TDD switching period will cause loop unlocking and reacquisition. The simulation shows that if the Doppler

Remote Sens. 2020, 12, x FOR PEER REVIEW 16 of 21

Excessive extrapolation errors of the Doppler frequency and ranging during the TDD switching Remote Sens. 2020, 12, 3340 16 of 21 period will cause loop unlocking and reacquisition. The simulation shows that if the Doppler 1 extrapolation error exceeds  and the distance extrapolation error exceeds 0.25Tcchip at the 41Tcoh extrapolation error exceeds and the distance extrapolation error exceeds 0.25Tchip c at the ± 4Tcoh ± · receivingreceiving and and transmitting transmitting switch switch time, time, the the tracking tracking loop loop will will unlock. unlock.

5.5. Discussion Discussion DueDue to to the the modulation modulation of of two two kinds kinds of of signals signals at at the the same same frequency frequency point, point, the the subpeaks subpeaks of of the the twotwo signals signals have have an a impactn impact on the on TT&C the TT&C and data and transmission data transmission performance performance of each of other. each This other. section This issection conducting is conducting simulation simulation research toresearch study theto study deterioration the deterioration of the performance. of the performance.

5.1.5.1. Ranging Ranging and and Doppler Doppler Measurement Measurement Performance Performance for for Integrated Integrated Signals Signals

WhenWhen the the simulation simulation parametersparameters areare TTsspsp = 1010 s,,σcece=10m10 andm and  DopplerσDoppler 780Hz= ,780 comparedHz, compared with the withpure the BOC pure signal, BOC the signal, measurement the measurement accuracy accuracyis reduced is mainly reduced due mainly to the dueinterference to the interference of the APSK’s of thesubpeak APSK’s, which subpeak, will whichreduce will the reduceCNR of the the CNR TT&C of signal the TT&C. The signal.degree Theof the degree decrease of the in decreasethe signal in is thethe signal same isas thethat same during as thatthe acquisition during the process. acquisition The process. measurement The measurement results are shown results in are Figure shown 7. in Figure7.

(a)

(b)

FigureFigure 7. 7.The The ranging ranging and and doppler doppler measurement measurement performance performance of of an an integrated integrated signal signal and and pure pure signal: signal: (a()a Doppler) Doppler frequency frequency measurement measurement results results of of integrated integrated signal signal and and pure pure BOCcos BOCcos signal; signal; (b ()b Ranging) Ranging resultsresults of of integrated integrated signal signal and and pure pure BOCcos BOCcos signal. signal.

Remote Sens. 2020, 12, x FOR PEER REVIEW 17 of 21

Remote Sens. 2020, 12, 3340 17 of 21 5.2. Bit Error Rate (BER) for Integrated Signals

5.2.It is Bit necessary Error Rate to (BER) obtain for Integratedthe data Signalstransmission signal from the integrated signal via filtering. In the separation process, the filter will reduce the CNR of the signal and eventually increase the BER. It is necessary to obtain the data transmission signal from the integrated signal via ﬁltering. In the The BER of the integrated signal and pure signal are shown in Figure 8. If the integrated signal separation process, the ﬁlter will reduce the CNR of the signal and eventually increase the BER. needs toThe achi BEReve ofthe the same integrated BER, the signal CNR and of pure the signalintegrated are shown signal in will Figure be8 4~5. If the dB integratedhigher than signal that of a singleneeds signal. to achieve the same BER, the CNR of the integrated signal will be 4~5 dB higher than that of a singleFor BOC signal. signal, the BER will be less than 1e-4 when success is acquired. Therefore, Figures 5 and 8 showFor BOCthe BER signal, of theTT&C BER signal will be is less higher than 1e-4than when the data success transmission is acquired. signal Therefore, at the Figures same5 CNR. Thus,and the8 show data the transmission BER of TT&C signal signal power is higher can than be the kept data 10~ transmission30 dB higher signal than at the the same TT&C CNR. signal. Thus, If the powerthe is data higher transmission than this signal level, power the cansubpeak be kept of 10~30 the data dB higher transmission than the TT&C signal signal. will Ifinterfere the power with is the measurementhigher than of this the level, BOC the signal. subpeak If it of is the lower data transmissionthan 10 dB, signalthe power will interfere of the BOC with thesignal measurement should keep a highof level, the BOC which signal. is not If necessary it is lower thanfor TT&C. 10 dB, the power of the BOC signal should keep a high level, which is not necessary for TT&C.

