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General Description

The High Definition Sounding System consists of dual Model ADD-9600 Automated Dropsonde Dispensers with a multichannel data receivers and several aircraft-mounted antennae. Each ADD system automatically prepares, deploys and collects data simultaneously from up to 45 in-flight devices dropped as frequently as once every ten seconds*. During free-fall, high resolution 4 Hz Pressure, Temperature, Humidity (PTU) and GPS winds are transmitted to the aircraft. At the end of the flight at splash, the XDD measures sea surface temperature via a calibrated IR SST sensor.

Principle of Operation

Up to 45 XDD expendable dropsonde devices can be loaded into an extended ADD magazine. The air blower provides two PSI of pneumatic force to help each XDD device clear the aircraft’s slip stream on ejection.

Simplicity is key to system reliability, and a single rotating drum with only one degree of mechanical freedom helps ensure reliable, jam-free operation. A rotary encoder monitors the precise position of the motor driven drum; XDD devices leave the magazine cartridge via one of three queues selected by the drum. Much like a revolving door, the drum creates a fail-safe pressure interlock, preventing loss of cabin pressure in the case of power loss or a mechanical jam on pressurized flights.

Proximity sensors provide cartridge load status to support dynamic mission changes. Remote interrogation and control of loaded XDD devices is made via non-contact bidirectional optical communications. Control functions include power on/off, GPS and meteorological sensor testing, and assignment of RF telemetry parameters. Before commencing a sounding, a band scan checks for the presence of interference.

* Even with four satellites in view, a cold GPS startup requires 30-60 seconds to acquire a 3D signal lock. Thus an initial “spin up” delay is required; thereafter, devices can be released as quickly as one every five seconds.


ADD-9600 interior view.


  • Simple and reliable automated operation has a single moving mechanical assembly
  • Provides real time data to web browsers
  • State-of-the-art 4 Hz Winds data rate
  • Sea Surface Temperature IR sensor
  • Accommodates up to 90 XDD dropsondes
  • Up to 80 XDDs can be in flight simultaneously


Model XDD-938 Digital Dropsonde devices telemeter calibrated 4 Hz GPS winds, and 2 Hz pressure, temperature, relative humidity and sea surface temperature (SST). Multiple channels, and time division multiplexing maximize the use of the RF spectrum. Prior to release, each XDD is assigned a time slot and frequency. Digital FEC telemetry ensures error-free 400 MHz UHF communications.

Mechanical Interface

An HDSS payload consists of a pair of 100 cm x 40 cm x 30 cm ADD-9600 dispensers, each of which physically mates with an aircraft drop tube. The dispenser assemblies are light enough to be dismounted and reloaded by a single operator, and on manned aircraft the magazine can be reloaded to facilitate continuous high resolution profiling.

Remote Control

A flexible control interface permits unattended upper air profiling, supporting a wide variety of mission scenarios spanning atmospheric research and operational hurricane profiling. XDDs can either be deployed “on demand” one at a time, as a sequential time series, or within an airspace based on preset coordinates. Once the release sequence is initiated, XDD preparation, release and telemetry functions are fully-automated. As telemetry is received, data are presented in real time to web browsers and stored locally in non-volatile solid state memory.

Instrument Development and History

Initiated in 2008, the R&D of the ADD was funded by the U.S. Navy’s Office of Naval Research; R&D of the XDD was funded by the National Oceanic and Atmospheric Administration in 2004. The HDSS system design is both simple and reliable, enabling very high time-and-spatial-resolution vertical atmospheric profiling for research and operational hurricane reconnaissance.


Block Diagram of System Electronics

Installation Requirements

The system was designed to support a variety of operational unpressurized aircraft, including the NASA B-57. Deployment aircraft must be equipped with:

  • Vertical 3” dia. round drop tube or adapter
  • External nadir-mounted 400 MHz antenna with low noise RF preamplifier
  • GPS signal replicator feed
  • 28 Vdc aircraft power (250 Watts maximum)

The system operates via standard 28 Vdc aircraft power and interfaces with an RF coax feed to an nadir-mounted external UHF antenna, top mounted GPS antenna and RJ-45 Ethernet to the aircraft’s 802.3 10/100 BaseT LAN. The system provides real time data access and control to any computer aboard with a web browser.


Side view of one ADD mounted in NASA P-3.

High Definition Sounding System

As shown in the following figures the NASA WB-57 HDSS payload deploys expendable XDD meteorological sensors via dual fully redundant Automated Dropsonde Dispenser (ADD-9600) units. Each ADD contains an embedded controller with network interface and dual multi channel telemetry receivers. Dual drop tubes are mechanically “deconflicted” such that neither ADD depends on the other, and they do not share RF spectrum as one spans 400-403 MHz and the other covers 403-406 MHz.. Equipped with magazine extenders, ADDs can be loaded with up to 45 XDD for a total of 90 drops per flight. Much like other aircraft systems, the HDSS provides reliability through complete hardware redundancy. Prior to flight XDD devices are loaded via the lower access doors, or by lowering the pallet itself to gain access to the top door.

