What is ARPA?
ARPA is the abbreviated term for Automatic Radar Plotting Aids. This system was developed by Rokycany Research Center in Czechoslovakia to replace manual radar plotting.
The ARPA consists of computer workstations, called operator terminals which are located next to each radar screen for each sector. The data about ownship are entered into ARPA computer by hand or automatically by data link.
The information about the airspace in front of ownship is gotten from the radar screen picture, and also from other systems like IFF or FCS.
Who Developed ARPA?
ARPA was first developed by the US Air Force. AFSC (Air Force Systems Command) together with Stanford University formed a group called Tacair in the early ’70s, later re-named AIL Systems in the early ’80s. The AIL Systems team was responsible for developing ARPA and later also for producing a military version called Tacair.
The Basic Principle of ARPA Operation
When an operator gives a current position to ARPA and enters a track number, the computer searches in its track database for this track. If it finds a match it displays ownship symbol at the appropriate position on the map. The operator checks if it is correct or not.
If the entered data are wrong, he does not have to re-enter all of them. He can delete the last digit of the x coordinate and enter a new position. ARPA computer will calculate correct x coordinate value, y coordinate value is corrected automatically too.
If the track number is not in the database, the operator has to enter it manually. If he enters data for an ownship symbol that already exists, the existing one will be replaced by this track information. The same happens when two different tracks cross. The newer track will replace the older one.
ARPA computer can be operated in three modes: full automatic, semi-automatic and manual mode.
In manual mode operator enters his own data and uses ARPA functions only to show current positions of other tracks on the map. In both other modes, besides plotting functions, the operator can also give commands to the ARPA computer like setting up route points into its memory or entering the current position of ownship manually.
ARPA has another mode called track monitor mode. This mode is used for showing tracks in real-time on the map without drawing symbols there. It is also possible to enter new data to any track displayed on the map.
When radar data are not available, ARPA can still be used to plot ownship’s track number by using data from other systems. It is possible to enter ownship position manually or read it into the computer via a data link.
The same procedure is done with other tracks too. These tracks will be shown using Xs as symbols.
ARPA is able to manage tracks in memory with an unlimited number of tracks, but the maximum value is set by the following criteria:
The more tracks are in memory, the slower ARPA will perform. The minimum useful number of tracks in memory was 20 to 30 during the 1980s-1990s; nowadays it should be at least 50.
ARPA can store up to 50000 track points in memory, and the total number of points is only limited by available computer storage. This maximum value is usually not reached while plotting tracks due to the programming of the software.
However, if more than 5000 track points are read into the ARPA computer through a data link (e.g. from an ATC computer), this maximum value will be reached and the plotting of all data might become impossible.
Software developers should take care of such cases when new versions are under development.
What is ARPA Used For?
ARPA can be used for different purposes. But usually, it is used as a primary radar display showing ownship position on the map together with other aircraft or objects.
Radar information, which is not displayed on this screen, is only available to the ARPA computer itself where they are saved for future reference. The data can be later analyzed using different methods like searching for closest points or analyzing separate tracks.
ARPA is a navigation aid that displays ownship position on the map together with other data from whatever source they are available:
The pilot has always a complete picture of the whole airspace around his aircraft. He can see if there are any other aircraft in front of him and what distance they are.
He also knows if there are any other planes at the same altitude, what is their distance and where they are relative to his own aircraft (above or below).
ARPA can be used to display enemy tactical data like SAM emplacements, radar sites, etc. These data will not be displayed on the screen together with other aircraft maps, but they will be shown on the screen in different symbols.
ARPA automatically saves all data that come into it for permanent reference. It is possible to view these data later either on a radar display or computer monitor.
All tracks are displayed in time sequence so it is easy to see what happened when and where each track was at any given time.
What is the Difference Between ARPA and Radar?
Radar and ARPA are different technologies, even though they do the same thing. Radar was first developed during World War II for military purposes. The technology has been continuously improved ever since it came into use. Though it still has its advantages and disadvantages like any technology.
Radar can be used as a primary radar display (showing ownship position on the map together with other aircraft or objects), but also as a navigation aid (showing ownship position on the map together with other data like GPS positions).
ARPA is still based on radar technology, but it is improved to make its use more user-friendly and versatile.
ARPA was originally designed for military aircraft, but nowadays can be found also in commercial airliners. Radars in modern military aircraft operate on different frequencies than ARPA.
What is the Main Purpose of ARPA?
ARPA can be used to display ownship position on the map together with other aircraft or objects. Like a primary radar display, it has a complete picture of the whole airspace around its own aircraft.
The pilot always sees if there are any other aircrafts in front of him and what distance they are. He also knows if there are any other planes at the same altitude, what is their distance and where they are relative to his aircraft (above or below).
