RF downlink serves ‘eye in the sky’
Public safety agencies adopt wireless video to provide airborne support for traffic monitoring and ground-based patrol units. Dispatch and control centers can view images in real time, which helps them direct operations more efficiently.
Just turn on the TV news, and you’ll see it. Live helicopter video feeds cover everything from beautiful sunsets to traffic jams, to the latest high-speed pursuit. It’s nothing new for the broadcast industry.
But the advantage of a bird’s-eye view is also of interest to public safety agencies. Law enforcement, in particular, has complemented ground-based patrol fleets with airborne support, especially through the use of helicopters. High intensity spotlights and night-vision devices enhance the pilot’s effectiveness even further.
Agencies that can afford to take things to the next level are equipping their helicopters with a video camera and downlink transmitter. By relaying aerial images to a central command and control center (CCC), operations officers can view and evaluate the downlink video in “real time.”
Public safety agencies had to look no further than the broadcast industry for answers on how to add downlink video to their arsenal of weapons.
The aircraft The helicopter is equipped with a gyro-stabilized video camera, as shown in Photo 1, below. Typical telephoto/zoom capabilities range from 143 to 243 operation. Cameras are capable of infrared detection for night operation and are most often manually controlled by an aircraft crew member. The camera’s video signal is fed to an onboard video monitor, a VCR and the downlink microwave video transmitter. The signal is then relayed to the ground-based CCC video receiver via a microwave antenna. The antenna housing may hold one or more antennas, each having different gain and pattern characteristics. This allows the airborne operator to select the antenna that performs best for a particular situation. The types of antennas usually found in downlink applications are omnidirectional, directional down-looking and directional side-looking.
Perhaps the most popular single antenna is the side-looking, high-gain antenna. Because the antenna is directional, it must be continually realigned as the aircraft moves to maintain an optimal signal to the CCC receiver on the ground. This task is accomplished by onboard antenna-control circuitry, an integral part of which is a GPS receiver. This receiver offers information relating to the aircraft’s current location at any point in time. When the control circuitry is programmed with the coordinates of the CCC, a reference azimuth is developed that allows the electronics to establish and maintain the proper aircraft antenna orientation.
The aircraft’s GPS information is also applied to a subcarrier input of the downlink transmitter. The transmitter is modulated with airborne video and the subcarrier (GPS) information.
The downlink transmitter is encased in a weathertight housing and attached to the top of the antenna assembly. Both the camera and antenna are mounted inside their respective radomes. This helps to minimize wind-loading effects and the detrimental effects of direct contact with the elements.
The command and control center The downlink video receiver at the CCC demodulates video and subcarrier signals. (See Figure 1 on page PS3). Both signals are routed to a device called a slave controller.
The slave controller uses the demodulated subcarrier (GPS) information to determine the current relative location of the transmitting aircraft. It then applies a voltage to the antenna rotator control line causing the ground-based antenna to turn directly toward the aircraft. The controller receives a feedback voltage from the antenna assembly, which it interprets and displays as antenna azimuth (in degrees) on its front panel.
The CCC antenna assembly most often comprises a directional antenna, rotator, RF bandpass filter assembly and low-noise amplifier (LNA). The assembly is typically mounted on the top of an antenna structure, to ensure an unobstructed 360 degree view, and encased in a weather-resistant radome, as shown in Photo 2 at the right.
The slave controller offers front-panel access to antenna azimuth and polarization (right-circular, left-circular, horizontal or vertical) controls, as well as LNA “on/off” settings. Downlink video receiver functions can also be accessed through the controller’s status and control lines. This allows remote manipulation of receiver operating parameters, such as RF channel and IF bandwidth selection, as well as relative display of the receiver AGC level.
If the downlink RF signal level is too low, the controller can be programmed to automatically remove the degraded video image and insert test pattern color bars. This is done so that viewers monitoring the feed at the CCC do not have to look at demodulated video noise. The controller then passes the downlink video signal (or inserted color bars) to a time base corrector (TBC).
