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Gpredict is a free, real-time satellite tracking and orbit prediction application for Linux, Mac OS X, and Windows. It is designed to show where satellites are now and when they will pass over your location, so it suits anyone who wants to follow amateur radio satellites, weather satellites, or other tracked objects without paying for software. The official site at oz9aec.dk is currently under construction, so the software itself is best obtained from its established distribution channels rather than from that page.

What Gpredict does

Gpredict combines two related jobs:

• Real-time tracking — it continuously calculates and displays satellite positions as they move.
• Orbit prediction — it projects future positions so you can see upcoming passes over your ground station.

This makes it useful for planning observations, radio contacts, or antenna pointing, because you can look ahead instead of only watching the current position.

Supported platforms

Gpredict runs on the three major desktop platforms named by the project:

Because the project website is under construction, use your operating system's package manager or the project's established download locations to get the current version.

How to track satellites with Gpredict

1. Install the application

Install Gpredict using the method appropriate to your platform. On Linux this is typically a package manager; on Windows and Mac OS X it is a downloaded build. The expected result is that Gpredict launches and shows a default ground station and module layout.

2. Set your ground station

Your location determines every pass prediction. Enter your latitude, longitude, and altitude so Gpredict can calculate when satellites rise above your horizon. If your position is wrong, every pass time will be wrong.

3. Add satellites and update orbit data

Satellites are tracked using TLE (Two-Line Element) data, which describes their orbits. You need to:

• Add the satellites you care about to a module.
• Update the TLE data regularly, because orbits change over time.

Stale TLE data is the most common reason predictions drift or look wrong. Refresh it before relying on any pass.

4. Read the tracking and prediction views

Once configured, Gpredict provides:

• A map view showing satellite positions and ground tracks.
• Pass predictions listing when a satellite rises, reaches maximum elevation, and sets for your location.
• Tracking displays for following a selected satellite in real time.

Use the pass list to plan ahead, and the map and tracking views to follow the satellite as the pass happens.

Common setup issues

• Wrong pass times — almost always a ground station location error or outdated TLE data.
• Satellite missing from predictions — the satellite may not be added to the active module, or its TLE may not have been updated.
• No passes shown — check that your location is set and that the satellite's orbit actually passes over your latitude.

Where to get help

Since the official site is under construction, look for the project's documentation, user community, and distribution channels for installation help and troubleshooting. The application's own help resources and the amateur satellite community are the practical places to resolve setup problems.

weather.com is a free-to-access weather site from The Weather Channel that combines national and local forecasts, interactive radar, severe-weather alerts, and hurricane coverage in one place. It's most useful when you want a quick local answer (will it rain at 3 p.m.?) plus the bigger picture (is a storm system or hurricane heading your way). The site also offers a paid Premium subscription, so some features sit behind a login and payment — the free site covers forecasts, radar, and alerts, but you shouldn't assume everything on the page is free.

National vs. local forecasts: which one answers your question

These are two different products, and mixing them up is the main reason a forecast can feel "wrong."

A practical rule: start local for today's decisions, then check the national view if the local forecast shows something unusual (a big temperature swing, a named storm, a multi-day rain event) and you want to know why.

Why your local numbers can differ from the national map

Local forecasts are generated for a specific point or small area, while national maps smooth data across large regions. Terrain, coastlines, and urban heat islands create real differences that a national map can't show. If the national outlook says "rain likely" and your local hourly forecast says 20%, trust the local one for your address — but treat both as probabilities, not guarantees.

Using radar and hourly/daily forecasts to plan your day

The radar and the hourly forecast answer different questions, so use them together.

• Radar shows where precipitation is right now and where it's moving. Use it to judge timing: if a band is 30 minutes west of you and moving east, you have roughly half an hour.
• Hourly forecast shows expected conditions hour by hour, including precipitation chance and temperature. Use it to pick a window for outdoor plans.
• Daily forecast gives the multi-day picture — highs, lows, and precipitation chance per day. Use it for weekly planning.

A workable routine:

1. Open the local forecast for your city.
2. Check the hourly view for the next 6–12 hours to find your best window.
3. Switch to radar to confirm whether precipitation is actually approaching.
4. Check the daily view before committing to plans more than a day out.

