A real time VHF downlink communication system is described for transmitting side-looking airborne radar (SLAR) data directly from an aircraft to a portable ground/shipboard receiving station. Use of this receiving station aboard the U.S. Coast Guard icebreaker Mackinaw for generating real-time photographic quality radar images is discussed. The system was developed and demonstrated in conjunction with the U.S Coast Guard and NOAA National Weather Service as part of the Project Icewarn all weather ice information system for the Great Lakes Winter Navigation Program

Chase, T. L.

Gedney, R. T.

Jirberg, R. J.

Kramarchuk, I.

Mueller, R. A.

Schertler, R. J.

English

NASA Technical Reports Server

General Disclaimer 
One or more of the Following Statements may affect this Document 
 
 This document has been reproduced from the best copy furnished by the 
organizational source. It is being released in the interest of making available as 
much information as possible. 
 
 This document may contain data, which exceeds the sheet parameters. It was 
furnished in this condition by the organizational source and is the best copy 
available. 
 
 This document may contain tone-on-tone or color graphs, charts and/or pictures, 
which have been reproduced in black and white. 
 
 This document is paginated as submitted by the original source. 
 
 Portions of this document are not fully legible due to the historical nature of some 
of the material. However, it is the best reproduction available from the original 
submission. 
 
 
 
 
 
 
 
