The ENTER key confirms data entries or menu selections. The NAV key accesses the Position and Navigation screens.
The GOTO key is used to create a direct route to any landmark stored in memory. The MENU key is used to access the route, landmark and setup functions. The PWR key turns the receiver on and off.

ENTER NAV

The MARK key is used to create landmarks and store the current position.
The LIGHT key turns the light on and off.
The ARROW pad is used to enter landmark names, and scroll through the series of screens and menu selections.

Getting Started

Installing the Batteries
The GPS Pioneer uses two AA alkaline batteries that are installed at the back of the receiver. To remove the battery cover, turn the ring of the battery door screw counterclockwise until the battery cover can be removed. Insert the new batteries as shown, being sure to respect the polarities, and replace the battery cover.
Replace the screw and turn the ring clockwise until the battery door is held in place securely. While the battery door does provide the waterproofing seal to the batteries, you should avoid overtightening the battery door screw.
Getting Signals from Satellites
Since the GPS Pioneer receives information from satellites orbiting the earth, the antenna needs to have a relatively unobstructed view of the sky. Large obstructions such as buildings, cliffs, and overhangs may interfere with signal reception causing your GPS Pioneer to take additional time to compute your location.
The GPS Pioneer is designed to fit comfortably in your hand. Hold the receiver with the antenna towards the sky.
Initializing the Receiver - EZStart
Before using your GPS Pioneer for the first time, the receiver needs to know its approximate location. Using Magellans EZStart procedure, the GPS Pioneer will prompt you for the information it requires when you turn it on for the first time. You do not need to initialize your receiver each time you use it. Follow these steps to initialize the GPS Pioneer if this is the first time you are using it. Press
to turn the GPS Pioneer on.
SELECT REGION. Use the ARROW pad to change the flashing text to select the appropriate region for your present location. Press ENTER. SELECT COUNTRY or STATE. Use the ARROW pad to change the flashing text to select the country or state for your present location. Press ENTER.
ENTER ELEVATION. Use the ARROW pad to enter the approximate elevation for your position. If unknown, leave the elevation at 0. Press ENTER. ENTER TIME. Use the ARROW pad to enter your present time. Press ENTER. ENTER DATE. Use the ARROW pad to enter the date. Press ENTER. The GPS Pioneer then displays the POSITION screen and automatically begins searching for satellites that it knows are in this area for the date and time you entered. The display shown here may differ from yours depending upon the information you selected in steps 2 and 3. As the GPS Pioneer scans the sky, the arrow in the circle will swing around pointing to a satellite and displays, in the center of the circle, that satellites elevation above the horizon.

Computing a Position

The GPS Pioneer will begin to acquire information from the satellites and use this information to compute your current position (called a position fix). Whether you have just
completed the initialization process or have just turned your GPS Pioneer on, the GPS Pioneer will display the following screens in the order shown. As the GPS Pioneer searches for satellites, some of the small circles around the large circle will become black indicating that the GPS Pioneer is tracking that satellite and receiving information. The number at the bottom right of the screen displays how many satellites are being tracked. After the GPS receiver has received positioning data from at least three satellites (approximately 2-3 minutes), it will begin computing a position fix based upon the information it is receiving. As soon as a position fix is computed, the receiver switches to the navigation screen displaying the moving compass. The word TRACKING is displayed in the lower right corner indicating that the receiver is computing position fixes. Note: If the receiver has not acquired a position fix in approximately 10 minutes, refer to the troubleshooting section of this manual. More information on these screens and the information they display can be found in the next chapter.

Basic Operation

Saving a Position Fix
Position fixes can be saved in memory for use later when you want to return to that position. Saved position fixes are referred to as landmarks or LMK. To save (mark) your current position press
Receiver-Generated Name: The GPS Pioneer prompts you to enter a name or accept the receiver-generated name for this landmark. To accept the receiver-generated name (LM01 - LM99), press ENTER.
User-Created Name: To create a name (up to four characters), use the UP/DOWN arrows to change the character and the LEFT/RIGHT arrows to move the cursor to the left or right respectively. After you have input the desired landmark name press ENTER. Press
to accept the current latitude and press again to accept the current longitude.
Shortcut: Press MARK at anytime while viewing this screen to quickly save the position.

Creating a Landmark

To create a landmark at a location which is not your current position, use the same procedure as saving a position with the additional step of inputting different latitude/longitude coordinates. To create a landmark press
The GPS Pioneer prompts you to enter a name or accept the receivergenerated name for this landmark. When the desired name is displayed at the top of the screen press ENTER. Use the ARROW pad to change the latitude and press ENTER to accept. Use the ARROW pad to change the longitude and press ENTER to accept. The newly created landmark is stored in memory and you are returned to the screen that was displayed when you pressed the MARK key. Shortcut: Press MARK at anytime while viewing this screen to quickly save the position.

Viewing the POSITION Screen
The POSITION screen displays the coordinates for your last computed position and information about any satellites that are visible. It can be accessed by pressing the NAV key.

Latitude and Longitude

North Indicator Satellite Pointer Number of satellites being used.
Satellite Elevation Degrees above the horizon of the satellite indicated by the satellite pointer. Satellite Icons Visible but not tracked. Satellite is being tracked.
Tracking Indicator Appears when the receiver has acquired enough information from the satellites to compute a position fix.
You may sometimes notice that the number of satellites being tracked differs from the number of satellites being displayed graphically. This is due to more than one satellite being tracked in the same general area.
Viewing the Navigation Screens
Without an Active Route. Without an active route the navigation screen displays your heading and the speed at which you are traveling. The lower portion of the screen displays a moving compass. The triangle at the top of the compass points in the direction you are traveling and the arrow points to north.
Navigation Screen without an Active Route (Moving Compass)

Current Heading

Current Speed Direction of Travel

North Indicator

Tracking Indicator Appears when enough satellites have provided information to compute position fixes.
The navigation screens display your speed of travel. For the speed to be displayed, you must be moving at a speed greater than 2 miles per hour.
With an Active Route. When you have an active route the navigation screen still displays your heading and speed but also includes the bearing and distance to your destination. The moving compass is then replaced with steering information. You can use the graphical representation or the steering indicator to assist in directing you towards your final destination.
Navigation Screen with an Active Route (Steering)
Destination Landmark Bearing to Destination Current Heading Destination Icon Distance to Destination Current Speed North Indicator Steering Indicator Indicates the number of degrees to turn, right or left, to destination landmark.
Destination Pointer Tracking Indicator Appears when enough satellites have provided information to compute position fixes.