Figure 8. The data transmission error rate of an integrated signal and pure signal. Figure 8. The data transmission error rate of an integrated signal and pure signal. 5.3. Summary of Error Suppression Methods 5.3. Summary of Error Suppression Methods According to the results of the software simulation, the measures used to improve the measurement accuracyAccording are as to follows: the results of the software simulation, the measures used to improve the measurement accuracy are as follows: (1) Improve the PN code rate, which will reduce Tchip.

(1) (2)ImproveIncrease the the PN length code ofrate, the which PN code. will The reduce method Tchip can. increase the integration period. (2) (3)IncreaseIncrease the thelength integration of the PN period. code. The method can increase the integration period. (3) (4)IncreaseReduce the the integration bandwidth period. of the loop. However, the stability of the loop will be weakened, so the (4) Reduceerror the and bandwidth SNR in the extrapolationof the loop. However, process should the bestability considered of the comprehensively. loop will be weakened, so the (5)errorReduce and SNR the in switching the extrapolation period of process the TDD should mode. be Thisconsidered method comprehensively. can effectively reduce the extrapolation errors. (5) Reduce the switching period of the TDD mode. This method can effectively reduce the extrapolationThese methods errors. can be adjusted in baseband signal processing. In addition, during the TT&C process between satellites, there will be measurement errors caused These methods can be adjusted in baseband signal processing. by the ionosphere. However, the error can be eliminated by two or three frequency links. Taking In addition, during the TT&C process between satellites, there will be measurement errors the S/X dual frequency measurement as an example, the effect of the residual ionospheric high-order causedterm by on the the ionosph group delayere. willHowever, be less thanthe 1error mm. can Moreover, be eliminated when the by flight two orbits or three of the frequency satellite are links. Takingat the the same S/X altitude,dual frequency the ionospheric measurement change rateas an will example, be smaller the than effect when of thethe flight residual orbits ionospheric are at highdi-orderfferent term altitudes. on the group delay will be less than 1 mm. Moreover, when the flight orbits of the satellite are at the same altitude, the ionospheric change rate will be smaller than when the flight orbits are at different altitudes.

