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干扰与异常 / Interference and anomaly


星上干扰 / Satellite interference

随着通信需求的增长,人们采用极化复用、空间复用和缩小轨位间距等手段,对静止卫星轨位和无线电频谱资源作了接近极限的开发利用。这种充分甚至略为过度的资源开发和利用,在不同极化、重叠服务区、或者相邻卫星的使用者之间引发难以避免的相互干扰。除此之外,设备故障和操作不当、以及人为因素,也是常见的干扰原因。任何有意无意的星上载波干扰都可能降低通信质量,甚至造成通信中断,给用户和卫星操作者造成很大的麻烦。为此,卫星通信业者有必要对星上干扰的原因作分类研究,并且总结排查经验。

With the rising demand ever, the orbital resource and frequency spectrum resource for GSO communication satellite has been almost fully exploited and utilized, by means of cross polarization, multi coverage, and orbital space reduction. Such overloaded exploitation will inevitably cause mutual interference between the users at different polarizations, overlapping coverage, and neighboring satellites. In addition, interference also appears because of device malfunction, improper operation, and other manmade reasons. Whether intended or not, any kind of interference may lower the quality of service, sometimes even interrupt the communication, and thus, it will bring serious trouble to the satcom users as well as the satellite operators. Therefore, it is necessary for us to analyze the satellite interferences, find out their reasons, and conclude the experience on how to deal with them.

要清除干扰,首先要寻找干扰源。通信卫星的服务区很大,一个点波束就能覆盖小半个中国,而上行干扰源可能存在于服务区内的任何地点。目前虽有可以准确查找干扰源地理位置的一些技术手段,但还难以普及应用到每个干扰分析过程。原因之一为检测手段需要辅之以某些客观条件,其次还在于较高的使用成本。

In order to remove a particular interference, we have to locate its source in advance. Normally, the service area of a communication satellite is very wide, even a spot beam can easily cover half of the landmass of China. However, the interference could be uplinked anywhere within the service area. At present, there are several technical measures to find the geographical location of the interference source. Nevertheless, it is not practical to apply such ways to every case. One of the reasons is that some particular conditions should be satisfied for the locating process, and another reason is its high cost.

传统的干扰排查主要依靠转发器管理者对各种干扰形式的了解、对用户业务的熟知、以及相关的分析经验。下文将粗略介绍常见的邻星干扰反极化干扰互调干扰由上行设备引起的载波干扰、以及非授权使用和恶意干扰等星上干扰,并对相关的分析处理方法作经验探讨。

Traditional measures in interference analysis and removal mainly rely on the engineer who managers the transponder, especially on his or her familiarity with various interferences, relative user carriers, and the incident removal experiences. The following text will briefly introduce common interferences from adjacent satellite, cross-polarization and inter-modulation, and the carrier interference caused by uplink device, and also launched unauthorized even intrusively. Meanwhile, the analysis and processing experiences are discussed.

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邻星干扰 / Adjacent satellite

静止通信卫星的轨位间距通常在2度到2.5度之间。工作频段相同的两颗邻星多半有共同的地面服务区。由于天线波束具有一定的宽度,地面发送天线会在指向邻星的方向上产生干扰辐射(上行邻星干扰),地面接收天线也会在邻星方向上接收到干扰信号(下行邻星干扰)。为了限制相互之间的干扰,两颗邻星的操作者会按照国际电联制订的《无线电规则》,对载波功率谱密度和地面天线口径作适当的限制。因此,在一般情况下,邻星干扰是可以容忍的。

The orbital gap between two GEO communication satellites is normally 2 to 2.5 degrees. Two adjacent satellites which operating at the same frequency band are usually covering the same terrestrial service area. Because an antenna beam has certain width, and the antenna on the earth will point to the adjacent satellites at its side lobe, the transmitting antenna may create interfering radiation (which is known as uplink adjacent interference), and the receiving antenna will also be interfered (which is known as downlink adjacent interference). In order to limit such mutual interference, the operators of the adjacent satellites will follow the Radio Regulations by ITU, to take appropriate measures and limit the power density of the carriers, as well as the antenna size. After such interference coordination, the adjacent interference will be normally tolerable.