The receiver prompts you for a starting landmark for the route. The first landmark, *POS, is your present position. Use the ARROW pad to scroll through the list of landmarks. When the desired landmark is displayed (and flashing) press ENTER. The display changes to allow the selection of the landmark to be used as the end of the first leg in the route. Use the ARROW pad to scroll through the list of landmarks. When the desired landmark is displayed (and flashing) press ENTER. Note that as you scroll through the list of landmarks, the display updates showing you the bearing and distance from the start of this leg to the displayed landmark. If the distance from the start of the leg to the end of the leg is below 0.10 miles, the message INVALID is displayed and you are prompted to select a different landmark. The screen changes to the next leg in the route with the destination landmark of the previous leg inserted as the start of the next leg. The message END ROUTE is displayed in the TO field. You may continue this route by using the ARROW
pad to select a landmark as the destination for this leg or press ENTER to signal the GPS Pioneer that this was the last leg in the route and you are finished creating a route. Continue this process for each leg in the route remembering to press ENTER with END ROUTE displayed in the TO field to finish up the route. If you accidently pressed the ARROW pad but you meant to end the route, you can still end the route by continuing to press the ARROW pad until END ROUTE is displayed again. A route may contain no more then ten legs and the GPS Pioneer automatically saves the route and returns to the Route Menu as soon as Leg 10 is entered. After the route has been created, the GPS Pioneer automatically activates the route and begins providing navigation information for the route.

Viewing a Route

You can view a summary of the route in memory as well as viewing the individual legs of the route. All editing commands are accessed from the View Route function as well. With a route in memory, press at the top of the display. Press

MENU ENTER

until ROUTE appears.
The first screen displayed is the ROUTE SUMMARY screen. This screen displays the start and end landmark for the entire route as well as the total distance of the route.

Use the LEFT/RIGHT ARROWs to view the legs in the route. The leg screen displays the FROM and TO landmark for the leg as well as the distance and bearing for the leg. The circle graphically displays the bearing for the leg. Continue pressing the LEFT/RIGHT ARROWs to step through the other legs in the route, eventually returning to the ROUTE SUMMARY screen.
Activating/Deactivating a Route
With a route in memory, press MENU until ROUTE appears at the top of the display. Press ENTER. Press the UP ARROW. If the route is currently active, the display indicates: ENT TO DEACTVATE. If the route is currently deactivated, the display indicates: ENT TO ACTIVATE. Pressing ENTER will activate or deactivate the route depending upon its present status.

Deleting a Route

With a route in memory, press at the top of the display. Press until ROUTE appears ENTER. While still viewing
the Route Summary screen, press the UP ARROW three times until the display indicates PRESS ENT TO DELETE. Press ENTER. You will be prompted to confirm the deletion. Use the ARROWs to select YES or NO and press
Appending a Leg to a Route
With a route in memory, press at the top of the display. Press
until ROUTE appears. While still viewing
the Route Summary screen, press the UP ARROW until the display indicates PRESS ENT TO ADDLEG. Press ENTER. The display changes to the Add Leg screen with END ROUTE flashing. As in creating a route, use the ARROW pad to select the new landmark for this leg. With the new TO landmark flashing, press ENTER. The newly created leg is added to the end of the route.
Activating a Leg in a Route
As you are navigating you may decide that you no longer wish to continue on the leg that you are now using. Instead you wish to use another leg of the route. You will need to activate the leg of the route with the desired destination (TO landmark). Press MENU until ROUTE appears at the top of the display. Press ENTER. Use the LEFT/RIGHT ARROWs until the leg you wish to activate is displayed. Press the UP ARROW. If the leg is not active, the display indicates ENT TO ACTIVATE. Press ENTER. The leg has become activated and the receiver begins to compute the necessary information to continue you on the route using the leg you selected. If the display indicates ENT TO DEACTVATE, it means that the receiver is using this leg to compute the navigational information. Pressing ENTER at this screen not only deactivates the leg, but deactivates the route as well.

Editing a Leg in a Route

Press Press until ROUTE appears at the top of the display. ENTER. Use the LEFT/RIGHT ARROWs until the leg

you wish to edit is displayed. Press the UP ARROW until PRESS ENT TO EDIT is displayed and press ENTER. The Edit Leg screen is displayed with the FROM landmark flashing. Use the LEFT/RIGHT ARROWs to select a new FROM landmark and press ENTER. The TO landmark begins flashing alerting you that the GPS Pioneer is ready for you to select a new TO landmark. Press ENTER to accept the TO landmark as it is or use the LEFT/RIGHT ARROWs to select a new TO landmark and press ENTER. The leg before and after the one you just edited will be automatically changed to reflect the changes that were made to this leg.
Deleting a Leg From a Route
Press MENU until ROUTE appears at the top of the display. Press ENTER. Use the LEFT/RIGHT ARROWs to find the leg of the route that you want to delete. Press the UP ARROW until PRESS ENT TO DELETE is displayed and press ENTER. The GPS Pioneer prompts you to confirm the deletion of the leg. Use the LEFT/RIGHT ARROW to select (flashing) YES to delete or NO to cancel. If you attempt to delete a leg that causes the route to contain a leg that is under the 0.1 mile leg distance limitation, the receiver will display the message INVALID DELETE.

Additional Features

Viewing the Time and Date
You can view the current time and date (obtained from the satellites) by repeatedly pressing the is displayed at the top of the screen.

key until TIME

Viewing Elevation
You can view the last computed elevation for your GPS Pioneer by repeatedly pressing the

key until

ELEVATION is displayed at the top of the screen.

Viewing Battery Life

You can view the estimated battery life remaining by repeatedly pressing the

key until POWER is

displayed at the top of the screen.

Full Battery Life

40-60% Battery Life

Low Batteries

Changing Coordinate Systems
You may wish to change the coordinate system that your GPS Pioneer uses to display the position and landmarks coordinates. You have three options: LAT/LON using degree/minutes (DEGMIN), LAT/LON using degree/minutes/seconds (DEGMINSEC) or Universal Transverse Mercator (UTM). The choice you make will depend upon the maps or charts you may be using. You want your GPS Pioneer to be displaying the coordinates in the same mode that your map or chart uses. The following example shows the same position in each of the three different modes.

LAT/LON (DEGMIN)

LAT/LON (DEGMINSEC)
Press the MENU key until SETUP is displayed at the top of the screen and press ENTER. Press ENTER again and the currently used coordinate system begins to flash. Use the RIGHT/LEFT ARROWs to scroll through the list of coordinate systems and press ENTER when the desired system is displayed.

Changing Map Datums

If you are using a map (or chart) in conjunction with your GPS Pioneer you need to insure that the datum used by the GPS Pioneer matches the one used in creating the map. The map datum can usually be found in the legend box of the map or chart. The GPS Pioneer offers the choices of WGS84 (default) or NAD27. Press the MENU key until SETUP is displayed at the top of the screen and press ENTER. Use the RIGHT ARROW until SETUP MAP DATUM is displayed. Press ENTER again, the currently used map datum begins to flash. Use the RIGHT/ LEFT ARROWs to scroll through the list of map datums and press ENTER when the desired datum is displayed.

Changing Distance Units

Your distance units can be in miles and miles per hour (MIMPH), nautical miles and knots (NM-KTS), or kilometers and kilometers per hour (KM-KPH). To change the units, press MENU until SETUP is displayed at the top of the screen and press ENTER. Use the RIGHT ARROW until SETUP UNITS is displayed. Press ENTER again and, the distance unit of measure begins to flash. Use the RIGHT/LEFT ARROWs to scroll through the list of units and press ENTER when the desired unit of measure is displayed.
Changing Time Display and Time
To change the way that time is displayed (12 HOUR default, 24 HOUR, or UT), repeatedly press MENU until SETUP is displayed at the top of the screen and press ENTER. Use the RIGHT ARROW until SETUP TIME is displayed. Press ENTER again and the time display begins to flash. Use the RIGHT/LEFT ARROWs to scroll through the list and press ENTER to select. The screen changes to TIME SET. Use the ARROW pad to set the time and press ENTER when done. (You are not prompted to set the time if you selected UT as the time format.) When you change your clocks because of daylight savings time, remember to change the time in your GPS Pioneer.

