Navigation and Data Transmission
By P H MARLAND BSc, CENG, M EE, Lt Cdr RN Rtd
Compass & Navigation
Procurement and Research & Development of compass and navigation equipment was led from ACO Slough, in-house manufacturing eventually ceased in the 1960's, and overall project management was moved across to DGSW(N) by the 1970's. The main technical reference is the Admiralty Manual of Navigation (BR45d series), plus Stanag 4222 and Defstan's 09-100 and 21-26 for data conventions, whilst Fanning1 covers the historical narrative.
In the immediate post-war period, ships were fitted with basic gyro compasses (typically AP5005) plus a transmitting magnetic or gyro-magnetic compass (ATMC5/AGMC6) for emergencies. Vertical attitude reference data came from wholly separate stabilisers (Gyro Stabiliser Type 1-12). Definitions of the three main categories of equipment may help the lay reader2:
- Gyro Compass, AP5005, Sperry Mk 23 and Arma-Brown Mk 12.
- Compass Stabilizer, Sperry Mk 19 and Elliot (now GEC) NCS 1.
- Inertial Navigation System, UK SINS Mk 2, and now NATO SINS.
Radio navaids were limited to the early Decca sets (QM10) and MF DF (FM12), with widespread use of astro navigation (by hand-held sextant), plus a 1930's vintage ARL table.
Apart from heading, pitch & roll (and acceleration, if available), the other references are:
- Speed – measure by the ships log. Post-war ships initially used either a Chernikeef, or a Pitometer Type D log, replaced by the ARL Teddington developed Electro-Magnetic (EM) log, and more recently by the Solid State or Agilog (Outfit SSL). UK has also dabbled with a doppler bottom-tracking log for SRMH, intended to capture motion over the ground, rather than through the water.
- Depth is measured by echo sounder, initially the ES 765, replaced by the general service 778, with more specialised sounders in the Survey flotilla, or as a covert sounder for SM.
- Position was fixed by visual bearings when inside coastal (pilotage) waters, whilst offshore navigation relied on sextant and astro-navigation, but has evolved via Decca and Loran A+C, to Transit satnav, and now to the universal GPS. As a special case, precise navigation for minewarfare was based on Hyperfix. Combining position with chart information has involved the older ARL table, replaced by SNAPS to input to the command system, and most recently by WECDIS with electronic charting (see SNAPS).
Compass Stabiliser
The first significant change was the adoption of US technology (the Mk19) to provide both North and vertical reference information. This was trialed in HMS Eagle and replaced a large number of dedicated stabilisers with a centralised system fed by Mk19's. In parallel, from 1964 the UK also adopted the Mk23 compass using the same floated gyroball technology. Mk19 went into all ships with either ADAWS or CAAIS based weapon systems (Leander refits, and new construction Type 21/22/42/82), whilst Mk23 replaced AP5005 in frigates with manual AIO that did not require a vertical (Rothesay's and Tribals).
1 AE Fanning, Steady As She Goes, HMSO, 1986.
2 Gyro Compass normally uses a single gyro to indicate true north. The element is often floated or suspended by wire, and uses the earth's rotation rate (15°/hour) to assist in north finding and keeping. Compasses require an input of latitude and ship's speed for proper accuracy.
Compass Stabilizer uses two separate gyros and an accelerometer to indicate true north, and give accurate pitch and roll outputs for weapon stabilization. The sensors are all mounted on a two-axis gimbal.
Inertial Navigation (IN) systems uses three gyros and accelerometers mounted on a three-axis stable platform. IN normally have no ability to fix absolute position, and require an initial Lat/ Lon in order to provide very accurate navigation thereafter, and continuous outputs of velocity and attitude. Inertial systems have many similarities with compass stabilizers, but require much more accurate gyros and accelerometers, and more complex control loops and computations.