Remote Sens. 2020, 12, x FOR PEER REVIEW 18 of 21

6. ConclusionsRemote Sens. 2020, 12, 3340 18 of 21 In this paper, for a network with a large number of satellites or aircrafts, we propose a BOCcos and 16APSK6. Conclusions integrated signal in the TDD mode. The signal can achieve TT&C and data transmission at the same frequency, which greatly improves spectrum utilization. The equipment can be easily In this paper, for a network with a large number of satellites or aircrafts, we propose a BOCcos and integrated and miniaturized. In addition, this paper improves the TDD mode, which is commonly 16APSK integrated signal in the TDD mode. The signal can achieve TT&C and data transmission at the used in mobile communication systems to increase spectrum utilization, so that it can be applied to same frequency, which greatly improves spectrum utilization. The equipment can be easily integrated the TT&C of intersatellite links. The combination of the two can significantly reduce the costs of and miniaturized. In addition, this paper improves the TDD mode, which is commonly used in mobile intersatellite links and inter-aircraft links. The acquisition and tracking process of BOCcos + 16APSK communication systems to increase spectrum utilization, so that it can be applied to the TT&C of is analyzed and simulated. Although the performance is reduced by approximately 2~3 dB, the intersatellite links. The combination of the two can significantly reduce the costs of intersatellite links acquisition and tracking process is fully applicable to high-CNR environments such as intersatellite and inter-aircraft links. The acquisition and tracking process of BOCcos + 16APSK is analyzed and links or inter aircraft links. In addition, to solve the problem in which the TDD mode is not received simulated. Although the performance is reduced by approximately 2~3 dB, the acquisition and tracking and received simultaneously, this paper uses an extrapolation method to cause the loop to keep process is fully applicable to high-CNR environments such as intersatellite links or inter aircraft links. tracking continuously without reacquisition, which reduces the constraint of the acquisition time on In addition, to solve the problem in which the TDD mode is not received and received simultaneously, the TDD switching period. The ranging extrapolation error threshold is less than 0.25Tc, and the this paper uses an extrapolation method to cause the loop to keep tracking continuouslychip without 1 frequencyreacquisition, extrapolation which error reduces threshold the constraintis less than of the. acquisition Finally, the timeranging on accuracy the TDD of switching the system period. 4T The ranging extrapolation error threshold is lesscoh than 0.25T c, and the frequency extrapolation ± chip· is lesserror thanthreshold 9 cm. In the is less future, than we will1 . improve Finally, the the ranging BOC signal accuracy acquisition of the system method is in less this than scenario. 9 cm. In the ± 4Tcoh In thisfuture, paper, we BPSK will-like improve is used the to BOC signal signal acquisition. acquisition Although method the in this method scenario. is simple, In this the paper, advantage BPSK-like is of narrowused correlation to signal acquisition. peak of BOC Although signal is the abandoned. method is simple, the advantage of narrow correlation peak of BOC signal is abandoned. Author Contributions: Conceptualization, X.L. and W.W.; methodology, Y.Y.; software, L.Y.; validation, L.X. and XAuthor.L.; formal Contributions: analysis, L.X.Conceptualization,; investigation, Y.Y.; X.L.resources, and W.W.; W.W methodology,.; data curation, Y.Y.; L.X software,.; writing— L.Y.;original validation, draft L.X. preparation,and X.L.; L.X formal.; writing analysis,—review L.X.; and investigation, editing, L. Y.Y.;X. and resources, X.L. visualization, W.W.; data curation,Y.Y.; supervision, L.X.; writing—original Y.Y.; project draft administration,preparation, W.W L.X.;. All writing—reviewauthors have read and and editing, agreed to L.X. the and published X.L. visualization, version of the Y.Y.; manuscript. supervision, Y.Y.; project administration, W.W. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Technology and Industry for National Defense, grant number No. 060301 and No.B0110Funding:. This research was funded by Technology and Industry for National Defense, grant number No. 060301 and No.B0110. ConflictsConflicts of Interest: of Interest: The authorsThe authors declare declare no conflict no conflict of interest of interest.

AppendixAppendix A A We giveWe the give simulation the simulation results results at aircraft at aircraft scenario scenario in Figure in Figure A1. And A1. we And get we the get same the sameconclusion conclusion as theas satellite the satellite scene. scene.

(a) The Doppler measurement result and tracking loop result at Tsp  10 ms ,  ce  0.01m and

 Doppler  78Hz .

Figure A1. Cont.

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(b) The Doppler measurement error at Tsp  10 ms , ce  0.01m and  Doppler  78Hz .

FigureFigure A1 A1.. TheThe measurement measurement results results at at aircraft aircraft scenario, scenario, CNR CNR = =6060 dBHz dBHz..

