MILITARY COMMUNICATIONS - LESSON
The nature of warfare has changed over time. However, certain aspects of battle have remained constant for centuries. Namely, the need to maintain effective command and control of troops, as well as the ability to communicate securely.

Modern day warfare requires military units to operate as smaller, more agile forces, across widely distributed geographic areas, in often rugged terrain.

These dismounted units frequently cannot maintain line-of-sight communications with headquarters (HQ), let alone with other units operating even a few miles away. 

This not only limits HQ ability to track dismounted soldiers but also imposes severe limitations on the warfighter’s situational awareness that is vital to both safety and mission success.

The tactical battlefield is now becoming a ground for extensive digital data exchange where many sensors, weapons, computers and command centers need to exchange high speed data in order to perform effectively and coherently. 

As the tactical domain is extremely fragile, with rapid mobility in a hostile and dynamic environment, modern day communication tools have reduced the gap between strategic, operational and tactical domains.

On the battlefield, a communications link is a lifeline-a vital connection to comrades & commanders. Whether one is signing on to radio or logging on to a laptop, one needs a network that works - the first time, every time.

Information dominance 

To function effectively, military command and control depends on a complex communication network of equipment, personnel, and communication protocols to relay information among forces. 

Communications networks are decisive in all aspects of global military responsibilities. Communication has always been important with in the military from day one. Without this communication no soldier would know where to be or when to be there. 

Military missions today are often focused on counter insurgency, nation building, and humanitarian aid, requiring warfighters to operate as small forward based units across ever increasing areas of responsibility.

Now more than ever before, commanders must know the location and status of their operational forces and warfighters must be able to call for additional resources, regardless of geographic location. The mission success heavily depends on it.

The dependence on information of all kinds has produced superior combat efficiencies and effectiveness. Today’s military uses significantly smaller and dispersed units to operationally control battlespace areas than in prior warfighting constructs. 

The shift to strategic small units is possible, in part, because of the significantly increased lethality of smaller units enabled by the use of ISR capabilities and precision weapons. This precision, however, depends largely on reliable communications. This overall change in operational concepts has become a fundamental shift in military thinking.

Due to highly multi-dimensional and unpredictable battlespace there is a need for quick decision making with a joint command and control structure to direct operations. 

Militaries require micro-processing and miniaturisation technologies complementing real-time intelligence and information sharing, distributed decision making, and rapid execution of orders from a wide variety of forces and systems for concentrated effect over the entire spectrum of operations. 

Tactical communication systems are of very high operational importance, providing integrated, secure, highly reliable and available communication services for forces, even in constraints like hostile environmental conditions, limited spectrum, time critical and non-linear progress of communication requirements, technical failure of equipment, partial destruction of mesh networks, and jamming by adversaries or events.

Throughout US history, advances in military capability have been fueled by innovation. All branches of the military consistently have managed to use technology in new and creative ways to gain an edge over the enemy.

The ability of US forces to gather, process, and disseminate battlespace information in a networked fashion has given them a tremendous advantage in major combat operations. This battlespace information has allowed US forces to move faster and apply military power more aggressively and effectively. 

Today’s networks enable shared situational awareness, unity of action, and enhanced shared understanding. Communication networks needs to be integrated seamlessly and interoperable to be of value despite the challenges of spectrum, bandwidth, low density and laws of physics.

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It really depends on the situation, mission, and planning. I assume your interest is mainly on the tactical side rather than the strategic side, so I will answer accordingly.

Here are short answers for each of your other questions listed in the question details: 

(1) The military pushes information to troops on the ground by various means: in person, through the radio, through video conference, phone, or through email; this largely depends on the availability of communication equipment, authorization, money, time, and qualified signal personnel to make this all happen. 

(2) Troops are typically notified of changing circumstances by those same means listed earlier in the form of Fragmentary Orders (FRAGOs) or a tasking order. They are typically given a task and purpose. The means in which these FRAGOs or tasking orders are given largely depends on technology and time. This is explained more fully below [1].

(3, 4, 5) For smaller units (squad to company size) with less equipment, radio is typically the primary means of communications. Small special units (such as the Special Forces, Long Range Surveillance teams, snipers, etc.) may have radio systems capable of using higher frequencies and satellites to communicate; these radio systems have a lot of capability (for example, a small team can set up a data and radio network with other radios and then stream video onto a connected computer with the proper equipment, software, and know-how). 