Produced by the NASA Center for Aerospace Information (CASI) 
https://ntrs.nasa.gov/search.jsp?R=19790015142 2020-03-21T23:56:02+00:00Z
NASA Technical Memorandum 79164
(NASA-TH-79164) VHF DCWNLINE COMMUNICATION
	 N79-23313
SYSTEM FOR SLAB DATA (NASA) 10 F
HC A02/MF A01
	 CSCL 172
Onclas
IP	 G3/32 20942
i
VHF DOWNLINK COMMUNICATION
SYSTEM FOR SLAR DATA
R. J. Schertler, T. L. Chase,
R. A. Mueller, I. Kramarchuk,
R. J. Jirberg, and R. T. Gedney
Lewis Research Center
Cleveland, Ohio
Prepared for the
•	 Thirteenth International Symposium on Remote Sensing
of Environment
sponsored by the University of Michigan
Ann Arbor, Michigan, April 23-27, 1979
1
VHF DOWNLINK COMMUNICATION SYSTEM FOR SLAR DATA
R. J. Schertler, T. L. Chase, R. A. Mueller
I. Kramarchuk, R. J. Jirberg and R. T. Gedney
NASA Lewis Research Center
Cleveland, Ohio
ABSTRACT
This paper describes a real-time VHF downlink
communication system for transmitting side-looking airborne-
radar (SLAR) data directly from an aircraft to a portable
ground/shipboard receiving station. Use of this receiving
station aboard the U.S. Coast Guard icebreaker Mackinaw for
generating real-time photographic quality radar images will
be discussed. The system was developed and demonstrated in
conjunction with the U.S. Coast Guard and NOAA National
Weather Service as part of the Project Icewarn all-weather
ice information system for the Great Lakes Winter Naviga-
tion Program.
INTRODUCTION
Prior to 1969 the Great Lakes-St. Lawrence Seaways system was traditionally
closed to navigation during the winter ice season from mid-December until early
Airil because of the adverse effects of weather and ice. Since 1970, Great
Lakes shipping companies and twelve federal agen^ies led by the U.S. Army Corps
of Engineers and the U.S. Coast Guard have participated in a federally sponsored
program to demonstrate the practicality of extending the navigation system on
the Great Lakes into the winter months. The collection, analysis, and timely
dissemination of accurate information concerning the location, aerial extent,
type and thickness of ice within the Great Lakes was recognized as an essential
element in extending winter shipping operations. Storms, winds, and currents
produce rapid changes in ice cover. Due to the dynamic nature of the Great
L ykes ice cover, data more than 24 to 36 hours old is often of limited value
for aiding vessel navigation.
2. PROJECT ICEWARN
Project Icewarn was instituted as a result of the need for all-weather
near real-time ice information in the Great Lakes. The system developed to
meet these needs collects and distributes sidelooking airborne radar (SLAR)
imagery of Great Lakes ice cover to vessels as they move through the ice. The
project is a cooperative one involving the NASA Lewis Research Center, the U. S.
Coast Guard and the NOAA/National Weather Service. The system was designed by
NASA and jointly demonstrated by NASA, USCG and NWS during the 1974-76 ice
seasons. Since 1976 the system has been operated solely by the USCG and NWS
with only technical assistance provided by NASA.
The various elements of the Project Icewarn system are depicted in figure
1. The radar data are recorded aboard a Coast Guard HC-130B aircraft. The
side looking airborne radar system is the Motorola model A _N/APS-94 (X-band).
This system scans out to 50 km on both sides of the aircraft and results in
an image 100 km wide centered on the aircraft ground track.
The radar data is digitized, recorded on magnetic tape and transmitted to
vessels by two different modes. In the first mode the data is transmitted to
the Coast Guard Ice Navigation Center in Cleveland, Ohio via a real time com-
munications relay through the NOAA GOES satellite. The satellite data is
received at the NOAA command and data acquisition station in Wallops Island,
Virginia. It is then connected and sent out over dedicated telephone lines to
Cleveland. At the Cleveland Ice Center, the radar data are used to generate
photographic quality radar images on dry silver paper. From these images Coast
Guard ice interpreters develop annotated charts depicting ice conditions con-
taining geographical reference lines. The ice product, consisting of a radar
image and an interpretative ice chart is transmitted by facsimile via the
Great Lakes VHF-F`1 marine radio network to vessels operating in the lakes.
In the second mode the data is transmitted directly to L.S. Coast Guard
icebreakers via a VHF downlink. This second mode which allows an icebreaker
to receive real time data or a fast dump of tape recorded data will be dis-
cussed in detail in this paper. For more information pertaining to the other
segments of the Project Icewarn System see reference 1.
3. VHF DOWNLINK COK4UNICATIOMS SYSTEM
3.1 INTRODUCTI0N
The U.S. Coast Guard in carrying out its mission of support for Great
Lakes shipping during the extended winter season maintains a number of vessels
which assist in icebreaking. During the 1978-79 extended winter season, this
fleet consisted of the Great Lakes icebreaker `1ackinaw ,
 (88.4 m lon22.7 m
beam, 5 . 8 m draft and 10,000 SHP), the Arctic icebreaker Westwind ^82 m long,
19.5 m beam, 8.2 m draft and 10,000 SHP) along with a new 42.7 m multi -purpose
ice breaking tug, five 55 m long buoy tenders ane five 33.5 m long harbor tugs.
During the first few years of Project Icewarn, a number of these vessels were
supplied with the standard SLAR image/ice chart facsimile products available
from the Coast Guard Cleveland Ice Center via the Great Lake VHF-Ftd communica-
tions network. The scale of the facsimile image as received aboard the vessels
was 1:752,000. This facsimile product proved to be adequate for many icebreak-
ing applications. However, from operational experience gained during the first
years of Project Icewarn with the icebreaker Mackinaw, it was determined that
for icebreaking operations normally undertaken by the larger icebreakers it
would be desirable to have a photographic quality radar image on a scale of at
least 1:250,000. In addition the requirement was established that the data
should be transmitted to the icebreaker in as near to real time as possible.
The portable, real-time, VHF downlink communications sy.tt.m meets these require-
ments.
3.2 AIRCRAFT TO SHIP DATA TRANSMISSI0N SYSTEM
The SLAB aircraft to icebreaker data link employs two radio frequency com-
munications systems. The first system is used to relay actual radar data in a
digital format while the second is used strictly for voice communications.
Figure 2 is a schematic of the radar image receiving system employed aboard
the icebreaker Mackinaw. This system uses frequency modulation (F24J and oper-
ates at a center frequency of 217.55 MHz (VHF). The transmitter, located aboard
the SLAR aircraft, has a power output of 5 watts (+37 dbm). The carrier is
deviated about 125 KHz for a TTL "1" level. The digital data is transmitted as
BiPhase L information at either 50,000 or 6250 bits per second.
The VHF receiver is setup for a predetection bandwidth of 500 KHz and has
a signal to noise ratio of -113 dbm. A bit a neronizer is used to recover the
digital data from the demodulated output of the receiver. This bit syncronizer
drives a data decoder. The deeoier generates the proper syncronization and
video signals to drive a Harris Laserfax image recorder. This recorder uses
heat sensitive dry-silver paper to generate a photographic quality (109 lines/