当邻星的某个上行站大幅增加上行功率、或者上行天线偏向我星时,将会产生突发的上行邻星干扰。相应的排查手段为对比检查两颗卫星在干扰发生频段的频谱图是否存在相似的载波。如果相似载波有着较大的电平差,而且在邻星上的频谱电平远高于平均值,则很可能是干扰站的上行功率超标;如果相似载波的电平差不大,则有可能是干扰站出现天线指向偏差,或者使用了过小口径的上行天线。录得上行干扰的证据后,卫星操作者有权要求邻星的操作者限期解决干扰问题。

When an uplink station in adjacent satellite system increases its uplink power, or its transmitting antenna inclines towards another satellite, some additional uplink adjacent interference will be accidentally happened. To find out such interference, it is suggested to compare the spectrum in the interfering frequency bands for both satellites and then check whether the similar carrier existed or not. If the power levels of the similar carriers at both satellite are much different, while the power level of the suspected interfering carrier is much higher than others at the adjacent satellite, it is likely that the uplink power transmitted by the interfering station is much higher than normal. Otherwise, if the power levels of the similar carriers at both satellites are quite close, it may be caused by the antenna pointing error, or by an uplink antenna with ultra small aperture. After recording the evidence of such interference, the operator of the interfered satellite can then request the operator of the interfering satellite to resolve the problem in a given time.

突发性下行邻星干扰的产生原因多为地面接收天线的指向因故偏向邻星。在排除其他干扰可能性后,被干扰地球站的技术人员应考虑试行重调天线指向。笔者经历过的偶有例外是,工作寿命终结前的邻星,因燃料耗尽而轨位失控,一天中有两次飘近被干扰卫星,其间会有大段频谱遭受干扰。发生类似情况时,可以从互联网查找邻星的轨位监测或预测数据,并可在验证邻星轨位失控后,与其操作者进行交涉。

The accidental downlink adjacent interference is often caused by pointing error of a receiving antenna. After excluding other interference possibilities, the engineer of the interfered station should consider re-pointing the antenna. A rare exception was experienced by the author: once a satellite, which was close to the end of its life, had lost its east/west station keeping because of lack of fuel. It approached its adjacent satellite twice within a day. During that time, a wide frequency band of the adjacent satellite has serious interfered. If similar incident happens, one may look up the monitoring and prediction data of orbital position of the suspect interfering satellite from the internet, and then negotiate with its operator after confirmed the lost control of station keeping.

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反极化干扰 / Cross-polarization

为了充分利用有限的频谱资源,卫星通信采用正交极化频率复用方式,在给定的工作频段上提供双倍的使用带宽。中国通信卫星都采用由垂直极化和水平极化构成的双线极化频率复用方式,国际卫星组织和俄罗斯的一些C频段卫星则采用由左旋极化和右旋极化构成的双圆极化频率复用方式。

In order to increase the efficiency on using limited frequency resource, orthogonal polarization is normally adopted for frequency reuse, thus offering double bandwidth at the certain working band. Binary linear (vertical and horizontal) polarization is used by all Chinese communication satellites, and binary circular (right-hand and left-hand) polarization is often used by some IntelSat and some Russian satellites in C-band.

反极化干扰为工作在不同极化的同频率载波之间的相互干扰。为了避免反极化干扰,卫星天线和地面天线都应该满足一定的极化隔离度指标。卫星公司通常要求入网的地面发送天线在波束中心的交叉极化鉴别率(XPD, Cross-Pol Discrimination),即预期极化的发送载波与其反极化泄漏的功率比值,不低于33到35dB。

Cross-polarization interference is the mutual interference existed between the carriers at different polarizations with the same frequency. In order to avoid the cross-polarization interference, the antennas on the satellite and the earth station should satisfy certain requirement in polarization isolation. The qualification of XPD (Cross-Polarization Discrimination, the power density ratio of the expected polarization and its cross-polarization) requested by satellite companies for the user’s earth station is usually not to lower than 33 to 35 dB at antenna beam peak.