Changing North Reference

The GPS Pioneer uses magnetic north as a default reference for all navigation computations. You can change this to true north (good if you are also using a map) or back to magnetic north (default, good to use if you are using a compass) under the SETUP menu. Press the MENU key until SETUP is displayed at the top of the screen and press ENTER. Use the RIGHT ARROW until SETUP NORTH REF is displayed. Press ENTER again, the north reference begins to flash. Use the RIGHT/LEFT ARROWs to scroll between MAGNETIC and TRUE and press

to select.

Initializing the Receiver (EZSTART)
If you desire to re-initialize the receiver, (for example, you have moved more than 300 miles since the last time the receiver was turned on) you can do so in the SETUP menu. Press MENU until SETUP is displayed at the top of the screen and press ENTER. Use the RIGHT ARROW until SETUP PRESS ENT TO EZSTRT is displayed. Press ENTER again and the receiver prompts you to enter the necessary data. SELECT REGION. Use the ARROW pad to change the flashing text to select the appropriate region for your present location. Press ENTER. SELECT COUNTRY or STATE. Use the ARROW pad to change the flashing text to select the country or state for your present location. Press ENTER. ENTER ELEVATION. Use the ARROW pad to enter the approximate elevation for your position. If unknown, leave the elevation at 0. Press ENTER. ENTER TIME. Use the ARROW pad to enter your present time. Press

ENTER DATE. Use the ARROW pad to enter the date. Press ENTER.

Activating the Demo Mode

To turn on the Demo Mode, press MENU until SETUP is displayed at the top of the screen and press ENTER. Use the RIGHT ARROW until SETUP DEMO is displayed. To toggle between ON or OFF, press ENTER. The present status, on or off, will flash. Use the LEFT/RIGHT ARROWs to switch between on and off and press ENTER. While in the Demo Mode, the receiver displays sample information on the POSITION and both NAVIGATION screens.

Setting Display Contrast

To adjust the contrast of the display, press SETUP is displayed at the top of the screen and press ENTER. Use the LEFT/RIGHT ARROW until SETUP CONTRAST is displayed and press. Use the LEFT/RIGHT ARROW keys to change the contrast to the desired level and press ENTER.

Troubleshooting

Does not turn on: 1. Check to insure that the batteries are installed correctly and that the battery terminals are clean. 2. Replace the batteries. Takes more than 10 minutes to get a position fix: 1. If there are large obstacles nearby or overhead, move to a new location with a clear view of the sky and turn the receiver back on. 2. Make sure that the antenna is pointing up and that it is a reasonable distance from your body. 3. Check that the time is correct. If not, reset the time following the instructions for Changing Time Display and Time on page 28. 4. If the receiver still does not get a position fix within 10 minutes, you may wish to repeat the EZSTART initialization procedure found on page 29. Cannot view the second navigation screen: 1. The second navigation screen is displayed only if you have an active route or GOTO. Activate a route or GOTO and use the NAV key to scroll to the second navigation screen.
Destination Pointer does not point to the destination: 1. Note that much of the navigation information is based upon your movement. If you are standing still the navigation information (destination pointer, etc.) is not updated until you are moving. (The receiver is unable to detect which way you are facing while you are stationary.) Position coordinates on your receiver do not match the location on your map. 1. Make sure that your receiver is set up to use the same datum as your map. The map datum is generally shown in the map legend. See Map Datum under Setup for instructions on selecting the map datum in your receiver. 2. Check your LAT/LON format. Make sure that the format selected in COORDINATE SYSTEM (DEG/ MIN/SEC or DEG/MIN.MM) is in the same format as the map you are using.

Commonly Asked Questions

Does the receiver adjust itself for daylight savings time? No. You need to reset the time for changes in your area. (See Changing Time Display and Time on page 28.) Will my receiver function correctly in the year 2000? Absolutely. Even though only the last two digits of the year are displayed, the full year designator is stored in memory.

Why wont the receiver accept the coordinates higher than 59 seconds when I am inputting coordinates? The most common cause of this is you are trying to enter coordinates that are in degrees/minutes while your receiver is set to degrees/minutes/seconds. Since the last two digits in degrees/minutes is in hundredths (00 - 99) and degrees/ minutes/seconds can be no higher than 59 (00 - 59), inputting a number higher than 59 while in deg/min/sec results in an error and the receiver does not accept the entry. Can I use NiCad Batteries in my GPS Pioneer? Yes. However, the battery life of your GPS Pioneer will be diminished with the use of NiCad batteries. Can I attach my GPS Pioneer to external power? Yes. However, this requires the optional external power cable available from your dealer or Magellan Systems. Will I lose all my landmarks when my batteries die? No. As long as you leave the batteries inside the GPS Pioneer, memory will be retained for up to one month, even with dead batteries. (With good batteries installed, you can store your GPS Pioneer for six months without losing any memory.) When you remove batteries, you have 30 minutes to install new batteries before memory is lost. Why does my speed and elevation sometimes jump around? For security reasons, the U.S. Government introduces small errors (selective availability) which can affect positioning information. These errors are most noticeable while viewing speed, heading, and elevation.
Provides a visual indication of whether the receiver is locked or unlocked on satellite signals. While the tracking icon is displayed, the receiver is updating its position and can be used to save landmarks and as a navigation tool. If the tracking icon is not displayed, you may need to reposition the GPS receiver to get a better view of the sky. Battery Warning. When this icon first appears, the receiver will operate for about an additional hour before automatically turning off. The Magellan GPS Pioneer will retain its memory (route, landmarks, last fixes, etc.) for 20 minutes with the batteries removed. Memory will be retained even with low batteries for approximately one month if the unit is turned off. Light. Displayed when the LCD backlight has been turned key. The backlight will cause the batteries to on with the run down much quicker and should be turned off when not needed. External Power. Displayed when the GPS Pioneer is operating from external power using the GPS External Power Cable.

Contacting Magellan

If after using the troubleshooting section, you are still unable to solve your operation problem, please call Magellans Technical Service at 800-707-9971. Representatives are available Monday through Friday, from 7 a.m. to 5 p.m., Pacific Standard Time. Faxes can be sent to 909-394-7070. If necessary, you can also return your unit to Magellan for repair. (Please call for assistance first.) Ship the unit to Magellan Systems by Parcel Post or UPS and include a description of the problem, your name and address, and a copy of your sales receipt. If your return shipping address is different, please include it. With all correspondence, please be sure to state the model of the receiver you have and if calling, please be sure to have your unit with you. Packages should be sent to: Magellan Systems Corporation 960 Overland Court San Dimas, CA 91773 Attn.: Warranty/Repair Overseas customers may send units for repair to: COMAR Unit 3, Medina Court Arctic Road Cowles Isle of Wight P031 7XD U.K.