inch, 9-12 gray scale levels image at a scale of 1:250,000. A magnetic taus
recorder is included as part of the receiver station and is used for recording
the initial data transmissions.
The communication equipment used in the data link system is commercial .
off-the-shelf equipment which is available at a very reasonable price. The
details pertaining to the equipment including manufacturers is given in figure
3. From the performance specifications for this equipment a link margin is
readily computed as follows:
Table I. Link Margin Calculations for VHF Downlink System
Receiver Sensitivity:
TE - 2900 (lONf/10_1) - (10 5/10_1) - 6270K
Rn - 10 loglO KTE B.W. - (1.38x10-23)(627)(500x103)
10-3	 10-3
Rn - -113 dbm
Free Space Loss:
A - c/f - 3x108/117.55x106 - 1.38 meters
FSL - 20 log10 OD - 411 322x10 3	-129 dbT
Link Margin:
LM - PO+Ll+AT+ FSL +AR +L2 +Rn
LM - 37 - 1 - 1 - 129 + 2 - 1 + 113 - 20 db
Where:	 PO - Power output of transmitter (dbm)
L 1 - Coax line loss - transmitter (db)
AT - Transmit antenna gain (00
FSL- Free space loss (db)
AR - Receive antenna gain (dbi)
L2 - Coax line loss - receiver (db)
Rn - Receiver sensitivity (dbm)
Lm - Link margin (db)
N f - Receiver noise figure (db)
TE - Effective noise temperature (OK)
K - Boltzmanns constant - 1.38 x 10-23
B.W. - Bandwidth NO
D - Maximum range of communications for aircraft at 1500 ft
altitude, meters
C -Re of 1 (H - 3x108(meters/sac)
f -Frequency z
X - Wave length (meters)
3
The second communication system used for voice communications operates at
219 MHz. Converted 1k meter FM amateur radios are used for this purpose.
These units are capable of an output up to 10 watts. Both the voice and
digital data communications systems employ belly mounted k wave monopole
antennas on the aircraft and vertically polarized dipoles at the receiver end.
In both cases the antennas are enclosed in weather proof fiberglass envelopes.
The effective range of both systems is limited to line of sight. Typically
the SLAR aircraft operates at an altitude of 3350 m (11,000 feet) and reliable
data transmissions are received out to a range of about 220 km (120 nautical
miles).
Figure 4 is a photograph of the portable package comprising the shipboard
data acquisition system. The equipment is rack mounted and is approximately
120 cm high, 61 cm wide and 64 cm deep.
	 It weighs approximately 90 kg.
4. USE OF D014NLINK SYSTE-"4 ABOARD THE ICEBREAKER MACKINAW
At the present time icebreaking services for the Great Lakes are provided
primarily by the two large icebreakers, Mackinaw, specially built for Great
Lakes use and the polar class Westwind. Icebreaking activities for the
Mackinaw are centered mainly in Whitefish Bay at the eastern end of Lake
Superior and the St. Mary's River which connects Whitfish Bay and Lake Huron.
Icebreaking activities for the Wastwind are centered in the Straits of Mack-
inac. The task of both icebreakers is to keep the shipping lanes open
primarily for ore boats carrying taconite from the parts in upper `iinnesota
(Two Harbors, Silver Bay and Taconite Harbor) to steel mills.
The portable downlink communications system was used aboard the icebreaker
Mackinaw during the last three extended winter navigation seasons (1976-77,
1977-78, 1978- 79). Figure 5 is a photograph of the icebreaker Mackinaw. The
antennas were located on the main mast while the equipment was located in a
room behind the bridge.
During the winter the Mackinaw is stationed at Sault Sainte Marie, Mich-
igan operating normally in the Saint Mary's River or in Whitefish Bay of Lake
Superior. The downlink communication system has a range of 220 km limited
primarily to a "line of sight" distance between the SLAR aircraft and the ice-
breaker. Therefore the Mackinaw can receive a real -time downlink transmission
from the aircraft as it images Whitfish Bay while the icebreaker is operating
in the St. Mary's River, Whitefish Bay or at anchor in Sault St. Marie. The
icebreaker image recorder is capable of generating an image of the 50 km swath
track on one side of the aircraft ground track or the other. For Whitefish
Bay one 50 km swath is adequate to map the navi ational areas. However, the
tspe recorder stores the complete 100 km swath 150 km on both sides of the
aircraft) ^f data. The other 50 km swath can therefore be played back and an
image generated at the completion of the downlink transmissions. The data
rate for real time transmission is 6250 bits per second (BPS). A high speed
data rate transmission of 50,000 bits per second (eight times faster than the
time to collect the original data) is also available. This allows radar images
of ice covered areas outside the 220 km range of the data link, previously
imaged by the SLAR aircraft, to be ; r layed to the icebreaker where it is
recorded on magnetic tape only. An image cannot be generated during the high
speed data transmission but only during low speei (6250 BPS) plavback at a
later time.
During a typical SLAB mission, once the aircraft has es-ablished com-
munications with the icebreaker it will begin the real-time downlink trans-
mission. The icebreaker will select either the 50 km right or left swaths of
the radar coverage depending on ice primary area of interest for generating a
real-time radar image. Radar data for the full 100 km swath is recorded on
magnetic tape. Once the aircraft has cospleted its coverage of Whitfish Bay
the real-time data transmission is terminated. At this time a fast playback
downlink transmission is initiated for data previously tape rp coided in Lake
Superior outside the 220 km range of the icebreaker.
The portable radar downlink communications system described in this paper
had an additional application. For instance in 1976, the downlink e-tipment
was located at Point Barrow, Alaska as part of an Arctic ice informaL.on
demonstration (Reference 2) in Support of the North Slope resupply sealift.
Vessel movements along the North Slope were controlled by Crowley Maritime
Corporation from their field operations center in Barrow. The SLAR aircraft
routinely surveyed the ice conditions along the north coast of Alaska from
Wainwright to Barter Island. Radar data were transmitted to Barrow via the
downlink communications system in both real-time and fast data playback modes.
5. CONCLUDING REMARKS
In conclusion, Project Icewarn has had as its objective the collection and
dissemination of ice infor=tion to Great Lakes vessels. Because time plays
such an important part in the usefulness of this information, communications
systems have been vital to its success. The VHF downlink communications system
described herein has successfully met the need to provide icebreakers with
optimum ice information to support tactical operations. Ung todays techno-
ogy, the system was readily implemented at a reasonable cost and has proved
to be easy to operate with a very high degree of reliability.
REFERENCES
1. Schertler, R. J., et al: Grat Lakes All Weather Ice Information System,
Proceedings of the Symposium on Remote Sensing of Environment. University
of Michigan, Ann Arbor, 1976 (NASA TM X-71815).
2. Gedney, R. T., et al: All-Weather Ice Information System for Alaskan
Arctic Coastal Shipping, Proceedings of Ninth Annual Offshore Technology
Conference. Houston, Texas, May 1977 (NASA TM X-73619)
GREAT LAKES ICE INFORMATION SYST
SLAR
AIRCRAFT
UHF UPL
VHF
't DOWNLINK
41.0 % 	O.&A0, 	
AST GUARD
 