由于天线的极化隔离度有限,地面发送天线、卫星接收天线、卫星发送天线和地面接收天线都可能产生反极化干扰。卫星发射上天后,星上的接收和发送天线的极化隔离度已无从调整。因此,常见的反极化干扰通常为,因反极化地面发送天线的极化角失配、或因反极化载波的上行功率严重超标而产生的上行反极化干扰,以及因地面接收天线的极化角失配而产生的下行反极化干扰。上行反极化干扰通常只出现在一个或某几个载波上,下行反极化干扰通常影响整个接收频段。

Because of the limited XPD, the transmitting antenna on the ground, the receiving and transmitting antennas on the satellite, and the receiving antenna on the ground, may all cause cross-polarization (cross-pol) interference. After a satellite was launched, the XPD of the antennas onboard could no longer be adjusted. Therefore, the uplink cross-pol interference that we meet is normally caused by the polarization angle mismatch of ground transmitting antennas, or serious overdriven of the uplink power at cross-polarization. And the downlink cross-pol interference is normally caused by the polarization angle mismatch of ground receiving antennas. Uplink cross-pol interference is usually occurred at one or several certain carriers, and downlink cross-pol interference usually affects the whole receiving frequency band.

排除反极化干扰的步骤通常为,判断干扰源自上行抑或下行,然后调整失调天线的极化角。极化角调整前可能需要调整天线的对星指向。上行干扰或许还要检查并调整过高的上行功率。

The procedure to remove cross-pol interference is usually to determine where the interference come from, and then to adjust the polarization angle of the relative uplink or downlink antenna. Before the polarization angle adjustment, the antenna pointing may need to be adjusted in advance. For uplink cross-pol interference, the power level of the uplink carrier is probably necessary to be checked and adjusted, if it is too high.

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互调干扰 / Inter-modulation

卫星转发器和地球站设备中的功率放大器均为非线性放大器,当它以接近饱和功率放大多个载波时,载波之间产生的互调分量将抬高噪声底,从而降低输出信号的载噪比。避免非线性放大器产生互调干扰的措施唯有限制输出功率,使放大器工作在线性区。

The power amplifiers in satellite transponders and earth stations are all non-linear ones. When a non-linear amplifier drives multi-carrier at a high output power level that close to the saturation point, it will create intermodulation products and raise the noise floor, thus the carrier-to-noise ratio (C/N) of the output signal will be decreased. The only way to avoid and lower the intermodulation created by a non-linear amplifier is to limit the output power, so as to keep the amplifier working linearly.

为了避免地球站设备产生的上行设备互调干扰,卫星公司通常会对用户地球站功放的互调特性预作验证测试,限定用户功放在多载波条件下的最大可用输出功率。为了避免转发器工作在非线性区而引入的转发器互调干扰,卫星公司通常会根据用户载波的占用带宽,限定每一个载波的上行功率。卫星公司一旦在转发器频谱的日常监测中发现互调干扰,将要求相关用户降低上行功率,从而使所有的用户都能合理有效地使用转发器的功率资源。

In order to avoid uplink intermodulation interference caused by earth station equipmentssatellite companies will usually arrange verification tests on intermodulation specifications for the power amplifiers. The test is prior to an amplifier becoming into use, to limit its maximum usable output power under multi-carrier condition. In order to avoid transponder intermodulation interference caused by non-linear operation, satellite companies will also determine the uplink power for each user’s carrier, relatively to its bandwidth. Once an intermodulation interference is found by daily spectrum monitoring, the satellite company will ask the related user to decrease its uplink power, thus insuring that all the users can use the transponder power resource reasonably and effectively.