Fig 1
 5005 Compass |
 Mk19 Compass |
 AB Mk12 Compass |
In order to reduce the US$ outlay, the successor to the Mk 19 was a national programme; the Naval Compass Stabiliser Mk 1 (NCS1). The prototype was trialled in HMS Antrim in 1975, systems entered service in 1977 and was accepted in 1981; it then went into later construction Type 22 and 42. NCS1 used relatively cheap sensors, with 'cluster rotation' of the vertical gyro to average out errors, modular construction, and digital computation of all control loops. It provided analogue (synchro) outputs of heading, roll and pitch (plus, in the Seabed Operations Vessel, digital outputs of velocity and acceleration for hover purposes).
The system had a mixed record in service with early difficulties as hybrid micro-circuits allergically reacted to their ceramic packaging, and later problems caused by inadequacies in the FIMS diagnostic handbook that led to very high spares usage (through not understanding the fault codes, or the reliability of the BITE). Refit intervals were addressed by in-situ inspection of condition and alignment, and it has subsequently become a good performer; despite having a 1975 design freeze, and incorporating TTL logic from the early 1970's:
Fig 2
 NCS Mk 1 Space Reference Unit |
 Electronics Pack |
NCS1 is likely to be in-service until somewhere between 2015 and 2035 (subject to GTTR), and cost £241k each in 1989 (for batches of 10), rising to £390k for single orders, plus £54k of onboard spares (prices ex VAT).
Ships Inertial Navigation System (SINS)
After technical exchanges with the US during 1954, the UK programme began in May 1956 with the approval of a Staff Requirement for SINS. Work started at ACO Slough in 1957 and purchase of 12 early Draper ball bearing gyros from Northrop led to a first laboratory run of the UK system in July 1961, followed by initial sea trials in September. Prototypes were fitted to HMS Dreadnought in 1963 and production equipment became available in 1966, but did not become fully operational until 1968. ACO also played a major part in developing the gas bearing gyro, and their detailed design for a gyro rotor/gas bearing produced an order of magnitude increase in accuracy. A total of 18 UK SINS Mk1 were built and fitted to SSN 01-11 and DLG 05-09. SINS Mk 1 was a completely analogue design, sensitive to temperature and power supply excursions and had significant limitations when operating at high latitudes.
The requirement for its successor, SINS Mk2, was re-endorsed in 1965 as NSR 7861, and development ran until 1971, featuring pulse torquing, integrated circuits and digital computation (Elliot 920M). Sea trials began in 1975 and production equipments with full polar capability became available in 1976. A total of 24 UK SINS Mk2 were built and fitted to all SSNs (except 01), three CVS and two H Class survey ships. Both UK SINS were ACO designs, with the production engineering expertise supplied by Sperry (later BAe) at Bracknell, and gyro production by English Electric (later BAe Dynamics) at Stevenage. In the late 1970s the MoD (PE) emphasis shifted from intramural to contractor development, and the ACO team gradually dispersed. SINS Mk2 had an excellent record and performed significantly better than specification, though requiring considerable grooming, maintainer and navigator expertise and a measure of luck. This performance required gyros with drift rates rather better than 0.001/hour to provide overall performance in the 1 NM/day class. The last system cost over £1.2M at 1985 prices and through-life support costs had escalated due to the low numbers and obsolete technology involved, and for repair of the precision gas bearing gyros.
NATO SINS
The process of developing a replacement began in 1983 as NST 7864 for SLINS - the Ships Low Cost Inertial Navigation System. This stressed the need for much smaller, lighter and cheaper units to allow a wider fit, including dual fits, which had been impractical with SINS Mk2. Pressure for a collaborative solution led to the NATO PG4 group, and after several changes, a new MOU and specification was established in 1987 for NATO SINS, with the UK undertaking programme management. An international procurement competition was run with all the bidders offering Ring Laser Gyro (RLG) technology:
Fig 3
 Conventional floated Mechanical Gyro |
 Ring Laser Gyro |
The unique features are strapdown ring laser gyros, indexing and Kalman filtering; the resultant NATO SINS is shown below. The two-cabinet configuration with separate Inertial Measurement Unit (IMU) and electronics pack was adopted by the UK for shock reasons. NATO SINS weighs 385 kg, compared to SINS Mk2 (2011 kg), Mk19 (618 kg) and NCS1 (410 kg); it does not require chilled water and consumes 2 kW less power. Each NATO SINS cost £334k at 1990/91 prices, plus £91.5k of onboard spares:
The system has also been referred to Outfit XZD, is commercially known as the Mk 49 Mod 1, and in USN usage as AN/WSN-7. A one-box compass stabiliser variant is Litton's Mk 39 AHRS.