Accessories

Accessories for your Magellan GPS Pioneer are available from your Magellan dealer or you can order directly from Magellan using the order card supplied with your receiver. Carrying Case: Protects your GPS Pioneer from the elements and allows you to carry your GPS Pioneer on your belt, keeping it handy for when you need it. Mounting Bracket: Mounts on a dashboard or other surface allowing you hands-free operation of your GPS Pioneer. Allows the use of the External Power Cable while the receiver is resting in the bracket putting your GPS Pioneer where you want it and always ready to use. External Power Cable: Connects your GPS Pioneer to a cigarette lighter allowing uninterrupted use without any drain on your batteries. (Do not connect the GPS Pioneer to external power without the External Power Cable.) Instructional Video: A 30-minute instruction video in VHS format that provides you with instructions on how to use and operate your GPS Pioneer.

Glossary

Active Leg Bearing The segment of a route currently being used to compute navigational information. The compass direction from your position to a destination, measured to the nearest degree. A unique numeric or alphanumeric description of position. Refers to the theoretical mathematical model of the earths sea level surface. Map makers may use a different model from which to chart their maps, so position coordinates will differ from one datum to another. The datum for the map you are using can be found in the legend of the map. If you are unsure as which datum to use, use WGS84. Distance above mean sea level. A single leg route with the present position being the start of the route and a defined landmark as the destination. (If the unit has been moved while turned off and has not yet acquired a new position fix, the start of the GOTO will be the position fix last recorded.) The compass direction in which the Magellan GPS Pioneer is moving.

Coordinates Datum

Elevation GOTO

Heading

Landmark
A location saved in the units memory which is obtained by entering data, editing data, calculating data or saving a current position. Used to create routes. The angular distance north or south of the equator measured by lines encircling the earth parallel to the equator in degrees from 0 to 90. Coordinate system using latitude and longitude coordinates to define a position on the earth. A segment of a route that has a starting (FROM) landmark and a destination (TO) landmark. A route may consist of 1 or more legs. A route that is from landmark A to landmark B to landmark C to landmark D has three legs with the first being from landmark A to landmark B. The angular distance east or west of the prime meridian (Greenwich meridian) as measured by lines perpendicular to the parallels and converging at the poles from 0 to 180.

Latitude

LAT/LON

Leg (Route)

Longitude
Magnetic North The direction toward the north magnetic pole from the observers position. Position Fix Position coordinates as computed by the Magellan GPS Pioneer.
Time To Go (TTG) is the measurement of how long it will take you to arrive at your destination. TTG is based on how fast you are moving towards the destination and the distance remaining. The direction to the geographical North Pole from an observers position. The north direction on any geographical meridian. Universal Time, formerly referred to as Greenwich Mean Time (GMT). Universal Transverse Mercator (UTM) is the metric grid system used on most large and intermediate scale land topographic charts and maps. Cross Track Error (XTE) is the distance, left or right, of the desired courseline. The courseline is a straight line from your present position to your destination.

True North

UT UTM
A Antenna 2; reception 3; troubleshooting 31 B Batteries installing 3; life 25; NiCad 33; warning 34 Bearing/Distance 11; for a landmark 15 C Coordinate Systems changing 26 Contrast 30 Cross Track Error (XTE) 12, 39 Customer Service 35 D Date see Time and Date Datums see Map Datums Demo mode activating 30 Distance units 27 E Elevation viewing 25 F Function keys 2 G GOTO creating a route 13-14; deactivate 14 I Icons 34; see light Initializing 4-5; 29 L Landmark (LMK) creating 8; deleting 16; editing 16; for route 17; naming 8; saving 8; viewing 15 LAT/LON 7; display 9; options 26, troubleshooting 32 Light 34 M Map Datums changing 27 Multileg route 17; see also Route, creating
N Navigation screen Moving compass (without active route) 10; Steering (with active route) 11, 12 North Reference changing 28 O On/Off 4; deactivation 14; troubleshooting 31 P Position computing a position 5-6; screen 9; saving 7; troubleshooting 31 Position fixes saving 7; troubleshooting 31; see also Landmark R Route create 17-19; viewing 19-20; deactivating/activating 20; deleting 21, 24, appending to (adding a leg) 21; activating a leg 22; editing 23; see also GOTO
S Sat status 5 Satellite signals 3 T Time and Date view 25; changing 28; daylight savings 32; Time To Go (TTG) 12, 39 Tracking 6, 34 Troubleshooting 31 X XTE see Cross Track Error

Specifications Performance: Receiver: AllView 12 technology, tracks up to 12 satellites to compute and update position information. Warm - Approx. 35 seconds Cold - Approx 2.5 minutes 1 second continuous Position - 49 feet (15 meters) RMS (without Selective Availability) Velocity - 0.12 mph RMS steady state (without Selective Availability) Physical: Weight: Housing: Features: No. of Landmarks: 100 stored landmarks No. of Routes: Legs per route: Power: Source: 2 AA alkaline batteries or 3.3 VDC (2%) 100 mA at receiver or 9-16 VDC with Magellan External Power Cable Approximately 24 hours continuous operation maximum 7 ounces Waterproof Construction
Acquisition Times: Update Rate: Accuracy:

Battery Life:

This product has been certified by Mission HOME, the official educational campaign of the U.S. space community.
960 Overland Court, San Dimas, CA 91773

22-60125-000

N93-29589
TDA Progress Report 42-113 le (\ May 15, 1.993
Precise Tracking of the Magellan and Pioneer Venus Orbiters by Same-Beam InterferometryPart II: Orbit Determination Analysis
W. M. Folkner, J. S. Border, S. Nandi, and K. S. Zukor Tracking Systems and Applications Section
A new radio metric positioning technique has demonstrated improved orbit de' termination accuracy for the Magellan and Pioneer Venus Orbiter orbiters. The new technique, known as Same-Beam Interferometry (SBI), is applicable to the positioning of multiple planetary rovers, landers, and orbiters which may simultaneouly be observed in the same beamwidth of Earth-based radio antennas. Measurements of carrier phase are differenced between spacecraft and between receiving stations to determine the plane-of-sky components of the separation vector(s) between the spacecraft. The SBI measurements complement the information contained in lineof-sight Doppler measurements, leading to improved orbit determination accuracy. Orbit determination solutions have been obtained for a number of 48-hour data arcs using combinations of Doppler, differenced-Doppler, and SBI data acquired in the spring of 1991. Orbit determination accuracy is assessed by comparing orbit solutions from adjacent data arcs. The orbit solution differences are shown to agree with expected orbit determination uncertainties. The results from this demonstration show that the orbit determination accuracy for Magellan obtained by using Doppler plus SBI data is better than the accuracy achieved using Doppler plus differenced-Doppler by a factor of four and better than the accuracy achieved using only Doppler by a factor of eighteen. The orbit determination accuracy for Pioneer Venus Orbiter using Doppler plus SBI data is better than the accuracy using only Doppler data by 30 percent.