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上行设备干扰 / Up-link device

上行设备在建站时的疏漏和运行中的故障都是载波干扰的常见原因。其表现主要有,因中频电缆屏蔽不佳而产生的串扰,因地球站上下行通道存在耦合而出现的中频转发,以及因上行设备频率源失锁而引致的跳频或扫频干扰。

Carrier interference is often caused by careless installation or malfunction of uplink devices. It is mainly appeared as the electromagnetic wave coupled into less shielded IF cable, the IF retransmit caused by the coupling between uplink and downlink in an earth station, and the frequency hopping or sweeping interference caused by an unlocked uplink frequency resource.

调频干扰:最常见的中频串扰。调频声音广播的频率范围为88到108MHz,与卫星通信常用的70MHz中频设备的频率范围(62到88MHz)相邻,与140MHz中频设备的频率范围(104到176MHz)部分重合。由于两种中频带通滤波器的带宽范围都会覆盖部分调频声音广播的工作频段,当中频电缆频蔽不良、或者电缆插头座耦合欠佳时, 串入的地面调频广播载波可能随同地球站的中频上行载波,经过变频放大,被发射上星。调频干扰的载波特征为,数十kHz带宽的调频波,三两个同时出现,其频率间隔通常为200kHz的整数倍。因为串扰的耦合程度时有变化,干扰间或出现,载波幅度也不稳定。查找干扰源的方法通常为,用含调频解调功能的频谱仪监听调频广播内容,根据调频台的地名,逐个排查干扰频段附近载波中的同城用户上行站;或者根据调频台频率、干扰载波频率、以及用户载波中频频率之间的相对关系,推算干扰出自哪个上行站。为了保证监听效果,必要时需要暂时关闭被干扰载波。

FM interference is the most common IF coupling through less shielded IF cable. The frequency band of FM broadcast is 88-108 MHz, which is adjacent to the frequency range of 70 MHz IF device (62-88 MHz), and partially overlapping with the frequency range of 140 MHz IF device (104-176 MHz). Since both kinds of IF bandpass filters cover parts of FM frequency band; if the IF cable is not well shield or connected, FM signal will couple into the cable, be up-converted and amplified with the IF carrier, and eventually up-linked to the satellite. The FM interference has the feature that, several FM carriers of some ten kHz bandwidth appear simultaneity, while their frequency spacing is usually integer times of 200 kHz. During the variation of coupling extent, FM interference appears occasionally with unstable amplitude. Normally, there are two ways to find FM interfering source. If the interfering location could be found by monitoring the content of FM broadcast by a spectrum analyzer with demodulation function, the interfering station can be found from the uplink stations at the same location and using the neighboring frequency of the interference. Otherwise, interfering resource can be also found out by checking the frequency relation among FM carrier, suspicious interfering carrier, and IF frequency setting of the suspicious uplink station. To ensure the monitoring effect, the interfered carrier might be asked to turn off temporarily.

其他中频串扰:偶见。由雷达等地面无线电信号串入地球站上行链路所造成的干扰。与调频干扰相似,载波幅度不稳定,干扰间或出现。曾有站点多位于机场的用户苦受类似干扰。当受扰业务转移到另一颗卫星后,干扰也随之而去。

Radar and other radio signals will accidentally cause IF coupling interference. Similar to the FM interference, IF interference appears occasionally and its amplitude varies unstably. Once there was a satcom network including some uplink stations located in airports suffered from such interference. When the network migrated to another satellite, the interference also migrated with the carriers to the new satellite.

中频转发:在某个或多个上行站中,带宽通常为36MHz的整段中频下行信号被耦合到上行通道,发送上星。其现象为,整个转发器或者相邻转发器的大段频率范围之中,所有载波的载噪比看似正常,但Eb/N0远低于正常值。上下行信道之间产生耦合的原因有多种,避雷设施的安装不当也是其中之一。判断转发器存在中频转发的方法为,让其中的某个上行站发射一个单载波,并且在与该载波对应的下行频段观察是否出现多个频率递增(或递减)、幅度递减的单载波。(频率递增或递减的原因为,地球站的上、下行频率差与卫星的上、下行频率差有着微小的差别。)若是,则需要逐个排查相关频段中的所有上行站。