ARPA can be used to display enemy tactical data like SAM emplacements, radar sites, etc. These data will not be displayed on the screen together with other aircraft maps but shown in a separate section. All tracks are displayed in time sequence so it is easy to see what happened when and where each track was at any given time.
ARPA System Display Characteristics
A typical ARPA display range is 30NM. The maximum displayed range depends on aircraft, traffic density and required resolution. Aircraft altitude will also have an influence on detection range.
Low flying aircraft have a better chance to stay undetected than high flying ones, being more difficult to discern from ground clutter at longer ranges. The minimum range is typically one mile but can be as small as 300ft.
ARPA displays are typically color-coded to provide information on the different types of aircraft. For example, friendly ones are displayed with blue symbols and unfriendly aircraft (hostile) with red symbols.
A word of caution: not all military planes carry ARPA systems. You cannot depend on the red symbols to be hostile!
Advantages of ARPA
ARPA system has much more to offer than a separate C-scope. The following are the main advantages:
- All radar information is available at all times and can be easily seen by the pilot without switching any displays. With a C-scope, it is not possible to have a complete picture of ownship position on the map.
- ARPA has all the information available in one display, it is not necessary to switch between different displays (e.g. MFD and C-scope) like with an old-fashioned C-scope system.
- Using ARPA makes data recording easier for permanent reference later on! Each ARPA track point is given a number, like 1797. It is easier to find an old track if it has been recorded as opposed to having used a separate scope for recording. ARPA provides much better quality data because the system itself does not miss anything, like a separate scope.
- ARPA is standardized and used by many countries and therefore is interoperable with almost any other country’s ARPA system.
Disadvantages of ARPA
· The information shown on an old-fashioned C-scope is still missing. The following are the main disadvantages:
- No ‘picture’ around township more, only aircraft symbols with data like altitude and distance to ownship. This makes it difficult to detect other planes on the map than with an old-fashioned C-scope.
- Ground clutter (radar returns from the ground) makes it difficult to determine aircraft against the background.
- No altitude information can be given for ownship track points (how high is ownship at the time of plotting).
- Old fashioned C-scope has no data recording capabilities (which is one of main advantages of ARPA system).
Limitations of ARPA
In real life, even though ARPA is a very useful tool for air defense, it has some limitations:
- The Scan mode cannot be used with the Attack mode of the radar. It will automatically switch to fighter intercept mode which will prioritize targets as “Track”. If an unknown Scan contact is plotted on the map it will be treated as a Track, which is an aircraft with friendly intentions.
- A target plotted in the Attack mode can only be seen when in range. Therefore it is possible to lose contact after passing it. However, if the system is set in Range-expanded mode (RANGE/EXP), targets will be visible at a greater distance.
- Radar can lose track of an aircraft if it turns away from the line of sight. However, if RANGE/EXP is used this will not happen.
- The ARPA system has no “look-ahead” capabilities and therefore cannot show contacts that are in front of ownship’s or ownship’s formation.
- Ownship is always tracked on the map, even when it is not in range of a target! This means that information about ownship position will be depicted on the map at all times. There will be no “holes” on the track-display like with an old C-scope system.
- · The system cannot handle more than 6 targets in range. When the number of targets exceeds 6, the lowest priority contacts will be discarded.
- SV Range-expanded mode is not available when using ARPA (the A/A radar set).
Typical ARPA Warning Messages
There are basically 3 different messages that can be given by the ARPA system:
- “Track” (green). Track is an aircraft path on the map and it is not a threat to ownship. Aircraft is friendly or unknown.
- “Tune” (yellow). A threat is detected but not yet fully identified. Ownship should tune the radar to identify it! This does not mean that ownship will engage in combat with this aircraft, it just needs further investigation to determine friend or foe. Tuning is achieved by pressing either MODE/RESET or C-scope TUNE button.
- “Ident” (red). An aircraft is within firing range and should be treated as a threat! This means that ownship should identify the target and after identification engage it if needed.
Errors of ARPA
- “Tuning” error: An aircraft is within range and should be treated as a threat but it has not been identified yet. Ownship enters the battle without knowing who will be the opponent!
- “Track error”: This occurs when ownship trackpoint does not cross ownship’s future position on the map, making it possible for ownship to have a threat unnoticed until it is too late.
- “Range error”: When the range scale is not set correctly the ARPA result might be altered. If the radar detects an unknown aircraft at x distance but the map shows y, chances are that target range has been set incorrectly!
- “Altitude Error”: When the radar detects an unknown aircraft at x distance but the map shows y altitude, chances are that target altitude has been set incorrectly.
- “Pos Error”: When ownship trackpoint does not cross ownship’s future position on the map, chances are that ownship position has been wrongly determined.
- “Aircraft ID failure”: This occurs when ownship has not been able to determine the target identity.
ARPA is very useful. It can give more information than a simple Attack or C-scope radar system, but it has some limitations and errors which must be considered when using it.