I. Introduction

Remote reconnaissance of planets in our solar system is conducted by NASA using unmanned space probes. A hyperbolic flyby of a planetary system may provide a few snapshots of geologic, atmospheric, and electromagnetic phenomena, which then reveal, through analyses, some Un22
derstanding of the underlying physical processes which are taking place. A spacecraft placed in orbit about a distant planet, on the other hand, will provide a much longer time history of measurements of various phenomena, leading to more comprehensive physical understandings. Navigation is one of the many critical engineering functions necessary to support the planning and operations of space flight
missions. This article presents results of a flight demonstration of a new technique for improving navigation for planetary orbiters. Radio antennas in the DSN provide communication links with distant spacecraft. Measurements of the microwave signal used for commanding the spacecraft and for relaying telemetry data from the spacecraft to Earth provide the basis for radio navigation. Any change in range between a Deep Space Station and a spacecraft affects the Doppler shift of the transmitted radio signal. Though many techniques, including ranging, radio interferometry, and onboard optical imaging, are used for interplanetary navigation, orbit determination for planetary orbiters has relied primarily upon Doppler data. The motion of an orbiter about a planet, induced by gravity, places a strong signature in the Doppler data received at Earth. Dynamic models allow the state of the orbiter relative to the central body to be estimated from a time history of the Doppler shift. The accuracy of navigation solutions and the ability to project the spacecraft trajectory forward may directly impact the quality of the science return. Pointing, scheduling, and configuration of onboard instruments rely upon predictions of the spacecraft trajectory. Interpretation and registration of images and other measurements rely upon reconstruction of the spacecraft trajectory. Determination of harmonic coefficients of the planet's gravity field depends directly on the orbit determination accuracy. Improvements to navigation, such as reducing the volume of tracking time necessary to maintain a specified level of orbital accuracy, predicting a trajectory further ahead within a specified error tolerance, or improving the accuracy of the final reconstructed trajectory solution, can simplify operations and enhance the science return.
In 1991 the Pioneer Venus Orbiter (PVO) spacecraft, launched in 1978, was in a highly eccentric orbit about Venus with a period of about 24 hours. The Magellan spacecraft joined PVO in orbit around Venus on August 10, 1990. During 1991, Magellan was in a less eccentric orbit with a period of about 3.26 hours. Same-Beam Interferometry (SBI) data sets were acquired in February and April 1991. Orbit determination solutions from these data sets have been obtained using various combinations of Doppler, differenced-Doppler, and SBI data. Formal errors associated with the solutions and solution comparisons for adjacent data arcs are examined to assess orbit determination accuracy. An overview of the simultaneous tracking technique is presented below, followed by discussions of the data scheduling, orbit determination strategy, and orbit determination results.

II. Radio Metric Measurements
Three types of radio metric measurements were included in this demonstration: two-way Doppler, differenced-Doppler, and SBI. Two-way Doppler is collected for a single spacecraft from a single Deep Space Station. Differenced-Doppler is collected by two widely separated Deep Space Stations for a single spacecraft. SBI is collected for two spacecraft simultaneously at two widely separated Deep Space Stations. The DSN Deep Space Stations used are located in California, Australia, and Spain.
Two-way Doppler (referred to below as Doppler) is collected when the Deep Space Station sends a stable carrier signal to the spacecraft and the spacecraft replies with a signal phase-locked to the uplinked signal. The frequency shift of the signal received by the Deep Space Station compared to the transmitted signal provides a measure of the rate of change of range to the spacecraft. The DSN currently tracks planetary orbiters at either S-band (2.3 GHz) For a short-period (2-24 hr) planetary orbiter, the or at X-band (8.4 GHz). During the time of interest for orientation of the orbit plane is the trajectory compothis demonstration, most of the Magellan Doppler data nent least well determined by line-of-sight Doppler meaacquired were derived from the station transmitting and surements. Doppler data acquired simultaneously at two receiving signals at X-band while PVO data were derived widely spaced DSN stations, and then differenced, profrom a station transmitting and receiving a signal at 5vide sensitivity to the orientation of the orbit plane [1]. band. For Doppler at S-band, the intrinsic data accuracy Differenced-Doppler has been used operationally during is limited by solar charged particle fluctuations to about the orbit phase of the Magellan mission to help meet strin1.0 mm/sec for the inferred range-rate for a 60-sec averaggent navigation requirements [2,3]. For the case when two ing time. For data taken at X-band, the accuracy is also spacecraft are in orbit about the same planet, an observable formed from Doppler measurements, differenced be- - limited by solar charged particle fluctuations, but at a reduced level. The X-band Doppler intrinsic data accuracy tween stations and differenced between spacecraft, is exis typically about 0.1 mm/sec for the inferred range-rate pected to provide further improvements to navigation [4]. for a 60-sec averaging time. A demonstration of this technique using the Magellan and Pioneer Venus orbiters at Venus took place in the spring Differenced-Doppler data are collected when the spaceof 1991. A detailed discussion of the data acquisition and craft carrier signal is measured at two Deep Space Stations. measurement error analysis has been given earlier [5].

The difference in the received carrier frequencies provides a measure of the difference in the range-rate from each Deep Space Station to the spacecraft. One component of the spacecraft velocity in the plane normal to the line-of-sight (plane of the sky) is inferred from this difference in lineof-sight range-rate, namely the component in the direction of the vector separating the two Deep Space Stations projected onto the plane of the sky. The intrinsic accuracy of X-band differenced-Doppler is typically 0.05 mm/sec for the inferred differenced line-of-sight range-rate for a 60-sec averaging time. Accuracy is improved relative to Doppler because station differencing reduces the effect of solar plasma fluctuations by removing fluctuations common to the two downlink ray paths. The accuracy with which the plane of sky velocity component is inferred is approximately the differenced range-rate accuracy times the ratio of the Earthspacecraft distance to the distance between the two Deep Space Stations. The SBI measurement of two spacecraft is depicted in Fig. 1. The two spacecraft in orbit about the same planet are so close angularly, as seen from Earth, that they may be observed in the same beamwidth of an Earth-based radio antenna. Each spacecraft carrier signal phase is recorded by two widely separated Deep Space Stations. Differencing the received carrier phases, first between stations and then between spacecraft, gives a measure of the separation of the two spacecraft in the plane of the sky along the projected baseline. The phase difference can be ambiguous by an integer number of cycles; the ambiguity must be resolved by a priori information (such as a sufficiently precise Doppler-only orbit) or by estimating a phase bias parameter for each SBI data arc. SB! data were taken at S-band for this orbit determination demonstration since PVO was tracked at S-band and Magellan was transmitting low-rate data at S-band in addition to the primary X-band signal. The SBI data accuracy corresponded to a doubly-differenced range accuracy of 1.5 mm for 5-mm integration times [5]. From this doubly-differenced phase the separation of the two spacecraft in the plane of the sky can be inferred with an angular accuracy of 180 prad for a baseline length of 8000 km (which is an average length of the separation vector between antennas from different DSN complexes projected onto the plane normal to the Earthspacecraft direction). At a distance of 1.5 astronomical units (AU's), the SBI data accuracy corresponds to a spacecraft-separation measurement accuracy of 40 m. X-band data are expected to be more accurate by an order of magnitude due to reduced sensitivity to solar charged particle fluctuations. All of the radio signals are affected by delays due to Earth ionosphere and troposphere as well as delays due to solar plasma. Calibrations for the troposphere were ap24

plied based on a seasonal model [6]. Calibrations for the Earth's ionosphere were applied based on daily measure- ments from Earth-orbiting beacon satellites [7].