IF transition is the interference that, in some uplink stations, IF downlink signals are coupled into the uplink channel, and then transmitted to the satellite. In the interfered frequency range at the whole transponder even the adjacent one, the C/N of all carriers seem normal, but their Eb/N0 are much lower than normal. Coupling can happen at different places of the uplink and downlink channels. One of the cases is caused by improper installation of lightening protection device. The method for checking IF transition interference is that transmitting a pure carrier, and monitoring it at downlink. If the pure carrier becomes several ones with increased or decreased frequency and decreased amplitude, such interference surely exists. (The frequency increase or decrease is because of the differences between uplink/downlink frequencies at the earth station and at the satellite.) To find out the interfering station, all the uplink stations in the related frequency band should be checked one by one.

跳频或扫频干扰:呈离散谱线形式、或者貌似正常数字调相波的干扰载波,以跳频方式干扰某几段频率范围,或以扫频方式干扰大段频率范围。跳频或扫频的原因大多出于上变频器中的频率源失锁。对于看似正常的干扰波,可以检查是否有频谱相似的同转发器或者邻近转发器载波,与干扰载波交替出现。若是,则应暂停该载波的上行,并且长时间观察载波频率是否稳定。对于频谱不规则的干扰波,则只能从最近工作不正常的站点、最近关闭的站点、乃至现有或过往用户关闭已久的站点中,大海捞针般逐个盘查。

Frequency hopping or sweeping interference presents at some discrete spectrum lines or a normal PSK carrier. The interference hops or sweeps at one or several frequency bands. All the carriers working in the frequency bands will be affected. It is usually generated by an unlocked oscillator of up converter. If the interference looks like a normal carrier, it is suggested that check whether there is a similar carrier in the same transponder or the neighboring transponder, appearing with the interference alternatively. If so, this carrier should be shut down, and its frequency stability should be observed for a long period. To remove an interference with irregular spectrum is a hard job. All the uplink stations operating at the relative frequency band should be checked one by one, especially the ones working improperly, the ones closed recently, and even the ones have been off for a long time and belong to a past customer.

为避免潜在的麻烦,用户在开通载波时,最好在现场测试上行频段是否存在无关载波,转发器管理者也应观察新开通用户的相邻频段是否随用户载波出现干扰信号;用户在关闭载波时,不能只关断中频载波,一定要关闭射频设备的电源。

To avoid potential troubles, when a new carrier begins its service, the neighboring frequency band should be observed to find any irrelevant carrier existed or not. Satellite control center could monitor it at downlink, and the uplink site could also check it at uplink frequency band. In addition, to remove a carrier, switching off IF carrier is not enough, but AC power of the RF devices must be turned off as well.

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非授权使用与恶意干扰 / Unautrorization and intrusion

通信卫星的服务区通常能覆盖一个大国、乃至几乎所有的可见陆地。由于使用者可在服务区内的任意地点发送上行载波,而转发器管理者却很难判定载波的上行站点,卫星转发器难免出现被未经授权者偷用、或者遭受恶意干扰的情况。非授权使用和恶意干扰都是非法行为。

非授权使用大多出于试验目的。有经验者通常将偷用载波发在两个转发器之间的保护带、或者转发器范围内的空闲频段上。除了会额外占用转发器的功率资源外,这类偷用载波对正常用户的使用并未产生太多的不利影响。作为例外,偷用载波可能误占业务量稀少的用户TDMA或DAMA频率池,也曾有研究单位在用户载波工作频段试发CDMA载波,这类偷用将对用户载波产生直接干扰。

为了保障自己的合法权益,卫星公司可以考虑对非授权载波施放干扰。但因部分不法偷用者会采用你扰我躲的手段。他们在仓促改变上行频率时,还可能无意中干扰到正常用户。为此,卫星公司往往不愿意多启事端。如果只需做短时间窄频段的实验,卫星通信行业中的潜在用户往往可以经与卫星公司商洽,将偷用转为试用,从而避免双方的冲突和麻烦。