Expected result: you get a timing decision (go now, wait an hour, move it indoors) rather than a vague "it might rain."

Common snag: radar shows what's happening, not what will happen. A gap on radar doesn't mean the rest of the day is dry, and a green blob over you doesn't always mean rain is reaching the ground.

Finding hurricane and severe-weather alerts — and reading them correctly

weather.com carries hurricane coverage and severe-weather alerts alongside its forecasts. When you're in or near a storm's path, the alert layer is the part to check first, because alerts are issued for specific areas and time windows rather than as general outlooks.

How to read them:

• Check the alert's area and timing, not just its headline. A warning for a county next to yours may not apply to you.
• Distinguish watches from warnings. A watch means conditions are possible; a warning means they're occurring or imminent. Treat a warning as the one that changes your plans.
• Cross-check the hurricane track against your location rather than relying on the cone's center line — the cone shows a range of possible paths, not a single predicted route.

If an alert and your local forecast seem to disagree, the alert is the more authoritative signal for safety decisions.

What the free site includes vs. what Premium adds

The site is built around free access to national and local forecasts, radar, reports, and hurricane coverage. It also promotes a Premium subscription through "Get Premium" / "Upgrade" links, which means at least some content or features are gated behind a paid tier and a login.

What that means in practice:

• Don't assume every feature you see is free — if a page pushes you toward Premium, that's the signal something is restricted.
• The specific features, price, and terms of Premium aren't detailed in the site's public description, so check the subscription page directly before paying.
• For most day-to-day needs (local forecast, radar, alerts), the free site is the starting point.

Recognizing forecast uncertainty

Every number on the site is a probability or a model output, not a fact. Two things follow from that:

• Precipitation chance is not intensity. A 40% chance of rain says nothing about how hard it will rain or for how long.
• Longer range means less certainty. A 10-day outlook is a trend, not a plan. The further out you look, the more you should treat the forecast as "likely direction" rather than "expected conditions."

The practical move: for anything within 24 hours, trust the hourly and radar views; for anything beyond a few days, use the daily and national views to spot trends, and re-check closer to the date.

Satellite tracking software predicts where a satellite is at any moment and when it will pass within range of your location, so you can point an antenna, schedule a contact, or just watch it cross the sky. The core inputs are simple: your observer coordinates and a current set of orbital data (TLEs). Gpredict is a free, real-time satellite tracking and orbit prediction program for Linux, Mac OS X, and Windows, and it is a useful concrete example of how these tools work. This explainer covers the mechanism, the setup steps, and the problems that most often break a tracking session.

What satellite tracking actually means

Tracking has two related meanings, and it helps to keep them apart:

• Position tracking — knowing where a satellite is right now, usually shown on a world map with its ground track and footprint (the area of Earth from which it is visible).
• Pass prediction — knowing when that satellite will rise above your local horizon, how high it will get (maximum elevation), and when it will set.

For radio work, a third layer matters: Doppler shift. As a satellite moves toward you and then away, the frequency you hear drifts. Tracking software calculates this offset so you can retune during a pass.

Why orbit data (TLEs) drives everything

Tracking software does not measure the satellite directly. It propagates an orbital model forward in time. The standard input is a TLE — a Two-Line Element set, a compact text description of a satellite's orbit at a given epoch.

Key consequences:

• TLEs go stale. Atmospheric drag and other forces make the real orbit drift from the prediction. A TLE a few days old may be fine for a rough look; for precise antenna pointing or Doppler tuning, fresher data is better.
• You need a current source. Most trackers can download TLEs automatically from a catalog such as Celestrak or Space-Track. The update interval is a setting worth checking.
• The epoch matters. If your software is using an old element set, your pass times will be off even if your clock and coordinates are perfect.

Core features to expect

Setting up a tracker like Gpredict

The general workflow is the same across most tracking programs:

1. Set your observer location. Enter latitude, longitude, and altitude. This is the single most common source of wrong results — a sign error or a swapped lat/long puts you on the other side of the planet.
2. Check your clock and time zone. Predictions are time-sensitive. Make sure the system clock is accurate and the time zone is correct, especially if you compare pass times with other people.
3. Load or download TLEs. Add the satellites you want and point the software at a current TLE source, or import a file you downloaded.
4. Refresh the elements. Set an update interval, or manually re-download before an important session.
5. Read the pass list. Look at start time, maximum elevation, and duration. Higher maximum elevation generally means a better, longer contact.
6. Optionally connect hardware. If you have a rotator or radio, configure the interface so the software can point the antenna and apply Doppler correction.