Fig 4
UK NATO SINS dual installation in SSN
Equivalent US Systems. The US have a longer record of developing IN and related compass stabiliser systems, in both strategic (SSBN) and fleet submarines (SSN), and in surface ships. There are several vendors (Autonetics, Litton, Sperry and Rockwell), with both US National and export variants:
- Mk 2, and the newer Electrostatic Gyro Navigator (ESGN) in SSBN, or as WSN-3 in SSN.
- Mk 3 and WSN-1, -2 and -5 in surface ships (carriers, and DDG with TLAM).
- Mk 29 for export, and WSN-7 (from the NATO SINS programme) in all new construction.
SNAPS
Ships Navigation Processing System was developed against SR(S)7972, covering the integration of radio navigations aids, automatic display of the calculated position on a chart table using a light spot, and a position feed to the command system for picture and grid stabilisation. The system replaced the venerable (1930's era) ARL table that only handled dead-reckoning with log and compass inputs. SNAPS included several configurations of plotting tables (JNA-B) and consoles (JNX-Z) that hosted the computer, PSU and data recorder. The user interface was via a Keyboard Display Unit (KDU).
Ship fitting of the console (as fit-to-receive for the full system) started in 1981, but problems with wire-wrap connections in the computer unit backplane delayed completion until 1983. SNAPS systems ranged from 1 to 3 tables (or a 2+2 linked configuration in CVS), or as TSNAPS in Trident submarines.
Fig 5
SNAPS table and console, KDU top right
The system was usually managed by the OOW from the bridge, with a second semi- independent system in the Ops Room as the General Operations Plot (GOP). The original requirement included a Towed Array Plot (TAPS) as JNC, later dropped. In addition to interfacing with navaids, other outputs included position to the command system, RN Outboard, and Flyco for Nav initialisation of Sea Harrier via MADGE. Part of the memory was radiation-hard Silicon on Sapphire (SoS), to allow a post-EMP restart from the last position.
In service, there was steady software development, however the were significant problems with the stability of the best-position (BP) algorithm designed by Abbott & Gent to combine multiple types of information, and this was dropped in favour of a simpler Computed Position (CP) based on a single sensor. The computer hardware had been derived from the airborne Jaguar NavAttack unit, and was effectively obsolete by 1993, requiring tailored operational programmes to overcome memory shortages. The last changes in 1995 incorporated dual NATO SINS in SSN, but the system then continued until almost 2008. PDS proposals to upgrade the CPU, to improve chart registration, and for an adjunct PC for off-line route planning, were all unsuccessful. SNAPS always relied on a paper chart atop the digitally driven chart table, and never moved into the electronic charting environment, though the table was subsequently re-used in Sandown class SRMH.
SNAPS attracted some attention of the US Coast Guard, but the USN chose to follow a separate route with NavSSI, and early vector electronic charting. UK has eventually replaced SNAPS by WECDIS, noting the widespread commercial adoption of both raster and vector chart products viewed in high-definition colour, that were not available in the 1990's
Radio Navaids
- MF DF used outfits FM12-FM16, and most recently CXB (Marconi Lodestar IIID). These were originally provided for navigation; the remaining function is SOLAS distress work on 500kHz and 2182kHz via the SQB autoalarm (and likely to be superseded by GMDSS).
- There were a series of Decca receivers as Outfits QM10-QM16, plus Omega (QW1), Loran A (QYA), and Loran C (QYC). From the late 1970's these were supplemented by Transit satnav, using the Magnavox MX1105 (as Outfit QZ3) interfaced to SNAPS. The wider fleet fit of a stand-alone Decca commercial model DS4 or DS5 Satnav receivers was then accelerated by the 1982 Falklands conflict.