Pneumatic XDD ejection reliably ensures XDDs are delivered well clear of the aircraft fuselage. We’ve reviewed the Rake Data report by Ru-shan Gao of NOAA’s Aeronomy Lab, dated Feb. 5, 2002. Based on that information, along with ground clearance data we are designing a 16” long by 4” diameter drop tubes that will be canted slightly at between 30-45° from vertical. The design of the tube is similar to tubes we have used on the Navy CIRPAS Twin Otter, the NASA P-3 and the NASA DC-8. Past experience we ensured XDDs would clear the fuselage outside the WB-57 slip stream and this was confirmed in November 2013 by the aft camera. A 5 PSI air line with cockpit air connect the HDSS air tank and pressurizes the magazines to about ~2 PSI at sea level. A blower enables clearing of loaded units and mass dummy test releases on the ground when the engines are off. Optical detectors in the ADD exit tubes enable the HDSS to measure both the speed and acceleration of each XDD drop. Users can manually set the master air control valve to modify airflow via the HDSS web control interface; enabling optimization at various altitudes.


HDSS payload mounted in WB-57 pallet

Two 16” long by 4” diameter drop tubes protrude at a slight canted angle from the bottom of the pallet with three blade antennae. Multiple 110 Vac 400 Hz blanket heaters regulate payload temperatures above the local dew point temperature.

Note while the HDSS is fully self-contained and remote controlled, if satellite communications are lost manual XDD release by the SEO cockpit operator can be effected via three switches wired to each ADD. One switch places the ADD in manual mode, which doubles as a drop-inhibit safety. The other two drop a device from either A or C lanes, enabling preloaded slow or fast fall mode XDDs to be deployed.

  • Overall HDSS weight i~255 lbs. less pallet
  • HDSS center of gravity (CG) location approximately the center of the pallet
  • Dimensional layout of the HDSS fits laterally within the 3’ x 5’ unpressurized WB-57 pallet, extending approximately 15” above sides
  • No laser, fluid, chemical, RF transmission, pressure/vacuum systems and no special handling requirements are required
  • XDDs weigh ~58g and ejected by air at ~7 meters/sec depending on KIAS and altitude

WB-57 HDSS high altitude test flights have simulated the flight environment of the NASA Global Hawk with wing mounted telemetry reception during maneuvers, ensuring compatibility with NASA Ku satcom links.


HDSS Network diagram layout - payload at left, satcom-linked ground equipment at right.

Network Description and Settings

The HDSS payload communicates via the WB-57’s satcom. NASA assigns static TCP/IP settings for the seven HDSS devices and can provided a portable VPN router box that enabling access during the flights to provide meteorological support. The necessary IP addresses that we need open bi-directionally to the WB-57 VPN during flights include the following addresses: TCP/UDP port 22 (ssh); port 80 (http); and port 3306 (mySQL replication). While the VPN provides dynamic addressing via a DHCP server, hosts aboard aircraft network require static Network addresses and connect to aircraft Ethernet via RJ-45.

HDSS Overstorm Observation Cameras

Dual low profile high definition fixed dome network cameras are mounted looking aft and forward to image XDDs leaving the drop tubes, viewed from forward/aft. Each camera has a static TCP/IP address and enables viewing drops to ensure adequate fuselage clearance is being achieved. Each camera connects to 802.3 Ethernet. The cameras combine wide-angle high resolution in dust/rain-sealed (NEMA-12/IPC-66) die cast metal housings. Built in DSP provides high dynamic range for good low level light performance in contrast limited viewing conditions. SD card storage stores imagery in case the satcom goes offline if satcom links go down or saturate. Key features include:

  • Megapixel high sensitivity MOS sensor w/ 720p HD images up to 30 frames per second (fps)
  • Full frame (up to 30 fps) transmission at 1,280 x 960 image size
  • MEGA Super Dynamic and Adaptive Black Stretch (ABS) technologies for 128x wider dynamic range
  • High sensitivity w/ Day & Night (Electrical) function: 0.8 lux (Color), 0.6 lux (B/W) at F2.2
  • Multiple H.264 (High Profile) and JPEG streams enable simultaneous real time monitoring
  • VIQS (Variable Image Quality on Specified Area) technology allows designated areas to retain higher image quality, enabling lower image file size and bit rate


Dual HDSS cameras provide simultaneous forward and aft looking views.