ReferencesReferences

1.1. Crisan,Crisan, A. A.-M.;-M.; Martian, Martian, A.; A.;Cacoveanu, Cacoveanu, R.; Coltuc, R.; Coltuc, D. Evaluation D. Evaluation of synchronization of synchronization techniques techniques for inter for- satelliteinter-satellite links. links. In Proceedings In Proceedings of the of 2016 the 2016 International International Conference Conference on on Communications Communications (COMM (COMM),), Bucharest,Bucharest, Romania Romania,, 9– 9–1010 June June 2016; 2016; pp. pp. 463 463–468.–468. 2.2. Wu,Wu, W.R.; W.R.; Dong, G.L.; G.L.; Li, Li, H.T. H.TEngineering. Engineering and andTechnology Technology of Deep of Space Deep TT&C Space SystemTT&C; China System Science; China Publishing Science Publishing& Media Ltd.:& Media Beijing, Ltd. China,: Beijing 2013;, China, pp. 143–194.2013; pp. 143–194. 3.3. Wu,Wu, W.; W.; Li, Li, H.; H.; Li, Li,Z.; Z.;Wang, Wang, G.; G.;Tang, Tang, Y. Status Y. Status and andprospect prospect of China’s of China’s deep deepspace space TT&C TT&C network. network. Sci. Sin.Sci. Inf Sin.. 2020 Inf., 502020, 87, –50127, 87–127., doi:10.1360/ssi [CrossRef-2019] -0242. 4.4. Veres,Veres, P.M.; P.M.; Gabányi Gabányi,, K.É. K.É; .;Frey, Frey, S. S. Very Very Long Long Baseline Baseline Interferometry Interferometry Observations Observations of of the the Proposed Proposed Radio Radio CounterpartCounterpart of of an an EGRET EGRET Source. Source. SymmetrySymmetry 20202020, 12, 12, 1516, 1516., doi [CrossRef:10.3390/sym12091516.] 5.5. KKinman,inman, P. P. Doppler Doppler tracking tracking of of planetary planetary spacecraft. spacecraft. IEEEIEEE Trans. Trans. Microw. Microw. Theor Theor.. Tech. Tech. 19921992, 40, 40, 1199, 1199–1204.–1204, doi:[CrossRef10.1109/22.141352.] 6.6. DelDel Portillo, Portillo, I.; I.; Cameron, Cameron, B.G.; B.G.; Crawley, Crawley, E.F. E.F. A A technical technical comparison comparison of of three three low low earth earth orbit orbit satellite satellite constellationconstellation systems systems to provideto provide global globalbroadband. broadband.Acta Astronaut. Acta 2019 Astronaut., 159, 123–135. 2019, [ CrossRef159, 123] –135, 7. doi:Foreman,10.1016/j.actaastro.2019.03.040. V.L.; Siddiqi, A.; De Weck, O.L. Large Satellite Constellation Orbital Debris Impacts: Case Studies 7. Foreman,of OneWeb V.L.; and Siddiqi, SpaceX A.; Proposals. De Weck, AIAAO.L. Large SPACE Satellite Astronaut. Constellation Forum Expos. Orbital2017 Debris, 5200. Impacts: [CrossRef Case] Studies of OneWeb and SpaceX Proposals. AIAA SPACE Astronaut. Forum Expos. 2017, 5200, doi:10.2514/6.2017- 5200.