恶意干扰通常出于经济或政治目的。曾有涉嫌商业利益冲突的用户载波被同行干扰、以及未完成干扰协调的转发器资源被其他卫星公司干扰的情况,也曾有用户载波被敌对国家或组织干扰的事例。

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地面干扰 / Terrestrial interference

为了有效利用无线电频谱资源,并且避免不同业务之间的相互干扰,国际电联在《无线电规则》中,对无线电频谱作了细致的分段应用安排。因为有限的资源难以满足众多的应用需求,卫星通信所常用的C和Ku等频段也被分配给地面微波等其它业务所使用。除了同频段不同业务之间的互扰外,其他频段业务的高次谐波也可能干扰卫星通信业务。潜在的干扰源包括常见的地面微波,以及电台雷达高频电气设备高压输变电设备和供电线路等等。为了避免卫星通信天线在主瓣或旁瓣方向上接收到这类地面干扰信号,地球站选址时应对微波环境作相关测试。

由于飞行物体的遮挡和反射等也可能干扰卫星通信系统的正常工作,地球站选址时还应使接收天线的指向避开可能有飞机频繁起降或经过的机场和航线。

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地面微波干扰 / Micro-wave

由于卫星通信与地面微波通信共用C频段(6/4GHz)和Ku频段(14/12GHz),卫星通信地球站发送的上行载波可能干扰同频段的地面微波通信线路,地球站接收的下行载波也可能受到同频段地面微波信号的干扰。为此,工作在C频段和Ku频段的地球站在选址建站时,应对视距范围内的地面微波站点作调查,或对天线指向的微波环境作测试,以求避免干扰。必要时,还需向当地的无线电管理部门提交干扰分析报告。

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电波衰落异常与日凌干扰 / Propagation fade and sun outage

在较低的天线仰角条件下,电波在地球站与卫星之间的单向传播距离可能大于三万九千公里,卫星通信的电波传播损耗以上行和下行链路的自由空间损耗(C频段接近200dB,Ku频段略高于200dB)为主。除了基本恒定的自由空间损耗之外,电波传播还会遭受衰落异常的影响。后者起伏不定,而且大多产生于距地球表面50到400公里的电离层、以及10公里以下的对流层。

电波传播的衰落与异常主要为电离层和大气闪烁、以及降雨影响。考虑得再细一些,C频段和Ku频段卫星通信存在着通常不高于0.5dB的大气吸收和损耗,低天线仰角的应用会受大气折射和散焦的影响,使用低增益天线的移动终端业务还可能发生多径衰落现象。

除此之外,卫星通信载波在每年的春分和秋分期间还会遭受两次日凌干扰

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电离层闪烁和大气闪烁 / Ionosphere scintillation and aerosphere scintillation

电离层闪烁是电波通过电离层时所受到的不规则变化影响。据资料记载,电离层闪烁多发于赤道附近和高纬度地区,1GHz以下的低频段较容易受影响,而且夏季子夜的衰落值较大。也有例外,我国的中纬度地区属于闪烁增强地带,日本冲绳也曾记录过12GHz信号的3dB衰落。

大气闪烁是因大气折射率的不规则起伏所引起信号强度的微小变化,大口径天线在低仰角时的变化量为零点几个dB。

笔者曾在1990年代末的夏季深夜,应深圳用户的要求,在香港观察到类似现象。与常见的信号衰落或干扰现象不同,整个转发器的载波频谱和噪声底同步缓慢波动,载波的载噪比在频谱波动期间未见明显改变,波动变化的周期约为数十秒钟到数分钟,波动范围约为-4dB到+2dB。载波和噪声底均出现比正常值高2dB的现象,似乎意味着电波的传输损耗在短时间内明显低于正常值,或者说传输通道短暂出现某种放大作用。在地域相近的用户和笔者连续两夜观察到同一现象后,曾要求位于京沪的另两家用户参加第三夜的协同观察。遗憾的是,由于该现象在事隔多日后才重复出现,四地同时观测的安排未获成功。