Battalion sized command posts (CP) may have the resources (attached signal units or an internal asset) to establish an internal LAN network with routers, switches, VX, call managers, VoIP phones, firewalls, Riverbed (acceleration), TACLANE (encryption), and the necessary cables to connect it all. They will then connect that network into their higher headquarters (HQ) and adjacent units by either satellite--if higher HQ is far beyond the horizon, or High Capacity Line Of Sight (HCLOS)--if higher HQ is close enough with no obstructions. If terrain allows and equipment is available, both satellite and HCLOS are set up for redundancy. 

Larger sized command posts typically will have the resources for satellite communications. They would also establish an internal LAN network and then tie that network into the cloud by satellite. This happens by the command post transmitting signal to the satellite; the satellite then relays that signal to a Gateway on the ground. From that Gateway, the signal is connected to the internet. The same thing happens the other way around.   

Now, if all these elements (including higher HQ) have to be moving, radio and satellite communications are typically used. Variants of the Blue Force Tracker (BFT) system (comprised of a hardened computer, keyboard, GPS, satellite antenna/terminal, mapping and texting software, and a touchscreen) uses satellites in order to communicate. By using the BFT, the ground troops can text each other and see their location on a digital map on the touchscreen. They can also plot symbols which represent located IEDs, enemy, boundaries, checkpoints, or anything else they choose to plot. Orders, MEDEVAC 9-line, and spot reports can be given through this very rapidly and precisely. Most vehicles now have the BFT system on board. If the BFT is not available, radios and precise orders are mostly used.

The reason why everyone doesn't just use radios or satellite comms are explained below as well [2].

(6, 7) Missions are typically explained through the orders process. This involves the Military Decision Making Process (MDMP) and Troop Leading Procedures (TLPs) depending on how high up the chain of command you are. I won't break this down, as that in itself would be a massive topic in itself. In short, both MDMP and TLP involve a Warning Order (WARNO); planning, preparation, and rehearsals for the mission at hand; reconnaissance; completing the order, briefing the full Operation Order (OPORD), executing the mission, and adapting as you go. All troops should receive the Operation Order, write it down, understand it, and be able to take charge and continue mission if the situation dictates. 

Both the WARNO and the OPORD contain a review of the Situation (including the enemy situation, friendly situation, weather, and terrain), Mission (the 5 Ws including task and purpose), Execution (how the mission will be conducted from start to finish), Sustainment (how the operation will be supplied, what the supplies are, and what assets are available), and Command and Control (where will the leadership be, what their frequencies are; what is the communications plan including the primary means, the alternate means, the contingency, and emergency).

Planning, preparation, and rehearsals call for contingency plans. It answers questions such as "If the enemy does this, how should we respond," and "If this communication method fails, how do we continue to communicate?" Troops practice how to respond to such situations prior to moving out so that they know exactly how to respond to those given situations. They also rehearse the mission itself to limit confusion, increase precision, and eliminate mistakes. 

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[1] Once they begin executing the mission, the situation is bound to change due to Murphy's Law. When that happens, they either execute the contingency plan if they foresaw such a problem occurring, or they adapt and improvise. The leadership makes a decision, assigns a task and purpose to the affected troops, and they carry out the order. If there is a change of mission or execution given by headquarters, that would come in the form of a Fragmentary Order (FRAGO). That is basically another OPORD, but the parts that do not change would be radioed in or typed in as "No change" instead of reiterating the same thing.    

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[2] Here, I'll explain why we use radios and satellites and what their capabilities are:

Tactical communications usually involve ground forces that need to communicate to higher levels using their own equipment (such at radios, satellite comms, and newly laid wire/cable) and not preexisting infrastructure (miles of fiber or copper trunks going between Tech Control Facilities, distributed at Area Distribution Nodes, and administrated/troubleshot at Network Enterprise Centers). 

Now, before we dive into the details, you'll have to understand a few things. LOS comms typically consist of tactical radios that transmit a short to medium distance, depending on frequency, power, and obstructions. Different frequencies allow the radio wave to either bounce off of materials, go through materials, or be absorbed by it. Typically speaking, the higher the frequency, the more data is being carried. The higher the power, the more distance is possible. However, when we say LOS comms, we really mean to say that the signal cannot go past the horizon. 

Additionally, you should probably know what frequency ranges are. Here are the various frequency ranges the military primarily uses:

(1) High Frequency (HF) is from 3-30 MHz. 
(2) Very High Frequency (VHF) is from 30 to 300 MHz. 
(3) UHF is from 300 to 3 GHz. 
(4) 3 to 30 GHz is Super High Frequency (SHF). 
(5) 30 to 300 GHz is Exteremly High Frequency (EHF).