Remote Sens. 2020, 12, 3340 20 of 21

8. Li, Y.; Wang, Z.; Tan, W. The frequency spectrum management for aerospace TT&C system. In Proceedings of the 2013 5th IEEE International Symposium on Microwave, Antenna, Propagation and EMC Technologies for Wireless Communications, Chengdu, China, 29–31 October 2013; pp. 595–600. [CrossRef] 9. Gu, X.; Bai, J.; Zhang, C.; Gao, H. Study on TT&C resources scheduling technique based on inter-satellite link. Acta Astronaut. 2014, 104, 26–32. [CrossRef] 10. Tianjiao, Z.; Jing, L.; Zexi, L.; Ming, X. An Algorithm Research of Ground-Space Integrated Scheduling TT&C Resources of Orbit Determination of GNSS Constellation. In Proceedings of the 2013 Fourth International Conference on Intelligent Systems Design and Engineering Applications, Zhangjiajie, China, 6–7 November 2013; pp. 69–73. 11. Yan, L. The Design and Implement of TT&C and Data Transmission Integrated System of Satellite; Nanjing University of Science & Technology: Nanjing, China, 2018. 12. Abdullah, A.; Elnoubi, S.; Banna, M.; Nasr, M. Implementation of multi-user detection for ds-cdma communications. In Proceedings of the Twenty-Second National Radio Science Conference, NRSC 2005, Cairo, Egypt, 15–17 March 2005; pp. 403–410. 13. Jovic, B.; Unsworth, C.; Sandhu, G.; Berber, S.M. A robust sequence synchronization unit for multi-user DS-CDMA chaos-based communication systems. Signal Process. 2007, 87, 1692–1708. [CrossRef] 14. Nie, S.J.; He, B.Z.; Wang, H.Z.; Qin, Y.F. Design of integration channel of ranging and data transmission. Mod. Electron. Tech. 2013, 36, 57–62. 15. Chen, S.; Sun, S.; Wang, Y.; Xiao, G.; Tamrakar, R. A comprehensive survey of TDD-based mobile communication systems from TD-SCDMA 3G to TD-LTE(A) 4G and 5G directions. China Commun. 2015, 12, 40–60. [CrossRef] 16. Aslam, M.; Jiao, X.; Liu, W.; Moerman, I. An Approach to Achieve Zero Turnaround Time in TDD Operation on SDR Front-End. IEEE Access 2018, 6, 75461–75470. [CrossRef] 17. Betz, J.W. Binary offset carrier modulation for radio navigation. Navigation 2001, 48, 227–246. [CrossRef] 18. Wang, H.-Y.; Juang, J.-C. Retrieval of Ocean Surface Wind Speed Using Reflected BPSK/BOC Signals. Remote Sens. 2020, 12, 2698. [CrossRef] 19. Pickholtz, R.; Schilling, D.; Milstein, L. Theory of Spread-Spectrum Communications—A Tutorial. IEEE Trans. Commun. 1982, 30, 855–884. [CrossRef] 20. Rong, L.; Yingfeng, X. A multi-aircraft cooperative UAV observe and control system. J. Terahertz Sci. Electron. Inf. Technol. 2016, 14, 706–711. 21. Liuqing, Y.; Qiangui, X. A UAV TT&C System for One Station Controlling Several Vehicles. Electron. Optics Control 2013, 20, 6–15. 22. Guo, S.; Huang, G.; Liu, B.; Gao, Y. Research of MAPSK modulation based on the nonlinear satellite channel. In Proceedings of the 2010 3rd IEEE International Conference on Broadband Network and Multimedia Technology (IC-BNMT), Beijing, China, 26–28 October 2010; pp. 1197–1201. 23. Manqian, Z.; Jian, L.; Bo, Y.; Guangnan, Z. Research on high order modulation for satellite communications. Electronic. Des. Eng. 2014, 22, 114–117. 24. Zhang, J.; Zhu, L.; Guo, Y.; Gou, X. A new method of demodulation for 16APSK/32APSK. In Proceedings of the Proceedings of 2012 5th Global Symposium on Millimeter-Waves, Harbin, China, 27–30 May 2012; pp. 477–481. 25. Saxena, T.; Jadon, J.S. Spectral analysis of Sine and Cosine BOC modulated signals. In Proceedings of the 2014 International Conference on Signal Processing and Integrated Networks (SPIN), Noida, India, 20–21 February 2014; pp. 734–738. 26. Lohan, E.S.; Lakhzouri, A.; Renfors, M. Binary-offset-carrier modulation techniques with applications in satellite navigation systems. Wirel. Commun. Mob. Comput. 2007, 7, 767–779. [CrossRef] 27. Machida, M.; Handa, S.; Oshita, S. Theoretical analysis of symbol error rates of (4,12) circular-signal-set constellation for some detection schemes. Electron. Commun. Jpn. Part I Commun. 2000, 83, 19–28. [CrossRef] 28. Bhuiyan, M.Z.H.; Söderholm, S.; Thombre, S.; Ruotsalainen, L.; Kuusniemi, H. Implementation of a Software-Defined BeiDou Receiver. In Proceedings of the China Satellite Navigation Conference (CSNC) 2014 Proceedings, Nanjing, China, 21–23 May 2014; Springer: Berlin, Germany, 2014; Volume 303, pp. 751–762. 29. Meng, S.; Yang, W.; Lu, W.; Liu, J.; Yu, J. Code tracking performance of DS/FH spread spectrum signal for TT&C. In Proceedings of the 2010 2nd IEEE International Conference on Information Management and Engineering, Chengdu, China, 16–18 April 2010; pp. 491–495. Remote Sens. 2020, 12, 3340 21 of 21