Ill. Estimation Models

The spacecraft trajectory was integrated from initial position and velocity conditions (epoch state) using models for the dynamic forces on the spacecraft. The largest force was due to the gravitational field of Venus, which was modeled as a point mass (gravitational mass [GM]) term and potential field composed of spherical harmonic terms to degree and order 21 estimated from several years of radio metric data for PVO and Magellan [8]. Other significant forces were due to solar pressure, the solar point mass perturbation and, for Magellan, atmospheric drag and momentum wheel desaturation thrusts which occur twice daily. The right ascension and declination of the Venus spin axis and the rotation period were derived from Magellan radar images of surface features [9]. The rotation angle of Venus about the spin axis at a reference epoch was estimated for this demonstration from a 10-day arc of PVO and Magellan Doppler data. The Deep Space Station locations were mapped from Earth-fixed locations to inertial space using models for precession, nutation, and solid Earth tides, and calibrations for polar motion and length of day variations. Computed values for measurements were derived from nominal values for the spacecraft epoch state, force models, and inertial Deep Space Station locations. A least-squares fit to the observations minus the computed measurements was made to estimate model parameters. For this demonstration, gravity field parameters were not adjusted since, for short data arcs, epoch state errors can be aliased into gravity field parameters. The uncertainties in the spacecraft trajectory caused by imperfect unadjusted model parameters were included through the use of consider analysis [10]. The derived uncertainty in the trajectories depends on the formal error covariance for the solved-for parameters (computed) and on the uncertainty assumed a priori of the unadjusted (considered) parameters. For short data arcs, the spacecraft trajectory uncertainty is usually dominated by the uncertainty in the unadjusted gravity field. Because of this, the optimal orbit determination solution may not be achieved by weighting all of the data at its intrinsic accuracy since the estimated epoch state is derived by neglecting the considered parameters. By neglecting the gravity field, the estimation filter will produce a solution based on an over-optimistic estimate for the spacecraft plane-of-sky velocity based on the Doppler data. Without taking this into account in some

manner the differential data types may not fully influence the solution. The effect of mismodeling can be reduced by deweighting the Doppler data and including differential data, weighted at its intrinsic accuracy, in the estimation. This strategy has been used in the operational navigation for Magellan [2,3].

IV. Data Arcs

During the spring of 1991, Magellan was conducting radar mapping operations. Magellan typically performed radar mapping for one hour of each orbit, during which there was no signal transmitted to Earth. Radio metric data could be collected for Magellan during the two hours of telemetry playback each orbit. For the same time period, PVO was in an orbit with a period of 24 hours. Sband Doppler data from PVO were collected for approximately 6 hours per day centered roughly about periapsis, which occurred during the CaliforniaAustralia visibility period. SBI data were acquired over an eight-day period beginning February 16, 1991, and over a ten-day period beginning April 6, 1991. Figures 2 and 3 show the orbits of the two spacecraft as viewed from Earth for these two time periods. Nominal orbital elements for the spacecraft are given with respect to the plane of the sky in Tables 1 and 2. For the SBI demonstration, data could be acquired only when both Magellan and PVO were transmitting to Earth and when stations were allocated at two DSN complexes. Because Magellan used differenced-Doppler operationally, stations from different DSN complexes were scheduled to simultaneously track Magellan for about five hours during each 48-hour period. SBI data could then be acquired at those stations, on a non-interference basis, when PVO was also transmitting. This scheduling resulted in an average of two hours of SBI data acquired every other day. One SBI bias parameter was needed for each hour of SBI data since the Magellan signal was interrupted by either a mapping cycle or an attitude calibration after each hour of telemetry. Because of the sparseness of the SBI and PVO Doppler data, 48-hour non-overlapping data arcs were chosen for orbit determination solutions. A typical data schedule for a 48-hour data are is shown in Fig. 4. Each of the data arcs contained approximately 13 orbits of Doppler data from Magellan, about 5 hours of differenced-Doppler for Magellan, and Doppler from two PVO orbits, each with about 6 hours of data centered about periapsis. Four two-day data arcs were formed for the period February 14 to February 22, 1991, as summarized in Table 3. Since no Doppler data were collected from PVO for
the orbit beginning on February 14, 1991, Doppler data from the previous-orbit were included to allow each solution to contain data from two PVO orbits. PVO Doppler data within 1 hour of periapsis were excluded to reduce sensitivity to gravity field mismodeling. SBI data were acquired on the CaliforniaAustralia baseline near the time of PVO periapsis. Five data arcs were formed from data acquired from April 6 to April 16, 1991, as summarized in Table 4. Most of the SBI data acquired in April were during the CaliforniaAustralia overlap period with some data also acquired from the CaliforniaSpain baseline.

V. Orbit Determination Strategy
For this demonstration, orbit solutions for each spacecraft were formed for each data arc using different combinations of data. Three combinations of data were used for Magellan: Doppler only, Doppler plus differenced-Doppler, and Doppler plus SBI. Solutions for PVO were formed using only Doppler data and using Doppler plus SBI data. Orbit determination accuracy was assessed by comparing the orbit solutions for adjacent data arcs. To do this, the orbit solution from each data arc was propagated forward to the first orbit in the succeeding data arc and differenced with the succeeding solution trajectory. This solution-tosolution consistency provides one measure of orbit determination accuracy for post-fit data analysis. Orbit prediction, while of interest for mission operations, is not addressed here because neither experiment (February 1991 or April 1991) was long enough to provide more than one or two orbit prediction comparisons for prediction times of approximately one week (which is the typical period of interest). The quantities estimated for each data arc were six epoch-state parameters for each spacecraft, an atmospheric drag coefficient for Magellan, and phase biases for the SBI data. The epoch for each spacecraft was chosen to be an apoapsis near the beginning of the 48-hour data arc. A priori uncertainties for the estimated state and phase bias parameters were very large so as not to significantly constrain the solution. The a priori uncertainty for the Magellan atmospheric drag was 100 percent of its nominal value; this uncertainty is consistent with variations in the Venus atmospheric density above 100 km [11]. (Because the gravity field is mismodeled and no gravity field corrections were estimated, the estimated atmospheric drag tended to absorb gravity field mismodeling and hence not represent the actual atmospheric drag. This estimation of atmospheric drag is used here to allow comparison with other Magellan orbit determination solutions [2,3].) The data weights used in the solutions varied depending on which combinations of data were used. The intrinsic
accuracies of the data for a 60-sec sampling time were assumed to be 0.1 mm/sec for the Magellan X-band Doppler data, 1 mm/sec for the S-band PVO Doppler data, and 0.05 mm/sec for the Magellan X-band differenced-Doppler data. The accuracy of the SBI data was 1.5 mm for 5-mm averaging times. When fitting only Doppler data for Magellan and PVO, the Doppler data were weighted at their intrinsic accuracy. When fitting Magellan using Doppler and differenced-Doppler data, first the Doppler data were fit. Next, the differenced-Doppler data were included, weighted at their intrinsic accuracy, and the Doppler data deweighted by an increasing factor until the root-meansquare Doppler residual increased by 10 percent over the Doppler-only case. The typical deweighting factor for the Doppler data was 10-20. This empirically derived procedure allowed the differenced-Doppler data to influence the solution without unduly weakening the Doppler data [2,3]. When fitting Doppler and SBI data for Magellan and PVO, the SBI and PVO Doppler data were weighted at their intrinsic accuracies while it was found necessary to deweight the Magellan Doppler data by a factor of 100 to allow the post-fit SBI residuals to be minimized. The orbit solutions for Magellan using Doppler plus differenced- Dopplerdata were similar to the operational orbit solutions. Operational orbit determination is performed using data arcs covering twelve orbits, using Xband Doppler and differenced-Doppler data. Consecutive operational Magellan orbit solutions use overlapping data arcs with four orbits of data in common between solutions. This has provided the sub-kilometer solution-to-solution consistency needed by the radar mapping instrument [2,3].