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降雨影响 / Rain fade

无线电信号的频率越高,波长就越短。频率为10GHz以上的Ku和Ka等高频段微波的波长仅为10到30毫米,与直径为数毫米的雨滴有一定的可比性。因此,Ku、Ka、以及更高频段的无线电波在穿越雨区中的密集雨滴时,将遭受严重的传输损耗。除了信号强度会因降雨而遭衰落外,地面接收系统的噪声温度会因降雨而增加,电磁波的极化角也会因降雨而被改变。

有关降雨对电波传播的影响以及降雨衰耗的估算方法可参见降雨衰耗估算页面。

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日凌干扰 / Sun outage

每年春分和秋分的前后几天,静止卫星所在的赤道平面与地球绕太阳公转的轨道平面大致重合,地球站、卫星和太阳会在一天中的某个时刻连成一条直线。这时,地面天线从卫星接收到的载波信号将被淹没在因太阳辐射而产生的热噪声中,这种现象被称为日凌干扰(sun outage)。

地面接收信号因日凌干扰而产生的载噪比恶化量在10dB以上。因此,日凌的最大持续时间大致可以估算为太阳角直径扫过接收天线10dB波束宽度的时间。太阳的角直径约为0.5度。天线的10dB波束宽度约等于125l/D(l为电波波长,D为天线口径)。口径较小、工作频率较低的天线,因为波束宽度较大,遭受日凌干扰的时间也较长。C频段3米天线的10dB波束宽度约为3.1度,受日凌干扰的最大持续时间约为14分钟。

各家卫星公司都在其公司网页上提供计算工具,供用户估算不同地理位置相对该公司某颗卫星的日凌发生时间。国内主要城市对常用卫星的日凌时间表及相关说明可参见中国十城市日凌预报表。预报表收录了2000年秋季和2001年春秋季受日凌影响日期及时刻的中点。在预报日期的前后几天,不同口径和工作频段的天线也会遭受数分钟到十数分钟不等的日凌干扰。预报表的数据虽然老旧,但对比表中的两份秋季预报可发现,前后两年的日凌发生日期和时刻变化不大。比较结果表明,老用户完全可以按照前一年的日凌日期和时段作准备,未必需要在每个春秋都找卫星公司重作计算。

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卫星工作异常 / Satellite anomaly

通信卫星采用高可靠性设计,有效载荷中的接收机和高功率放大器等关键部件还备有用于故障替换的备份电路。一般说来,只要渡过风险最高的发射、(太阳能电池板和天线)展开和定轨过程,再经过半年到一年的正常使用(即通过一到二次地影期间的充放电考验),卫星所面临的问题只在于太阳能电池的效率降低和燃料的消耗,出现致命风险的可能性已大大降低。但是,转发器增益有可能受某种原因而得到改变,卫星的轨位姿态控制问题会影响用户网络的正常运行,复杂而不稳定的空间环境也有可能影响卫星的工作状态。

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转发器增益变化 / Transponder gain setting

转发器增益档复位:通过调整通信卫星转发器中的信道控制单元(CCU, Channel Control Unit)(又称通量控制衰减器,FCA, Flux Control Attenuator)的放大量,可以调整转发器的增益。当通信卫星受到某种空间环境的影响时,可能发生单粒子翻转事件,存储某个转发器增益档设置值的状态锁存器可能因干扰而被复位。在一般情况下,这将使该转发器的增益下降到最小值。由于卫星的遥测信号通常不会反映转发器受干扰而引起的上述状态变化,卫星测控人员无法从遥测数据中得知相关转发器的工作状态已被改变。