Expected result: a list of upcoming passes for your location, and a map showing the satellite's current position and footprint.

Common problems and how to spot them

• Stale TLEs — pass times drift by minutes. Symptom: the satellite is not where the software says. Fix: refresh the elements.
• Wrong observer coordinates — every pass looks wrong or the satellite never rises. Symptom: predictions that make no sense for your area. Fix: re-check latitude, longitude, and sign conventions.
• Time-zone or clock errors — pass times are offset by whole hours. Symptom: consistent shift between your software and a reference. Fix: verify system time and zone.
• Confusing UTC with local time — most tracking tools display UTC. Symptom: you show up an hour or more early or late. Fix: convert deliberately.
• Elevation mask too high or low — you either miss low passes or get unusable ones. Fix: adjust the minimum elevation setting to match your horizon and antenna.

A note on the source

The site at oz9aec.dk, which describes Gpredict as free, real-time satellite tracking and orbit prediction software for Linux, Mac OS X, and Windows, currently shows only an "under construction" page. Treat the description as the project's stated purpose rather than a live download or documentation source; for current files and instructions, look for the project's own distribution channel.

GPS (Global Positioning System) is a satellite-based navigation system that lets a receiver determine its position on Earth by timing radio signals from orbiting satellites. It is the positioning layer underneath map apps, fitness trackers, and photo geotagging — not the apps themselves. You can use this explanation to understand why your phone knows where you are, why it sometimes doesn't, and what "geotagged" means for a photo.

GPS vs. the things built on top of it

These terms get used interchangeably, but they describe different layers:

A map app needs GPS (or another positioning source) to know where you are. A geotagged photo needs a device that recorded a position at the moment of capture. Neither is GPS itself.

The three parts of the system

GPS works as a coordinated whole, not as a single device:

• Satellites — a constellation orbiting Earth, each broadcasting a signal that includes its position and the time the signal was sent.
• Ground control — a network of monitoring and control stations that track the satellites, correct their clocks and orbital data, and upload updated information back to them.
• Receivers — the chips in phones, watches, cameras, and car units. A receiver only listens; it does not transmit. That is why GPS works without a data connection, though map apps often need one to download map tiles.

How a receiver calculates your position

The core idea is timing. A radio signal travels at a known speed, so the delay between when a satellite sent a signal and when the receiver got it reveals the distance to that satellite.

1. The receiver picks up signals from multiple satellites.
2. For each satellite, it computes a distance from the signal's travel time.
3. Knowing the distance to several satellites narrows the possible location to a single point.
4. With signals from at least four satellites, the receiver can also solve for its own clock error, which is necessary because receiver clocks are far less precise than the atomic clocks on the satellites.

More satellites and better signal geometry generally mean a more stable fix. This is why a position can drift or jump when you are surrounded by tall buildings.

What people commonly use it for

• Navigation — routing while driving, walking, or cycling.
• Location sharing — showing your position to friends or family in a messaging or map app.
• Fitness tracking — recording distance, pace, and route for runs and rides.
• Photo geotagging — writing coordinates into a photo's metadata so it can later be placed on a map. Sites that display photos on an interactive world map rely on this kind of data to position each image.

Limitations worth knowing

• Indoors and underground — satellite signals are weak and easily blocked by roofs, walls, and earth, so accuracy drops sharply or fails entirely.
• Urban canyons — tall buildings reflect signals, causing errors and position jumps.
• Hardware dependence — accuracy depends on the receiver's chip and antenna, not just on the satellites. A better receiver in an open area will outperform a weak one in a difficult location.
• It is a position, not a map — GPS gives coordinates. Turning those into a usable route, address, or map pin is the job of separate software and data.

If you only remember one thing: GPS is a timing-based positioning system, and everything from turn-by-turn directions to a photo pinned on a world map is an application built on top of that position fix.