the considered gravity field uncertainty. The uncertainty in Venus' GM was similarly taken to be a value which gave approximately the observed variation in determination of the Magellan and PVO semi-major axes from solutions using only Doppler data. These assumptions for the gravity field uncertainty were adopted only to give an appropriate spacecraft trajectory uncertainty for solution-to-solution differences for this demonstration. Solar pressure forces were considered with an uncertainty of 10 percent of their nominal value. The zenith ionosphere uncertainty was taken to be 1017electrons/m2 which is a typical uncertainty in the daily ionosphere calibration [7]. The zenith troposphere uncertainty was taken to correspond to 4 cm of path delay due to observed variation in water vapor content compared with the seasonal model employed.' Small thruster firings occurred twice daily for Magellan to desaturate momentum wheels used to control the spacecraft attitude. The effect of these thruster firings on the spacecraft trajectory was modeled as an impulsive maneuver. The magnitude of the velocity imparted to the spacecraft from each maneuver was typically - 3 mm/sec. Calibrations for the thruster firings are provided on the spacecraft telemetry from which the magnitude of the maneuver can be determined to a few percent [13]. The uncertainty in each maneuver was assumed to be 0.1 mm/sec. Uncertainties in station frequency and timing standards affect station-differenced data types more strongly than single-station Doppler data. The uncertainty in station frequency calibrations was important for solutions containing differenced-Doppler data. The station frequency calibration uncertainty was assumed to be 5 x 10-14 sec/sec [14]. Uncertainties in station clock epoch were important for solutions containing SBI data. The effect of an unknown offset in the time-tags for the SBI data at the two Earth receivers is discussed in [5]. For analysis of the SBI data, nominal values for the difference in station clocks between the DSN stations were taken from Very Long Baseline Interferometry measurements made routinely for maintaining knowledge of Earth orientation.' The uncertainty of this determination of the station-differenced clock epoch uncertainty was 0.2 /sec.
VI. Orbit Determination Covariance
In addition to comparing successive orbit solutions to measure orbit determination accuracy, the solution-tosolution differences are compared below to a nominal orbit covariance. This orbit covariance was formed using a priori uncertainties for a number of consider parameters. Table 5 lists the a priori uncertainties assumed. The gravity field uncertainty was a major error source for all solutions and dominated the orbit uncertainty for solutions using only Doppler data. Due to computational limitations, the considered gravity field covariance was of degree and order.6 rather than the covariance of the field of degree and order 21. A diagonal covariance of degree and order 6 was taken from a previous gravity field determination [12] scaled by an empirically determined factor of 1.5. With this assumed gravity field uncertainty, the observed solution-to-solution variations for Magellan solutions using only Doppler data approximately agreed with

S. E. Robinson, "Errors in Surface Model Estimates of Zenith Wet Path Delays Near DSN Stations," JPL Interoffice Memorandum 335.4-594 (internal document), Jet Propulsion Laboratory, Pasadena, California, September 3, 1986. H. Oliveau, L. Sung, and J. A. Steppe, "TEMPO Group Clock Synchronization and Syntonization Report from the DSN VLBI Mark 1V-85 System," JPL Engineering Memorandum 335-192 (internal document), Jet Propulsion Laboratory, Pasadena, California, February 25, 1991.
Figures 5 and 6 show the root-sum-square (rss) position covariance for Magellan and PVO at apoapsis using several combinations of data types. The rss position uncertainty is usually largest at apoapsis due to the fact that the uncertainty in determination of the longitude of the orbit ascending node with respect to the plane of the sky dominates the uncertainty in spacecraft position determination. The position uncertainty for both spacecraft when only Doppler data are included is dominated by the considered gravity field uncertainty. This is in contrast to the case for PVO when using a one-day data arc where the data noise dominated the position uncertainty and the position determination uncertainty was much larger [15]. The position determination uncertainty for Magellan using Doppler plus differenced-Doppler data contains nearly equal contributions from data noise, troposphere, clock rate, and gravity field uncertainties. The position uncertainties for solutions containing SBI data are dominated by gravity field uncertainty but at a reduced level. With the 48-hour Doppler data arc the position improve- ment for PVO when SBI data are included is much less than if a one-day data arc is used [15]. These orbit determination covariances are nominal only for this demonstration period, especially for data types other than SBI. No attempt has been made to optimize orbit determination performance by altering data scheduling, elevation cutoff, or different data weighting algorithms [16]. The orbit covariances are used primarily to check that the observed solution-to-solution differences are understood in terms of known mismodeled parameters.

adjacent data arcs using Doppler plus differenced-Doppler data. The Magellan mission requirement is for adjacent solutions to differ by less than -.'1.4 km (0.15 km radial, 1 km cross-track, and 1 km down track) over the mapping period of the orbit, which is approximately the central 1-hour period shown in Figs. 7, 8, and 93 It can be seen that Doppler data alone would not satisfy the mission requirements. The Doppler plus differenced-Doppler solutions generally satisfy the mission requirements (and could be improved by using overlapping data arcs as is done operationally). Note that the trajectory differences for the Doppler plus differenced-Doppler solutions are consistently less than the value expected from the covariance analysis. This implies that considering a constant zenith troposphere uncertainty of 4 cm overestimates the effect on the trajectory, possibly because the deviations from the calibrations for the two sample time periods were smaller than normal. Figure 9 shows the orbit solution differences for Magellan solutions using Doppler plus SBI data. The Doppler plus SBI solutions are seen to be significantly better than the Doppler-only or Doppler plus differenced-Doppler so- lutions as expected. This is true even though the SBI data were acquired at S-band while the differenced-Doppler data were acquired at X-band. SBI data acquired at Xband are expected to be more accurate by about one order of magnitude [5]. Figure 10 shows the rss position differences and the expected position difference for PVO orbit solutions from adjacent data arcs using only Doppler data. Figure 11 shows the orbit solution differences for PVO solutions using Doppler plus SBI data. The PVO Doppler-only solution differences are much smaller than the Magellan solution differences for solutions with either Doppler-only or Doppler plus differenced-Doppler. This is due to the PVO periapsis altitude being much higher than Magellan's periapsis altitude, which makes the PVO orbit determination much less sensitive to gravity field mismodeling. The addition of SBI data only slightly improves the PVO orbit solutions because, with a two-day data arc and low sensitivity to gravity field mismodeling, the Doppler data determine the longitude of the ascending node of PVO's orbit with accuracy comparable to the SBI data. Table 6 lists the time-averaged orbit position difference and an overall average for Magellan and PVO for each combination of data types studied. This figure of merit is introduced to quantitatively compare the orbit determination performance. Using the seven solution-to-solution
S. N. Mohan, Magellan Navigation Plan, JPL Document 630-51, Rev. B (internal document), Jet Propulsion Laboratory, Pasadena, California, March 23, 1988. 27
VII. Orbit Determination Results
Figure 7 shows the rss position differences between solutions for Magellan from adjacent data arcs using only Doppler data, plotted over one orbit. The expected differences are also shown. The expected position difference curves are derived by assuming each solution is an independent sample from a distribution characterized by the formal covariance given in the previous section. Thus the expected difference between two solutions that use similar data schedules is just the position uncertainty for either solution times the square root of two. The Doppler-only solution statistics are dominated by the (considered) gravity field uncertainty. The gravity field uncertainty was determined in such a way as to get approximate agreement between the expected position difference and the actual solution differences for Doppler-only solutions for Magellan. Figure 8 shows the rss position differences and the expected position difference for Magellan orbit solutions from