30. Yang, W.; Meng, S.; Wang, J.; Liu, J. Acquisition performance analysis of a synchronization scheme of DS/FH hybrid spread spectrum signals for TT&C. In Proceedings of the IEEE 2011 10th International Conference on Electronic Measurement & Instruments, Beijing, China, 16–19 August 2009; pp. 4–395. 31. Wen, L.; Yue, X.; Zhongliang, D.; Jichao, J.; Lu, Y. Correlation combination ambiguity removing technology for acquisition of sine-phased BOC(kn,n) signals. China Commun. 2015, 12, 86–96. [CrossRef] 32. Burian, A.; Lohan, E.S.; Renfors, M. BPSK-like Methods for Hybrid-Search Acquisition of Galileo Signals. In Proceedings of the 2006 IEEE International Conference on Communications, Istanbul, Turkey, 11–15 June 2006; Volume 11, pp. 5211–5216. 33. Xie, G. Principles of GPS and Receiver Design; Publishing House of Electronics Industry: Beijing, China, 2009; pp. 349–393. 34. Benedetto, F.; Giunta, G.; Lohan, E.S.; Renfors, M. A Fast Unambiguous Acquisition Algorithm for BOC-Modulated Signals. IEEE Trans. Veh. Technol. 2012, 62, 1350–1355. [CrossRef] 35. Wang, C.; Zhou, M. Novel Approach to Intersatellite Distance Measurement with High Accuracy. J. Guid. Control. Dyn. 2015, 38, 944–949. [CrossRef] 36. Kuang, K.; Zhang, S.; Li, J. Real-time GPS satellite orbit and clock estimation based on OpenMP. Adv. Space Res. 2019, 63, 2378–2386. [CrossRef] 37. Maciuk, K.; Lewińska,P. High-Rate Monitoring of Satellite Clocks Using Two Methods of Averaging Time. Remote Sens. 2019, 11, 2754. [CrossRef] 38. El-Mowafy, A. Impact of predicting real-time clock corrections during their outages on precise point positioning. Surv. Rev. 2017, 51, 183–192. [CrossRef] 39. Liu, G.; Guo, F.; Wang, J.; Du, M.; Qu, L. Triple-Frequency GPS Un-Differenced and Uncombined PPP Ambiguity Resolution Using Observable-Specific Satellite Signal Biases. Remote Sens. 2020, 12, 2310. [CrossRef] 40. Jia, X.; Zeng, T.; Ruan, R.; Mao, Y.; Xiao, G. Atomic Clock Performance Assessment of BeiDou-3 Basic System with the Noise Analysis of Orbit Determination and Time Synchronization. Remote Sens. 2019, 11, 2895. [CrossRef] 41. Xiangqiang, M. Research and Application of Satellite Orbit Prediction Method Based on osculationg Kepler Element; University of Chinese Academy of Sciences: Beijing, China, 2017. 42. Wenbing, W.; Rongfang, L. A new method of orbit prediction for LEO satellites using empirical accelerations. Chin. J. Space Sci. 2015, 35, 715–720. 43. Liu, X.; Wang, D.; Zhong, X.; Meng, Y. Research on the Inversion Method of USO Frequency Stability Joining GNSS and Inter-satellite Distance Measurement. In Proceedings of the China Satellite Navigation Conference (CSNC) 2016 Proceedings, Changsha, China, 18–20 May 2016; Springer: Berlin, Germany, 2016; Volume 390, pp. 3–13. 44. Hao, F.; Yu, B.; Gan, X.; Jia, R.; Zhang, H.; Huang, L.; Wang, B. Unambiguous Acquisition/Tracking Technique Based on Sub-Correlation Functions for GNSS Sine-BOC Signals. Sensors 2020, 20, 485. [CrossRef] 45. Kaplan, E. Understanding GPS:Principles and Applications, 2nd ed.; Artech House: Boston, MA, USA, 2006; pp. 153–240. 46. Parkinson, B.W.; Enge, P.; Axelrad, P.; Spilker, J.J., Jr. Global Positioning System: Theory and Applications; American Institute of Aeronautics and Astronautics (AIAA): Washington, DC, USA, 1996; pp. 409–433. 47. Divsalar, D.; Net, M.S.; Cheung, K.-M. Acquisition and tracking for communications between Lunar South Pole and Earth. In Proceedings of the 2019 IEEE Aerospace Conference, Big Sky, MT, USA, 2–9 March 2019; pp. 1–14.

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