Currently, the ground forces mainly use VHF or UHF for radio communications. They survive atmospheric noise and electrical interference well enough and they both go through walls decent enough (UHF likes to bounce a lot, though). But because it doesn't reflect upon contact with the ionosphere, it's not well suited to travel beyond the horizon. In some cases, UHF can skip off some charged particles in the atmosphere, so there are some over-the-horizon capabilities; however, the skip distance depends on how high the ionosphere is. VHF, on the other hand, likes to hug the ground. Because of these characteristics, VHF radio is typically used by ground forces to communicate in short distances or in open areas. This is suited for communications between ground forces in the immediate area--from squad leader to platoon leadership, from platoon leadership to company command, and from company command to the battalion CP. This all depends on how far all the units are from each other and whether or not the radios are in a location where they can transmit and receive the signals. The downside is that it doesn't survive obstructions too well.  Sometimes retransmit, or "retrans," signal units have to be used in order for the signal to be transmitted over a distance or over major obstacles (such as a tall mountain or hill) in order to reach other units or HQ. Since I'm not very familiar with UHF, I'll only expand a bit on VHF.

VHF is popular mainly because it is very easy to use. Ground forces really only need a radio, antenna, power source (battery, vehicle, or generator power), frequency hop (FH) channels or frequencies, and COMSEC to use it (I'll explain both below). Once everything is set up, no further adjustments are necessary until the COMSEC key expires and a new one has to be loaded. Since VHF somewhat goes through buildings, it works well enough in urban environments. The power can be adjusted to either lower the electronic footprint or raise the transmission distance. Additionally, it works very well on the move. Ground troops typically use an omnidirectional whip antenna (it propagates in all directions), and it works well that way--they don't have to worry about what direction to point the antenna and how it is polarized. The quality of audio transmission is high, so there's no need to talk slow and overly clear (like in HF). There's no satellite to track and no specific angle to use. All you have to worry about is whether or not your equipment is in working order and if your radio operators are well trained. Your fill (COMSEC key and FH channel) may get deleted due to operator error or equipment error. If this happens, you won't be able to talk to anyone until you get it refilled.

Now, you must be wondering what COMSEC and FH are. COMSEC is what encrypts the radio transmission and allows your radio to decrypt a signal with the same COMSEC loaded in a distant end radio. This prevents the enemy from hearing your transmissions. FH basically makes your radio jump a number of frequencies really fast. If another radio has the same timing as your radio along with the same list of frequencies to jump around to, they can be synced up and hear each other. This prevents the enemy from jamming your transmission and hearing you.  

Now, how does a unit communicate with their headquarters beyond the range of VHF or UHF? This consists of communications beyond the horizon. This is made possible by radio frequencies in the HF range that can bounce off of the ionosphere and ground at certain angles. This is also made possible by using satellites to relay signals. 

HF radio can go beyond the horizon by bouncing off the ionosphere at given angles, but the angle changes depending on many factors including season, solar activity, time of day, etc. Additionally, the maximum usable frequency changes because of those same factors. Typically, HF is seen more as a back up since it is difficult to set up, and you can't quite move very much. Additionally, it can't transmit much data and the audio quality isn't the best.

Satellite comms allows a lot of data transmission along with high audio quality due to the high frequencies. The military uses UHF, SHF, and EHF ranges for satellite communications. This mainly depends on equipment and availability of satellites. Typically, geostationary satellites are used--those are satellites that move along with the rotation of the earth above the equator; that way, they stay above the same area of the earth for the duration of the satellite's life. Because they do that, the ground personnel with the satellite dish has to point their dish at a specific point in the sky (where the satellite is), conduct a peak-and-pull to connect to and track the satellite with the correct amount of power and polarization. Once that happens, the operator can then begin transmitting and receiving data.

Since satellite communications are capable of transmitting a large amount of data due to the extremely high frequency, they can truly connect to a data network on the far end and truly provide decent-speed internet access, video streaming, you name it. The only issue is the availability and cost. 

Cost depends on the availability of the type of satellite being used. X-band satellites are military satellites that are technically free of charge for the military to use; however, they do not cover the entire planet. Because of this, sometimes the military has to use either Ku-band or Ka-band. Since these are mostly commercial, the military has to pay for such services. They also have to divvy up the connection for multiple users to use by either Time Division Multiple Access (TDMA) or Frequency Division Multiple Access (FDMA). FDMA is faster, but is more expensive. TDMA is slower. I haven't seen the military really use Code Division Multiple Access (CDMA) for their primary means of transmitting data; however, they do use CDMA with Iridium satellite phones. I can explain all forms further, but I do not believe that is necessary for this topic.