comparisons possible for this demonstration, Table 6 indicates that orbit solutions using SBI data are significantly more accurate for Magellan and slightly more accurate for PVO. The quantitative ratios will, in general, depend on orbit geometry, data arcs, and estimation strategy.

VIII. Conclusion

Orbit determination results have been obtained for Pioneer Venus Orbiter and Magellan using same-beam interferometry data, which is a new data type for plane-
tary orbiter navigation. The orbit determination accuracy using this data type, based on solution-to-solution consistency, has been explained in terms of nominal error models. For the particular orbit determination strategy and observational geometry used for this limited data set, the orbit determination accuracy for Magellan using SBI in combination with two-way Doppler data is better by a factor of four than orbit determination accuracy using two-way Doppler plus differenced-Doppler data and better by a factor of eighteen than orbit determination accuracy using Doppler alone. This new data type should find much application in the future as more missions with multiple orbiters and/or landers are flown to Mars.

Acknowledgments

The authors thank Doug Engelhardt of the Magellan navigation team and Neil Mottinger of the Pioneer Venus Orbiter navigation team for their assistance in spacecraft modeling.

References

[1] S. R. Poole, M. P. Ananda, and C. E. Hildebrand, "Radio Interferometric Measurements for Accurate Planetary Orbiter Navigation," in AAS Advances in the Astronautical Sciences, vol. 40, part 1, pp. 93-111, San Diego, California: Univelt Inc., 1980. [2] D. B. Engelhardt, J. B. McNamee, S. K. Wong, F. G. Bonneau, E. J. Graat, R. J. Haw, G. R. Kronschnabl, and M. S. Ryne, "Determination and Prediction of Magellan's Orbit," paper AAS-91-180, presented at the AAS/AIAA Spaceflight Mechanics Meeting, Houston, Texas, February 11-13, 1991. [3] J. D. Giorgini, E. J. Graat, T.-H. You, M. S. Ryne, S. K. Wong, and J. B. McNamee, "Magellan Navigation Using X-Band Differenced Doppler During Venus Mapping Phase," paper AIAA-92-4521, presented at the AIAA/AAS Astrodynamics Conference, Hilton Head, South Carolina, August 10-12, 1992. [4] W. M. Folkner and J. S. Border, "OrbiterOrbiter and OrbiterLander Tracking

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[6] C. C. Chao, The Troposphere Calibration Model for Mariner Mars 1971, JPL Technical Report 32-1587, Jet Propulsion Laboratory, Pasadena, California, pp. 61-76, March 1974. [7] H. N. Royden, D. W. Green, and G. R. Walson, "Use of Faraday-Rotation Data from Beacon Satellites to Determine Ionospheric Corrections for Interplanetary Spacecraft Navigation," Proceedings of the Satellite Beacon Symposium, edited by A. W. Wernik, Warszawa, Poland, pp. 345-355, May 1980. [8] J. B. McNamee, G. R. Kronschnabl, S. K. Wong, and J. E. Ekelund, "A Gravity Field to Support Magellan Navigation and Science," J. Astron. Sci., vol. 40, pp. 107-134, 1992. [9] M. E. Davies, T. R. Colvin, P. G. Rogers, P. W. Chodas, W. L. Sjogren, E. L. Akim, V. A. Stepanyantz, Z. P. Vlasova, and A. I. Zakharov, "The Rotation Period, Direction of the North Pole, and Geodetic Control Network of Venus," J. Geophys. Res., vol. 97, pp. 13,141-13,151, 1992.
[10] G. J. Bierman, Factorization Methods for Discrete Sequential Estimation, San
Diego, California: Academic Press, 1977. [11] G. M. Keating, J. L. Bertaux, S. W. Bougher, T. E. Cravens, R. E. Dickinson, A. E. Hedin, V. A. Krasnopolsky, A. F. Nagy, J. Y. Nicholson III, L. J. Paxton, and U. von Zahn, "Model of Venus Neutral Upper Atmosphere: Structure and Composition," Adv. Space lies., vol. 5, pp. 117-171, 1985. [12] N. A. Mottinger, W. L. Sjogren, and B. G. Bills, "Venus Gravity: A Harmonic Analysis and Geophysical Implications," J. Geophys. Res., vol. 90, supplement, pp. C739C756, February 15, 1985. [13] D. B. Engelhardt and S. N. Mohan, "Deterministic Errors in the Magellan Orbit Due to Attitude Control Thruster Activity," paper AIAA-89-0349, presented at the 27th Aerospace Sciences Meeting, Reno, Nevada, January 9-12, 1989. [14] P. A. Clements, A. Kirk, and R. Unglaub, "Results of Using the Global Positioning System to Maintain the Time and Frequency Synchronization in the

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Fig. 8. Magellan solution-to-solution trajectory differences using Doppler plus differenced-Doppler data. The dashed curves are for solutions In February 1991. The dotted curves are for solutions In April 1991. The dark solid curve Is the expected trajectory difference based on the covariance analysis.
Fig. 9. Magellan solution-to-solution trajectory differences using Doppler plus SBI data. The dashed curves are for solutions in February 1991. The dotted curves are for solutions in April 1991. The dark solid curve Is the expected trajectory difference based on the covarlance analysis.

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Fig. 10. PVO solution-to-solution trajectory differences using only Doppler data. The dashed curves are for solutions in February 1991. The dotted curves are for solutions In April 1991. The dark solid curve Is the expected trajectory difference based on the covariance analysis.

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Fig. 11. PVO solution-to-solution trajectory differences using Doppler plus SBI data. The dashed curves are for solutions In February 1991. The dotted curves are for solutions In April 1991. The dark solid curve is the expected trajectory difference based on the covariance analysis.