因为载波电平大幅下降,发现业务中断或者系统余量降低的用户会在第一时间投诉。从频谱监测图可以发现,整个转发器的载波电平和噪声底电平都将明显低于未受影响的相邻转发器,但载噪比大体维持不变。相应的故障处置为,发送遥控指令,先使增益档复位,然后再将其调回应有的档位。如果判断正确的话,在作增益档复位操作时,转发器的频谱不受改变;在将增益档调回原位的过程中,转发器中的载波电平和噪声底电平都将同步提高。故障恢复后,还应逐个检查载波频谱,要求在故障过程中提高上行功率的用户将电平调降回正常值。

转发器边带受相邻转发器的影响:设在自动增益控制状态的Ku频段转发器增益,将随上行载波功率而改变。当用户开关载波或者大幅度调整载波功率、以及卫星接收的载波功率在强降雨过程中急剧变化时,转发器的增益都将随之而大幅波动。当通信卫星转发器中的带通滤波器的滚降特性不够理想时,转发器的放大作用可能影响相邻转发器的边缘频段。因此,位于转发器边带的载波电平和噪声底电平,会在相邻转发器的增益急剧变化时,随之产生高低变化。虽然受影响载波的载噪比未遭改变,但是大幅波动的载波电平可能超出接收设备的动态范围,从而影响接收效果。

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卫星轨位异常 / Station keeping

因受太阳和月亮的扰动、以及地球重力分布不均匀的影响,静止卫星轨位在不受人工控制时,会在南北方向产生周期性变化,并在东西方向出现缓慢漂移。因此,卫星发射重量的相当部分为用于轨位控制的推进器燃料。

在卫星寿命末期,为了节省燃料以延长工作年限,卫星操作者可能采取放弃南北控制的手段。这时的卫星将处于倾斜工作状态。从地面看上去,卫星好像以二十四小时为周期,在南北方向上走8字。如果地面天线的指向不变,因为卫星周期性地偏离天线波束中心,载波幅度将呈波动变化。地面天线的口径越大,波束越窄,越容易受影响。为此,大口径天线必须工作在自动跟踪状态。长此以往,地面天线驱动装置中的丝杠,有可能出现局部范围的严重磨损。

当接替卫星因故迟迟不能发射时,应退役的卫星有可能在寿命终止前的短时期内放弃东西控制。这时,不但会加重上述载波波动影响,还将对邻星业务产生相当严重的下行干扰。

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卫星姿态异常 / Attitude control

现代通信卫星多采用三轴稳定的姿态控制方式,使赋型波束按照设计要求,准确地覆盖在服务区上。三轴稳定卫星的俯仰轴(Pitch)垂直于轨道平面,滚动轴(Roll)与轨道相切,偏航轴(Yaw)正交于俯仰轴和滚动轴。当卫星姿态不稳定时,天线波束在地球表面的覆盖情况会遭受改变。卫星的俯仰轴偏差将使服务区产生东西偏移,滚动轴偏差将使服务区产生南北偏移,偏航轴偏差将使服务区产生轴向转动。姿态轻微异常时,只有服务区边缘用户受影响;严重时,全体用户都会受影响,甚至造成业务中断。

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空间环境引起的卫星故障 / Space environment

通信卫星工作在很不稳定的空间环境中,空间灾害性天气可能对卫星通信产生严重影响。NASA于1996年8月出版的参考文献《Spacecraft System Failures and Anomalies Attributed to the Natural Space Environment》罗列了1973年6月至1995年3月的100多宗在轨卫星和航天飞机因空间环境影响而产生的故障和异常事件。文献对因受空间环境影响而引起的在轨卫星姿控系统故障非姿控系统故障作了分类统计

下表为上述NASA文献内容摘要中的一些常用缩写词汇。

AKM 远地点发动机 P-Plasma 等离子体
CPE 控制处理电子 M/OD-Meteoroid & Orbital Debris 流星体/轨道碎片 
P/L PayLoad 有效载荷 N-Neutral thermosphere 中性热电离层
SAA 南大西洋异常 R-Radiation 辐射
SEU 单事件翻转 S-Solar 太阳
T&C 遥测遥控 T-Thermal 热
  G-Geomagnetic 地磁
 

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