- Interim Transit satnav systems were superseded by the RN GPS programme, under SR(S)7812 for a full military P-Y code GPS receiver (Outfit QYF), though the controlled reception pattern antenna (CRPA) as an anti-jam measure was dropped in favour of a simple fixed reception pattern antenna (FRPA). All the Decca and Loran A systems were abandoned for GPS, with Loran C as a backup..
- Precise navigation for mine warfare initially used the HiFix commercial HF hyperbolic chain, then moved onto Hyperfix under NSR7866, as the QX series receivers, or Trisponder (Outfit QTA), all fed into the MCMV's NAVPAC processor. All the hyperbolic radio aids have now been replaced by differential GPS based systems.
Fig 6
QZ3 Transit satnav aerial
Data Distribution
Initially, heading, roll, pitch and speed information was distributed by mechanical re-transmission units that handled the 'fan out' of analogue information. Post-war, the data distribution methods have evolved via the following sequence:
- M type DC stepping system (+24v/0v/-24v) used to pass rotation, and capable of working data transmission down to a few minutes accuracy for fire control, with 'hunters' as part of the coarse/fine switching for the power drive follow-up. The version in commercial marine usage was 'S Type' (+12v/0v DV only), at lower torque.
- These were replaced by Magslip (UK) or Synchro (US) for AC data transmission, with coarse-fine chains capable of working to sub-minute level. Synchro retransmission units (TRA series, CRU, SCTS and finally by Data Distributors) were succeeded by the Outfit PDM series of solid state units.
- Digital links - the early Ferranti 'Christchurch B' interface also known as parallel B or P(B), was supplemented by RS232C, RS422, and the bespoke S3 serial signalling link. The slightly later ASWE Serial Highway (ASH, Defstan 0019) grew into a shipwide Combat Systems Highway (CSH) after re-engineering by J&S Marine, whilst the submarine WSDB used 1553B (Defstan 0018). Other excursions included Stanag 4156 for Navigation systems (NATO SINS), plus NMEA for integrating commercial navaid equipment.
- Recent systems are Ethernet, then ATM or token-ring, running on copper or fibre optic, and most recently Gb Ethernet for the Data Transfer System (DTS) in Type 45 and QEZ.
Electro-mechanical units were replaced by solid-state data distributors (mostly from Thorn EMI at Rugeley) which used digital multiplexing between the nodes, and were software reconfigurable, only generating synchro line voltages at the point where the information was required: PDM(1) Upholder, (2) T23, (3) Vanguard and (4) SRMH. This also required an additional Outfit INA to cater for systems like GSA8 with analogue inputs, run off the CSH, or INB to interface synchro (Heading, Roll, Pitch and ships speed) onto the CSH, in ships that did not have a PDM.
Wartime wiring involved lead sheathing or braided wire armouring, replaced by Tough Rubber Sheathed (TRS) wiring with screw terminals, in heavy cast aluminium watertight junction boxes in 1950s ships like Type 41/61. Replaced by silicon insulation (leading to the 'green-spot' scare) then PVC twisted pairs, until the Falklands led to renewed emphasis on Low Fire Hazard (LFH) cable to minimise smoke and toxic combustion products during Damage Control. For lower power signalling, taper pin connections became the norm, and multi-pole Plessey Mk 5 were replaced by the more recent Type 608 series connectors.
Fig 7
Solid-State Data Distributor
Navigation System Engineering
After SNAPS, there was significant effort applied in the early 1990's to design a 'navigation sub-system' for The Future Frigate (TFF); this was not realised, due to the multiple changes in the target platform: TFFCNGFType 45, and the loss of SWSE staff as ACO was absorbed into MoD PE at ABW, leading to a prime contractor solution. Coherence was maintained for NAVPAC, but formal system engineering was only re-studied under Project HINE, and then with Geospatial and Temporal Referencing (GTTR) which included replacement of QYF and NCS1, and an overall Defstan (09-100). HINE considered the total error budget on an end-to-end basis, including the conversion algorithms, but (like the similar SIAP initiative for air picture) has failed to deliver anything concrete.
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