When receive signals from the air unit are weak, the video image will be poor and it may also cause the picture to “roll” due to degradation of the recovered vertical synchronization signal. Because this can be annoying to the viewer, the TBC is used to strip away the demodulated synchronization signal and reinsert its own. This allows the picture to remain stable with regard to synchronization, even when the recovered video signal quality is poor. The processed video is then distributed within the CCC.
Audio from the air crew can be downlinked with the video signal by using a separate subcarrier input of the downlink video transmitter. Although this may be sufficient for broadcast applications, tactical operations typically require a two-way communications channel, such as that of an existing radio system. Audio from designated air-ground tactical channels can be mixed or applied separately to the left (L) and right (R) audio inputs of the VCRs and monitors at the CCC.
Remote receiver sites Whether the aircraft is behind a range of hills or just too far away, there can be times when it is not possible to establish an adequate point-to-point link directly to the CCC. When this occurs, remote hilltop receiver sites may be required. Like the CCC, remote sites are equipped with a downlink video antenna assembly, receiver and slave controller.
Although the equipment layout at the remote receiver site is similar to that of the CCC, additional equipment and considerations are involved.
First, there must be a method of transporting the demodulated video signal from the remote receiver site to the CCC. This is usually accomplished by a microwave link.
Second, if the video feed from the aircraft is analog and the microwave system is digital, an analog-to-digital conversion must be performed at the remote receiver site, prior to transport.
With today’s technology, full-bandwidth video can be transported over a digital network on two T1s. Moving Picture Experts Group video compression format 2 (MPEG-2), a current compression method used in the video industry, is incorporated into the process. The analog video signal is digitized, compressed and interleaved onto two bonded T1s. The T1s receive proper conditioning and line coding for transport over the microwave system.
Next, the airborne electronics controlling the helicopter antenna must be programmed for the coordinates of each remote receiver site. When the aircraft crew member selects the desired downlink site, the controls must orient the antenna accordingly.
Another consideration is that of remote control. Although remote control operation of local equipment at the CCC is desirable, it is not a requirement. When remote receiver sites are involved, however, it becomes necessary to develop some method of remotely controlling critical system components. See Figure 2, above.
A control station PC can be located at the CCC to communicate with local and remote-site equipment. Local communications can be accomplished through serial data lines or a common connection to an “in-house” LAN. Communications between the CCC and the remote site equipment is accomplished over the microwave system. Multiplex channels are used, with data modems on each end. Use of statistical multiplexers (stat mux) allows several remote, serial devices to be accommodated over the same multiplex channel.
The aforementioned capabilities of the slave controller are also valid for remote-control operation. The MPEG2 encoders can be accessed remotely for adjustment of picture resolution, video processing controls, noise reduction settings and pause/resume controls of the encoding process.
‘In-house’ processing & distribution At the CCC, downlink video from remote and local receivers can be monitored and recorded simultaneously. Multiple aircraft can cover the same event by selecting different downlink receiver sites. From the CCC, the various video feeds can be routed to separate monitors, VCRs, and finally to the inputs of a video switcher.
The switcher not only allows selection among downlink video feeds, but it also provides effects such as wipes, fades or inserts. A master or house sync source is required to synchronize all video sources feeding the switcher. This keeps the picture from “jumping” during the switching process.
The switcher output is connected to a program monitor and VCR, and distributed as desired. Because the downlink video can be of value to more than one public safety agency, it can be distributed over an interagency network. If the transport mechanism between agencies is digital, it must once again be encoded on the transmit-end and decoded on the receive-end.
Adding ‘eyes’ to ‘ears’ Although the broadcast industry has used downlink video for many years, public safety agencies are now just beginning to realize its potential for their own applications.
Law enforcement has been the first to begin using this high-tech tool. This is understandable, because it offers invaluable aid in surveillance and the making of crucial tactical decisions. Other agencies equipped with air fleets, that need to make their own tactical decisions for the well being of the public, will no doubt follow.
While current implementation of public safety downlink video systems has been limited to just a few of the larger organizations, long-term evidence of public benefit will no doubt result in increased federal or state funding for smaller agencies.