,/ICE/REAKER
„•	 5-BAND
DOWNLINK
ICE NAVIGATION
ORE
	
CENTER WSCGINWSI
VESSEL
VHF•FM
WALLOPS ISLAND
STATION INCAAI
FIGURE I	 SCHEMATIC OF PROJECT ICEWARN,
GREAT LAKES ICE INFORMATION SYSTEM
 TA Pt
217. 55AIRCRAFT FROM	
r'" RECORDER ^1AIRCRAFT
L---J I
I	 I	 j
I	 i
VHF RECEIVING	 DIGITAL	 SIGNAL
SYSTEM	 CONDITMI N>ER	 DCODDER
RADAR	 DRY SILVER
IMAGE
	
~- PAPER IMAGE
RECORDER
FIGURE 2	 SCHEMATIC OF THE VHF RADAR DOWNLINK
COMMUNICATIONS SYSTEM
0v
6
- - - - 
=	
y.^^. 	^_-t mlmm.: 4_ _	
- - --
r
. J
7
OF TELEMETRY LINK
Ll •	 -1 db	 -
VECTOR T-12D6 PO	 • 37 dbm
STAR DATA FM TRANWITTER
CON'PRESSOR IO	 •	 217.55 M11Hz
AND ENCODER BID L Al •	 125 KHZ RG141 AlU
DIGITAL DATA PO •	 51VATTS LO-LO SS COAX
AT 6250 OR
50,000	 BITSISEC
Rp	 -113 dbm
AT B. W. • 500 KHZ
N, F.	 • 5 d COM1VINT IND.CI	 171
n '
AT••Idb,
COM1IANT IND.
C1-189
W MONOPOLE
VERTICAL POLAR
FS L • -129 db
AT 200 MILES
LINE OF SIGHT
DISTANCE
M1tICRODYNE CPR 101
	
ANTENNA y v^^
TELEMETRY
RECEIVER	 RG 141 AIU	 I	
AR	 2dbi
BANDWIDTH•500KHZ
	 10-1055
COAX	 - L 2 ' I db
F161'RE 3	 VHF TELEMETRY LINK
7
i
IN
f+ -50 km--+.^
-A
	
A	 f+
^
	 At
OftC7	
µ MM7
IMAQRERD(R
(DRY SAVER PAP(R, 	 S^
RADAR IMAGE
i
7 IMICI DECODING UNIT
IM Yt11TAG(
DIUSIAVNI UNIT
IIG*l
DND I1101R R
u-+tat
FIGURE 4 PHOTOGRAPHY OF THE PORTABLE VHF RADAR
DOWNLINK COMMUNICATIONS EQUIPMENT
US COAST GUARD ICEBREAKER WXKINAW
FIGURE 5 PHOTOGRAPH OF THE U.S. COAST GUARD
ICEBREAKER MACKINAW
i
9


VHF downline communication system for SLAR data

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server